How to Check Motor Winding Resistance on Single & Three-Phase Motors

For a quick review on this topic, please click this link. We cover Groundwall Insulation testing, how to test your windings for connection issues including open & shorts. 

What Is a Motor Winding Resistance Test?

Testing windings on a 3 phase motor is very easy with Motor Circuit Analysis™ (MCA™). Winding resistance measurements detect various faults in motors, generators, and transformers: shorted & open turns, loose connections, and broken conductors & resistive connections problems. These issues may be the cause of wear or other defects in a wound rotor motor. Winding resistance measurements detect problems in motors that other tests may not find. Instruments such as megohmmeters and ohmmeters will detect direct ground faults but will not indicate if the insulation is failing, turn to turn faults, phase unbalance, rotor issues, etc. If the motor is grounded, the megohmmeter & ohmmeter will solve your issue when you ohm a motor but if the motor problem is not a ground issue, you are going to need to use another tool or instrument to troubleshoot the problem since the motor maybe still operational but having issues such as tripping the VFD or circuit breaker, overheating, or underperforming, etc.

Motor Circuit Analysis™ (MCA™) is a test method that determines the true state of health of 3 phase & single-phase electrical motors. MCA™ checks the motors coils, rotor, connections, and more. MCA™ can verify ac motor winding resistance as well as dc motor resistance and determine state of health.

Motor Winding Resistance Unbalance or Connection Issues

MCA™ instruments give you results on screen and the test takes less than 3 minutes to perform and does not require additional interpretation and or calculations. The motor health is determined quickly with high accuracy and ease. All components of the single and three-phase motors are evaluated to determine the health of the complete motor.

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Connection issues create current unbalances between the phases in a three-phase motor, which causes excess heating and premature insulation failure. Resistance unbalance indicates connection issues that can be caused by loose connections, corrosion, or other buildups on the motor terminals. High Resistance connections can also occur which can cause excessive heat at the connection point that could lead to a fire damaging equipment and causing a safety hazard. A second test at the motor leads is required to pinpoint the issue if the initial test was performed at the motor control center (MCC). This direct test at the motor leads will confirm the motor state of health and will either condemn the motor or determine the associated cabling as the root issue. Many healthy motors are rewound and put back into operation only to have the same preliminary issue unresolved.

MCA™ testing technology gives in-depth information about the state of the motor’s components, including the insulation and windings. Plus, it works with single-phase and three-phase motors and AC and DC testing.

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Testing AC Motor Windings

The AT34™ & AT7™ instrument’s on-screen instructions guide you through the process. The measurements are automatic, and the test leads do not have to be moved once connected. This means that you can check single phase motors and three phase motors accurately and without additional steps to perform the test. Software suites (single user to enterprise suites are available) that are easy to use enabling you to tend, track and share information on all your motor assets and additional equipment.

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Testing DC Motor Windings

DC motors can have windings arranged in series, shunt or compound configurations.

When testing a DC motor with a  standard ohm meter multiple tests are generally required to ensure accurate and consistent results. The technician is required to compare the values from the test to those published by the motor’s manufacturer to determine if a problem exists. By using MCA™ technology, testing the windings does not require knowledge about the motor’s specific published values or extensive electrical information. In fact, MCA™ products allow for entry-level technicians to get accurate, clear results in three minutes that do not require any interpretation. The DC motor winding testing procedure is the same as the AC motor testing procedure. The recommended method is to take a baseline test of a new or freshly rebuilt motor. Once the motor is reinstalled the baseline test can be trended with future tests to determine a change in the motor system which will eventually develop into a motor fault. ALL TEST Pro’s line of deenergized instruments has simple on-screen instructions and data saving features that eliminate errors, calculations, and reference values required for troubleshooting and trending motors.  ATP uses Test Value Static™ (TVS™) as an indicator to track the lifecycle of individual motors. This value tracks the motor asset from cradle to the grave (installation to decommissioning). This value changes as the asset ages and will help you trend the motor and its current state of health.

Motor Circuit Analysis testing is a deenergized method that will thoroughly assess the health of your motor. It is easy to use and quickly delivers accurate results. The ALL-TEST PRO 7™, ALL-TEST PRO 34™, and other MCA™ products can be used on any motor to identify potential issues and avoid costly repairs. MCA™ fully exercises the motors winding insulation system and identifies early degradation of the winding insulation system, as well as faults within the motor that lead to failure. MCA™ also diagnoses loose and faulty connections when the tests are performed from the motor controller. Find out more ways MCA™ outperforms other testing equipment in our video.

The ALL-TEST PRO 7™

The ALL-TEST PRO 7™ conducts deenergized testing of a single-phase or three-phase motor. With its broad range of testing capabilities, this portable device can test AC and DC motors, motors above and below 1 kV, generators, transformers, and any other coil-based equipment.

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THE ALL-TEST PRO 34™

The ALL-TEST PRO 34™ is ideally suited for deenergized testing of AC induction squirrel cage rotor motors that are rated for less than 1 kV. This model offers the same high-quality, simple testing capabilities as the ALL-TEST PRO 7™, including an easy-to-read screen that displays instructions and a health assessment of the motor’s components.

Both units have ATP’s patented rotor dynamic test for determining rotor condition & Test Value Static (TVS™) for tracking motor health from the initial start-up to termination or repair. Features include portability, in-the-field design (no AC power required, no additional laptop, weighs under 2 lbs., weatherproof, easy to use, long battery life, & safe and easy to operate. 

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Buy MCA Motor Testing Equipment Today

ALL-TEST Pro ONLY develops, designs, and manufactures motor testing equipment. We serve all industries worldwide that utilize electrical motors. Our customers range from small shops to fortune 100 & 500 companies, government, military, and EV auto manufacturers. Find out why our customers are relying on ALL-TEST Pro to pinpoint the problem and as the final say when it comes to motor status.

In under three minutes, you get the answers you need to troubleshoot single and three phase motors, as well as trending capabilities. Check out our video for more about our motor winding testing products.

To get pricing information for any of our motor testing options, request a quote today or contact our team online at ALL-TEST Pro

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