SimPowerSystems

Chopper-Fed DC Motor Drive

The example described in this section illustrates application of Power System Blockset to the operation of a DC motor drive in which the armature voltage is controlled by a GTO thyristor chopper.

The objective of this example is to demonstrate the use of electrical blocks, in combination with Simulink blocks, in the simulation of an electromechanical system with a control system. The electrical part of the DC motor drive, including the DC source, the DC motor, and the chopper, is built using blocks from the Elements, Machines, and Power Electronics libraries. The DC Machine block models both electrical and mechanical dynamics. The load torque-speed characteristic and the control system are built using Simulink blocks.

Description of the Drive System

A simplified diagram of the drive system is shown in the next figure. The DC motor is fed by the DC source through a chopper that consists of the GTO thyristor, Th1, and the free-wheeling diode D1. The DC motor drives a mechanical load that is characterized by the inertia J, friction coefficient B, and load torque T_L (which can be a function of the motor speed).

Figure 2-9: Chopper-Fed DC Motor Drive

In this diagram, the DC motor is represented by its equivalent circuit consisting of inductor La and resistor Ra in series with the counter electromotive force (emf) E.

The backreaction EMF is proportional to the motor speed

where K_E is the motor voltage constant and is the motor speed.

In a separately excited DC machine, the motor voltage constant KE is proportional to the field current I_f

where L_af is the field-armature mutual inductance.

The torque developed by the DC motor is proportional to the armature current Ia:

where K_T is the motor torque constant.

The DC motor torque constant is equal to the voltage constant.

Thyristor Th1 is triggered by a pulse width modulated (PWM) signal to control the average motor voltage. Theoretical waveforms illustrating the chopper operation are shown here:

Figure 2-10: Waveforms Illustrating the Chopper Operation

The average armature voltage is a direct function of the chopper duty cycle .

Note that this relation is valid only when the armature current is continuous. In steady state, the armature average current is equal to

The peak-to-peak current ripple is

where is the duty cycle and r is the ratio between the chopper period and the DC motor electrical time constant.

In this case study, a variable-speed DC motor drive using a cascade control configuration is considered. Here is a block diagram of this drive:

Figure 2-11: Variable-Speed DC Motor Drive

The motor torque is controlled by the armature current I_a, which is regulated by a current control loop. The motor speed is controlled by an external loop, which provides the current reference I_a* for the current control loop.

Transient Performance for a Fault at Bus B2

Modeling the DC Drive