The power-electronics equipments, such as rectifier, inverters and choppers have switching devices and their operation produces current and voltage harmonics into the system from which they are working. These harmonics affect the operation of other equipments connected to the same system through conduction or by radio interference. The harmonics present in the inverter system lead to the following disadvantages,

(i) Harmonic currents will lead to excessive heating in the induction motors connected with the thyristor system. This will reduce the load carrying capacity of the motor.

(ii) If the control and regulating circuits are not properly shielded, harmonics from power side can affect their operation and malfunctioning can result.

(iii) On critical loads, torque pulsation produced by the harmonic current can be harmful.

(iv) Harmonic currents cause losses in the a.c. system and can even sometime produce resonance in the system. Under such resonant conditions, the metering and instrumentation may also be affected because of the distortion, etc.

These effects can be minimized by reducing the harmonic content.

There are various methods available to reduce the harmonic content. The reduction of harmonic contents or the improvement in waveshape can be done by following methods:

(i) Single-pulse width modulation

(ii) Transformer connections

(ii) Multiple commutation in each half-cycle

iv) Stepped wave-inverters

Normally, a single-phase bridge inverter produces a square wave. This square- waveform contains 33$\frac{1}{3}$ percent third harmonic, 20 percent fifth harmonic and 14$\frac{1}{2}$ percent seventh harmonic. In some applications, harmonics at the output should be less than 5 percent. It is customary to reduce the lower order harmonics by some technique and use filter for higher order harmonics. This procedure not only decreases the cost of the filter but also improves the transient response very much.