Speed Control of Universal Motor

• The speed control of universal motor is shown in the figure A. 
• The single phase bridge rectifier converts alternating voltage into direct voltage.
•  The zener diode provides constant voltage to RC circuit. 
• The charging of capacitor is done through variable resistor R. 
• When the voltage across capacitor becomes equal to diac break over voltage, the diac turns on. 
• The gate terminal of SCR receives gate pulse and therefore it turns on. As soon as the SCR turns on, current passes through universal motor. 
• The discharging of capacitor is done through diac and gate cathode circuit. The speed control of DC motor is done through variable resistor R. 
• If the variable resistor R sets at higher value, the charging rate of capacitor decreases and firing angle increases. 
• This will result in speed of universal motor decreases. There are others several method to control speed of universal motor.

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