Motor damage

[mbuffer97370]

For a three phase motor, what is the average time to motor damage once the motor loses one phase? Assume it is operating at full load.

 

[motorsdirect]

Hi
I dont think you could assume an average...but it would be measured probably in seconds rather than minutes.....and probably only very few seconds to cause serious damage!!(as little as 15 secs )

jeff

http://www.motors-direct.co.uk

 

[justinn4]

If the rotor stalls, the remaining lines will take the locked rotor current (Ilr). NEMA motors must allow 20 seconds with the locked rotor current with no permanent damage. If the load does not stall the motor current will be lower than the locked rotor value, consider that the allowed loss energy liberated in the windings is proportional to the current (i) squared and time (t).
W = (i^2)t = Ilr^2 * 20 sec

or t = W/(i^2)

 

[RRaghunath]

If I read correctly, the issue is what is the time available before motor is damaged when single phasing (loss of one phase) takes place.
When there is single phasing, the withstand time should be much less than 20sec (the NEMA value is for stalling with three phase power available, I presume)if the motor stalls.

It can be calculated for motor stalled condition as well as 'coninues to run' condition if the motor (I2SquaredT)value and the motor currents (at the time of single phasing) are known.

The (I2SquaredT) value is part of the data sheets for alternators, I am not sure about the motors. May someone in the forum can help.

raghunath_n00@rediffmail.com

 

[UKpete]

It certainly depends on the size of the machine. The larger it is, the greater the specific heat capacity and the longer it takes to overheat.