Chopper Control of Separately Excited DC Motor

The chopper converts the fixed DC voltage to variable DC voltage. Self-commutated devices (directly on or off devices via gate) like MOSFET, IGBT, power transistors, GTO and IGCT are used for making choppers because they can be commutated by low power control signal and do not need commutation circuit.

The chopper was operated at high frequency due to which it upgrade the motor performances by decreasing the ripple and removing the discontinuous conduction. The most important feature of chopper control is that the regenerative braking is carried out at very low generating speed when the drive is fed from a fixed voltage to low DC voltage.

Motoring Control

The transistor chopper controlled separately excited DC motor is shown in the figure below. The transistor T_r is operated periodically with period T_r and remains open for a duration T_on.The waveforms of motor terminal voltage and armature current are shown in the figure below. During on the motor terminal voltage is V and operation of the motor is described as

In this interval, the armature current raises from i_a1 to i_a2. This interval is called duty interval because the motor is directly connected to the source.

At t = t_on, T_r is turned off. Motor current freewheels through diode D_f and motor terminal voltage is zero during interval t_on≤ t ≤ T. Motor operation during this interval is known as freewheeling interval and is described by

Motor current decreases from i_a2 to i_a1 during this interval.The ratio of duty interval t_on to chopper period T is called duty cycle.

Regenerative Braking

Chopper for regenerative braking operation is shown in the figure below. The transistor T_r is operated periodically with a period T and on-period of t_on. The waveform of motor terminal voltage v_a and armature current i_a for continuous conduction is shown in the figure below. The external inductance is added to increase the value of L_a. When the transistor is on, i_a increased from i_a1 to i_a2.

The mechanical energy is converted into electrical energy by the motor, now working as a generator, partly increased the stored magnetic energy in the armature circuit inductance and the remainder is dissipated in armature and transistors.

When the transistor is turned off, the armature current flows through diode D and the source V and reduces from i_a2 to i_a1. The stored electromagnetic energy and the energy supplied by the machine are fed to the source. The interval  0  ≤ t  ≤ t_on is called energy storage interval and the interval t_on ≤ t ≤ T called the duty interval.

Forward Motoring and Braking Control

The forward motoring operation of the chopper is obtained by the transistor T_r1 with the diode D₁.The transistor T_r2 and diode D₂ provide the control for forward regenerative braking operation.

For the motoring operation, transistor T_r1 is controlled, and for braking operation, the transistor T_r2 is controlled. Shifting of control from T_r1 to T_r2 shift the operation from motoring to braking and vice versa.

Dynamic Control

The dynamic braking circuit and its waveform are shown in the figure below. During the interval between 0 ≤ t ≤T_on, i_a increases from i_a1 to i_a2. The part of the energy is stored in inductance and rest is dissipated in R_a and T_R.

During the interval T_on≤ t ≤ T, i_a decreases from i_a2 to i_a1.The energies generated and stored in inductances are dissipated in braking resistance R_B, R_a and diode D.Transistor T_r control the magnitude of energy dissipated in R_B and therefore control its effective value.