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.

 

[waross]

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

When you look at a speed over load chart for a squirrel cage induction motor, the speed drops from synchronous speed to rated speed linearly as the load is added.
No load speed is slightly below synchronous speed due to bearing friction, windage and rotor losses.
These losses are small enough that they may be ignored in almost all cases.
Your loaded speed is synchronous speed minus the slip speed, in this case 1000 RPM minus 990 RPM equals 10 RPM slip.
If we assume (WAG) 3% losses at no load, then the no-load slip will be !0 RPM x 3% = 0.3 RPM slip.
You may expect a no load speed of 1000 RPM minus 0.3 RPM slip equals 999.7 RPM no-load speed if this were a squirrel cage induction motor.
Are wound rotor induction motors that much different?.
By The Way, it doesn't take much rotor current to develop 3% load torque.

 

[edison123]

1% speed variation from no-load to full load is what makes an IM basically a constant speed motor.

 

[LionelHutz]

but what about the speed in this case, should we have a lower speed when make the test of motor turns alone?

It will probably run at rated speed, or near rated speed.

Using rotor resistance, the speed of the motor depends on the rotor resistance and the load. Both are an inverse relationship. Resistance goes down then speed goes up. Load goes down then speed goes up.

With no load, the whole resistance range which can be applied to the rotor via your liquid rheostat will likely be low enough to accelerate the motor to full speed.

 

[FacEngrPE]

At zero torque load, slip is nearly identical for all rotor resistance selections.
As the torque load increases, the impact of increasing resistance on motor speed becomes large.

figure from http://www.allaboutcircuits.com/textbook/alternating-current/chpt-13/wound-rotor-induction-motors/