OT--Electrical Question

Has anyone seen a motor switch phase? Specifically, this is a blower motor on a 10 ton A/C unit. Twice within the last 4 months, service was called because of no heat. Upon inspection, they find that the blower is running backwards (switched phase). Now, yes it is possible that someone has tampered with the unit, but there is another unit next to it that has not had any problems and nothing else in the building has changed. Any thoughts?

Mark

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Mark
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is it posible there's a reverse air flow when the blower is not energised? is the blower motor itself a single phase motor? if it's a single phase motor it will run either rotation once started rotating, depending on the type of motor. good luck, sammmm

Reply to
SAMMM

SAMMM

Motor is 3-phase. As far as the air flow reversal; I think that there would have to be a very high positive air pressure in the building for that to happen. But, at this point, anything is possible.

That's for the input, though.

Mark

Reply to
Mark

check for an open/missing phase.

Reply to
jeff

Yup.

Good possibility the fan contactor is going bad else there's a loose terminal someplace nearby.

Reply to
PrecisionMechanical

Ill elaborate some here, since I have a few minutes to spare....

Where you have fresh make up air requirements, and a multi staged system in operation....what can happen is that one unit ends up drawing in outside air through the damper belonging to the other unit, or similar.....maybe natural draft because a main bay door is opened...thus "kick starting" the second stage fan (that has the dropped leg)....in essence, it becomes its own "mini rotary converter"...

Other similar scenarios exist....for instance, where it concerns the condensor instead of an evap fan, then high winds can often provide enough momentum where a phase is dropped....

Handy tool to use here might be a 'laser pointer thermometer'....beings that a bad termination or contact usually runs decidedly warmer...

I once had a rpc connected that was simply too small...what happened was that I HAD to let the Bridgeport motor come to a complete stop...else upon reversal, the idler motor would reverse instead....

==

And to the OP, do appreciate let us know what you find out.

Reply to
PrecisionMechanicaL

And it takes surprisingly little air pressure differential to get a fan (well balanced) spinning pretty darned good. With sequental defrosts in large freezers, even with dampers closed, the fans that have been turned off often get driven in reverse by the remaining fans. There is enough momentum once a large radial gets going that it acts on the airflow like a fan in reverse...you'd swear there was power to the motor but it's only the small air differential that keeps it going (or sucking) far more air than one would expect. I would anticipate that there would be enough momentum to start backwards easily under the right power conditions.

Koz

Reply to
Koz

Sam is right, the only way this could happen is if you lose 1 phase.

Gary H. Lucas

Reply to
Gary H. Lucas

Hi Gary,

Actually that was Jeff's initial assessment.....

I merely added more info as to whys it ometimes go one direction, sometimes the other....less noticable in fact might even be where it's basically inoperative....repeatedly locking out instead on thermal or other safety trip...

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Okay...here's the deal....imagine having quit a few units upon the rooftop...all are fed from a common buss...lets say 200 amps...there's a separate disco at each individual one..the tech leaves...all of em are running fine....

But there's a bad connection someplace in that 200A main buss...

Now, a few days later....you get a particularily hot ( or cool ) day....so at some point all units eventually will shut off....

Now fun begins when they go to restart...much depends upon the staging sequence...anything located before the bad spot in the wiring is gonna start up just fine...but those others will have problems, repeatedly tripping then getting reset on thermostat recall....only to trip out again...

Now with all this happening, imagine that some dampers also get opens up, for fresh air makeup etc, with these being under strict timer control and so not necessarily dependant upon a call for heating /cooling...( this occurs under low voltage control, usually a separately fed buss or from t transformer installed at the unit)....well...suddenly you now can get maybe a pair of fans spinning ...maybe one or even both em the right direction, or maybe not...but at any rate now you have a pair going, so easy enough to get a few more spinning then, eventually...finally, a compressor or two can get going... and so on it goes....till comes the occasion where the initial call for heat or cooling has satisfied and so all the units again shut down...

Reply to
PrecisionMechanical

To All

First, thanks for the responses. Now to fill in some facts that I should have included. First, all the A/C units service separate zones. There is no common duct work; that's why I didn't consider a back flow of air from another unit as there would have had to been a large positive pressure in the building. Second, and probably most important, the blowers are belt driven "squirrel cages". Not easy to get spinning in the opposite direction with just air flow. And, from what I was able to find out from 3 different electricians ( over 120 years of experience, total ), these type blowers will still flow air in the same direction even if spinning backwards, BUT at a reduced ( < 75% ) efficiency. Also, without physically changing legs, they say that the motor will always run in the same direction.

Long story short. Unit ran 12 years with no problem of "swapped phase". Needed unit serviced in January. No air flow. Belt was broken, but at this time the "tech?" says the motor is spinning the wrong way. Keep in mind that the unit will flow air even if spinning the wrong way. Only conclusion, tech didn't know what he was doing, so he physically swapped legs to reverse motor. Now, no air flow again in April. Tech finds belt broken and the motor is turning the wrong direction. He swaps legs and everything is back to what it was for the previous 12 years before the tech came in January.

Again, thanks to all.

Mark

Reply to
Mark

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