Alright, let’s talk about using a digital multimeter on a three phase motor. I’ve done this so many times that it’s almost second nature to me. You’ll need a decent digital multimeter, and you’re ready to go. First things first, you want to check for continuity. Set your multimeter to the continuity setting, which usually looks like a small microphone symbol. Touch the probes to the motor windings. You should get a reading close to zero ohms. If the reading shows OL (open loop), it means the winding is damaged. Once, I was diagnosing a Three Phase Motor at a factory, and the winding read OL. Swapped it out, and the motor ran smoothly again.
Next, voltage is crucial. You want to measure the voltage of each phase. Set your multimeter to the AC voltage setting. Ideally, you should see voltages around 220V or 480V, depending on your motor’s spec. One day, I was working in a workshop where the motor specs required 400V, but we were only getting 350V on one phase. Turns out, it was a faulty connection that needed tightening. Once fixed, bam, the motor spun at the right speed again.
Current measurement is next. If you want to measure the current in each phase, you’ll need a clamp meter accessory or a multimeter with a clamp function. During one of our projects, we were troubleshooting a motor that kept tripping. Using a clamp meter, we found out that one phase was drawing 10 amps while the others were at 7 amps. It pointed us to a mechanical issue causing uneven load distribution. Adjusting the motor fixed the issue, saving the company significant downtime.
Let’s move on to insulation resistance. This is a bit more technical. Use a megohmmeter to measure insulation resistance between windings and from windings to the motor body. When I first started in this field, I undervalued this step. But one day, we had a motor with frequent breakdowns. Checking the insulation showed it had degraded to below 0.5 megohm. We replaced the motor, eliminating the problem entirely. Always aim for readings above 1 megohm to ensure good insulation.
Lastly, rotation direction is something you can’t overlook. Before switching on the motor, use a phase rotation meter to ensure proper phase sequence. During an installation job, I once skipped this and ended up running a conveyor belt in reverse. Nowadays, checking the phase sequence is habitual for me. Proper phase sequence ensures your motor runs in the intended direction. Get a reading and if it’s incorrect, swap any two of the three-phase wires to correct it.
Temperature readings can also offer insights into motor health. Using an infrared thermometer, you can measure the motor housing temperature. During high load conditions, one of our motors was overheating. We measured close to 90°C, which was way above its 75°C rating. This temperature spike led us to find a ventilation issue. Once resolved, the motor temperature stayed within the acceptable range, ensuring its longevity.
Bear in mind, these checks are essential but not exhaustive. A three phase motor, like any piece of machinery, has its quirks and quirks. Over time, you’ll get a feel for what’s normal for your specific setup. I always keep a log of motor readings in the field. It helps to refer back to historical data during troubleshooting. Continuous monitoring through digital multimeters and other tools can be your best defense against unexpected motor failure.