This unique millivolt testing technique provides a basic check in troubleshooting motor breakdown situations.
As a maintenance electrician, I'm sure you've been in the high-pressure situation of having to get a "failed" motor running ASAP. And, you probably had to use all the troubleshooting techniques you acquired and mastered over the years. Well here's a tried-and-true technique you can use that involves testing the contact quality on live starters. Sound odd or intriguing? It shouldn't.
Remember, your typical 3-phase manual or magnetic motor starter has three sets of power contacts and three overload (OL) relays. When you energize a motor through these types of starters, equal currents theoretically should flow through each contact and OL relay. So, you can measure the voltage drop across each contact or OL relay pole and compare the readings with one another.
Then, by using Ohm's Law, you can calculate the respective contact resistance values (technically, impedance values) and compare them. These readings will certainly give you a "heads up" as to a poor contact and potential downtime.
This technique will not set a precise benchmark, and it's not supposed to. It also won't substitute as a preventive maintenance program or replace current-injection OL relay testing or thermographic surveying. What it does do is provide a basic check for use in breakdown troubleshooting situations.
Here's how to go about it. "Safety first" should be your motto here. Before you start, remove your watch and rings, put on your safety glasses and rubber gloves, and make sure you have adequate lighting to see the interior of the starter. Also, make sure your multimeter has internal protection circuitry that prevents damage, should you accidentally apply line voltages to the input terminals while the multimeter is set at lower voltages.
Now you can begin:
1. Connect your multimeter's lead set to the correct input terminals, and set the multimeter to read AC millivolts.
2. With the starter energized, begin with the left-hand pole and place one probe tip on the line-side terminal, but electrically upstream of the OL relay and the other on the load-side terminal. Then, note the reading.
3. Repeat the process with the other terminals and OL relays and compare the readings.
Try to place the test lead probes in the same relative position at each contact and OL relay.
Remember, even though you're making a fraction-of-a-volt measurement, you're connecting your multimeter to points that are energized by line voltage. So, make sure you use the utmost caution and exercise all required safety practices. Also, be aware of the circuit voltage, available short-circuit current, immediate surroundings, working clearances, and the capabilities and limitations of your test instrument.
How about an example? Suppose you complete the testing as described above and note the measurements listed in the Table. When reviewing these readings, remember that those of the OL relays are pretty close, and the starter contact reading for Phase C is much higher than those of the other contacts. This could be due to internal problems such as excessive wear, loose hardware, or poor contact pressure.
After taking phase current readings, use Ohm's Law and calculate the respective resistance values, which should provide a relatively high value for Phase C starter contact. If you leave the starter as is, the heating will continue, and the starter will eventually fail.
You can use this online millivolt-drop test on contactors that serve other loads as well. In fact, with a little experience, you can measure the voltage drop on a cable termination. Where there's a small amount of bare cable exposed, place one probe on a strand and the other on the corresponding lug or bus bar and measure the millivolt-drop. Then take current readings and calculate the resistance at the termination.