0195136047.pdf

774 BASIC CONTROL SYSTEMS

L C M

L C M

PWM

inverter

Vd

PWM

inverter

Filter

+

−

Filter

Dc link

(b)

(a)

Ac

supply

Diode

bridge

Figure 16.1.20PWM inverter drives.(a)Supply is dc.(b)Supply is ac.

class of slip-power control. In order to implement these techniques using power semiconductor

devices, some of the schemes are static rotor resistance control; the static Scherbius drive; and

the static Kramer drive, of which only the first is considered here.

The rotor circuit resistance can be smoothly varied statically (instead of mechanically) by

using the principles of a chopper. Figure 16.1.21 shows a scheme in which the slip-frequency ac

rotor voltages are converted into direct current by a three-phase diode bridge and applied to an

external resistanceR. The self-commutated semiconductor switchS, operating periodically with

a periodT, remains on for an intervaltonin each period, with a duty ratioδdefined byton/T.

The filter inductor helps to reduce the ripple in currentIdand eliminate discontinuous

conduction. Figure 16.1.22 shows input phase voltage waveforms of the diode bridge and a

six-step rotor phase-current waveform, assuming ripple-freeId. For a periodTof the switch

operation, the energy absorbed by resistanceRis given byId^2 R(T−ton), while the average power

consumed byRis

R

S

Id

Vd

+

−

Ld, Rd

Wound-rotor

motor

Diode bridge

3-phase

ac supply

Figure 16.1.21Static rotor resis-

tance control of a wound-rotor in-

duction motor.