To make long story short, I have this bad air handler blower problem, a technician came and determined the motor is bad, so he replace old motor with new motor and new capacitor. The unit work for little while then it got overheat and shut down. So we are back to square one, i.e. the blower will run for 10 minutes or so, then shut it off, ice would build up on the evaporator coil.

The tech came and could not figure out the reason; then I notice the replaced motor is rated 1 hp vs the old 1/2 hp, but they both has same 1075 RPM, 230 voltage. Would that cause this problem ?

Old motor: GE 5KCP39PG
New motor: Dayton 3LU92J

Many thanks

What protective device is causing it to shut down? I doubt there is a thermal in the motor. Not sure what might shut just the blower down. Protection wise.

The only thing that makes sense is a bad contactor. The current that the motor is pulling needs to be checked as well as the voltage across the contactor when the motor is running.

The only thing a 1 HP motor will do is cost you a little more to run the unit.

Does the new capicitor match the new motor? Could be a good cap. But the wrong cap

Myself I would never exchange motors that was a 1/2 horse to big,but that's me

I believe it's thermo shutdown, because the motor is pretty hot even after a few minutes run, capacitor should match the motor but I will double check. Also when blower is on, air flow is rather weak, you can hardly feel it at the vents, specially from the intake vent.

All most all HVAC style motors have a thermal safety built into the motor windings.

My best guess is that there is something obstructing the air flow causing the motor to go out on overload/thermal safety.

In a residential setting there should never be a need for a one HP motor. The reason is the duct system cannot handle the amount of air that the motor could produce. There is also a limit on the air movement since the blower wheel can only move so much air at a given RPM to start with. I would be looking for a system problem and if you can look at the underside of the A/C coil. I bet you find a lot of dirt and debris there.

Example

One more item. Is the motor running in the correct rotation. If the motor is running in the wrong rotation it will cause the problem you have also.

Thanks for the suggestions, underside of coil is indeed pretty dirty, but how would that affect the air flow through coil ? I also noticed today that the coil is icy half way before motor shut down.

Also how can I determine whether the motor running on correct rotation ?

Thanks again.

Hint:

How does a dirty air filter restrict the air through the system. Dirt clogs the holes in the evaporator coil, thus little air will pass. You do have a filter, right? And you change it monthly, right?

If air comes out the return, the motor is turning backwards orit's the wrong squirrel cage. In a squirril cage, blower air must enter he center and exhaust radially on the outside surface. They pump no air, if installed backwards.

A small piece of newspaper, say 1" x 6" can act as a teltale and give you the direction like a flag if you hold one end with your fingers in the air stream.

You can tell by the pitch of the blasé, but that's harder.

The air filter is electric one, I took it out to give it a good clean; the coil itself is relatively clean. So I give it another try without the air filter, same thing is happened again, motor running weak and shut itself out after 40 minutes. :(

I think the motor is running at right direction, because when it running I can attach a tissue paper and it would stick to the intake vent. Capacitor match the motor according to the spec.

It's getting frustrated, it's only a blower system with 3 parts (motor, capacitor, relay). Not a nuclear device, how hard can this be ?

I believe you are going to have to lower the blower speed. Since you do not have a Amprobe to check the current draw situation I recommend you lower the speed one setting and see how that works.
Now the person who came and installed should have checked all of this when they were there and did the work. I also know you are smart enough to have already check to be sure all your registers are open and there is not much else to check.

According to the link below your motor is a 4 speed design.

Motor,1hp,D/D Blower - Direct Drive Blower Motors - HVAC Motors - Motors : Grainger Industrial Supply

And you said you have the 20 MFD capacitor installed already and as long as it is a 20 MFD in capacity that is not the problem.

What colors are now being used for the motor speed operation?

The capacitor is 20 uF as required.

Only white and black wires are connected to the relay, I think it's wired at highest speed. Though it still does not feel like move much air.

When it work properly, you should have too air and noise. IF POSSIBLE could you run it with fan ON ONLY, with the access door open, to by pass the coil, provided that it won't draw dirt into the fan... or don't put it in the unit but run it outside(brace it with something heavy)

Another item the motor installer should check is the A/C interior coil. If it is plugged up with dirt it will cause the exact problem you are having. SEE MY NOTE BELOW on this subject. You can try to adjust the speed down to compensate for the restriction BUT this will not solve the problem if the coil is dirty.

Standard A.O. Smith 4 speed wiring diagram for speed selection.

White to one side of the 220/240 volts power supplied for motor

The other side of the power supplied to the motor is a selection of ONE of these colors/speed wires.

Black for high speed

Yellow for medium high speed

Blue for medium low speed

Red for low speed

I believe you are trying to make the unit blow harder by trying to force a bigger motor to run at higher speed. THIS will not work due to duct work limitations or the AC coil if plugged up.

I would pick a lower speed like the yellow for starters. If the motor still trips on the internal overload then go to blue if it still trips you will be forced to use the red.

Over many years of service work and field rep for manufactures I have seen a bigger motor installed trying to compensate for a problem elsewhere. As I mentioned in a previous post the installer should have used an Amprobe to determine the maximum speed/correct wire color selection to avoid this entire problem. The installer/service person could have also used the Amprobe reading to detect a plugged up coil. NOTE: You have to look on the side of the coil where air enters to see if it is dirty and on a standard furnace install that is about impossible to see so if you looked at the coil as you said you did then you were probably looking at the wrong side. The dirty side is usually placed right on top of the furnace/air handler and is not readily accessible.

At this point I would have to call the professional installer/service person an IDIOT for doing this to your system and you can feel free to tell them I said so.

Thanks for the suggestions.

1) I have run the blower with fan only with same result.

2) both side of the "coil plate" are accessible, and I can see it through with flash light. (blower is by A/C only)

3) original motor of 1/2 HP had enough blowing power, but the repair man order the 1 HP unit because he thought the original one was 1 HP.

I am thinking the new motor is probably bad right out of box.

I agree. U have an idiot maquerating as a professional . Duct, dirt, speed, wrong pitch fan, wrong direction, both, bad contactor. This is trivial stuff. Squirrel cage blowers blow hard. What happened from the very beginning of the start of problems?

Note: take any fan, put a board in front of it. Which way does it blow? Sure way to tell, piss into both air streams. It blows opposite the side u get the least wet.

Quote:

---

Originally Posted by KeepItSimpleStupid

i agree. u have an idiot maquerating as a professional . duct, dirt, speed, wrong pitch fan, wrong direction, both, bad contactor. this is trivial stuff. squirrel cage blowers blow hard. What happened from the very beginning of the start of problems?

---

Original problem was blower blowing weak, and shut itself down after few minutes run, due to overheating, same as what I have now.. :(

Dirt, lots of it or bearing friction or bad contactor in that order. This assumes direction is right.

New motor had been replaced today, now the motor can run for hours without tripping thermo protection. However the air flow still so weak that it's hard to notice at vent. Now we have new motor, new relay, new capacitor, clean filter, what else is there ? I cann't wait for arrival of fall season.

If the squirrel cage was not replaced, try reversing the motor direction.

Electric Forced Air Furnace - InterNACHI Message Board

It's the only thing hat makes sense.