Q14Electrical Machines and Drives

Chopper Controlled DC Drive

A chopper (DC-DC converter) controlled DC drive is used to obtain smooth, continuously variable DC motor armature voltage directly from a fixed DC source (such as a battery, or a rectified and filtered DC bus), by rapidly switching a power semiconductor device (IGBT, MOSFET, or thyristor) ON and OFF at a high chopping frequency (typically several hundred Hz to several kHz), with the ON-to-total-period ratio, called the duty cycle d, determining the average voltage delivered to the motor armature.

The basic single-quadrant (Class A/Type A) chopper drive consists of the chopper switching device in series between the DC source and the motor armature, with a freewheeling diode connected across the armature to provide a path for the armature current to continue circulating (due to the armature circuit's inductance) during the chopper's OFF interval, preventing dangerous voltage spikes from the sudden interruption of an inductive current.

where Va is the average armature voltage, Vs is the fixed DC source voltage, and d = Ton/T is the chopper's duty cycle (ratio of ON time to total switching period T). Since the DC motor's steady-state speed N is directly proportional to (Va - Ia*Ra)/Kphi, continuously varying d from 0 to 1 provides smooth, stepless speed control from zero up to base speed, entirely electronically and without the switching transients, discrete steps, or power loss associated with resistive armature control.

During the chopper's ON period, the source directly supplies the armature; during the OFF period, the armature current, which cannot change instantaneously due to the armature circuit's inductance, freewheels through the diode, maintaining continuity of current flow (provided the chopping frequency is high enough, or the armature inductance large enough, that the current does not fall to zero before the next ON period begins - the continuous-conduction mode). If the load or motor inertia causes the current to fall to zero before the next switching cycle, the drive enters discontinuous conduction mode, in which the average armature voltage becomes a more complex, current-dependent function of duty cycle rather than the simple linear relationship above, and the motor's effective torque-speed characteristic becomes noticeably softer (more drooping) in this mode.

More elaborate multi-quadrant chopper configurations (Class C, Class D, or full four-quadrant chopper bridges built from two or four switching devices with associated diodes) extend this basic single-quadrant, forward-motoring-only capability to also provide regenerative braking (returning armature kinetic energy back to the DC source when the chopper duty cycle is arranged so the motor's back-EMF exceeds the average applied voltage, forcing current to flow back into the source through an appropriately-positioned diode or switch) and reverse-direction motoring, making chopper-controlled DC drives a versatile, efficient, and widely used technique for battery-powered traction (electric vehicles, forklifts, trolleys) and other DC drive applications requiring fast dynamic response, high efficiency, and smooth speed control.