How to Test a Motor with a Multimeter? (3-Way Guide)

A faulty motor can cause a lot of trouble. So, you never know when you may have to test your motor. That is why today we are going to look at how to test a motor with a multimeter. However, for this process, you need some DIY skills. With some DIY skills and proper execution, you can get the task done pretty easily.

In General, to test the motor, first, you need to set the multimeter to resistance mode. Then test the motor terminals and wires. The aim is to check the windings for open or shorts circuit.

Apart from the method above, there are two other different methods by which we can test an electric motor. Here we are going to discuss all three motor tests. So, let’s get started.

Test 1: Compare the Capacitor’s Terminal Voltage with Applied Voltage

When connected properly the voltage of the capacitor’s terminal should be 1.7 times the voltage of the power supply. If you get a reading according to the ratio mentioned above, that means the motor is getting proper voltage. For this motor test, we are going to use two multimeters; Circuit Tester A and Circuit Tester B.

applied voltage - diagram

Step 1 – Check the Power Supply Voltage with Circuit Tester A

Just like in the diagram shown above, first, connect the red probe of the multimeter to the red wire; connect the black probe to the black wire. This is the process for Circuit Tester A. The multimeter should be in AC voltage mode. You should apply the required settings for the multimeter before connecting it with the motor. If you follow these steps correctly, then you should get the voltage of the power supply. If you use a 100 VAC motor, you will get 100V reading on the multimeter.

Step 2 – Check the Capacitor’s Terminal Voltage with the Circuit Tester B

Now, use Circuit Tester B to check the capacitor’s terminal voltage. Link the red probe to the red wire. Then connect the black probe to the white wire. Now check the voltage of the multimeter. If all the connections are good, you will get a reading of 1.7 times the power supply reading.

For instance, if you are using a 100V motor for this test, you will get a 170V reading on the multimeter.

When you get a reading that is equal to 1.7 times the power supply, that means the motor is working properly. However, if you don’t get such a reading, there might be an issue with your motor.

Test 2: Check the Electricity That Is Conducted Through the Cable

Any type of faulty wires or connectors can be the reason for a bad motor. So, it is always better to check the wires and connections, before jumping to any conclusions. With this method, we are going to check whether the motor circuit is closed or open with a simple continuity test.

conduction through extension cable - diagram

Step 1 – Cut Off the Power

First, turn the power off. When executing a continuity test, there’s no need for power.

Step 2 – Make the Connections According to the Diagram

Check the diagram above and connect Circuit Tester C and D accordingly. To do that, you have to connect C’s red probe to the black wire and D’s red probe to the red wire. Now, connect the remaining two black probes of the C and D to the end of the extension cable. If there are any breaks on the checked circuit, the multimeters will start beeping.

Note: While testing wires, always choose a location that is exposed and close to the motor. When connecting the probes to the wires make sure to connect them properly.

Test 3: Check the Resistance of the Motor Winding

man checking the resistance of the motor winding

In this test, we are going to measure the winding resistance of the motor. Then, we will compare it to the originally designed values of the motor winding. Afterward, we can check the motor’s condition according to the two values.

winding resistance - diagram

Step 1 – Remove All the Additional Components

First, remove the additional components such as capacitors and extension cables from the motor’s circuit.

Step 2 – Set Up the Multimeter

Now set the multimeters to resistance mode. If you remember, in the previous two tests, we set the multimeters to voltage mode. But not here.

Step 3 – Connect the Probes

Connect both of the black probes to the black wire. Now connect the red probe of Circuit Tester E to the red wire. Then, connect the red probe of Circuit Tester F to the white wire. If you are still confused, study the diagram shown above. (1)

Step 4 – Check and Compare the Readings

The readings of the multimeters should be 170 ohms, given that is if, we use a 100V motor. Sometimes, these readings can be smaller than 170 ohms like when there is an internal short circuit, the readings can be smaller than 170 ohms. However, if the windings are broken, the readings should be over a few thousand ohms.

For the example above, we used a 100V motor. But, when it comes to other motors, you will have to find out the designed values according to the model. Try and find it on the internet or ask the manufacturer. Afterward, compare the two values. (2)

What Should I Do When the Motor Does Not Pass Above Tests?

If your motor does not pass these tests, that means there is something wrong with your motor. The reason for this issue could be a bad motor, or it could be the bad components such as; bad relays, switches, cables, or the incorrect voltage. Whatever the reason, you’ve got yourself a faulty motor.

However, according to each test, the solutions may vary. For instance, if the motor does not pass the 1st test, the problem is in the wiring or the capacitors. On the other hand, if the motor does not pass the 2nd test, the problem is in the connector or the cable. For a good understanding, here is a simple guide.

If the motor fails Test 1, you might have to replace the wiring and the capacitors.

If the motor fails Test 2, you might have to replace the connector and the cable.

If the motor fails Test 03, you might have to replace the motor.

Mechanical problems such as faulty ball bearing can trouble the operation of your motor. This kind of situation happens because of the excessive axial or radial load. You might have to test for these kinds of issues too. So, follow these steps.

Step 1: First, remove the gear head and the motor.

Step 2: Then, turn the shaft clockwise and counterclockwise.

Step 3: While rotating the shaft, if you can hear abnormal friction or a sound, it is a signal for misalignment or damage. In this case, you might have to replace the motor.

Wrapping Up

These three methods are the best solutions for electric motor testing. If you follow these steps correctly, you can identify the condition of any motor. However, if you still have some doubts, feel free to go through the article one more time. 

Take a look at some of our related articles below.




References
(1) diagram – https://www.computerhope.com/jargon/d/diagram.htm
(2) internet – https://www.livescience.com/20727-internet-history.html

About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gI realized early on carpentry was a huge passion for me and I’ve stayed in the industry for over 20 years now. This gives me a unique ability to really be able to tell you what the best tools and recommendations are. I’m not only a carpenter but I also like machinery and anything to do with electrics. One of my career paths starting off was as an apprentice electrician so I also have a lot of experience with electrical products and anything related.