Negative Slip in motor running backwards
Negative Slip in motor running backwards
(OP)
Can someone explain to me how negative slip occurs in a motor that is spinning backwards. I have seen a motor that is freespinning backwards trip the breaker when starting. I know this is not a function of regenerative voltage due to the fact there is no field in the rotor when the motor is freespinning. I even confirmed this with a measurement of about 2V while the motor was frespinning.
I believe I have heard before that the reason for the breaker tripping was to high current with the motor being in a negative slip state when spinning backwards. Can someone please show me how this negative slip effects current by using the motor model and torque equations. Thanks.
I believe I have heard before that the reason for the breaker tripping was to high current with the motor being in a negative slip state when spinning backwards. Can someone please show me how this negative slip effects current by using the motor model and torque equations. Thanks.





RE: Negative Slip in motor running backwards
The reason that it can trip a breaker is simply that you first have to bring the motor to slip 100 % (n=0). That alone takes current and energy. So, when it comes to spinning from 100 % slip towards 1 - 3 % or whatever the normal slip is, the protection has already been preheated. It then trips before full slip is reached.
Sometimes, a reverse spinning motor draws more than normal starting current - it depends of rotor type - and then the current in itself can trip the overcurrent element.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Negative Slip in motor running backwards
Knowing you as I do, I'm also surprised at the brevity!
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RE: Negative Slip in motor running backwards
Bill
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Jimmy Carter
RE: Negative Slip in motor running backwards
If if you're talking about a load-center breaker with time overcurrent element, then Gunnar's explanation is very credible. Basically the motor has to accelerate a wider speed range to get to full speed, so the starting current lasts longer. Could trip a thermal element whose setting had small margin to trip to begin with.
But if it's a typical MCC setup with combination starters, then breaker trip is only initiated by instantaneous element. In that case I would be inclined to look at the fact that the motor L/R ratio increases as slip increases. So the transient dc offset component does not decay as fast during the first quarter-cycle before the highest first peak is reached. Could trip an instantaneous element whose setting had small margin to trip to begin with.
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RE: Negative Slip in motor running backwards
It sounds like the amount of current needed to first stop the motor to zero speed and then accelerate it would be a greate deal.
Is it indeed true that without a field in the motor the motor will not generate any voltage while freespinning backwards? I measured about 1.6V and just wanted to make sure that I was taking the measurment correctly by measuing the voltage with an AC meter function.
RE: Negative Slip in motor running backwards
Under the equivalent circuit model, the current as function of slip is typically increasing slightly passing slip=1 in the increasing slip direction , so there may be a little more current drawn. There is also the inductance / decay affect I mentioned.
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