2 fan motors went up in smoke


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Old 05-29-20, 04:46 PM
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2 fan motors went up in smoke

That was a catchy title, eh?

I'm scratching my head as to why 2 motors both burned out the low speed windings on a 2 speed blower motor. One new and lasted for a full 4 months with very light heating (low speed) operation, and a week of high speed (AC) before the "uh-oh" I smell smoke moment. Low side was burned out while in an AC mode. Weird.

Its a 30 y/o Bryant air handler unit with heat strips and an outside ground mounted (30 y/o) Whirlpool compressor that literally refuses to die. I've got great cooling.

This new motor was replaced in Jan only because of a bad diagnosis (slow blow fuse blowing), and I had the old motor (a solid 20 years old GE version) on hand that I pulled out at that time. I dodged the heat in the Phoenix attic and installed the old motor again on Monday, and life is good. Only for a week. Then smoke. My local electrical motor shop was kind enough to confirm it was the low side on both motors that failed.

I'm confident its not low voltage to the supply box in the attic, that was the cause of the slow blow fuses failing from heat (not voltage spike) and have fully diagnosed the amp draw on the motor and the heat strips. The box failed to have adequate spring tension and the terminal lugs heated up, heating up the fuse. Box replaced, problem solved, no blown fuses.

Separately, today I called Packard, who makes the fan relay that was replaced in Jan (only for sake of not wanting to climb up there again), and the old one bench tests fine. I've not bench tested the new (currently installed) relay, but after understanding the inner workings, it is probably not the culprit of what I feel is the root cause of motor failure. It just switches high and low speeds based on signals it's been given. It would have to fail in a "both circuits connected" mode, and that would surely cause immediate plumes of acrid insulation smell if that was the case. Agreed?

My hypothesis is both the low speed and high speed are being activated at the same time.

I have a Honeywell programmable (10 years old) Tstat. It has a definite heat or cool setting switch. Could it be somehow calling for heat (low speed fan) and cooling (high speed fan) at the same time (at random) if it has or is failing?

I've band aided the problem by only hooking up the high side of the fan motor (#3), and what I heard today from the motor shop is that this is common for AC techs to do that to avoid fan failures, and return service calls on their dime. I'm 24 hours into this "new normal" of a one speed fan and no smoke yet. While I have plenty of documentation on the wires in the box to let others know what the capped off wires are for, I sure want to solve/diagnose the root cause of short motor life.

How say ye of great knowledge?

Wow, sorry for the long post on a Friday



 
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Old 05-29-20, 05:49 PM
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My hypothesis is both the low speed and high speed are being activated at the same time.
Ok.... so did you check that hypothesis out ?

Possible problems......
miswired motor.
undersized motor.
Fan relay...... extremely doubtful. A fan relay uses the NC contacts to run the heat speed and when that relay is activated..... the NC contacts open and the NO contacts close.... actviating the high speed for cooling. It's almost impossible for both sets of contacts to be active at the same time.
 
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Old 05-29-20, 08:06 PM
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Without being in the attic with a DVOM monitoring the situation, I am at a loss to diagnose. Any thoughts there?

Miswired motor? For #1 (new unit), while anything is possibly, highly improbable with my pictures and notes taken during the install. Besides, it lasted 4 months. For #2 (prior used unit) being out of service for the same 4 months. Absolutely zero possibility of miswiring. Again, it operated for a week before failure. I'm thinking a miswire would be a more immediate failure.

Its a squirrel cage motor. All units (2 new, and 1 prior installed) are 1/2 hp. The prior motor during the mis-diagnosis drew 4-5 amps depending on the leg at start up, then down to 2 to 2.8 amps during running.

 
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Old 05-29-20, 08:17 PM
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So is the normal running current within nameplate spec for amperage ?
Try running the blower for 15-20 minutes.
Turn off and feel the motor. Is it real hot ? That would not be good.
 
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Old 05-30-20, 02:52 AM
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Thats a great next step, and I would need to climb up into the attic very early before things heat up here (quickly) in the desert southwest, where the overnight low is 80+, and inside the attic is probably 90, then will quickly climb to who knows what temp in order to do a temp check of the fan casing. I have a digital heat gun, so it is possible.

My measurements in Jan revealed an amp draw of 4 or 5 (depending on the lead) at startup and 2 to 2.8 when running. The nameplate says 3.2 amps. I'm probably good there.

I guess I can do some testing on the Tstat output and see if by playing with the temp settings, and operating the heat/cool switch if I can get the Tstat to output signal voltage (guessing here) to both the yellow (cool) and red (heat) at the same time that the green (fan) gets the signal. Am I right with that theory that the fan would then give both high and low inputs through to the relay at the same time, then on to the motor resulting in high and low speeds trying to happen at the same time?
 
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Old 05-30-20, 11:01 AM
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Many blowers are timer controlled from a control board.

If you have a fan relay.....
When the relay is off...... the normally closed contacts are engaged. This is the circuit for the heat blower speed. One N/C contact goes to blower slow speed winding and the other contact goes to typically a heat operated fan/limit switch. When the furnace gets hot...... it supplies power to that NC contact.

When the G wire is activated..... it opens the normally closed contacts and closes the normally open contacts. One of the NO contacts is always live and the other goes to the high speed blower winding. There is no power sent out to the low speed blower winding even if the furnace is heating.

Once the G wire is powered...... the blower is at high speed and connecting the heating wire will do nothing.
 
 

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