A Real Learning Experience – Electrical Diagnostics

The other day, I had a phone call with a service tech on a rooftop unit.  He had made a diagnosis of a bad motor and went and got a new motor.  He put the new motor in and “it only runs slowly and sounds like it is laboring” and wanted to know why?  Let me give you the whole story……

Phone call # 1. The first thing I ask is for the specs on the motor and he gives me the info off the motor data plate.   The first thing I notice is it is a dual-voltage motor (208-230/460 Volts 3 phase) and explain that most manufacturers send out those motors on the higher voltage internal wiring to be safe rather than sorry. I ask what the voltage is to his unit and he says, “208 volts”. I ask him how his motor is wired and did he set it up for the correct voltage?  He says, “I’ll call you back!”

Phone call #2.  He says he changed the wiring for 208/230 volts, but it is still doing the same thing. He then says,  “It’s got to be a bad motor -right?” I asked him, at this point, why he thought the original motor was bad and he said, “it was doing the same thing. It would not come up to speed and would trip the internal overload eventually”. So I ask him what were the odd of both motors doing the same thing?

So, what is he missing here?  Why would a 3 phase motor run slow, sound like it is laboring, and eventually trip an internal overload? Let me ask you — what are the symptoms of a 3 phase motor missing a power leg?

So , while I still have him on the phone, I tell him just what I said above and tell him he needs to check to make sure he did not “lose a leg” of power. So he gets his meter and says he has 113/118/218 volts at each leg at the disconnect. So now I ask him what he has at the motor?  He says he doesn’t want to run it for fear that he’ll burn it out.  I tell him, ” take the 3 wires off the motor, turn the unit on and energize the blower contactor and then see what he has on each leg.  He says he’ll call me back.

Phone call # 3. He calls me back and says all his power is good!  I ask him, “where did you  measured it”? He says, “at the contactor”.  I ask, “which side”? And he says, “the line side!” OK, I say, “what do you have on the load side going to the motor”?  GUESS WHAT–on the load side he has 118, 218, and 10 volts when he read each leg to ground..  Then he says, “but when I read between the pairs, I read 203 volts.”  I tell him he was reading through the windings.

Well after all was said and done, and 3 phone calls later, he changed the contactor and , lo and behold, the motor runs fine!  He thanked me for the assistance!

The point I’m making here is to always do proper diagnostics.  In this case, he never checked each power leg to the motor to ground  and just read between the pairs.  Because they were still attached to the motor, he was reading a “back feed” through the motor windings and thought he had proper voltage and condemned the first motor.

Now that original motor may have been bad since it had been single phasing for a while.  Despite this “common” diagnostic mistake, he had enough sense to ask for help instead of condemning the replacement motor.  What was good about this is the fact that he was willing to do what was asked of him instead of coping an attitude.  He wanted to solve the problem.  I truly feel this was a great learning experience for him and he won’t ever make this mistake again. Hopefully you readers have learned a little something here also!