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When a winding fault is detected from the motor control center or disconnect using MCA, a test must be performed at the motor, as motor circuit analysis measurements of Test Value Static, phase angle and current/frequency response, & insulation to ground can detect cable faults as well.

If the winding tests good at the motor, then the cable has a fault; If the test improves but a fault still shows, it is both a cable and winding fault; and if the test shows the same results at the motor, the fault is in the stator windings.

ALL-TEST Pro electric motor testing instruments testing in place and via bench testing

The multi-technology approach to motor diagnostics means you are utilizing different testing technologies that will complement and validate each other. One example is your vibration technician suspects a possible rotor problem in a critical application, but the cost to replace means a shutdown of production, where the motor’s cost is small compared to the incurred costs of the
shutdown.

In a situation like this, many people would be reluctant to make the call for replacement, for if the diagnosis is wrong, the cost is very high. Therefore, this motor may be run to failure, due to the uncertainty of the diagnosis. In this case, to put the multi-technology approach in practice, use Electrical Signature Analysis (energized testing) to confirm or rule out the preliminary findings (bad rotor). If the shaft of the installed motor can be turned or the load quickly disconnected, then a Motor Circuit Analysis test (deenergized) can be performed to assess the condition of the rotor, stator, and connections. By utilizing the multi-technology approach you will have more confidence in your findings and hence, a greater degree of certainty that you have determined the real fault(s).

Multi-Technology Approach to Motor Diagnostics

Winding insulation degrades over time. MCA™ (Motor Circuit Analysis) detects these developing faults very early.Early detection of these faults allows for corrective action before they become catastrophic and result in a major rebuild or replacement.

These internal winding faults are the beginning of the end for most motors.Using Motor Circuit Analysis (MCA™) can help identify these types of internal winding faults. MCA™ is a deengerized test method and the test can be initiated from the Motor Control Center (MCC) or directly at the motor.

tech-tip-8-2017 Winding Failures in Motors

To a technician evaluating a motor, a meg-ohmmeter is like a Doctor’s blood pressure cuff. It is a measurement you have to make. It provides important information, and when it’s bad, it’s bad. But it is a one dimensional test, evaluating only the integrity of the insulation system to ground. By itself, it does not provide enough information to diagnose overall motor health. A high meg-ohm reading does not rule out motor electrical problems any more than a normal blood pressure reading rules out serious illness.

In addition, a meg-ohm test will miss inter-turn faults in the windings, it will miss poor connections, it can miss an open phase, and is totally unaffected by rotor problems. So should you measure insulation resistance? Of course, but recognize that much more information is needed to assess motor electrical health. Combining insulation resistance with other AC based tests such as Motor Circuit Analysis can give you a complete picture of motor electrical health,whether for troubleshooting or condition monitoring.

tech-tip-1-2019 MCA

For AC Induction motors, winding failures can start and end as turn and coil shorts that do not break through the ground-wall insulation, regardless of the root-cause of the failure.Therefore, if you are only performing an insulation to ground test then you will miss these types of faults. Insulation to ground tests only detect resistive paths between the stator core and the conductors adjacent to the stator core.

Motor Circuit Analysis (MCA™) is a deenergized, non-destructive test that evaluates the condition of the motor connections, stator, and rotor. MCA™ testing can be performed from the output side of the motor starter or motor drive, therefore, no need to open up and disconnect phase leads at the motor for routine testing purposes.

Limitation of Insulation to Ground Fault Detection

An Ohmmeter is used to measure the electrical resistance between two points.A Micro-ohmmeter is used to measure low resistance circuits. A Megohmmeter is used to measure high resistance circuits. The unit of measure for resistance is an ohm.

When testing electric motors, it is useful to know the insulation resistance between motor winding(s) and the frame ground.This value will normally be in the range of tens, or hundreds or millions of ohms.

However, motor winding faults can also occur within the winding and is not detectable using the Meg or Micro-Ohm-meters. For these types of tests, other types if instruments must be used , such as the portable, light weight, hand-held, deenergized motor testers offered by ALL-TEST Pro.

ALL-TEST Pro provides hand-held,battery operated, field portable test instruments designed to evaluate the entire electrical health of the motor. This included detecting developing coil-to-coil,turn-to-turn, and phase-to-phase short circuits before they become catastrophic. These instruments will enhance troubleshooting efficiency,improve your electric motor maintenance program, and help avoid unplanned production outages.

tech-tip-3-2018 Resistance Issue

When a deenergized MCA™ motor testing program is first implemented it is not unusual to have between 10-30% of the motor systems tested to exhibit one or more alarm condition(s) when testing is performed from the output of the motor starter or motor drive. When a motor system is in an alarm condition, this does not necessarily mean that the motor will fail or that it should be stopped immediately, but that the measured values have exceeded predetermined limits.

One of the first considerations should be motor criticality. Obviously, the most critical motors should be afforded a higher priority than less critical motors. The second consideration is the type and location of the alarm (is it related to the connections, cable, motor winding, etc.?)

Blue electric motor ready to be tested

Our last MCA™ data analysis tip stated that it is not uncommon,for new users that begin a MCA™ motor testing program, to have between 10-30% of motor systems tested to exhibit some alarm condition. It is important to note that a motor system exhibiting an alarm condition should not be condemned(or the motor replaced), if the test was performed from the motor control center (output of the motor starter or motor drive). Motor connections and cables between the test point and the motor itself may be the root-cause of the alarm.

Therefore, the next step is to perform another test at the next connection point, whether a disconnect or at the motor itself,with incoming phase leads disconnected. If the alarm condition clears, then the problem is upstream of the test point. If the alarm persists then it is the motor. Lastly, non-repeatable test results should be considered suspect and investigated further.

data-analysis MCA

Our last two MCA™ data analysis tips stated that it is not uncommon for new users that begin a MCA™ motor testing program can have between 10-30% of motor systems tested to exhibit some alarm condition. Tip 2 discussed the importance of performing additional testing to confirm the source of the alarm. I.e. is it related to connections, cables,or motor windings?

Moreover, with respect to AC induction squirrel-cage rotor motors <1000V, many new motors will exhibit an inductance and impedance imbalance, due to motor design/construction.Therefore, a healthy motor can exhibit an impedance and inductance alarm (even though it is in good condition). MCA™ measurements include impedance and inductance measurements, but phase balance is not used for assessing the condition of the motor windings.

tech-tips-6-2018 MCA

An important distinction between RCL meters and MCA™ meters is the ability to fully exercise the entire winding insulation system. Using resistance alone, the I2R loss can be determined across a circuit, but the system electrical reliability, developing winding faults or efficiency cannot be determined. Inductance,which is variable, depending on the winding design and rotor to winding position* also can’t be used for these purposes.

Unfortunately, systems using inductance as a base will often fail good electric motors and windings. In order to obtain the true condition of a motor winding, one must view all of the motor circuit components, including resistance, impedance,inductance, phase angle current frequency response (I/F)and insulation resistance, DF & Capacitance to ground.

tech-tip-1-2019 MCA

Motor Circuit Analysis™ (MCA™) is a deenergized, non-destructive testing method to assess the complete electrical health of a motor.

Patented Test Value Static™ (TVS™) is calculated from the 3-phase MCA™ static test and is used as a Reference value for the motor. Common types of faults in the rotor and stator winding will change TVS™. TVS™ is trended over a period of time to detect changes in the condition of the stator and rotor. TVS™ can also be used to compare motors of the same exact manufacture to insure you are receiving good, quality motors.

tech-tip-2-2019 Test Value Static

Traditional Megohmmeter testing will only detect faults to ground. Not all motor electrical stator winding failures begin as ground faults. Failures can start between turns in the same coil, between coils in the same phase, and phase to phase. If the only motor test you perform is with a Megohmmeter, you will miss detecting crucial stator and rotor faults.

Motor Circuit Analysis provides a complete view of the motor in just minutes. The test can be initiated from the Motor Control Center (MCC) or directly at the motor.

Motor Circuit Analysis is a deenergized, non-destructive testing method to assess the complete electrical health of a motor.

tech-tip-5-2019 Motor Circuit Analysis

Motor Circuit Analysis (MCA™) uses three unique tests IND, Dynamic (DYN), and Z-Fi to test both the winding insulation and insulation resistance to ground. Dissipation Factor (DF), Capacitance (C) to ground, and insulation resistance to ground (INS) are used for testing the ground wall insulation. Capacitance is the capability of a body, system, circuit, or device to store an electric charge. DF is the ratio between the resistive power loss and the reactive power loss of the insulation material. This is used to detect contaminated or overheated windings. The primary reason for the INS test is safety. INS is performed by applying a high DC voltage between deenergized current-carrying conductors (windings) and the machine casing or Earth.

The IND Test Mode is used to test AC three-phase squirrel cage induction motors with rated voltage of less than 1000V. This test mode performs the Static and optional DYN tests on the winding insulation and insulation resistance to ground. Use the IND test during routine condition-based maintenance (CBM) on equipment that has a previously stored Test Value Static™ (TVS™) Reference. TVS™ reference values are a quick and easy way to determine if the motor condition is changing. The DYN test can determine both rotor and stator condition if the motor is decoupled from the driven load.

The Z-Fi Test Mode is used on all types of AC motors (of any voltage), generators, and transformers. The low voltage tests automatically perform all the Static tests: DF/C, INS, impedance, induction, phase angle, current frequency response (I/F), and calculates a TVS. The Z-Fi test mode
must be used on all medium or high voltage equipment (greater than 1,000V) and should be used on installed equipment with no TVS™ Reference. The reason the Z-Fi test is used on motors with no prior TVS is because you want to determine the current health of the motor. Once you generate a TVS™ value you can begin to trend the data. In the Z-Fi test mode you do not perform a DYN (dynamic stator & rotor test) because the load or drive is attached to the motor and the test is not practical.

Can you perform a DYN test in the Z-Fi mode?  A DYN test is not offered in the Z-Fi test mode. Think of it as a baseline to find out what the condition of the motor is without a reference test. This equipment is usually already
installed without having the ability to rotate the motor shaft i.e. connected to gear box, submersible, or a pump.

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IND Mode vs. Z-Fi

Tracking your motor assets from “cradle to grave”; whether it’s routine maintenance, rewind or replacement, TVS™ keeps an eye on your motor assets and creates a culture of “Can Do!” that is easy to adopt at any facility where motors are present.

Any changes in the condition of the winding insulation or the rotor occur, it will be reflected in the TVS™.  A technological advantage of implementing and using TVS™ is that it eliminates errors caused by inductance unbalances that can occur due to the position of a squirrel cage rotor. TVS™ is independent of rotor position. ATP has the only motor testing tools (instruments) in the world that provide a TVS™ value.

  • TVS™ starts with either a baseline or an incoming inspection test on spare and replacement motors.
  • The secondary and following TVS™ tests should be taken after the equipment is installed from the output controller or any easily accessible point. Subsequent readings should be taken from the same location and should be compared with the installed TVS™ value.
  •  Should the remote TVS™ value indicate an issue, another test direct from the motor should be taken. If the motor tests good, we know it’s the cabling to the control. If the motor is bad, generally we know the motor is bad and the cabling is good. Both could be bad, but it is rare.

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AT7P Testing Motor
AT7P Testing from Control panel

Resistance testing in Motor Circuit Analysis™ (MCA™) is used primarily to find high resistance connections. These tests are taken directly at the motor junction box. A resistance test can reveal a miss-connected motor, or cold solder joints.

A phase resistance test performed in a Motor Control Center (MCC) or at a controller tests the entire motor circuit. This test can reveal high resistance connections in intermediate junction boxes, local disconnect switches and issues in the motor junction box itself. These high resistance connections generate heat, never get better, always get worse and almost always lead to unscheduled production losses.

In addition to spot heat damage and potential phase to phase or phase to ground faults which can cause expensive catastrophic damage, high resistance connections cause voltage unbalances which in turn lead to motor overheating and decreased operating efficiency.

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MCA Resistance
AT7 on Control Panel

Electrical Signature Analysis (ESA) is an energized test method where voltage and current waveforms are captured while the motor system is running to assess the health of the motor system. Energized testing provides valuable information for AC induction and DC motors, generators, wound rotor  motors, synchronous motors, machine tool motors, and more.

ATPOL Application IMage

Motor Circuit Analysis (MCA™) is a deenergized test method to assess the health of the motor and motor circuit. This method can be initiated from the Motor Control Center (MCC) or directly at the motor. The advantage to testing from the MCC is that the entire electrical portion of the motor system, including the connections and cables between the test point and the motor is evaluated.

ALL-TEST Pro produces its ESA and MCA™ instruments as discreet, handheld, battery operated units that are all extremely field portable. The data analysis and storage elements are WINDOWs based and are easily shared between computers. Along with providing flexibility to a reliability department the use of individual instruments provides users with the ability to choose how much of which technology is best for their electric motor maintenance program. Both instruments & software provide dependable, quick answers so maintenance staff and managers can make reliable decisions and keep their maintenance team working simultaneously on different motor applications.

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Testing At The Motor Junction Box: As with many motors a simple way to test the six lead motor involves going directly to the motor junction box. After confirming that all Lock Out / Tag Out requirements have been complied with and the motor leads have been checked for the presence of voltage, the motor junction box can safely be opened.

If the motor leads from the controller and the internal motor wires are labeled, make note of that connection. If they are not marked then mark them with colored tape or other identification so that they can be properly reconnected when testing is complete.

Disconnect the motor leads from the starter from the internal motor wires, or from the terminals in the box.

The internal motor wires or terminals should be numbered, one through six. As a check, you should be able to test for electrical continuity between terminals/wires 1-4, 2-5, and 3-6. These are your phase wires (A, B, C, or 1, 2, 3).

To test the motor in the WYE configuration you must short together terminals/wires number 4, 5, and 6. The wires can either be bolted together or significantly sized shorting jumpers used. The tester(s) can then be connected to terminals/wire numbers 1, 2, and 3. Only one INS to ground test is necessary in this configuration.

The 4, 5, and 6 leads need to be shorted together. This can either be done with jumpers at the bottom of the DELTA or WYE contactors or the WYE contactor can be somehow forced. With this shorting accomplished the instrument can be connected to cables 1, 2, and 3 at the the instrument can be connected to cables 1, 2, and 3 at the bottom of the RUN contactor.

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The ALL-TEST PRO 7™ brings predictive maintenance to DC motor testing. Preventive Maintenance Tasks for DC motors such as commutator & brush inspections, lubrication, are very important for their long-term successful operation. However, these inspections fail to determine the condition of the electrical winding or insulation. Adding periodic electrical tests, such as measurements of the winding resistance and Insulation Resistance to Ground (IRG) provide some insight to possible connection issues & weaknesses in ground wall insulation, but still fail to determine the overall condition of the equipments insulation condition.
ALL-TEST PRO 7™ motor testing instrumentBy adding MCA™ readings to DC motor testing provides early indication of developing problems within the motors electrical system beyond those detected using a megohm and ohm meter. MCA tests can be performed quickly from the drive and can confirm or eliminate faults in DC machines.

Several key points quickly determine the condition of DC Machines

  1. Take Series winding and armature windings readings together
  2. Test motors and generators the same
  3. I/F reading outside of the range of -15 to -50 indicates a winding fault
  4. An increase in temperature corrected winding resistance, accompanied by changes in impedance indicates loose connections
  5. A decrease in temperature corrected resistance accompanied by changes in impedance, inductance, phase angle & current frequency response (I/F) indicates developing winding shorts
  6. Deviations of phase angle or I/F of more than 2 points between like motors indicates the need for a MCA complete analysis
  7. Changes in MCA reading in the armature circuit between test intervals prompts a bar to bar armature test
  8. Changes in MCA readings in the armature circuit taken back to back indicates carbon build up in the armature

By following these simple guidelines using the AT7P™ provides early fault detection before the DC machine fails during operation. Recommended testing intervals should be at least those shown in Table 1.

Table 1: DC Motor Frequency

Once a developing fault is detected, it is recommended to reduce the time intervals between tests until the machine can be removed for repair. A complete armature test is recommended in conjunction with preventive maintenance tasks.

Conclusion

Preventive electrical testing of direct current machines is much easier using the DC mode function of the AT7P™. Step by step detailed easy to follow procedures are provided on the large backlit LCD display to make the testing quick and easy to perform from the motor drive in less than 5 minutes. Additional tests and features are available for troubleshooting at the motor to quickly pinpoint the source of the problem. MCA™ testing dramatically improves DC machine testing by saving time and providing more details as compared to traditional techniques and methods.

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For MCA trending and analysis of three phase electric motors, three motor leads are connected to the MCA instrument. When new motors are tested the technician may encounter motors with multiple motor leads.  This allows for the motors to be used in multiple applications. Normally the connection diagrams are provided by the Original Equipment Manufacturer (OEM). This guide is provided if the manufacturers diagram is unavailable. These guidelines do not supersede the OEM connections.   Generally, the coils all use standard numbering schemes so connecting them for MCA testing is straight forward. It is assumed that the technician has basic electrical skills and access to the proper wire connections materials such as wire nuts, split bolts, lugs, assorted machine screws or bolts, and insulating materials that may be needed to make temporary or permanent connections to the motors under test.

Three phase motor winding each have a start to the phase and the end to the phase. These phases are then connected in a DELTA or WYE configuration. Any unbalance in test results will show up regardless of the connected configuration. If the test result is to be used as baseline data, any subsequent testing should be done in the same configuration for trending and comparative purposes. A note about the test configuration can be entered into the relevant computer analysis
software test data file. Example- MCA PRO™ computer software.

Six Lead Motor Diagram

To test the motor in the DELTA configuration the start of each phase is connected the end of another, and the motor leads T1, T2 & T3 are connected to this junction of the phase leads. Firmly connect leads T1 to T6, T4 to T2, and T5 to T3 and use these connections as the test points 1, 2, and 3. To connect the motor in the WYE  configuration, firmly connect the end of the phases together to form a “wye” connection and insulate leads T4, T5, and T6 and then use the start of the phases as the test points 1, 2, and 3 as phase connections.

Six Lead IEC Motor Diagram

Nine Lead Motor Diagram

Nine lead motors will come from the OEM or repair facilities with some of connections internally connected in either a DELTA, or WYE configuration.  To complete the connections, connect motor leads T4 to T7, T5 to T8, and T6 to T9 with wire nuts or other suitable means and use motor test points 1, 2, and 3 as phases  connections.

Nine Lead IEC Motor Diagram

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For MCA trending and analysis of three phase electric motors, three motor leads are connected to the MCA instrument. When new motors are tested the technician may encounter motors with multiple motor leads.  This allows for the motors to be used in multiple applications. Normally the connection diagrams are provided by the Original Equipment Manufacturer (OEM). This guide is provided if the manufacturers diagram is unavailable. These guidelines do not supersede the OEM connections.   Generally, the coils all use standard numbering schemes so connecting them for MCA testing is straight forward. It is assumed that the technician has basic electrical skills and access to the proper wire connections materials such as wire nuts, split bolts, lugs, assorted machine screws or bolts, and insulating materials that may be needed to make temporary or permanent connections to the motors under test.

Three phase motor winding each have a start to the phase and the end to the phase. These phases are then connected in a DELTA or WYE configuration. Any unbalance in test results will show up regardless of the connected configuration. If the test result is to be used as baseline data, any subsequent testing should be done in the same configuration for trending and comparative purposes. A note about the test configuration can be entered into the relevant computer analysis
software test data file. Example- MCA PRO™ computer software.

Twelve Lead Motor Diagram

12 Lead Motor wye and delta

Twelve lead motors offer the highest flexibility of any motor.  They can be connected in a WYE or DELTA configuration, and are used for “high” or “low” voltage operation or multiple speed operations.  However, this versatility does not complicate the testing procedure for testing motor spares or those motors returning from repair.

Twelve Lead IEC Motor Diagram

12 Lead Motor IEC wye and delta

To test the motor in a DELTA configuration, firmly connect leads T1 to T12, T2 to T10, T3 to T11, T4 to T7, T5 to T8, and T6 to T9.  Then use the pairs containing T1, T2, and T3 as phases 1,2, and 3 for testing.

To test the motor in a WYE configuration, firmly connect and insulate leads T10, T11, and T12.  Then connect leads T4 to T7, T5 to T8, and T6 to T9 and use 1,2, and 3 as phases connections for testing.

There are other configurations which may apply for specific applications.  For example:  WYE start, DELTA run, or for high voltage or low voltage. For MCA testing the main importance is that all coils get tested during the test and the recommended connections accomplish this. If an unbalance is detected then individual coils can be tested as discussed below.

Individual phases or coils can be tested by performing single phase measurements from the start of a phase or coil to the end of the same phase or coil.  For example, in a DELTA connected 12 lead motor, A phase can be measured from 4 to 9, B phase to 5 to 7, and C phase 6 to 8.  For a WYE connected motor, A phase 1-10, B phase 2-1, C phase 3-12.  Individual segments can be compared using single phase measurements of individual coils, 1-4, 2-5, 3-6, 7-10, 8-11, 9-12.

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How to Tell If Electric Motor Is Bad

What do you do when a motor fails or trips a drive? What tools do you currently use to determine if the motor is “good” or “bad”? If you are like most technicians, you probably use a Megohm Meter and a Digital Multi-meter.

Looking at an actual motor test on an installed motor where the drive had tripped.
The electrician, using a Megohm Meter and Digital Multi-Meter, acquired these results.

So, what does this indicate about the condition of this motor? Based on these readings the problem is obviously, with the Drive and not the Motor, right? So,
what would you replace the VFD or the Motor? The service technician was relying on a megohm meter Insulation-to-Ground test which indicates that the ground wall insulation has no weaknesses to ground, and a Digital Multi-meter (Resistance test), which indicates there is continuity in the windings and all connections are good. The service technician was only looking at 2 factors that affect the motor. Both instrument measurements indicate that there is nothing wrong with the components tested but fails to provide a complete picture of the motor’s condition. As far as these instruments can tell this motor is in good condition.

These methods of testing are very reliable in determining if your motor is “alive” or “dead” (i.e., shorted to ground) or has connection issues, but will not give you the motor’s current state of health. Did you replace the Drive or the Motor?

Using Motor Circuit Analysis™ (MCA™), this is what that same electrician found: by performing MCA™ testing. Phase angle (Fi) and Current/Frequency (I/F) both indicate no evidence of existing or developing winding shorts.

If you replaced the motor, you cost your company time and money, both in the cost of the motor and the fact that you will have to replace the drive when it trips again.

The same electrician had an identical motor trip the drive on a different line.

Now what? Is it the Drive or the Motor? If you said Motor, you are correct.  Since these readings are the same as the on previous motor it would suggest that the motor is good so the fault must be in the drive.

The MCA™ instruments clearly shows unbalances in both phase angle and current frequency response which are indications of winding shorts. So in this case the fault is definitely in the motor.

MCA™ instruments offer fast reliable answers to motors state of health.

• Fast test under 3-5 minutes.
• Easy on screen directions.
• Answers displayed on screen as GOOD, BAD, WARN.
• Available with phone APP or MCA™ Software suites.

What are your motor testing tools measuring?

What is MCA™ technology? MCA™ (Motor Circuit Analysis) is a deenergized low voltage test method that exercises the motors winding insulation system to assess the health of the entire motor and the associated cabling.

**Winding coil faults: turn-to-turn & coil-to-coil.

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Resistance is a fundamental property of a materials ability to resist the flow of electricity through it. The units of resistance are ohms and uses the Greek symbol omega (Ω) and the mathematical symbol is (R). All materials have some amount of resistance, most metals have low resistance and are known as conductors. The Specific Resistance of a material is resistivity and is represented by (ρ).  The resistance of a material is dependent on the type of material, the length and shape of the material. The resistance of an object or material determines how much work or heat is created as current flows through the material. For example, a material with a high resistance will consume a large amount of energy as current flows through the material. The current that produces work and creates heat is known as resistive current (Ir).

The resistance measurement is named after Georg Simon Ohm a 19th century, German physicist who studied the relationship between voltage, current, and resistance.  He is credited for formulating Ohms’ Law which is the resistance of a circuit (R) is equal to voltage (E) applied to the circuit divided by the current flow (I) through a circuit.  R = E/I

Materials in electrical circuits are classified as either conductors or insulators.

Conductors are materials that have loosely bonded electrons in the outer most shell of the atoms making up the conductive material and offer very little resistance to current flow.  Electrons flow easily through conductive material.  Examples of conductors are copper, steel, iron, bronze, and many other metals.

Insulators are materials that have very tightly bonded electrons in the outer most shell of the atoms that make up the insulating material and resist the free flow of current through the material. Insulators present a high resistance and restricts the flow of electrons.  Examples include rubber, glass, wood, and many plastics.

The fundamental of electricity is that current takes the path of least resistance, therefore insulators are used to direct the of current flow through the intended path and prevent the flow of current through unwanted paths.

In motors conductors are formed into coils or windings to create the magnetic field required to convert electrical energy into mechanical torque.  To maximize the strength of the magnetic field current needs to flow through each turn of the winding. Therefore, the conductors that are used build the windings are coated with multiple layers of insulation to direct the current through the winding. This insulation is referred to as winding or turn insulation.

When the insulation between conductors begins to break down, current will still not flow between conductors until the resistance of the insulation falls below the resistance of the conducting material around the conductor. Therefore, the resistance measurement of the individual windings will remain unchanged until the insulation is has completely failed.

Resistance is directly proportional to the overall length of the conductor, the size of the conductor (in circular mills), and the temperature of the conductor.  For example, it is much easier for water to flow through a wide, short pipe then it is for water to flow through a more narrow, longer pipe.  Current through an electrical conductor reacts the same way.  Current will flow much easier through a large, short piece of wire then it will through a more narrow and longer piece of wire, because there is less resistance of flowing electrons in the larger conductor than the smaller conductor.

Therefore, when measuring the winding resistance in a deenergized three phase motor electric motor, any resistance unbalance is usually the result of connection issues. The resistance of all three phases should be balanced in relationship to each other.  Any unbalance of 5% is a warning and indicates there are issues in the motor circuit.

When testing from the MCC a resistance unbalance could be anywhere from the connection in the MCC (Motor Control Cabinet), the cabling or the motor itself. Additional testing needs to be performed progressively closer to the motor to locate connections that are causing the unbalanced resistance.

If resistance measurements at the motor are balanced this verifies the issue is somewhere between the MCC and the motor cables.  If resistance values directly at the motor are unbalanced this confirms there is an issue inside the motor.  Examples of things that can cause unbalanced resistances are loose connections, cold solder joints either in the motor or at the MCC, frayed or broken wire, dirty terminals or oxidation of the connections anywhere in the motor circuit.

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MCA™ is a field proven very easy to use and safe method of evaluating the condition of electrical equipment while the equipment is deenergized. The basic premise of MCA:
In equipment with three phase coils all phases should be identical. Consequently, all the electrical characteristics of the winding should be the same. If any change in the condition of insulation occurs, it is never good, (windings do not “fix” themselves). So, any change in the winding insulation system is “bad”. The phase coils have 2 separate and independent insulation systems The groundwall insulation system and the winding insulation system; the condition of one insulation systems  doesn’t indicate the condition of the other, therefore each insulation systems needs to be tested thoroughly and independently. The groundwall insulation system isolates the coils from the equipment’s frame or other exposed parts of the equipment while the winding insulation system directs the current through the  conductors to create the magnetic field. MCA™ performs a series of tests on both insulation systems.

Groundwall Insulation: Breakdown of the groundwall  insulation system is a safety issue and requires immediate action. MCA™ measures the insulation resistance to ground measurement to locate any weaknesses in the groundwall insulation but doesn’t provide the overall condition of the insulation. Dissipation Factor (DF) and Capacitance to Ground (CTG) reading provide additional indication of the overall condition of the ground wall insulation system, but none of these provide any indication of the winding insulation system.

Winding Insulation: A breakdown in the winding insulation system will result in shorts between tuns in the windings which results in weakened magnetic field, unbalanced current flow, increased heating, and eventual atastrophic equipment failure. MCA™ performs a series test by applying low voltage AC & DC voltage to the three phase windings while the motor is deenergized. The DC voltage measure the winding resistance using especially designed Kelvin leads to provide very accurate winding resistance measurements to identify connection issues.

When the winding insulation begins to degrade, it undergoes a change in the chemical makeup of the insulating material surrounding conductors. The AC current flowing through the windings exercises the entire winding insulation. The very small changes that occur because of the chemical makeup are measured and evaluated. By analyzing the amount and relationships the causes and severity of developing winding failures can be identified and the proper action recommended.

MCA™ can be used for:
1) Incoming inspections on all new & repaired motors
2) Spares testing
3) Pre installation testing
4) Troubleshooting
5) Routine predictive maintenance testing

MCA™ Tests
Static Test – tests all three AC motors windings, performs a series of tests at different frequencies on all three phases of the motor’s windings from the motors line leads, T1, T2, T3. The results of the test are input into a proprietary algorithm to create the Test Value Static (TVS). The TVS is a dimensionless number that serves as a baseline value to defines the condition of the equipment. Any changes in this value > 3% indicates a fault. This value may be compared to other identical  equipment (must be the same HP/KW rating, speed, frame size, and manufacturer).

Dynamic Test – is performed on squirrel cage induction motors < 1000 V. While the motor shaft is smoothly and slowly manually rotated, stator and rotor signatures are created. The stator and rotor signatures are automatically analyzed to identify and report faults in either the rotor or stator.

Phase Comparison Test – tests three phase coils in all types of three phase equipment, including motors, generators, and transformers. The phase comparison or “Z” test measures, DC winding resistance (R), impedance (Z), inductance (L), phase angle (Fi) and current frequency response (I/F).

The results of the tests are recorded and provided to determine any differences in the phases. These differences are compared to pre-determined guidelines created through many years of field testing the condition of the winding insulation. These values can be trended over time, used to determine the type and severity of developing fault, and provide an estimate of time to failure.

The following guidelines have been developed from over 35 years of field testing, but they are simply guidelines and are a good starting point, however, as with any guideline’s failure will not occur immediately if these guidelines are exceeded.


The basic procedures for MCA™ testing static and dynamic tests are performed all on new equipment to evaluate the new motors condition and establish base line or reference values for future testing. New baselines are established from the motor control center (MCC) once a motor has been installed. All future readings can be taken are if all measurements from the MCC are balanced all connections in the motor circuit are tight and the winding insulation surrounding the conductors in all the phases are in good condition. If an unbalance occurs, analysis and perhaps further testing may be same required to evaluate the type and severity of the fault. The AC tests measure impedance (Z), inductance (L), phase angle (Fi) and the current frequency response (I/F) to evaluate the condition of the winding insulation.

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Experience has shown that 20 to 40 percent of motor systems tested may have some sort of alarm condition.  Just because a motor has exceeded the alarm limits in MCA Basic™ or MCA PRO™ it does not necessarily mean the motor will fail or should be stopped immediately.  For over 30 years the dedicated staff at ALL-TEST Pro have gathered data and resources to determine when a motor will most likely fail on the most common 3-phase squirrel cage motors.  Some motors may have a special design that may cause the measured values to be outside the standard limits but still operate correctly.  In some cases, even a brand-new motor can receive an inductance and impedance alarm due to the Rotor Bar/Winding ratio.  The following analysis tips will help you determine when a motor should be condemned and should be taken out of service.

Never condemn a motor from the Motor Control Center (MCC).  Faulty cables and bad connections between the test point and motor can cause unbalanced readings and produce an alarm.  If an alarm is received at the MCC another test should perform directly at

the motor with the motor leads disconnected from the motor cables from the MCC.  If the alarm remains, a problem with the cables and connections from the MCC can be ruled out and the motor can be investigated further.  If the alarm clears, then the motor cables and connections should be inspected for possible failures.

Another thing to think about is what types of faults are identified during a test.  Winding shorts are generally more severe than contamination or rotor faults.  Developing motor faults are first indicated by changes between the baseline TVS value and a newly obtained TVS value, Stator Signature, or unbalances in Phase Angle (Fi) and Current Frequency Response (I/F).  Motors that receive these faults should be taken into consideration before motors with inductance/impedance or resistance faults.

A motor should never be condemned from one test.  If there is any residual voltage on the motor, you may get a result that could show a warn or bad alarm.  It is recommended to take a second and even a third test of the motor to verify alarms.  It is also important to isolate the motor from any other induced voltages from other electrical equipment while conducting a test.  An induced voltage on the motor can cause inconsistent and unreliable readings that do not repeat.  It is important not to condemn a motor that is giving unrepeatable readings because of this.

When testing a motor that is currently installed in a machine it is good practice to conduct a test directly at the Motor Control Cabinet (MCC). Not only are you testing the health of your motor, but you are also testing for problems in the motor’s cabling assembly as well as connection points at the MCC and at the motor. If an alarm is received while conducting a test with one of ALL TEST Pro’s deenergized instruments at the MCC than the next course of action would then be to conduct a test directly at the motor with the motor cables disconnected.
Depending on if the alarm clears or stays you are then able to pinpoint the location of your alarm between the MCC and the motor. If an alarm is received at the MCC and is consistent at the motor, then it is confirmed the motor is the root cause of the problem. If you receive an alarm at the MCC and it clears directly at the motor, then the cause of the alarm lies between the MCC and motor cables.

Some potential problems could be:
• Fraying and weakening of cable insulation material
• Poor or loose connections either at the MCC or motor
• Contaminated/oxidized contactors at the MCC
Always remember to make multiple tests at each location to confirm repeatability and accuracy of your test results.

For decades, ALL TEST Pro has been an industry pioneer in offering easy to use, portable, and battery-operated instruments for all your deenergized motor testing needs. In some facilities it can be quite challenging finding a standard 120-volt AC outlet to connect a device to, even more so if an extension cord is needed to reach the location where testing is being performed.
AT7 key pad batteryThe most important thing to remember with a battery operated instrument is to keep it fully charged while not in use. It may seem simple, but nothing is more frustrating than needing to conduct a motor test, but you must wait because the battery is fully discharged.

It is recommended to keep the instrument on the charger when not in use because of this. The battery will slowly discharge while sitting idle so if you do not use the instrument for an extended period the battery will eventually fully discharge and not turn on. While the instrument is plugged into the supplied charger the charging circuit in the instrument will automatically turn on once the battery falls below the preset threshold. Meaning there will not be power on battery if it is at full charge. Lithium-Ion batteries do not develop memory and don’t require full discharge before charging. To increase the life of the battery it is recommended to perform more frequent partial discharges instead of a full discharge. Make sure to only use the supplied charger with your instrument as using an aftermarket or 3rd party charger can damage the charging circuit and or battery if the charger has the incorrect polarity or supply voltage.

Many medium to high voltage electric motors are equipped with a Capacitor bank or Surge Arrester to protect equipment from unexpected transient power surges due to external lightning strikes, internal switching events or other transient voltage surges. These devices are crucial to protect equipment that could easily be damaged by these unexpected surges in power. When conducting a motor test with one of ALL TEST Pro’s line of deenergized motor testers it is particularly important that these capacitor banks or surge arresters are disconnected and isolated from the motor.

These capacitors and surge arresters will filter test results and create incorrect and inconsistent readings which can lead to false diagnoses of the motor. When testing a motor with a capacitor bank or surge arrester it is recommended to take a test directly at the motor with the incoming motor cables disconnected. You can also disconnect the motor cables at the load side of the capacitor bank or surge arrester and conduct a motor test at that point. Always remember to conduct multiple tests before condemning a motor to verify consistency of results. Please refer to the Motor Circuit Analysis manual for more details on condemning criteria.

Not only is MCA (Motor Circuit Analysis) a great way to determine developing winding faults of a motor at the earliest stage but it also can be used to pinpoint the exact location of a fault in a motor system from the MCC (Motor Control Center) all the way to the motor.  One of the most crucial factors to find early-stage faults is conducting two baseline tests when installing the motor.  The first baseline test should be performed directly at the motor completely disconnected from any motor cables or other equipment.  Future tests can then be compared and trended to this baseline test to look for changes which will signify a motor fault.

Once the motor is installed into the machine a second baseline test should be performed directly from the MCC.  This will establish a baseline test all the way from the MCC to the motor and again can be referenced when taking future tests.

With both baseline tests it will be quite simple to determine the exact location of a fault if a motor is starting to fail or intermittently tripping a drive or circuit breaker. First a test should be performed directly at the MCC and then compared to the initial reference test from the MCC.

If there is a deviation between test results or a WARN or BAD indicator is displayed on the results screen the technician should then conduct a test directly at the motor with the motor cables disconnected.  If there is still a deviation between the new test and the initial baseline test taken directly at the motor or a WARN or BAD indicator the technician can conclude the motor is the root cause of the failure and should be addressed appropriately. If the deviation between tests clears and no WARN or BAD indicators are established, then the motor cables and connection points at the MCC can be investigated further until the root issue is found.

(MCA™) Motor Circuit Analysis™ takes the guesswork out of rebuilt and new stock motors. By performing a quick, less than 3-minute motor test as soon as a motor arrives, you can put your mind at ease knowing the motor is perfectly healthy and will function properly once installed or you can reject the motor directly at the shipping dock if it fails to meet your criteria.

Depending on the application, a motor install can take up to an entire day of work so performing a test prior to installation eliminates the chance of the motor not working properly.  The process and goals are the same for rewound or new motors: save time, ensure safety, get the replacement installed the first time & improve morale. Never again go through the struggle of a tough motor installation to just to have to pull it out because the motor trips as soon as power is applied.  By implementing this single strategy your company will save money and prevent unnecessary extra work by installing and uninstalling a defective motor.

MCA™ is used to test inbound and outbound motors (new and used). Motor tags with MCA™ information help communications between vendors and customers as well as maintenance staff.  MCA™ determines the motor’s health and status eliminating motor inventory ambiguity whether a motor is being shipped outbound (vendor or customer) or being received inbound for stock or immediate use.

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A belted system can be evaluated by inputting information regarding pulley size and belt circumference into the Electrical Signature Analysis (ESA) software.  The ESA software then automatically calculates the belt frequency and will generate cursors to help evaluate the condition of the belted system.  A belted system that is not properly installed can cause problems such as misalignment, sheave/belt wear, and can end in bearing failure.  These results can be trended over time.  ESA evaluates the current and voltage spectrums using a Fast Fourier Transform (FFT), which converts the time waveform to a frequency spectrum.  The FFT highlights amplitude and frequencies to identify belt and pulley issues.

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Current Spectrum (RMS) chart for tech tip 6

As an example, the low frequency data above is from a fan driven by a 150kW, 400V, 260A, 1485RPM induction motor.  The peak labeled BLT is the belt frequency which is the speed of the belt.  Additionally, there are multiples of the BLT and these are shown in both spectra.  The lower spectra show the Line Frequency Peak and then that there are sidebands on either side of the Line Frequency that are at the BLT frequency.

The multi-technology approach to motor diagnostics means you are utilizing different testing technologies that will complement and validate each other. One example is your vibration technician suspects a possible rotor problem in a critical application, but the cost to replace means a shutdown of production, where the motor’s cost is small compared to the incurred costs of the shutdown.

In a situation like this, many people would be reluctant to make the call for replacement,for if the diagnosis is wrong, the cost is very high.Therefore, this motor may be run to failure, due to the uncertainty of the diagnosis. In this case, to put the multi-technology approach in practice, use Electrical Signature Analysis (energized testing) to confirm or rule out the preliminary findings (bad rotor). If the shaft of the installed motor can be turned or the load quickly disconnected, then a Motor Circuit Analysis test (deenergized) can be performed to assess the condition of the rotor, stator, and connections. By utilizing the multi-technology approach you will have more confidence in your findings and hence, a greater degree of certainty that you have determined the real fault(s).

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tech-tip-5 ESA Multitechnology Approach

Motors are designed to operate between 50 & 100% of rated load. The best operating efficiency for most motors is around 75% of rated load. Power factor (PF) is a measurement that can quickly determine the amount of load on a motor. Typically, motors with low PF during normal operation are oversized for their current application and will cost more to operate than a more correctly sized motor.  Motors operating with low PF will contribute to low system PF, which could result in high PF charges by the utilities and higher energy loss within the motor.  Using PF to correctly size motors in the plant will result in increased electrical reliability and less wasted energy.

Electrical Signature Analysis (ESA) evaluates both voltage and current, giving a broad view of motor system health that includes incoming power quality. Combining this information with knowledge of the application can indicate opportunities for energy cost savings.

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tech-tip-2-2018 Low Power Factor

A Stator Mechanical fault is created when the stator core becomes loose within the motor frame, or if the windings are loose between the stator slots.  A loose stator core or winding over a period of time will cause a breakdown in either the winding insulation system or insulation system to ground.  Electrical Signature Analysis (ESA) evaluates the current and voltage spectrums using a Fast Fourier Transform (FFT), which converts the time waveform to a frequency spectrum.  The FFT highlights amplitudes and frequencies identify mechanical faults such as problems with the stator.

When there are peaks at the same frequency in the current and voltage spectrums they are related to the incoming power.  When there are peaks only in the current and not the voltage, then the fault is coming from either the motor or the driven load.  In the ESA example on the right a Stator Mechanical problem is indicated by Line Frequency sidebands of running speed multiplied by the number of stator slots.  The red arrows identify stator mechanical frequency peaks in the current spectrum and not in the voltage.

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Static Eccentricity (air gap) is a fault that is created when the rotor is not in the magnetic center of the stator.  Static eccentricity can cause an increase in operating current, overheating, energy losses, and overloading of bearings.  Electrical Signature Analysis (ESA) evaluates the current and voltage spectrums using a Fast Fourier Transform (FFT), which converts the time waveforms to a frequency spectrum.  The FFT highlights amplitudes and frequencies identify faults such as static eccentricity.

When there are peaks at the same frequency in the current and voltage high frequency spectrums they are related to the incoming power.  When there are peaks only in the current and not the voltage, then the fault is coming from either the motor or the driven load.  In the ESA example above a Static Eccentricity problem is indicated by Line Frequency (50Hz) and twice Line Frequency sidebands of running speed multiplied by the number of rotor bars.  The red arrows identify static eccentricity frequency peaks in the current spectrum and not in the voltage.

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ESA Software confirming static eccentricity (air gap) issue.

A broken or fractured rotor bar fault can occur with excessive starts, large loads, manufacturing processes, etc.  When a broken rotor bar occurs, there is no longer a path for current to flow. This creates stress on the neighboring bars in the form of increased current and heat.  Eventually these rotor bars fail over time.  Electrical Signature Analysis (ESA) evaluates the current and voltage spectrums using a Fast Fourier Transform (FFT), which converts the time waveform to a frequency spectrum.  The FFT highlights amplitude and frequencies to identify mechanical faults such as broken or fractured rotor bars.

Generally broken or fractured rotor bars are found as elevated Pole Pass Frequency (PPF) sidebands of line frequency (LF).  PPF is calculated using the synchronous speed minus the running speed times the number of poles.  In this ESA sample there are PPF sideband spacing around LF (3600 RPM or 60 Hz) in the current -1 spectrum.

AC motor example:
460V, 1200 RPM (synchronous speed), 6 Pole motor, 1183.1 RPM (running speed), 60Hz (LF).
1200 RPM synchronous speed – 1183.1 RPM running speed = 16.9 RPM
16.9 RPM x 6 (# of poles) = 101.4 RPM or to work in hertz use 101.4RPM / 60 seconds = 1.69 Hz
PPF = 101.4 RPM or 1.69 Hz

Vibration and infrared may indicate an initial problem. Using ESA technology, you can pinpoint your motor’s actual problem or verify its condition.

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The voltage unbalance between phases will impact the operation of an electric motor. An electric motor may be de-rated when operating on unbalances under 5%. The effects of voltage unbalance are:

  1. Reduced locked rotor and breakdown torques for the application.
  2. Slight reduction of full-load speed.
  3. Current will also show significant unbalance that is related to the specific motor design.
  4. Significant operating temperatures may result. For instance, a 3.5% voltage unbalance will result in a 25% increase in temperature rise.
tech-tip-1-2016 Voltage Unbalance

One of the primary causes of premature electric motor and insulation failure is contamination. A key, often overlooked, part of any electric motor maintenance program is to ensure the motor is clean. Air passages, fan and surfaces of the motor should be cleaned periodically. Contamination buildup on these surfaces will reduce the electric motor’s ability to cool itself resulting in a shorter insulation life. The area around the motor shaft should be kept clean to reduce the chance for contamination incursion into the bearings,as well. Monitoring the electrical insulation condition with the motor circuit analysis will allow early detection of winding contamination buildup within the electric motor on the windings.

tech-tip-2-2016 Motor Cleanliness

Electrical machine (motors & generators) storage will affect the life of the equipment. Over time, conditions such as moisture, dirt, dust, rodents and general vibration will have a negative impact on the electrical and mechanical condition of the equipment.

When storing machines for any significant amount of time, a number of requirements should be considered:

  1. Store the electric motors away from sources of significant vibration,contamination and moisture.
  2. Rotate the shaft of the motor at least quarterly, if not monthly.
  3. If the storage area ever reaches the dew point, install heaters or dehumidifiers to prevent condensation.
  4. Perform Motor Circuit Analysis™ periodically, to ensure that winding degradation has not occurred. Some plants place a tag on each motor showing the last dates for inspections on condition of the motor, with different colors representing the schedule for turning the shaft (green for the first month of each quarter, red for second month and yellow for the third).
tech-tip-4-2017 Motor & Generator Storage

A number of fault conditions will cause a variable frequency drive (VFD) to ‘nuisance trip.’ VFD related winding shorts can occur in the end-turns of the electric motor’s coils between individual conductors. This type of fault cannot be detected with an insulation resistance tester or Ohm-meter and the motor may still operate satisfactorily in bypass for some time. VFD faults, input voltage and cable faults will also cause nuisance tripping. If incoming power is satisfactory (+/- 10% to the VFD voltage rating), check the motor windings and cables with motor circuit analysis in order to isolate the location of the fault (motor, drive or cable). This practice will reduce troubleshooting time in terms of hours(or longer), avoiding costly unplanned downtime of the associated equipment.

tech-tip-6-2107 Variable Frequency Drive

SOFT FOOT occurs on machines when the feet of the machines and the platform they are mounted on are not on the same plane. In electric motors soft foot distorts the frame which in turn can distort the stator magnetic field. This creates unbalanced electrical forces between the rotor and the stator magnetic field. These faults are often diagnosed by vibration personnel as unequal air gaps or static eccentricity (by users of Electrical Signature Analysis – ESA). Static soft foot is best checked using a dial indicator to determine the amount of soft foot and feeler gauges to determine the type of soft foot.

Dynamic soft foot requires a more detailed method of testing. Soft foot in motors can lead to premature bearing failure and loose and broken rotor bars. ESA quickly and easily identifies static and dynamic soft foot.

Static soft foot can be detected using the deenergized Motor Circuit Analysis Dynamic test mode.

How "soft foot" can damage electric motors over time

As a general rule of thumb, operating a motor more than 10°C above the motor’s rated insulation class may decrease the life by half. Excessive heat will accelerate the degradation of a motor’s insulation system. Heating of a motor can be caused by overloading, too frequent starts, high ambient temperature, to name a few. For example, a motor with a class F insulation system is rated for 155°C. If the motor exceeds this temperature more than 10°C then the life of the insulation system maybe cut in half.

tech-tip-9-2017

Winding and motor insulation systems follow the Arrenius Equation: A chemical reaction rate doubles for every temperature increase of 10° C, since insulation systems are dielectrics they follow these rules. This means that a motors life decreases by 50% for every 10° C increase in motors temperature.

Many electric motors use thermal convection to keep the motor cool. The greater the surface area of contact, the greater heat transfer ability. Fins on the motor enclosure increase the surface area of the motor housing which increases the heat dissipation capabilities of the motor, thereby maximizing the life of the motors insulation systems.

Allowing build-up on the motors exterior limits the motors ability to adequately dissipate heat, dramatically reducing the life of the motors insulation system and the life of the motors.Frequent cleaning of the motors exterior will allow the motors to achieve its expected life.

tech-tip-7-2018

Single Phasing is a condition that occurs when one of the three phases that is supplying voltage to a three phase motor is lost. When this occurs the current across the remaining two legs can go to 1.73 times (173%) the normal FLA (please refer to the figure on the right).

During this condition the excess current flowing thru the other windings will cause those windings to overheat. This could permanently damage the winding insulation and possibly cause a fire inside the motor. Caution should be implemented to ensure that overloads on the motor are rated as to prevent this condition.

tech-tip-6 Single Phasing

Before performing any electrical or electronic test it’s important to check the functionality of your
instrument.

For example, before using a voltmeter to check for life threatening voltages, it is a maintenance best practice to verify it is reading properly before using to insure functionality.  Similarly, it is good practice to check electric motor test instruments and test leads for soundness and functionality before taking them to the field. Use of a test motor such as the ALL-TEST Pro Demonstration Motor of known condition is ideal.

Functionality can be also be proved by simply shorting the test leads by connecting clips to a common piece of unfinished metal.
On most ALL-TEST Pro instrument the Meg Ohm Insulation test takes place between the Blue #2 test lead and the Yellow ground lead. With both leads clear, or open, the reading should be off the scale high (>XXX Mohms). With the leads connected to a common piece of metal (shorted) the reading should be near zero.

With all three (Black, Blue, and Red) test leads connected to a common piece of metal (shorted) and any auto test performed,
phase Resistances should be zero.

Instrument Functionality Check

With instrument functionality confirmed, you can be confident that any abnormal readings you see in the field originate in your test
object.

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Dissipation Factor is an electrical test helps define the overall condition of an insulating material.

A di-electric material is a material which is a poor conductor of electricity but an efficient supporter of an electrostatic field. When an electrical insulating material is subjected to an electrostatic field, opposing electric charges in di-electric material form di-poles.

A capacitor is an electrical device that stores an electrical charge by placing a dielectric material between to conductive plates. The Groundwall Insulation (GWI) system between the motor windings and the motor frame create a natural capacitor. The traditional method of testing the GWI is to measure the value of the resistance to ground. This is a very valuable measurement for identifying weaknesses in the insulation but fails to define the overall condition of the entire GWI system.

The Dissipation Factor provides additional information regarding the overall condition of the GWI.

In the simplest form when a dielectric material is subjected to a DC field the diploes in dielectric are displaced and aligned such that the negative end of the dipole is attracted toward the positive plate and the positive end of the dipole is attracted toward the negative plate. Some of the current that flows from the source to the conductive plates will align the dipoles and create losses in the form of heat and some of the current will leak across the dielectric. These currents are resistive and expend energy, this is resistive current IR. The remainder of the current is stored on the plates current and will be stored discharged back into system, this current is capacitive current IC.

When subjected to an AC field these dipoles will periodically displace as the polarity of the electrostatic field changes from positive to negative. This displacement of the dipoles creates heat and expends energy.

Simplistically speaking, the currents that displace the dipoles and leaks across the dielectric is resistive IR, the current that is stored to hold the dipoles in alignment is capacitive IC.

Dissipation Factor is the ratio of the resistive current IR  to the capacitive current IC, this  testing is widely used on electrical equipment such as electric motors, transformers, circuit breakers, generators, and cabling which is used to determine the capacitive properties of the insulation material of the windings and conductors.  When the GWI degrades over time it becomes more resistive causing the amount of IR to increase.  Contamination of the insulation changes the dielectric constant of the GWI again causing the AC current to become more resistive and less capacitive, this also causes the dissipation factor to increase.  The Dissipation Factor of new, clean insulation is usually 3 to 5%, a DF greater than 6% indicates a change in the condition of the equipment’s insulation.

When moisture or contaminants are present in the GWI or even the insulation surrounding the windings, this causes a change in the chemical makeup of the dielectric material used as the equipment’s insulation.  These changes result in a change in the DF and capacitance to ground.  An increase in the Dissipation Factor indicates a change in the overall condition of insulation, comparing DF and capacitance to ground helps determine the condition of insulation systems over time.  Measuring Dissipation Factor at too high or too low temperature can result in unbalanced results and introduce errors while calculating.  IEEE standard 286-2000 recommends testing at or around ambient temperature of 77 degrees Fahrenheit or 25 degree Celsius.

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If a motor has been disassembled for any reason, it is recommended to verify the health of a motor’s winding system prior to assembly. For example, after replacing a bearing, removing the rotor for inspection, cleaning the winding or even a complete stator rewind, it is always a good idea to test the stator for potential faults before reassembly. ALL-TEST Pro’s line of deenergized instruments are perfect tools for this but there are a few things that need to be considered when analyzing the test results.

AT34 on Demo Motor

When the rotor is removed from the motor, any mutual inductance unbalance, that is caused by any variance in Rotor Bar/Winding ratio, is removed as well.
Therefore, the only portion of the motor winding system that is responding to the AC signal, from the instrument, is the self-inductance from the stator windings and back iron.

This means the fault tolerance guidelines of an unassembled motor should be tighter than the criteria of a full assembled motor. It is recommended to follow the unassembled motor tolerance table below.

AT34 Test Result Graph

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