Handbook of Electrical Engineering

HARMONIC VOLTAGES AND CURRENTS 421

The effect of the commutation angleuon the 180◦,60◦and supply current waveforms is the
same as found in sub-section 15.3.2 for the 120◦waveform. Therefore each coefficientbnbecomes,

bnu=

2 bn

un

sin

(un

2

)

Figures 15.7 and 15.8 show the following waveforms foru= 5 ◦for the first 25 and 43
harmonics included, some being naturally zero.

• Star primary line current or 120◦waveform.


• Delta primary line current or the sum of the 180◦and 60◦waveforms.


• Total primary line current.

15.4 Inverters

15.4.1 Basic method of operation

Inversion is the process by which a DC voltage is changed into an AC voltage by the use of a set of
switches. The following illustrates the method of operation of a simple single-phase ‘square-wave’
inverter. Consider Figure 15.9.

The four switchesT 1 ,T 2 ,T 3 andT 4 , are controlled in their fully ‘on’ and fully ‘off’ modes, in
a sequence that causes the currentIacand hence voltageVacto flow in one direction, to fall to zero,
to flow in the opposite direction and again to fall to zero. The conduction of current in the load from
A to B is achieved by closingT 1 andT 2 , and keepingT 3 andT 4 open. The conduction from B to
A is the reversed process,T 3 andT 4 are closed andT 1 andT 2 are kept open. The capacitors, diodes

Figure 15.9 Circuit diagram of a single-phase square-wave inverter.