Motor Stall Time

[luis7006]

Hello,

I would thank your advice about the following:

From the technical data sheet of a slip ring motor, we have:

Starting time of 25 seconds
Stall time (cold/warm) 3/2 seconds

We think there is an error because the stalling time should be longer than the starting one, isn't it?

Any comment will be welcome!

Best Regards

 

[dpc]

The stall time can be less than the starting time, but the difference is usually not as great as your example.

When starting, a motor is turning, so the heat transfer and actual current are not the same as under locked rotor conditions.

I'd verify the numbers with the manufacturer.

You will need to take a close look at the motor protection in order to provide adequate Locked Rotor protection. Newer digital relays can often handle situations where safe stall time is less than the starting time.

How big is this motor and what is the nominal speed?

 

[luis7006]

Hello dpc

Thank for your answer!

The motor is 1510 kW at 1187 rpm.
As you can see it is a large one that suppossely will be started with "a rotor resistance".

I will contact the manufacturer to see if there is an error or how to understand the given values.

Best Regards

 

[electricpete]

As dpc said, locked rotor is not a good place to be, since there is absolutely no air flow to remove the heat. So allowable stall time is expected to be shorter than allowable time to get to full speed.

The rotor of a wound rotor motor is particularly vulnerable to thermal damage during starting since it has inuslated conductors (unlike squirrel cage motor rotors).

The reason the starting time is so long is likely that the starting sequence involves several steps of decreasing rotor resistance.

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[LionelHutz]

The starting time is likely using a varying resistance as the motor accelerates. This will result in the current being much lower than what the stalled current is.

The stalled current is likely considered the current that flows when there is no rotor resistance while the rotor is locked. This current will be much higher than you will see once you add resistance.