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Equivalent Motor Impedance Multipliers

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?

RE: Equivalent Motor Impedance Multipliers

According to ANSI C37.010-1999 "Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis" the subtransient reactance of an induction motor Xd" is to be multiplied by factors in order to state the circuit breaker ratings for 1/2 cycle [presumption this is the minimum time for circuit breaker closing] and 1.5~4 cycles for comparison with medium voltage breaker interrupting capabilities.
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].

 

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