Any motor guys here? Odd issues with 7.5Hp 1P motor

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fastline

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midwest usa
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I will try to be brief. Have this motor on a compressor. It tripped a breaker today. Investigated and could find no apparent issues and reset breaker. It is pulled 250A and popped the breaker again in 1sec. The motor was completely disconnected from driven loads.

it did something similar with high current trips many months ago but randomly started working again. This is a cap start/cap run motor. All caps bench tested good and look healthy, and start switch was investigated and cleared as functioning perfect. Contacts are nice, everything there is like new.

the aux winding shows 1.0ohms and main winding shows 0.0 ohms.


I don't know whether we should replace the start caps as a test regardless? I have never seen a motor short a winding, pull this kind of amperage, and not be obvious! there is no odor of a failure and windings appear to look fine. Megger test did not show any leaks to ground.

Puzzled.....
 
fastline said:
I will try to be brief. Have this motor on a compressor. It tripped a breaker today. Investigated and could find no apparent issues and reset breaker. It is pulled 250A and popped the breaker again in 1sec. The motor was completely disconnected from driven loads.

it did something similar with high current trips many months ago but randomly started working again. This is a cap start/cap run motor. All caps bench tested good and look healthy, and start switch was investigated and cleared as functioning perfect. Contacts are nice, everything there is like new.

the aux winding shows 1.0ohms and main winding shows 0.0 ohms.


I don't know whether we should replace the start caps as a test regardless? I have never seen a motor short a winding, pull this kind of amperage, and not be obvious! there is no odor of a failure and windings appear to look fine. Megger test did not show any leaks to ground.

Puzzled.....
Click to expand...
A megger would not show a turn-to-turn short in a winding. But it would have to be a pretty solid one to cause that current increase.
 
Our Fluke meggers have a pretty good ohms scale on them. We found the winding with shorted coil on a 100 Hp a few weeks ago. It should do well on a 7.5 Hp.

Did you zero your meter before testing? Does the motor pass the smell test? Our 100 did.
Does it have a thermal overload reset button on it?
Disconnect the motor and test the wires between the cb and the motor.
 
That is where I am at guys. No odor at all! The motor is open drip and I would expect to hear or see something with that sort of draw. The motor certain spins up. I would guess to 1000rpm but I even bumped the switch while it was running that speed and it still was pulling well over 200A. Running at that speed, even if the start circuit did not work right, it should run up to speed without a load on it.


I just megger tested as a formality. I only used my clamp meter for ohm test. I realize it is not precise but I know my meter and there was just no resistance showing up! Aux winding appeared fine at 1.0ohms.

I was wondering if something else was pulling the power but I had the clamp meter right on the wire INSIDE the motor.

It really sort of acts like start circuit issues but that device is very robust. I really like how it is built and I know the caps are in circuit at startup. If it was pulling high after getting to speed I could see a disengagement issue but it won't drive up. The electrical noise from the motor is odd.


Any chance if the start caps meter test good they could have issues? I do have an ESR meter so I guess I can check em with that. They usually just blow up.\

My general thoughts are if a winding tried to weld to itself, with that much power, that should blow up. that wire can't be but 18ga wire in each winding, or less.
 
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Could be bad bearings or a worn endbell.

If the caps test good then they likely are. They are also cheap so there is no harm in Changing them. First put your amp meter on one of the leads that clips on the starting cap and see if it is getting any current when the motor tries to start.
 
fastline said:
I will try to be brief. Have this motor on a compressor. It tripped a breaker today. Investigated and could find no apparent issues and reset breaker. It is pulled 250A and popped the breaker again in 1sec. The motor was completely disconnected from driven loads.

it did something similar with high current trips many months ago but randomly started working again. This is a cap start/cap run motor. All caps bench tested good and look healthy, and start switch was investigated and cleared as functioning perfect. Contacts are nice, everything there is like new.

the aux winding shows 1.0ohms and main winding shows 0.0 ohms.


I don't know whether we should replace the start caps as a test regardless? I have never seen a motor short a winding, pull this kind of amperage, and not be obvious! there is no odor of a failure and windings appear to look fine. Megger test did not show any leaks to ground.

Puzzled.....
Click to expand...
Some questions/observations.

First if it does run some of the time then the windings should be good, and if it does run with the compressor loading it then the run capacitor is also likely good.

You mentioned checking out start winding switch, but did you also check out centrifugal mechanism. If it hangs up and doesn't release when motor stops it won't actuate the switch next time you start it and you won't have start capacitors in the circuit. With compressor coupled to load it won't have enough torque to move at all, with uncoupled motor you may get some rotation out of it, but that is from phase shift from the run capacitor only, and it may not be enough to accelerate very fast.

Most of the time you get at least a few seconds of time before breaker trips, but if breaker is marginally sized to begin with you may get pretty quick trip in such conditions. Source capacity and circuit impedance do have an impact also. Stout supply and short length of conductor (service, feeder, branch circuit combined) run can deliver more current and will have faster trip time when such conditions occur. Longe runs and/or higher impedance source results in current limiting effects and lower current will mean longer time until breaker trips.

How long is your mentioned second of time before breaker trips. Long enough to say one thousand one is closer to a real second and may mean something different than if that second is closer to "instantaneous" after closing control switch. That "instantaneous" situation is a bigger indication of a short circuit or ground fault, but testing you said you performed seems to eliminate this, in what you tested, make sure you have checked supply conductors between motor and controller also if you are having short circuit/ground fault conditions.
 
190210-1133 EST

fastline:

Numerous good points have been mentioned. But I will start from a base point. Assume, but it is clear, that this is a single phase motor. Output power is 7.5*746 = 5595 W. Assume 80% efficiency at full load. Thus, input power at full load might be 6994 W. On a capacitor run at full load possibly a PF = 0.9 . Obviously lower at no load.

1. Disconnect the motor from any external mechanical load.

2. Open one lead to the start capacitor, and one to the run capacitor.

3. Hand rotate motor and listen or feel for any abnormal condition.

4. Externally drive the motor with another small motor, like 1 HP, at the nominal speed of the 7.5 HP motor. In this case with the run capacitor circuit opened. This is a test to determine power loss in the 7.5 HP motor from bearing friction and windage. Measure the small motor power input unloaded, then with the 7.5 HP motor as a load. The change in power is the power loss in the 7.5 HP motor from windage and friction.

Someone else may be able to tell you what to expect. My guess might be 5% or less of 7.5 HP. Possibly not below 1%. Most motors have a cooling fan.

Bearings can possibly be considered as constant torque. Thus, bearing friction power loss is about proportional to speed.

Windage possibly follows a cube law curve vs speed.

5. If rotational frictional losses, includes windage which is a friction, seem ok, then reconnect the run capacitor. And try starting the unloaded motor.

In a typical motor the run capacitor should produce enough starting torque to quickly start an unloaded motor.

If you pass this start test, then load the motor. May not be feasible in a simple way.

6. If in 5 the motor seems to run OK, then measure power input. I don't mean current, but real power. This should equal the windage-friction losses measured in 4 above plus the I^2*R stator losses plus the estimated rotor losses.

If the actual power input is much greater than the above estimate, then there is the likelihood of shorted stator turns.

7. If the above tests seem good, then the starting capacitor and associated components are the likely problem area.

8. Oscilloscope plots of both current and power during startup with and without mechanical load would be useful.

.
 
ActionDave said:
Could be bad bearings or a worn endbell.

.
Click to expand...

Siezed up 5T Emerson scroll compressor in heat pump. All electrical good.

CUt it open and found the bottom bearing had come loose (3 bolts holding bushing had bibrated out)
Rotor magnetically locked to stator when power applied. LRA.

Sent photos in to Emmerson, they did come back and said that was so rae it was not even on their failure analysis charts -- set it should have been, folks just ascribe the LRA failures of their motor to electrical when all it was is loose screws!. Likely notmay HVAC gurus take the time to cut a compressor apart. Just change it out and scrap.
 
Ha, someone that thinks like me. Even if we swap the motor, that motor is coming apart to be examined because I have never seen such an issue. I did note the end bell came off pretty easy, and the rotor rides VERY close to the stator. This is open and drip and I did not detect any grinding noises though. Certainly spins free as a bird with no load but I guess possible when magnetized, it binds up.
 
junkhound said:
... Cut it open and found the bottom bearing had come loose (3 bolts holding bushing had vibrated out) ...
Click to expand...
I find it quite annoying when my entire problem boils down to: I have a screw loose.

But seriously ... While you have it apart, clean everything, lubricate everything that moves and tighten everything that doesn't. I know it sounds trivial but it's often all that's necessary to put a machine back into service.
 
fastline said:
Ha, someone that thinks like me. Even if we swap the motor, that motor is coming apart to be examined because I have never seen such an issue. I did note the end bell came off pretty easy, and the rotor rides VERY close to the stator. This is open and drip and I did not detect any grinding noises though. Certainly spins free as a bird with no load but I guess possible when magnetized, it binds up.
Click to expand...

Too much play in a bearing or bearing holder in the end housing may not obviously show up until belt tension pulls it into stator.

You will see marking on stator as well as rotor if they have touched while running.

If too much play in end housing a good machine shop can fix that.
 
Checking the Starting Circuit

Checking the Starting Circuit

A good place to start is checking the starting circuit. One easy way to do this is to place a clamp-on amp probe on one of the leads going to the starting cap. You should see current for a brief moment, until the motor comes up to 70% or so speed. This will determine if there is continuity in the start winding (will not tell you if you have shorted turns), if the centrifugal starting switch is working, and the cap will allow current to pass in and out of it. If current does not stop after motor comes up to speed, the switch may be shorted. You could also check the running cap current to make sure it is working. Current should be there the whole time the motor is powered up.

Checking electrolytic caps can be a little tricky. Usually the ESR (equivalent series resistance) of the cap will go bad before the capacitance does. This means that if you are simply checking with a capacitance meter, it may give you a false indication that the cap is good. You can purchase meters that check ESR. Another method of checking a cap is to connect a doorbell transformer across the cap (this will limit the current), measure the voltage across the cap, and measure the current. With this information you can calculate the capacitance (which will also check ESR) by the following formula Capacitance = Current/2 x pi x frequency x voltage
 
I’m with kwired on this, the centrifugal switch may not be working. Checking the contacts for wear is one thing, checking that they actually change state at 80-90% speed is another. There are numerous things in the mechanism that can go wrong, and so many different versions of the mechanism that it’s impossible to list them all. I had one a long time ago like this on a big bench grinder, everything APPEARED normal but the motor would not accelerate before the fuses cleared. I eventually resorted to listening for the switch, couldn’t hear it and replaced it. Everything worked after that. I never found out what was wrong with that switch, I couldn’t see anything wrong with it and I didn’t have a way to test it on a bench at that time. After the fact I thought of rigging a way to connect it to a drill motor to spin it up, but it was long gone by then. The real problem of the grinder not starting had been solved though.
 
OK, I did more testing. I am convinced the main winding is fine. Just no way a shorted winding would allow this much repeated hits. No smoke, no smell, nothing. I ran tests on the start circuit and wanted to share to see what you guys though. My inclination is the start circuit has issues either not enough capacitance or won't unlock.

I tested the voltage at startup at 210V.... Likely normal. I then tested the current on the start caps. 15A. Doing the math, that should be closer to 40-50A. The start caps are 216-240uf, 2 in parallel.

The issue is I am still seeing 220-250A and if I deduct the current on the start circuit, that is still too much. i forgot to test the rpm of the motor but only a 1725 motor and seems to to get close unloaded. Still, the current is way too high. If I took a typical cap start motor, unloaded, and threw the juice to it at 80% of full speed, it would just accel to full speed. This one does not care and continues with very high current and motor sounds like it is fighting. Like a 1P motor that has no caps. It just buzzes and gets pissed. This one sounds the same but spins the rotor.
 
190211-2432 EST

fastline:

500 ufd at 60 Hz is about 5.3 ohms by some online calculator.

Alone at 240 V this would be 45 A. Voltage will be higher because it is in series with an inductor. Thus, current is somewhat higher. This is not directly your high current cause, nor if it never dropped out, would it be the cause of your high current.

In some way your main winding is the cause.

I don't believe you have told us directly what happens with no external mechanical load.

.
 
Again, we have disconnect all loads. When we put power to the motor, it spins up to what we believe to be very close to design speed, 1725rpm. Did not check with a tach yet but prob not needed. However, the motor continues to pull very high amp even near full speed and has a growl that sounds like a motor IMO, that you are trying to start without start caps. How they just hum, but this one certainly spins up.


Its just amazing that there is not magic smoke, not odor, no instant trip, it takes 1sec to trip to breaker.

Puzzled but I would like to determine if the start circuit is to blame. I really thought I found something but even without the start circuit current, it is still pulling too much. The current is near locked rotor current.
 
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