Uncoupled slip-ring induction motor speed

[freetown]

Hi all,

A slip-ring induction motor of 2700kW and 165A (stator) 990rpm, 1340V and 1250A (rotor). When we start the motor with no mechanical load (uncoupled motor so motor turns alone) we have 40A current and it's normal but what about the speed in this case, should we have a lower speed when make the test of motor turns alone?
We use liquid resistance starter to start the motor. This is because using a tool that monitoring device measures the rotor currents (and other parameters), it says low current and i assume it's normal and also low speed

Thank you.

 

[edison123]

With rotor resistance fully shorted out, the motor decoupled, coupled no-load and coupled load speed is the same at rated RPM.
Rotor currents at rated speed are at slip frequency, which is very low (in your case 0.5 Hz if your supply is 50 Hz).
Your normal commercial clamp-ons and ammeters will not measure such low frequency rotor currents very accurately.
Monitor the stator current. No-load stator current will be around 30 to 50% of rated stator current.

 

[crshears]

Quoting edison123: With rotor resistance fully shorted out, the motor decoupled, coupled no-load and coupled load speed is the same at rated RPM.

With respect, that doesn't make sense to me; uncoupled speed will be some nominal amount slower than synchronous speed, coupled but unloaded speed will typically apply some minimal but measurable mechanical and/or windage load to the shafting and therefore result in a slightly lower speed, and loaded speed will be yet lower than that, no?

It is my understanding that the added load slows down an induction motor, which increases the frequency of the currents in the rotor which, by motor reaction, then stabilizes the motor speed at some lower value. Indeed, I recall once being next to the conveyor motors when coaling operations were to commence; when the belts were started empty, the speed rose to a certain value, then, over the course of a few minutes, could be heard to slow slightly but discernibly as coal hoisting began.