There is a 100 HP machine , 460V, 3 phase 60 Hz with 4 poles, and star connected

Question

There is a 100 HP machine , 460V, 3 phase 60 Hz with 4 poles, and star connected. Rs=Rr=0.05 Ohm, Xls=Xlr=0.5 Ohm, rated speed is 1700 rpm. Assume the load demand 85 Nm torque at 50% speed,

1)Calculate the slip speed using V/f control method,

2) Calculate the applied voltage using V/f control assuming Vo=0, verify if the torque can meet the load demand using V/f control and Vo=0.

3) Assume the calculated applied voltage of V/f control has been used assuming Vo=0, and calculate what is the needed slip if slip control can be used.

(Assume Xlm is big enough, and the simple equivalent circuit can be used)

Explanation / Answer

Slip speed ,s = (Ns-Nr)/ Ns

Ns = 120f/P

=(120x 60)/4 = 1800

Rated = 1700

s= (1800-1700)/ 1800 = 0.0555555.

T= 3/(2pi Ns) X (sE2^2 * R^2) / (R2^2 +(s*X2^2)

E2 is the applied voltage and Ns is the sychronous speed(50% means put 1800/2 =900). T torque 85Nm.

Solving the equation the applied voltage = 1365V which is higher than the operational volt. So it is not possible to run the motor at given torque at 50 % speed.

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