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Motors In Tandem


bob

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Hi,

 

We have got on site two 750 k W 400 V wound motors driving a common load. One of the motor has been damaged and cost of repairs is very high. We are looking for an alternative solution, like replacing the motor with another motor one. But I trust that the motor should be an identical one due to sharing issues.

What are the issues that need to be addressed ?

Thanks.

 

Bob

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Hi Bob

 

I assume that these two motors are wound rotor??

 

If they are wound rotor and you leave resistance in the rotor circuit, then the requirement for them to be identical is reduced. If they are not identical and the rotors are bridged, the imbalance will be more severe than a standard cage motor.

 

You need to match the rated full load slip (with the rotor shorted).

If the frame (rotor) voltage is not the same, this can be accommodated by different resistors in the start circuit, but this will not correct a run time imbalance.

 

Best regards,

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Hi Marke,

 

Thanks for the reply. You are right , the two motors are wound rotor. I trust that the damaged one would need to be replaced by a similar(twin) wound rotor motor with the same torque/speed characteristic and slip. What would be the possible outcomes by replacing the damaged motor with a 500 kW wound rotor motor?It is also important to note that the load is a variable one and the issue to match a fixed resistor to compensate for the difference in slip is out of question.

 

Bob

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Hi Bob

 

The critcial thing is the full load slip.

If both motors have got identical full load slip, then they will share the load equally. If the full load slip of one motor is less than the other, it will take the higher load.

Adding rotor resistance is one way to balance the full load slip, but it does add slip losses which are dissipated in the rotor resistors. This will drop the running efficiency.

 

If the replacement motor has the same full load slip with the rotor shorted as the existing motor, there should be no issues at ful speed and all loading conditions.

If the start characteristics are different, it may be necessary to alter the start resistors to balance it up during start.

 

Best regards,

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Hi MARKE,

 

Thanks for your prompt reply. I am quoting "If both motors have got identical full load slip, then they will share the load equally". I believe this holds true provided the k W of boths motors are identical.

 

Regards.

 

Bob

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Hello Bob

 

If you have two motors with different power ratings, but identical full load slip, the shaft power will be distributed in accordence with their power ratings.

i.e. two motors:

motor a) has a full load slip of 1480 RPM and a power rating of 100 KW

motor B) has a full load slip of 1480 RPM and a power rating of 200KW

 

If the load is increased until the slip is 1480 RPM, then motor a) will be providing 100KW shaft power and motor B) will be providing 200KW shaft power.

 

Best regards,

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