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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.
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
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 (de-energized) 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).
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.
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.
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.
An Ohmmeter is used to measure the electrical resistance between two points.A Micro-ohmmeter is used to measure low resistance circuits. A Meg-Ohmme-ter 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 trouble-shooting efficiency,improve your electric motor maintenance program, and help avoid unplanned production outages.
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 pre-determined 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.?)
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.
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.
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.
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.
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 de-energized, non-destructive testing method to assess the complete electrical health of a motor.
If the only motor test you perform is with a Megohmmeter,you will miss detecting crucial stator and rotor faults. Megohmmeter testing will miss inter-turn faults in the motor windings. It also will not detect poor connections between phases or rotor problems.
Motor Circuit Analysis™ provides a complete view of the motor in just minutes. Equip your technicians with the correct tools to perform their job in an efficient, accurate way. Proper tools help you improve machine reliability,increase technician productivity, and reduce energy consumption in your facility.
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).
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.
Electrical Signature Analysis (ESA)
Read more about the ESA testing method and how it is used by ALL-TEST Pro’s motor testing instruments.
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:
- Reduced locked rotor and breakdown torques for the application.
- Slight reduction of full-load speed.
- Current will also show significant unbalance that is related to the specific motor design.
- Significant operating temperatures may result. For instance, a 3.5% voltage unbalance will result in a 25% increase in temperature rise.
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.
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:
- Store the electric motors away from sources of significant vibration,contamination and moisture.
- Rotate the shaft of the motor at least quarterly, if not monthly.
- If the storage area ever reaches the dew point, install heaters or dehumidifiers to prevent condensation.
- 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).
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.
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.
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.
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.
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.