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Discussion Starter · #1 ·
A couple of days ago I was testing a standby generator at a water pump station. When I transferred the load to generator line to line voltage was about 195 and <50 hz. (Single phase 120/240 generator.) I went ahead to start checking the engine when about 2 min later I noticed the pump room was filling with smoke. Turns out a 3hp baldor pump was smoking away. I transferred back to utility power and everything was back to normal, the pump seemed to be fine.
I'm just wondering why is it that a motor would smoke under low voltage? Lower voltage should equal lower current flow.
I wonder if the lower voltage and frequency prevented the centrifugal switch from transferring from the starting winding to the run winding, and the smoke was from the starting winding running too long. Would appreciate anyone who has insight into this.
 

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A couple of days ago I was testing a standby generator at a water pump station. When I transferred the load to generator line to line voltage was about 195 and <50 hz. (Single phase 120/240 generator.) I went ahead to start checking the engine when about 2 min later I noticed the pump room was filling with smoke. Turns out a 3hp baldor pump was smoking away. I transferred back to utility power and everything was back to normal, the pump seemed to be fine.
I'm just wondering why is it that a motor would smoke under low voltage? Lower voltage should equal lower current flow.
I wonder if the lower voltage and frequency prevented the centrifugal switch from transferring from the starting winding to the run winding, and the smoke was from the starting winding running too long. Would appreciate anyone who has insight into this.
Why is the generator at 195 volts instead of 240 volts?
 

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Motors do not work that way. Lower voltage does not equal lower current. The motor needs power to do its work. If the voltage drops, the current has to go up to do the same amount of work (power).

I just reread your post. Are you saying you had a motor designed for 60 hz running at 50 hz?. If so the work being done by the motor would go down do to the lower speed, and the volts/per hertz would be about the same at 195 volts and 50 hz as it would be at 240 volts and 60 hz. That would not damage the motor.

I think you may be correct that the starting winding was damaged.
 

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Lowered voltage is going to cause higher current draw all by itself, and if I'm not mistaken, lower frequency will cause the motor to turn slower, reducing back emf and causing even more current draw and overheating

Had to go check out my motors book, I guess the lowered frequency would offset the lowered voltage problem. So I got nothing on this one
 

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Lowered voltage is going to cause higher current draw all by itself,
That is correct for motor loads.
and if I'm not mistaken, lower frequency will cause the motor to turn slower, reducing back emf and causing even more current draw and overheating
Motors are run every day below their base frequency using VFDs without problems. Running the motor at a reduced speed will reduce the work and reduce the current.
 

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That is correct for motor loads. Motors are run every day below their base frequency using VFDs without problems. Running the motor at a reduced speed will reduce the work and reduce the current.
When speed is reduced with a VFD, voltage is also reduced or the stator iron will saturate.
 

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Discussion Starter · #10 ·
Out of curiosity is there a way to calculate current when supplying a motor with other than rated voltage? For example how many amps would a 240v rated motor pull when being fed with 200v
 

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Out of curiosity is there a way to calculate current when supplying a motor with other than rated voltage? For example how many amps would a 240v rated motor pull when being fed with 200v
A motor with a fixed load will consume the same amount of watts. So changes in voltage inversely change current draw. You cut the voltage in half and the current will double.
 

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Discussion Starter · #12 ·
If that's true I don't think that was the reason my motor was smoking. The math works out 240v X 17fla = 4080 / 195v = 20.9 . Basically a 3hp 240v motor fed 195v will pull 21 amps. I doubt 4 more amps will make a motor smoke up a room in two three minuets.
 

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If that's true I don't think that was the reason my motor was smoking. The math works out 240v X 17fla = 4080 / 195v = 20.9 . Basically a 3hp 240v motor fed 195v will pull 21 amps. I doubt 4 more amps will make a motor smoke up a room in two three minuets.
I'm going with your starting circuit not disengaging idea on this one.
 

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120/240 generator implies this is single phase. A single phase motor means some sort of starting capacitor system. If it was a Cap-Start /Induction-Run motor, the lower frequency would have not allowed the centrifugal switch to take the starting capacitor out of the circuit. Starting capacitors and the wires feeding them are not designed to be in the circuit for more than a few seconds, so they start to fry pretty quick.
 

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If that's true I don't think that was the reason my motor was smoking. The math works out 240v X 17fla = 4080 / 195v = 20.9 . Basically a 3hp 240v motor fed 195v will pull 21 amps. I doubt 4 more amps will make a motor smoke up a room in two three minuets.
What did the overloads say?
 

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out of curiosity is there a way to calculate current when supplying a motor with other than rated voltage? For example how many amps would a 240v rated motor pull when being fed with 200v
ohms law...
 

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pretty sure there is no external overlaod on that motor, it probably has internal thermal protection like a lot of single phase motors. i would check cap (about the only part that can smoke) . if wiring was bad motor would not restart
 
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