PF and Motor Overload

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At 100% slip (stall) all we have is essentially stator losses, a small part of which is heat so power factor is nearly zero. As it accelerates current remains very nearly LRA until we reach breakdown torque at around 15-25% slip at which point current drops but power factor has increased significantly even though current is still nearly LRA:
 
paulengr said:
At 100% slip (stall) all we have is essentially stator losses, a small part of which is heat so power factor is nearly zero. As it accelerates current remains very nearly LRA until we reach breakdown torque at around 15-25% slip at which point current drops but power factor has increased significantly even though current is still nearly LRA:
Click to expand...

I think much of the heat losses are actually in the rotor (which is essentially the equivalent of a transformer secondary, the stator being the primary).

A typical locked rotor failure is evidenced by melted rotor bars.
 
retirede said:
I think much of the heat losses are actually in the rotor (which is essentially the equivalent of a transformer secondary, the stator being the primary).

A typical locked rotor failure is evidenced by melted rotor bars.
Click to expand...

Highly HP dependent. Above 100-200 HP a little known fact is that the rotor rather than the stator becomes the thermal limit. At stall you have a big hunk of steel that is surrounded by air that is not moving. Only appreciable heat transfer is conduction via the bearings. On smaller motors during stall with a small thermal mass the stator coils fail first.

Hence under 200 HP a typical thermal overload relay (eutectic, bimetallic, RC, etc.) is fine. At 200+ HP you really should switch to a microprocessor overload relay specifically for stall protection.


@electrical/documents/content/ct_237869.pdf

Error on my part...at stall current through rotor is quite high because there is no counter EMF. Slip is 100% so R2 x (1-s)/s goes to 0 so the rotor is shorted leaving only the rotor impedances, X2, R2 (the static part), etc. I was thinking in terms of the IEEE 112 tests.

However the thermal damage still depends on motor size so just stating that rotors melt on stall is true but also size dependent. On small motors typically it looks like overload (uniformly burned stator) irrespective of whether it was turning or stalled.
 
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