Question:

Characterisation tests have been performed on a four pole, 415 V (line to line), three phase, star connected, 50Hz induction motor, with the following results:

• No-load test : 415V_l-l, 3.76A, 234.3 W/phase.

(obtained at synchronous speed with a drive motor supplying the rotational losses of the drive system)

• Locked rotor test: 163.8 V_l-l, 30A, 603 W/phase.

a)  Draw the per phase approximate equivalent circuit model of the induction motor, with all elements "stator-referred". Identify the physical significance of each element in the model.

b)  Using the characterisation data, calculate the "stator-referred" induction motor parameters. Assume that the shunt elements are much larger than the series terms. Remember that the measured input voltages are "line-to-line", not phase voltages.

c)  A load test is performed on the induction motor, which is fed from a 415V_l-l 50Hz supply, and the rotor speed is measured to be 1410rpm. Calculate the output mechanical power and mechanical torque for this test if the rotational losses constitute 880W for this condition. [Hint : Assume that the stator and rotor winding resistances are split equally].

d)  Calculate the induction motor efficiency for the load test described in (c).

e)  It is required to run the induction motor at a no-load zero slip speed of 900 rpm. Neglecting rotational losses, what must the supply voltage and frequency be in order to achieve this speed?