Page 438 - Handbook of Electrical Engineering

HARMONIC VOLTAGES AND CURRENTS 427

Table 15.4. Harmonic rms currents and voltages in a star-wound induction motor that is fed from a voltage
source inverter
Harmonic Stator current Rotor current Magnetising Air-gap voltage
number current

Mag. Angle Mag. Angle Mag. Angle Mag. Angle
(Amps) (Degrees) (Amps) (Degrees) (Amps) (Degrees) (Amps) (Degrees)
1 392.28 −25.60 371.51 −14.27 78.56 −89.07 230.27 −3.93
5 62.14 −89.35 59.86 −89.33 2.27 −89.72 33.32 0.28
7 31.70 −89.46 30.54 −89.45 1.16 −89.83 23.80 0.17
11 12.84 −89.69 12.37 −89.69 0.47 −89.88 15.14 0.12
13 9.19 −89.72 8.86 −89.71 0.37 −89.90 12.81 0.09
17 5.38 −89.80 5.18 −89.79 0.20 −89.92 9.80 0.08
19 4.30 −89.81 4.14 −89.80 0.16 −89.93 8.77 0.07
23 2.94 −89.85 2.83 −89.85 0.11 −89.94 7.24 0.06
25 2.49 −89.86 2.40 −89.85 0.09 −89.95 6.66 0.05
29 1.85 −89.88 1.78 −89.88 0.07 −89.96 5.74 0.05
31 1.62 −89.89 1.56 −89.88 0.06 −89.96 5.37 0.04

If the above calculations are made for all the active harmonics then their results can be added
and the waveforms synthesised. Table 15.4 summarises the results.
Figure 15.12 shows the synthesised currents and air-gap voltage using the first 61 harmonics.

15.4.3.2 Worked example

The same motor as used in the ‘worked example’ of sub-section 15.4.3.1 is fed from a current source
inverter. Find the currents and air-gap voltage in the circuit.
The equivalent circuit fed from a constant current source is shown in Figure 15.11, wherein
I 1n is the source current instead of V 1n .
◦
Assume the inverter output line current consists of a 120 rectangle wave and that the
commutation angle u is small enough to be ignored. The complete waveform has a harmonic con-
tent of,

1 πn 5πn 7πn 11πn
b n = cos − cos − cos + cos
πn 6 6 6 6
3.464
= for n = 1, 5, 7, 11, 13 etc.
nπ
1.1026
=
n

The rms value of the fundamental line current is 392.26 amps. Therefore its peak value is
√
392.26 2 = 554.74 amps which corresponds to b 1 having a value of 1.1026. The peak values of the
harmonic components of the line current are given below in Table 15.5.