Slip Losses

[ram14375]

hello everybody

we have a cement mill driven by twin drive

These motor are identical slipring induction motors of 2100kw/11kv/rotor voltage 1455v and 1105 amps

 

There is a power difference of 100-150kw between the two motors.

 

The motors are started thru LRS and shorted by short ckt contactors.they are running at full speedonly always

The rotors cables are also of uniform size and equal length

 

can anyone comment on this

how to find the slip losses

ram

[jraef]

hello everybody

we have a cement mill driven by twin drive

These motor are identical slipring induction motors of 2100kw/11kv/rotor voltage 1455v and 1105 amps

 

There is a power difference of 100-150kw between the two motors.

 

The motors are started thru LRS and shorted by short ckt contactors.they are running at full speedonly always

The rotors cables are also of uniform size and equal length

 

can anyone comment on this

how to find the slip losses

ram

No two motors are ever exactly the same, even if they are manufacturerd side-by-side. But I agree, 7% difference seems a bit excessive. The motor slips should be the same or close enough to account for maybe a 1 or 2% differential.

 

If the condition starts out bad, i.e. 150kW, then slowly gets better, i.e. 100kW, then it could be a factor of heating during startup. In other words, significant differences in the starting resistance is causing one motor to drag the other as a load rather than getting help, so the windings in that prime mover get hotter, changing the current draw and thus the kW. But as I said, I would expect it to even out over time as the motors equalize.

 

If that condition stays the same when both motors are running with their rotors shorted, I would look for some sort of difference in the motor connections, wire lengths etc., then if that checks out, start looking at the motor winding resistances thenselves.

[ram14375]

dear jraef

i checked both the rotor cable sizes and length ,it is of uniform size

I decoupled the motors and ran it individually and checked the no load current.both of them are same

 

any possibilities from the load side..

 

how to measure the slip losses

ram

[marke]

Hello ram

 

There are a number of issues that can influence the full load slip of a WRIM, but the two major factors are the turns ratio between the rotor and the stator, and the impedance seen by the rotor.

The total impedance includes the impedance of the rotor, the bushgear, the cables and the shorting contactors.

I would look closely at the brushes and ship rings, and the shorting contactors. All it needs, is one slightly loose connection or lower pressure contact.

Essentially, the slip losses between the two motors is different, and that should show up as heat.

If the slip losses are due to external factors, that may show up as a rise in temperature. Perhaps an IR camera may help to find the solution for you.

 

Slip losses are essentially the slip times the shaft torque.

 

If you have a 2% slip, then you have a 2%slip loss which can be caluclated from your shaft power.

ie if your shaft power is 98KW and your slip is 2%, then your slop losses are 2KW.

 

Best regards,

Mark.