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

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Wouldn't shorted turns in the run windings create these symptoms by grossly lowering the impedance, and with no smell due to the short running time?
 
fastline said:
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.
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fastline said:
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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Sounds like possibly centrifugal switch isn't operating when up to speed and start caps are in circuit too long. Especially if this has been happening intermittently.

If testing with no driven load connected, try removing one of the start capacitors from the circuit and then trying to run it. It likely won't accelerate as fast but won't draw as much current either. Can even try removing both start capacitors from circuit, with no load the run capacitor will probably be able to get it started but will be a slow start in comparison. If it will get to normal speed and normal no load current - your issues is with the centrifugal switch.
 
fastline said:
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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If you place a clamp-on ammeter on one of the leads of the starting cap, does the current stop after it comes up to speed? If not, the switch is not opening.
 
I made a manual switch to open the start cap circuit once it is spinning. Amps still stayed high. We are going to open the motor just to see if there is anything visible but likely we will have to shelf it and convert the compressor to 480/3p. At least it will be more reliable on 3p.

I don't want to pitch the motor unless confirmed bad because they are insanely expensive. I think $1100 for that one.
 
fastline said:
I made a manual switch to open the start cap circuit once it is spinning. Amps still stayed high. We are going to open the motor just to see if there is anything visible but likely we will have to shelf it and convert the compressor to 480/3p. At least it will be more reliable on 3p.

I don't want to pitch the motor unless confirmed bad because they are insanely expensive. I think $1100 for that one.
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Once you have replaced it, you can take it to a motor shop for actual testing. They can detect shorted turns very easily. Then they can quote you on the cost of rewinding and you can decide to repair or scrap. No point at all in keeping it on the shelf without finding out what is wrong with it.
 
Can you use a current-limiting power source, such as in series with a very high-wattage incandescent bub (500w halogen?), and measure the current through each isolated winding section? If the run winding has a short, as I surmise, it should exhibit a much lower impedance because of the AC, where an ohmmeter wouldn't show a difference.
 
190212-2141 EST

fastline:

You are hung up on the start winding. Almost certainly it is not your problem.

I scanned back thru the posts and could not determine whether this is just a capacitor start motor, or if it is also a capacitor run. If run is included, then both run and phase shift coils will be nearly identical.

If only a capacitor start, then less space and smaller wire is used for the start winding providing more space for the run winding. I did not quickly determine what might be the ratio of start to run resistance for the capacitor start only type of motor. But your resistance measurements might indicate shorted turns on the run winding.

.
 
ptonsparky said:
It did in the 100 HP I referenced. Mine happened to be on a VFD that would shut down on drive overcurrent. Megged fine and no smell.
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Similar but different problem, I had a 30HP Hitachi that would show a ground fault on a PF700. Megged the T leads, and the motor, it was fine. No smell either. Hooked the motor up to an adjacent VFD, also showed ground fault.

Replaced motor, all was well.

I was told a megger can't pick things up that a VFD can?

Well, that makes it a little tough for a guy troubleshooting in the field sometimes. Fortunately, I've can only remember seeing this once in the 10 years or so I've been megging.
 
Yes, this is cap start/cap run. Obviously run cap stays in circuit all the time and start caps are switches out.

I knew when I switched out the caps and saw no change in current that something was wrong for sure. We pulled the motor and tore it down and everything in it looks flawless. There has been some minor contact of the rotor and stator at the back of the motor but very minor. Nothing melted, but I did note the main winding color is not the bright copper color, it is a bit more dark to brown ish. Not black and charred though..

I have the stator assy in the truck and looking to visit a motor shop to see if they can run a quick scan to determine a shorted winding. It certainly is not leaking to ground so a megger is of little use here.
 
fastline said:
Yes, this is cap start/cap run. Obviously run cap stays in circuit all the time and start caps are switches out.

I knew when I switched out the caps and saw no change in current that something was wrong for sure. We pulled the motor and tore it down and everything in it looks flawless. There has been some minor contact of the rotor and stator at the back of the motor but very minor. Nothing melted, but I did note the main winding color is not the bright copper color, it is a bit more dark to brown ish. Not black and charred though..

I have the stator assy in the truck and looking to visit a motor shop to see if they can run a quick scan to determine a shorted winding. It certainly is not leaking to ground so a megger is of little use here.
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Bet you have short between turns.
 
Cow said:
Similar but different problem, I had a 30HP Hitachi that would show a ground fault on a PF700. Megged the T leads, and the motor, it was fine. No smell either. Hooked the motor up to an adjacent VFD, also showed ground fault.

Replaced motor, all was well.

I was told a megger can't pick things up that a VFD can?

Well, that makes it a little tough for a guy troubleshooting in the field sometimes. Fortunately, I've can only remember seeing this once in the 10 years or so I've been megging.
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IGBT reflected wave damage will do that. Connect that motor "across the line" and your winding damage will become obvious. Done that before, and it goes boom immediately, meg tests done prior didn't indicate any issues, but VFD kept indicating ground fault.
 
gar said:
190212-2141 EST

fastline:

You are hung up on the start winding. Almost certainly it is not your problem.

I scanned back thru the posts and could not determine whether this is just a capacitor start motor, or if it is also a capacitor run. If run is included, then both run and phase shift coils will be nearly identical.

If only a capacitor start, then less space and smaller wire is used for the start winding providing more space for the run winding. I did not quickly determine what might be the ratio of start to run resistance for the capacitor start only type of motor. But your resistance measurements might indicate shorted turns on the run winding.

.
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My experiences most anything over about 2 or 3 HP the aux winding is obviously smaller conductor than the main winding. Most anything over about 3 HP also is capacitor start/capacitor run. Have seen an occasional 7.5 -10 HP motor intended to be used for fan motor (in particular one certain make of crop drying fans with lightweight fan blades use an OEM designed motor) that are PSC motors - but those don't require high starting torque is why they can get away with that.
 
190213-1013 EST

kwired:

I can not speak from experience, but my trusty textbook indicates that a capacitor run motor will have both windings about the same, possible even identical.

On this basis, then the 0 ohm reading on the run winding vs 1 ohm on the capacitor winding indicates a problem with the run winding.

.
 
gar said:
190213-1013 EST

kwired:

I can not speak from experience, but my trusty textbook indicates that a capacitor run motor will have both windings about the same, possible even identical.

On this basis, then the 0 ohm reading on the run winding vs 1 ohm on the capacitor winding indicates a problem with the run winding.

.
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2 HP and below, and maybe even up to 5 HP all the connecting leads are typically same size conductors, but larger motors the main winding leads are larger than the aux winding leads. They definitely are not intended to carry same current. Pretty certain I have seen smaller magnet wire on the aux windings as well on larger single phase motors.

If they are indeed the same, one should be able to reconnect the capacitor to the main winding and it should still work.
 
190214-1108 EST

kwired:

One has to distinguish between a capacitor start motor and a capacitor run motor. Both are two phase motors.

The start type is designed for high starting torque (enough to meet needs) and minimum cost. This leads to smaller wire and less stator space for the start capacitor winding to get more run power from the run winding.

In the capacitor run motor you want roughly the same power fed in thru each winding. Thus, both windings will be about the same wire size, slot size, and resistance.

See Chapter 17, Capacitor Motors, starting at page 303, in "Alternating-Current Machinery", Bailey and Gault, McGraw-Hill. 1951.

I suspect you are seeing capacitor start motors, and not start and run combined..

.
 
gar said:
190214-1108 EST

kwired:

One has to distinguish between a capacitor start motor and a capacitor run motor. Both are two phase motors.

The start type is designed for high starting torque (enough to meet needs) and minimum cost. This leads to smaller wire and less stator space for the start capacitor winding to get more run power from the run winding.

In the capacitor run motor you want roughly the same power fed in thru each winding. Thus, both windings will be about the same wire size, slot size, and resistance.

See Chapter 17, Capacitor Motors, starting at page 303, in "Alternating-Current Machinery", Bailey and Gault, McGraw-Hill. 1951.

I suspect you are seeing capacitor start motors, and not start and run combined..

.
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wrong, On smaller motors up to about 1.5 HP max some are capacitor start only. Over 2 HP almost all of them are capacitor start/capacitor run. The run capacitor is always in the circuit on those, the centrifugal switch just puts additional start capacitors in parallel with the run capacitor during starting.

Capacitor run only is a PSC motor. Those are not all that popular over about 1 HP, have low starting torque and are mostly only found on fans or other low starting torque loads. I have seen some 10 HP PSC motors, but they are OEM designed for specific application - which happens to be crop drying fan motors- but a type with lightweight fan propeller compared to other similar fans with heavier propeller and CS?CR type motors being typical.
 
190214-2953 EST

kwired:

I think you misunderstood part of what I was saying.

If a motor is designed as a capacitor run motor, then it is likely that both the run winding and the capacitor winding will be somewhat comparable in size. In turn this means approximately the same resistance, number of turns, wire size, and slot size. Of necessity this same coil is used as the capacitor coil for both capacitor start and run functions. A larger capacitor is added in parallel with the run capacitor for starting to provide the greater starting torque.

In the run state the two coil currents are about the same, and design tends toward a 90 degree phase shift. And unity power factor is approached.

For a capacitor start only motor the capacitor winding can be smaller leaving more room for the run winding.

How a particular motor is designed will depend upon desired performance vs cost.

.
 
gar said:
190214-2953 EST

kwired:

I think you misunderstood part of what I was saying.

If a motor is designed as a capacitor run motor, then it is likely that both the run winding and the capacitor winding will be somewhat comparable in size. In turn this means approximately the same resistance, number of turns, wire size, and slot size. Of necessity this same coil is used as the capacitor coil for both capacitor start and run functions. A larger capacitor is added in parallel with the run capacitor for starting to provide the greater starting torque.

In the run state the two coil currents are about the same, and design tends toward a 90 degree phase shift. And unity power factor is approached.

For a capacitor start only motor the capacitor winding can be smaller leaving more room for the run winding.

How a particular motor is designed will depend upon desired performance vs cost.

.
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I understand what you are saying, but is not what I am seeing most of the time when I work on them, especially on the larger motors. Connecting leads are not even same size, aux winding leads will be 14 or 12 AWG when main winding leads are 10 or 8 AWG on a lot of 10 HP motors. Clamp ammeter on them while running, aux winding only draws about 10 amps max while main winding is drawing 30+.

A lot of these motors are somewhat definite purpose motors and are crop drying fan motors and TEAO rated, but there are a lot of centrifugal fans as well as conveyors I run into as well that are using general purpose motors, and still seems to be a lot of what I mentioned in anything over 5 HP. Under 5 HP I won't argue with you too much, haven't seen as many obvious things to be able to say otherwise with those.
 
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