Locked rotor

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gar said:
210225-2206 EST

Besoeker3:

Rotor resistance is very much a part of the question on locked rotor.

For everything else held constant on an induction motor the amount of rotor resistance will determine locked rotor current, and this also influences the locked rotor torque.

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This:
"The above is not a very good discussion, but does provide some information. In this discussion external resistors are used with a low resistance wound rotor to provide essentially a constant motor except for the ability to adjust rotor resistance during operation."

Adjust rotor resistance during operation??
 
210226-0517 EST

Besoeker3:

It is much easier to evaluate experimentally the effect of rotor resistance with a wound rotor motor compared with building a different fixed resistance rotor for each test on rotor resistance.

In an actual application you may want to adjust the motor characteristic during its cycle. For example: there are also times in an application where you might want to use a wound rotor motor to allow high starting torque, and then at full speed change rotor resistance to have improved speed regulation vs load.

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A typical submersible pump will, of course, have low load and torque requirements at low speed, but will need to accelerate fairly quickly to the speed at which their fluid bearings reach operating speed.
 
gar said:
210226-0517 EST

Besoeker3:

It is much easier to evaluate experimentally the effect of rotor resistance with a wound rotor motor compared with building a different fixed resistance rotor for each test on rotor resistance.

In an actual application you may want to adjust the motor characteristic during its cycle. For example: there are also times in an application where you might want to use a wound rotor motor to allow high starting torque, and then at full speed change rotor resistance to have improved speed regulation vs load.

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Click to expand...
"and then at full speed change rotor resistance"
The locked rotor, and that's what we are talking about, is zero speed.
 
210226-1227 EST

Some references:


"Alternating-Current Machinery", Bailey and Gault, McGraw-Hill, 1951.

From page 188:
"From this it is apparent that the maximum torque of an induction motor is independent of rotor resistance and occurs at such a speed that the rotor resistance equals the reactance. By varying the resistance, therefore, the maximum torque can be made to occur at any desired value of slip or speed."

From page 189:
is a set of plots for varying rotor resistance. Also comments on "high starting torque with small starting current". And "speed for a given torque can be varied through quite a wide range."

For a given motor with everything held constant except for rotor resistance:
1. Maximum zero speed torque, therefore maximum acceleration, is obtained with a high resistance rotor.
2. But a high resistance rotor results in an inefficient motor at some reasonable higher speed.
3. Also a high resistance rotor results in very poor speed regulation under varying load conditions.
4. A low resistance rotor provides good speed regulation, and efficiency, at near synchronous speed.
5. A low resistance rotor motor must have sufficient torque throughout it speed range to accelerate to near synchronous speed.
6. High torque at zero speed will produce the greatest initial acceleration.

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Inrush can be up to 3x LRA. It depends on core loss and stator reactance. Locked rotor current itself depends on how much reactance is allocated to the rotor vs the stator. Higher stator currents imply greater losses but also boost starting torque. Look at a NEMA torque/speed curve chart but realize there is a lot more to it than that. You can trade off starting torque for break down torque.
 
It would be best if the OP specifies what criteria he is looking for when trying to know "which is better"! Is he talking about economy of operation or what?
 
210301-2341 EST

paulengr:

Display an induction motor starting current time plot from an oscilloscope that shows any substantial inrush current. You will display starting current, but nothing like the inrush current of a transformer, or an incandescent bulb, or transient current to an RC circuit from a DC supply.

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