## Motor Contribution in Short Circuit Current

## Motor Contribution in Short Circuit Current

(OP)

I have a question about the contribution of motor in short circuit current. For a simple circuit of a 1500kW 13.8kV generator, which feeds a 1000kVA 13.8kV/480V transformer, and the transformer feeds a 480V 250HP motor. We have to calculate the short circuit current at the primary terminals of the transformer. If the full load current of the motor is 270A and the impedance of the transformer is 5.75%, then which formulas should be used to calculate that how much short circuit current this motor will produce, and how much of this short circuit current (produced by motor) will pass through from the secondary to the primary of the transformer? Thanks for help!

## RE: Motor Contribution in Short Circuit Current

## RE: Motor Contribution in Short Circuit Current

But I am going to start another thread for a discussion on induction motor contribution to a non bolted fault.

Bill

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"Why not the best?"

Jimmy Carter

## RE: Motor Contribution in Short Circuit Current

## RE: Motor Contribution in Short Circuit Current

The attached Bussmann document includes some short circuit calcs including motor contribution.

Also available here: http://www.cooperindustries.com/content/dam/public...

## RE: Motor Contribution in Short Circuit Current

IEC 60909-0 Short-circuit currents in three-phase a.c. systems -Part O: Calculation of currents.

Chapter 3.8.2 Contribution to short-circuit currents by asynchronous motors

The impedance ZM = RM + jXM of asynchronous motors in the positive- and negative-sequence systems can be determined by:

ZM=1/(ILR/IrM)*UrM/√3/IrM=1/(ILR/IrM)*UrM^2/SrM (26)

where:

UrM is the rated voltage of the motor;

IrM is the rated current of the motor;

SrM is the rated apparent power of the motor SrM = PrM/(TM.COSTM)

TM=efficiency ; COS(TM)=power factor

If RM/XM is known, then XM shall be calculated as follows:

XM=ZM/√(1+(RM/XM)^2)

The following relations may be used with sufficient accuracy:

RM/XM = 0.10, with XM = 0.995 ZM for medium-voltage motors with powers PrM per pair of poles>= 1MW;

RM/XM = 0.15, with XM = 0.989 ZM for medium-voltage motors with powers PrM per pair of poles <1MW;

RM/XM = 0.42, with XM = 0.922 ZM for low-voltage motor groups with connection cables.

For the calculation of the initial short-circuit currents according to 4.2, asynchronous motors are substituted by their impedances ZM according to equation (26) in the positive-sequence and negative-sequence systems. The zero-sequence system impedance Z(0)M of the motor shall be given by the manufacturer, if needed (see 4.7).

## RE: Motor Contribution in Short Circuit Current

## RE: Motor Contribution in Short Circuit Current

## RE: Motor Contribution in Short Circuit Current