3p motor: report of locked rotor amps but motor free to turn

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wyreman

wyreman

Senior Member
Location
SF CA USA
Occupation
electrical contractor
Got a message that I have to check out later in the day that a pump motor is down.

Owner reports measurement of locked rotor amps yet the motor is free to turn.
:roll:

I don't know the size [probably big], if there is a vfd or not, what is the voltage imbalance or anything more than that right now.

Did you ever hear of such a thing?
 
Wha??s

Wha??s

Still haven't seen it.
More info: 1/2hp 3p motor drives pump
Normal current 1.8a
Locked rotor is around 9?a
It is drawing the 9a but you can look right in and spin the rotor with your finger.

Mechanical contactor, the heaters didn't open yet a 20amp 3p opened before it.

It just hums and draws the locked rotor current but you can spin the rotor.
Could it be a high resistance short in the windings that happens to produce the locked rotor current?

also- two motors have done the same thing
 
The two pumps are next to each other with sep contactors and disconnects.
The actual LRA is not 9A but unknown at this time until I get there, may not be til tomorrow...
 
wyreman said:
Still haven't seen it.
More info: 1/2hp 3p motor drives pump
Normal current 1.8a
Locked rotor is around 9?a
It is drawing the 9a but you can look right in and spin the rotor with your finger.

Mechanical contactor, the heaters didn't open yet a 20amp 3p opened before it.

It just hums and draws the locked rotor current but you can spin the rotor.
Could it be a high resistance short in the windings that happens to produce the locked rotor current?

also- two motors have done the same thing
Click to expand...

1. Are you certain you have all three-phases available (you said 3p)? What voltage is being used?

2. As for the breaker tripping before the overload heaters, remember the breaker is most likely a thermal magnetic device (quick reacting). The heaters react slower than it and are not intended to deal with shorts or ground faults.

3. Was the motor changed or energized for the first time just before the problem started? If yes, open the pecker-head and look for bad splices AND if a multi-voltage motor, be sure it it connected correctly for the voltage being used.
 
catchtwentytwo said:
1. Are you certain you have all three-phases available (you said 3p)? What voltage is being used?

2. As for the breaker tripping before the overload heaters, remember the breaker is most likely a thermal magnetic device (quick reacting). The heaters react slower than it and are not intended to deal with shorts or ground faults.

3. Was the motor changed or energized for the first time just before the problem started? If yes, open the pecker-head and look for bad splices AND if a multi-voltage motor, be sure it it connected correctly for the voltage being used.
Click to expand...

So it is starting to look like it may be a short / ground fault because two pumps went out same time. I think the motor has been there for a long time... But whatever it is, why does it hum and produce big current while the rotor turns freely. I hope to see it tomorrow.
 
nakulak said:
did you lose a phase ?
Click to expand...

Brownout, single phasing...
This begs the question of the only interesting feature of this motor failure:
Motor hums and draws 5x load current while rotor spins freely...​

Can't tell till I put my hands on it but right now two 20+ yr veteran HVAC guys are on it and I am pretty sure they have a Fluke. Actually, I will have to recreate it off a whip because the new pumps are going in... Just don't want to see them lose two more $900 motors...
 
If the motor was being fed with single phase, then spinning the shaft would cause it to "start" and run. This would definitely be dangerous if you used your hand to do the spinning.
 
wyreman said:
http://www.afcisafety.org/files/Parallel-Arc-email.mov

The only thing that I can imagine so far would be a high resistance fault in the windings that is stable! and draws 9amp. Maybe not...
Click to expand...

I'd go along with the others.
Check the phase to phase continuity and do it from the starter end. That way, you will also check the continuity of the cable going to the motor.
Also do an insulation to ground (megger) check.
If that all looks OK, check that all three voltages are present.
And, if all else fails, how much would a replacement 1/2 hp motor cost?
 
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