As mentioned before, when we do a motor test it is important to include the mechanical side as well. Let’s look at performing a simple motor test. For this test I will work on the basis that the motor has been removed from the installation and is sitting on your workbench.
1. Take the motor shaft and turn it to make sure it spins free. If it does, listen to the sound of the bearings. Be on the lookout for a grinding noise which will indicate failed bearings. As a rule of thumb for me, if the motor has been removed from service, I will be changing the bearings anyway providing it passes the test.
2. Check for any cracks on the mounting feet, flange (if flange mounted), endshields terminal box etc. This sort of damage will lead to failure when you least need it and should be repaired before putting the motor back in service
3. Check the condition of the fan and fan cover. Always ensure the fan cover is in a good state and covers the fan completely. This is a very important safety aspect as you don’t want anyone getting a finger in there when the motor is running.
4. Only now will I move to the Electrical part of the test which I do as follows:
4.1. Connect one lead of your insulation tester to ground. Ensure you have a good connection by pressing the test button and touching the other lead onto a different spot of the motor casing. Remember to set your meter to the 500V scale. If your connection is good, you will get a deflection on your instrument the same as you would by touching the two leads together, ie. short circuit
4.2. Leave the one test lead connected to earth and touch the other test lead on each end of the different motor windings. Write down the reading obtained. Although Wiring Regulations state that the reading needs to be higher that 1Mega Ohm, I prefer the reading to be above 2 Mega Ohm and best case scenario, a reading of infinity. Now also test between windings for a short. Connect one test lead to a winding and the other to another winding. Again I will be looking for a reading above two Mega Ohm and still prefer the infinity reading. Proceed to do this between all windings. If the readings obtained indicates that there is no short down to earth or between windings, proceed to the next step.
4.3. Change the scale on your insulation tester to 200 Ohm, disconnect the test lead from the earth position and now we will test the continuity of each winding. This is done by testing between the two ends of the winding and recording the reading. Remember, you are now reading on the Ohm Scale! Once you have recorded the readings of all three windings (this test is based on a three phase motor), you need to compare the readings obtained. I will be looking for the readings to be at least within 10% of each other. If it is within this range, I will deem the motor Electrically Sound and ready to be put back in service providing all mechanical areas are also repaired and bearings replaced
Just as a short explanation, each winding has a starting and ending point. These are numbered as 1 and 2. You will see on the winding or the terminal block that the are marked as U1, V1, W1 and the other side U2, V2 and W2. Make sure you keep all the ones and two’s in the right spot! You don’t want to get one of them the wrong way round when connecting the motor to power. So what do you do if you get handed a motor with six wires dangling loose in the terminal box and they are not marked? Don’t despair, I will show you a very easy way to test them and get the numbers sorted in another post.
For now, I hope you find this post useful. Please feel free to make comments or ask questions.