Negative Slip in motor running backwards 1

[rockman7892]

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.

 

[Skogsgurra]

I wouldn't say that the slip is negative. Instead, it is more than 100 %. Negative slip is when you drive the motor faster than synchronous.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

PLS for you Gunnar, that's the best explanation I have ever seen on that issue!

Knowing you as I do, I'm also surprised at the brevity!


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[waross]

Great Gunnar. I wish that I could say that in less than a chapter or two. Grin.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I have seen a motor that is freespinning backwards trip the breaker when starting.

What type of breaker and do you have any means to know what element tripped?

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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[rockman7892]

The breaker that tripped is a 250A magnetic only instantaneous breaker with trip setting set to 2500A.

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.

 

[electricpete]

Yes, it's true the voltage from motors free spinning backwards is typically small, since residual flux is a miniscule fraction of rated flux.

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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