Equivalent Motor Impedance Multipliers
Equivalent Motor Impedance Multipliers
(OP)
In the ANSI world, there are multipliers specified for motors to convert motor subtransient reactances to equivalent 1/2 cycle (for momentary) and 1.5~4 cycle (interrupting) network impedances.
My question is, if the X/R ratio for the associated motor is known, is this multiplier going to be applied to the motor short circuit P.U. Resistance as well?
My question is, if the X/R ratio for the associated motor is known, is this multiplier going to be applied to the motor short circuit P.U. Resistance as well?






RE: Equivalent Motor Impedance Multipliers
No multiplier is required for Resistance but as X/R indicated in the same standard does not depends upon the time indirectly the resistance would get the same multiplier.
IEC 60909 does not modify the impedances but multiply the I"kM in order to compare with breaker interruption capabilities.
In IEC 60909, a factor named "q" is stated for the time in seconds and motor in MW/no.of pole pair.
No.of pole pair= frq*60/rpmsynchron.For instance for 0.02 sec[less than 1/2 cycle at 50 Hz] :
q=1.03+.012*ln(m) m=MW/p [pole pairs]
Ibreaker=Ik"*miu*q miu =decay of symmetrical short-circuit current.
miu=0.84+0.26*exp(-0.28)*I"kM/IrM for t=0.02 sec where: I"kM=c*Un/sqrt(3)/Zm
Zm=1/(ILR/IrM)*Un^2/SrM Zm=sqrt(Xm^2+Rm^2)
ILR=locked rotor current IrM=motor rated current Un=rated motor voltage SrM=rated motor apparent power.
If it is a system breaker you'll take X"d=Zm that means miu=1 q=1.
Constant c is a correction factor for voltage calculating the greatest short-circuit current [e.g. for medium voltage c=1.1]
and the smallest short-circuit current[ c=1].