Chopper Control of Separately Excited DC Motor:

Motoring Control : A transistor Chopper Control of Separately Excited DC Motor drive is shown in Fig. 5.41(a). Transistor T_r is operated periodically with period T and remains on for a duration t_on. Present day choppers operate at a frequency which is high enough to ensure continuous conduction. Waveforms of motor terminal voltage v_a and armature current i_a for continuous conduction are shown in Fig. 5.41(b). During on-period of the transistor, 0 ≤ t ≤ t_on, the motor terminal voltage is V.

The operation is described by

In this interval, armature current increases from i_al to i_a2. Since motor is connected to the source during this interval, it is called Duty Interval.

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, known as freewheeling interval, is described by

Motor current decreases from i_a2 to i_a1 during this interval.

Ratio of duty interval t_on to chopper period T is called duty ratio or duty cycle (δ). Thus

From Fig. 5.41(b)

Equation (5.2) and (5.7) are also applicable here

From Eqs. (5.7), (5.8). and (5.114)

The nature of speed torque characteristic is shown in Fig. 5.43.

Regenerative Braking:

Chopper Control of Separately Excited DC Motor for regenerative braking operation is shown in Fig. 5.42(a). Transistor T_r is operated periodically with a period T and on-period of t_on. Waveforms of motor terminal voltage v_a and armature current i_a for continuous conduction are shown in Fig. 5.42(b). Usually an external inductance is added to increase the value of L_a. When T_r is on, i_a increase from i_a1 to i_a2.

The mechanical energy converted into electrical by the motor, now working as a generator, partly increases the stored magnetic energy in armature circuit inductance and remainder is dissipated in armature resistance and transistor. When T_r is turned off, armature current flows through diode D and source V, and reduces from i_a2 to i_a1. The stored electromagnetic energy and energy supplied by machine is fed to the source. The interval 0 ≤ t ≤ t_on is now called energy storage interval and interval t_on ≤ t ≤ T the duty interval. If δ is again defined as the ratio of duty interval to period T, then

From Fig. 5.42(b)

and from Fig. 5.42(a)

Since l_a has reversed

From Eqs. (5.8), (5.118) and (5.119)

The nature of speed torque characteristic is shown in Fig. 5.43.

Motoring and Regenerative Braking:

Chopper circuits of Figs. 5.41 and 5.42 can be combined to get a two quadrant chopper of Fig. 5.44, which can provide motoring and regenerative braking operations in the forward direction. Transistor T_rl with diode D₁ form a chopper circuit similar to that of Fig. 5.41, and therefore, provide control for forward motoring operation. Transistor T_r2 with diode D₂ form a chopper circuit similar to that of Fig. 5.42, and  therefore, provide control for forward regenerative braking operation. Thus, for motoring operation transistor T_rl is controlled and for braking operation transistor T_r2 is controlled. Shifting of control from T_rl to T_r2 shifts operation from motoring to braking and vice versa.

In servo drives where fast transition from motoring to braking and vice versa is required, both T_rl and T_r2 are controlled simultaneously. In a period T,T_rl is given gate drive from 0 to δT and T_r2 is given gate drive from δT to T, where δ is the duty ratio for T_rl. Therefore, from 0 to δT motor is connected to source either through T_rl or D₂ depending on whether the motor current i_a is positive or negative. Since V > E, during this period the rate of change of current is always positive. Similarly from δT to T, motor armature is shorted either through D₁ or T_r2 depending on whether i_a is positive or negative and during this period rate of change of current is always negative. Motor terminal voltage and current waveforms are shown in Fig. 5.44 (b).

From Fig. 5.44(b)

Above equation suggests that motoring operation (+ve I_a) takes place when δ > (E/V) and regenerative braking operation takes place when δ < (E/V) and transition from motoring to braking and vice versa occurs when δ = (E/V). The above equations are similar to those obtained for chopper of Fig. (5.41), and therefore, given the same numbers

Dynamic Braking:

Dynamic braking circuit and its waveforms are shown in Fig. 5.45. During the interval 0 ≤ t ≤ t_on, i_a increases from i_a1 to i_a2. A part of generated energy is stored in inductance and rest is dissipated in R_a and T_r. During interval t_on ≤ t ≤ T, i_a decreases from i_a2 to i_a2. The energies generated and stored in inductance are dissipated in braking resistance R_B, R_a and diode D. Transistor T_r controls the magnitude of energy dissipated in R_B, and therefore, controls its effective value.

If i_a is assumed to be rippleless dc, then energy consumed E_N by R_B during a cycle of chopper operation is

Average power consumed by R_B

Effective value of R_B

where

Equation (5.122) shows that the effective value of braking resistor can be changed steplessly from 0 to R_B as δ is controlled from 1 to 0. As the speed falls, δ can be increased steplessly to brake the motor at a constant maximum torque as shown in Fig. 5.8 by chain-dotted line.