Chopper Fed DC Drives

The DC voltage is changed from fixed to variable by the chopper. Because they can be commutated by a low power control signal and do not require a commutation circuit, self-commutated devices (directly on or off devices via gate) like MOSFET, IGBT, power transistors, GTO, and IGCT are used in the construction of helicopters. The chopper was run at a high frequency, which improved the performance of the motor by reducing ripple and eliminating discontinuous conduction. The most crucial aspect of chopper control is that when the drive is fed from a fixed voltage to a low DC voltage, the regenerative braking is performed at a very low generating speed.

Motoring Control (Chopper Fed DC Drives)

The diagram below depicts a transistor-controlled, separately excited DC motor. With a period T_r and an open state for a duration Ton, the transistor T_r is periodically operated. The graph below displays the waveforms of the armature current and motor terminal voltage. When turned on, the motor’s terminal voltage is V, and its operation is described as

Chopper Fed DC Drives

The armature current increases from ia1 to i_a2 during this time period. Due to the fact that the motor is connected to the source directly, this period is known as the duty interval. T_r is stopped at t = t_on. During the ton-t-T interval, the motor terminal voltage is zero and the motor current freely oscillates through the diode D_f. Freewheeling interval, the term for this phase of motor operation, is defined 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

In the illustration below, a choppers for regenerative braking operation is shown. With a period T and an on-period of t_on, the transistor T_r is periodically activated. The graph below displays the waveform of the motor terminal voltage v_a and armature current i_a for continuous conduction. Inductance from outside the system is added to raise L_a‘s value. The transistor’s on state causes i_a to rise from ia1 to i_a2.

The motor, which is now acting as a generator, converts mechanical energy into electrical energy by partially increasing the stored magnetic energy in the armature circuit inductance and dissipating the remaining energy in the armature and transistors.

When a transistor is turned off, the armature current reduces from i_a2 to i_a1 and flows through the source V and diode D. Both the machine’s energy and the electromagnetic energy that has been stored are fed into the source. The duty interval is denoted by the interval t_on ≤ t ≤ T and the energy storage interval by the interval 0 ≤ t ≤ t_on.

Forward Motoring and Braking Control (Chopper Fed DC Drives)

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.

Transistor T_r1 is controlled for operation during motoring, and T_r2 is controlled during braking. When the control is switched from T_r1 to T_r2,. The operation changes from driving 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.

I_a decreases from i_a2 to i_a1 over the T_on≤ t ≤ T interval. Braking resistance RB, R_a, and diode D are used to dissipate the energies produced and stored in inductances. Transistor T_r regulates the amount of energy lost in RB, thereby regulating its actual value.