Should an Electric Motor Have Continuity?

Last week I was teaching motor control to a group of apprentices, when it came time for testing the motor, one student asked me, should a motor have continuity?

Each individual motor winding should have continuity or a low ohm (Ω) resistance, for example, one test lead is on the terminal marked U1 and the other lead on U2 with the multimeter set to continuity or a low ohm setting. There should be no continuity or infinite resistance between phases or phase to ground.

In this article we will take a look at which test procedure you should be getting continuity with and well take a look at why you may be getting continuity in places you shouldn’t be.

testing motor continuity

In the context of an electric motor, continuity of the windings is essential for the motor to work and functioning correctly. It ensures that there are no breaks or open circuits in the motor windings or electrical connections.

motor maintenance pdf

To check for continuity in an electric motor, you can use either a multimeter set to the continuity or resistance mode or an insulation resistance tester set to 500 volts DC for a three-phase motor or 250 volts DC for a single-phase motor.

By placing the probes of the meter on different points of the motor, such as the motor terminals or windings, you can determine if there is a continuous electrical path. If the motor has good continuity, it means that electrical current can flow smoothly through the motor windings and other components.

However, when carrying out continuity or resistance testing on an electric motor there should be continuity, but only between the ends of each individual winding, meaning if you have one test lead of your multimeter on the terminal marked U1 and the other lead on U2 there should be continuity between these two terminals, and the same with any other common terminals such as V1 and V2 and W1 and W2.

motor terminals marker u1 v1 w1

It’s important to note that continuity alone doesn’t guarantee optimal motor performance. Other factors such as proper voltage supply, appropriate insulation, and mechanical integrity also play significant roles. By using the correct protective devices, regular maintenance, inspections, and professional testing can help ensure the motor’s continuity and overall health, thereby maximizing its efficiency and longevity.

Remember to exercise caution and follow appropriate safety protocols when working with electric motors or conducting continuity tests., and always ensure the motor is electrically isolated (disconnected) from the supply before carrying out any maintenance work.

Should a motor have continuity between phases?

When testing a motor there should be no continuity between phases. Continuity between phases would indicate a fault or a short circuit within the motor windings.

The windings in an electric motor are typically insulated from each other to prevent electrical short circuits and ensure proper operation. The insulation between windings is achieved through various methods and materials.

One common method is the use of insulation coatings or varnishes applied to the individual wires or coils comprising the windings. These coatings, often made of materials such as enamel or resin, provide a layer of insulation to prevent direct contact between adjacent windings

testing between phases 866 mega ohm reading
866 MΩ between phases at 500 volts DC

When performing a continuity test, there should be no continuity between the different phases of the motor. If continuity is detected between phases, it suggests a potential issue, such as a short circuit or damage to the insulation, which requires further investigation and repair by a qualified technician.

It is crucial to ensure that the motor’s electrical connections are secure and that there are no unintended paths for current flow between phases, as this can cause damage to the motor and affect its performance.

Why would there be continuity between phases?

There are many different things taking place inside a motor, but the insulation between the windings of a motor are specifically designed to prevent electrical contact and maintain separation.

However, there are instances where continuity between windings can occur. This typically indicates a fault or failure in the insulation system between the windings. Several factors can lead to insulation damage, including:

  • Age and wear: Over time, the insulating materials can deteriorate due to exposure to heat, vibration, moisture, or other environmental factors. This can cause cracks, breaks, or weakened insulation, potentially leading to continuity between windings.
  • Overheating: Excessive heat generated within the motor can crack degrade the insulation and dramatically decrease the longevity of the motor. This can be caused by factors such as high operating temperatures, overload conditions, incorrect protective devices or inadequate cooling systems.
  • Mechanical stress: Excessive mechanical stress or vibration on the windings can result in insulation damage or abrasion. This can occur due to improper handling, transportation, or operating conditions. Often if a motor is mounted incorrectly on a weak rusted platform the vibration can cause premature wear on the motor.
  • Mechanical overload: Overloading the motor will need to be compensated by increased torque output of the motor which leads to over current being drawn. If the windings are subject to a current higher than they are rated for the insulating material may overheat and break down leading to continuity between phases and tripping of the protective device.
  • Incorrect starting method: Motors can be started using many different methods depending on the motor type, load, and application. If the wrong starting method is used the motor may draw an overcurrent, leading to the breakdown of insulation.
  • Incorrect protective device: If the motor is protected by an overload relay (OLR) it is important that the relay is set correctly. If you have a small three-phase motor that draws a max of 3.5 amps, as stated on the motor’s spec plate, your overload relay must also be set to 3.5 amps. this will protect the motor in case of overload and high current.
  • Contamination: The presence of foreign substances, such as oil, dirt, or moisture, can compromise and significantly break down the insulation properties. Contaminants can weaken the insulation and create conductive paths between windings leading to motor damage and failure.

So as you can see, there are quite a few reasons why a motor may be getting continuity between phases, but when it does happen it suggests a potential insulation breakdown or fault, and it is important to identify and rectify the issue as it can lead to motor malfunctions, reduced efficiency, or even electrical hazards.

Regular maintenance, proper operating conditions, and periodic inspection of the motor’s insulation system can help prevent insulation damage and ensure the motor’s reliable performance over its lifespan.

Should a motor have continuity to ground?

A motor should not have continuity to ground under normal operating conditions. Continuity to ground would indicate a fault or an electrical earth leakage path between the motor’s windings and the motor frame or housing.

Continuity between phase and ground would suggest that the insulation between the windings and the motor’s grounded components has been compromised, likely from one of the reasons listed here.

Proper insulation is crucial in electric motors to ensure the safe and efficient operation of the motor. The insulation system is designed to prevent electrical contact between the energized windings and the grounded components, such as the motor frame or housing.

testing a motor phase to ground continuity
IR testing 3 phases to ground with a reading of 610 MΩ

Continuity to ground can occur due to insulation degradation, physical damage, moisture ingress, or other factors that compromise the insulation integrity. It can pose a risk of electrical shock, motor damage, or equipment malfunction.

Regular maintenance and periodic testing, such as insulation resistance tests, can help identify any insulation breakdown or leakage issues. If continuity to ground is detected, it is essential to investigate and address the cause promptly.

This may involve repairing or replacing the damaged insulation or addressing any other underlying issues that may have caused the fault.

Ensuring that a motor does not have continuity to ground is important for electrical safety and the reliable operation of the motor and associated equipment

How many ohms should a motor read to ground?

The resistance reading from a motor to ground can vary depending on the motor type, size, and its specific design. However, as a general guideline, the resistance reading from a motor’s windings to ground should typically be in the order of several megaohms (MΩ) or higher.

The N.S.A.I minimum acceptable insulation resistance must be greater than 2 MΩ between phases and phase to ground/earth.

When conducting a resistance measurement from the motor’s windings to ground, a high resistance reading or infinity indicates that the insulation between the windings and the motor’s grounded components is intact. It also suggests that there is no significant electrical leakage path to ground.

It is important to note that resistance readings can be influenced by various factors, such as the measurement device used, temperature, humidity, and the specific conditions of the motor. Therefore, it is advisable to refer to the manufacturer’s specifications to determine the acceptable resistance range for a particular motor.

If the resistance reading from the motor to ground is significantly lower than expected or near zero ohms, it indicates a potential insulation breakdown or electrical fault. In such cases, it is essential to have the motor inspected and serviced to address the issue and ensure safe and reliable motor operation.

How do you check continuity on an electric motor?

When speaking in electrical terms, continuity refers to the presence of an uninterrupted path of low resistance for electrical current to flow, continuity can be tested using a multimeter set to continuity or a low ohms (Ω) setting.

To test continuity of a motor, follow these simple steps:

  • Power off: Ensure the motor is disconnected from any power source before performing the continuity test.
  • Set the multimeter: Set your multimeter to the continuity or resistance mode. If using a digital multimeter, select the continuity symbol (usually represented by a sound wave or diode symbol) and the resistance setting is represented by the ohm symbol Ω .
  • Test leads: Connect the test leads of the multimeter to the two points you want to check for continuity. For example, to test continuity between phases, connect one lead to terminal U1 and the other lead to V1 and test, continue this until all are tested. To test between phase and ground you connect one lead to the frame of the motor and the other to each terminal starting at U1 and ending at W1. To test the continuity of each winding connect one lead to U1 and the other lead to U2 and test and continue this procedure till all windings are tested.
  • Check reading: When the test leads make contact with the motor terminals or windings, the multimeter will provide a reading. If the reading is close to zero ohms or the multimeter emits a beep, it indicates continuity, meaning there is an electrical connection between the tested points.
  • Repeat for other connections: If necessary, repeat the process to test continuity between other points or windings of the motor.

It is important to refer to the motor’s documentation for specific testing procedures and recommended readings for a particular motor. Additionally, ensure you are following proper safety precautions when working with electrical equipment to prevent accidents or injury.

Conclusion

So in conclusion, it is clear to see that the only time a motor should have continuity is when testing the individual windings of the motor, this would indicate good electrical contact between point a and point b, meaning no breaks or open circuit in the winding coil.

Gavin

I'm Gavin and Iv been teaching electrical science to apprentice electricians in a local technological university since 2022. I hold an Electrical Level 6 QQI Qualification along with several NZEB Certifications.

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