6-27. A wound-rotor induction motor is operating at rated voltage and frequency with its s...

6-27. A wound-rotor induction motor is operating at rated voltage and frequency with its slip rings shorted and with a load of about 25 percent of the rated value for the machine. If the rotor resistance of this machine is doubled by inserting external resistors into the rotor circuit, explain what happens to the following:

(a) Slip s

(b) Motor speed nm

(c) The induced voltage in the rotor

(d) The rotor current

(e)

(f)Pout

(g) P-RCL

(h) Overall efficiency ?

Solution

(a) The slip s will increase.

(b) The motor speed nm will decrease.

(c) The induced voltage in the rotor will increase.

(d) The rotor current will increase.

(e) The induced torque will adjust to supply the load's torque requirements at the new speed. This will depend on the shape of the load's torque-speed characteristic. For most loads, the induced torque will decrease.

(f) The output power will generally decrease:

(g) The rotor copper losses (including the external resistor) will increase.

(h) The overall efficiency n will decrease.