SOLID STATE CONTROL OF DC MOTOR DRIVES

• Power electronic converters used in DCmotor control are

Rectifier (AC to DC)Chopper(Fixed DC to variable DC)

Advantages

• Chopper offers greater efficiency,

• faster response,

• lower maintenance,

• smaller size and

• smooth control.

Applications

• Choppers are widely used in trolley cars, battery operated vehicles, traction motor control, control of large number of dc motors, etc…..

• They are also used in regenerative braking of dc motors to return energy back to supply.

Control strategies

1.Time ratio control (TRC)

Constant frequency system

Variable frequency system

2.Curent limit control method.(CLC)

Constant frequency system

• Ton is varied but chopping frequency (f) of chopping period T is constant

• This is known as Pulse width modulation

Variable frequency system

• The chopping frequency (f) of chopping period T is varied and either Ton is kept constant

• Or Toff is kept constant

• This is known as frequency modulation

• Chopper is on when current is equal to lower limit

• The chopper is off when the current exceeds the upper limit.