Handbook of Electrical Engineering

424 HANDBOOK OF ELECTRICAL ENGINEERING

For motors that normally operate at slips in the order of 0.5% to 3.0% the values ofsnforn,
greater than unity are approximately given by,

sn

n− 1

n

forn= 7 , 13 ,19 etc.

= 0. 8571 , 0. 9231 , 0 .9474 etc.

sn

n+ 1

n

forn= 5 , 11 ,17 etc.

= 1. 2 , 1. 0909 , 1 .0588 etc.

The rotor resistance that represents the winding and the load isR 2 /s 1 for the fundamental.
For the harmonic resistance to be found simply replaces 1 bysn.

15.4.3.1 Worked example

A 250 kW three-phase, four-pole, 50 Hz, 415 V, induction motor is fed from a voltage source inverter.
The motor has the following parameters for its star-wound windings. Find the currents and air-gap
voltage in the circuit.

R 1 Stator resistance 0.0053 ohms

X 1 Stator leakage reactance 0.0470 ohms

R 2 Rotor resistance at full-load 0.0045 ohms

X 2 Rotor reactance at full-load 0.1113 ohms

XmMagnetising reactance 2.9310 ohms

s 1 Full-load slip 0.00738 pu

Full-load efficiency 0.983 pu

Full-load power factor 0.902 pu

Full-load current 392.26 amps

The equivalent circuit fed from a Thevenin voltage source is shown in Figure 15.11.

Figure 15.11 Equivalent circuit of an induction motor when fed from a voltage source that contains harmonics.