You’ve no doubt heard the expression, “If all you have is a hammer the whole world looks like a nail.” This expression also applies in maintenance and troubleshooting.
Maintenance electricians typically carry a digital multimeter (DMM) with them. Considering what a DMM can do, this makes great sense. But it also can trap them into thinking in that hammer-and-nail fashion.
Suppose a motor opens its overload strips. You replace those and use your DMM to verify the correct voltage. No low voltage or voltage imbalance here, so it’s all fixed now!
But the motor can be vibrating. You don’t have a vibration tester with you, so should you check this? Absolutely. And you should use a higher-end tester so you can tell what’s going on other than “lots of vibration.” The meter should tell you the velocity, displacement, and acceleration. Even before using this instrument, you should put your DMM away and visually inspect the motor base and pedestal.
Vibration is only one of many potential causes your DMM isn’t going to detect. And it is not even a root cause; you will need other test equipment to prove and correct a root cause such as a misaligned load.
That vibration tester might not help you do anything other than eliminate vibration as a cause. What other tests might you perform? What about any or all of the following?
• Thermographic scan of the motor and of its immediate environs. A nearby heat source might be the culprit.
• Harmonic analysis of the branch-circuit and/or feeder supplying the motor. Good thing you found that big 5th harmonic!
• Power quality analysis of that circuit. Hmm, that flat-top does not look like a healthy sine wave.
• Bearing temperature measurements. If you fix other problems but don’t detect hot bearings, how are you going to look the next day when they fail?
• Ultrasonic analysis. This will reveal impending bearing failure and/or other problems.
• Insulation resistance tests. Is the insulation on those windings OK?
Although the DMM is indispensable, you need to be thinking of what tests you can perform to diagnose the problem rather than what tests your DMM can perform. If you’re troubleshooting critical equipment, good methodology requires running a full battery of tests. If you stop when you “find the problem” you may be stopping only when you find one of two problems.