Why your electrician may not be the best person to call with your motor problems

Posted on: December 18th, 2013 by admin No Comments

Why your electrician may not be the best person to call with your motor problems

If your lights won’t come on you’d call an electrician, right? You wouldn’t call a plumber, a roofer or, in all likelihood, a motor technician. So why, if you have a motor problem, would you call an electrician?

Only a few electricians understand the electric motor. If you need a piece of conduit bent, they’re your best bet. Don’t know what size and type of wire is needed for your 15 amp receptacle within 3 feet of a water faucet? Call your friendly neighborhood electrician!

But when the motor doesn’t start on your air compressor, or the return air fan is making noises that it has never made before you need to rely on the person who has the experience and knowledge dealing with problems similar to yours.

I’m sure that if you are a licensed electrician your blood pressure just went up. You may know the difference between a Wye/Delta connection and a PWS connection but, odds are, you don’t. I could not count the number of times an electrician has checked a three phase motor suspected of being single phased by measuring the voltage to ground at the load side of the contactor. (Hint – that won’t find the problem. If you don’t know why, you just proved my point). Does the average electrician, or even an above average one for that matter, understand the ramifications of using a Design C motor to replace a Design D motor? Let me give you an example to demonstrate.

A customer has a 60HP Lincoln motor on a Return Air Fan. One morning, after replacing filters and doing the usual preventative maintenance on the system, the staff maintenance personnel note that the fan did not start when they flipped the switch. So they call their electrician.

The electrician comes to the job site, opens the cover on the disconnect switch and sees a six pole knife switch. The leads across the top are color coded with tape, with three colors used, left to right and then repeated. The six leads coming off the bottom of the switch contacts are not color coded, but they are numbered. The electrician places the leads of his multi-meter across the different combination of motor leads and declares the motor winding to be bad – it is “open”. The motor should be pulled and sent to a motor shop.

The customer looks at the application. In order to enter the space where the motor is mounted he must boost himself about four feet into the air and crawl on his hands and knees into a space that is not tall enough to stand up. Then he is faced with a motor that is mounted vertically, shaft up, belt-driven to the shaft that supports the fan. There is no means of supporting the 500 pound motor while the mounting bolts are removed. In service man parlance – “It ain’t going to be pretty!”

The unanimous decision of the maintenance staff was “We want a second opinion”.

A second licensed electrician was called to the job site and asked to analyze the situation. It didn’t take long for the diagnosis – the motor has an open winding and has to be pulled. There is no continuity through any of the phases (it was a three phase motor).

Still hoping for a better result the customer called us; “We need you to come and check out a motor on our air handler” was their plea. The maintenance man told me “all I want to hear you say is that I don’t have to pull that motor out”.

We always try to get the background before we begin a diagnosis. We learned that the unit had been running until it was shut down for PM. The disconnect switch had been thrown to the “OFF” position, the work done inside the air handler, and the switch turned back on. The control for the system was remotely mounted so it was necessary to walk down to the control room and reset the control. Much to their chagrin, the motor would not restart.

This information in itself would lead one to think it was not a motor issue as the motor was running fine when it was shut down. A quick continuity check confirmed that the winding appeared to be completely open. This also would usually be incongruous with a motor that was running fine upon de-energizing. So what’s up?

A simple explanation comes from an understanding of how an electric motor works and, in a related discussion, how a generator works.

When a voltage is present across a closed electrical circuit an electric current is passed through the circuit. When that current is flowing through the circuit it sets up a magnetic field. If a conductor (an element capable of conducting electricity) is moved through the magnetic field a voltage is induced across the conductor, the polarity of which is opposite in polarity to the polarity of the original applied voltage. The magnitude of the opposing voltage is dependent on the strength of the magnetic field and the direction and velocity of the motion of the conductor. What we have is a very basic generator.

An electrical meter, in order to measure resistance/continuity, must supply a voltage across a closed circuit. The magnitude of the current flow is monitored and interpreted by the meter in units of resistance (ohms). A known DC voltage across an unknown resistance will produce a known current flow, according to Ohm’s Law (E=IR). Most meters operate on a relatively low voltage (typically 9 volts DC). Even this low a voltage is sufficient to set up a small magnetic field in the motor’s stator (field winding). So the potential exists to generate electricity. Now all we need is relative motion between a conductor (like the cast aluminum rotor bars in the motor’s rotor) and our small magnetic field!

Because the fan is a Return Air fan it typically takes air from the space it services and “returns” it to the system. Because the Supply Fan was still running the air was being pushed into the room, building static pressure and creating air flow across the fan, creating rotation of the fan and therefore the motor’s rotor.

VOILÀ! We have a generator! Not a large generator. Not large enough to power your TV while watching Soap Operas all day, but a generator none the less; sufficient in output to overcome the small voltage of your meter and give a false reading. We had only to stop the rotation of the fan blade to confirm our suspicions. The motor winding was fine.

“You don’t have to pull that motor out” I told the customer.

Following a round of applause from the maintenance staff we were able to find a defective disconnect switch – one that had eluded the detection of two electricians. (Not to point fingers, but they had also missed the fact that the phasing tape was also misapplied, causing the power leads to be misidentified).

I am not asserting that we have never made a mistake. But the fact is that a license from the state does not, in and of itself, make someone qualified to troubleshoot. Go with the experience – Go with Gem State Electric!

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