Speed and torque control  (Page 3/19)

Figure 10.3 Shunted-armature method of speed control applied

to (a) a series motor and (b) a shunt motor.

Armature-Terminal Voltage Control Armature-terminal voltage control can be accomplished with the use of power-electronic systems.

• In Fig. 10.4a, a phase-controlled rectifier in combination with a dclink filter capacitor can be used to produce a variable dc-link voltage which can be applied directly to the armature terminals of the dc motor.
• In Fig. 10.4b, a constant dc-link voltage is produced by a diode rectifier in combination with a dc-link filter capacitor. The armature terminal voltage is then varied by a pulse-width modulation scheme in which switch S is alternately opened and closed.
• When switch S is closed, the armature voltage is equal to the dc-link voltage $V_{\text{dc}}$ , and when the switch is opened, current transfers to the freewheeling diode, essentially setting the armature voltage to zero. Thus the average armature voltage under this condition is equal to

$V_a={\text{DV}}_{\text{dc}}$ (10.7)

Figure 10.4 Three typical configurations for armature-voltage control.

(a) Variable dc-link voltage (produced by a phase-controlled rectifier)

applied directly to the dc-motor armature terminals.

(b) Constant dc-link voltage with single-polarity pulse-width modulation.

(c)Constant dc-link voltage with a full H-bridge.

where

$V_a=$ average armature voltage (V)

$V_{\text{dc}}=$ dc-link voltage (V)

D = PWM duty cycle (fraction of time that switch S is closed)

• Figure 10.4c shows an H-bridge configuration. Note that if switch S3 is held closed while switch S4 remains open, this configuration reduces to that of Fig. 10.4b. However, the H-bridge configuration is more flexible because it can produce both positive- and negative-polarity armature voltage.
• For example, with switches S 1 and S3 closed, the armature voltage is equal to $V_{\text{dc}}$ while with switches S2 and S4 closed, the armature voltage is equal to $-V_{\text{dc}}$ .
• Using such an H-bridge configuration in combination with an appropriate choice of control signals to the switches allows this PWM system to achieve any desired armature voltage in the range .
• Advantages of armature-voltage control: because the voltage drop across the armature resistance is relatively small, a change in the armature terminal voltage of a shunt motor is accompanied in the steady state by a substantially equal change in the speed voltage. Thus, motor speed can be controlled directly by means of the armature terminal voltage.
• Frequently the control of motor voltage is combined with field-current control in order to achieve the widest possible speed range.
• With such dual control, base speed can be defined as the normal-armature-voltage, full-field speed of the motor.
• Speeds above base speed are obtained by reducing the field current; speeds below base speed are obtained by armature-voltage control. The range above base speed is that of a constant-power drive.
• The range below base speed is that of a constant-torque drive because, as in armature resistance control, the flux and the allowable armature current remain approximately