Page 191 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

POWER ELECTRONICS, RECTIFIERS, AND PULSE-WIDTH MODULATION INVERTERS

9.18 CHAPTER NINE

FIGURE 9.27 (a) A circuit controlling the voltage to an ac load by phase angle control.
(b) Voltages on the source, the load, and the SCR in this controller.

AC Phase Control for an AC Load
An ac load can be controlled as shown in Fig. 9.27a. The load voltage and current are
shown in Fig. 9.27b. A simpler ac power controller can be made by substituting the PNPN
diode with a DIAC and the SCR by a TRIAC (Fig. 9.28).

THE EFFECT OF INDUCTIVE LOADS ON PHASE
ANGLE CONTROL

Real machines are inductive loads normally. This introduces new complications to the
operation of the controller. Since inductors store magnetic energy, the current in an inductive

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