-It is generated by comparing a triangular wave signal with a DC
signal.The DC signal can range between the maximum and
minimum voltages of the triangular voltage
When triangle waveform voltage is greater than the DC level, the output of the op-amp swings high, and when it is lower, the output swings low the way it controls the motor by the timing ON and OFF duration that can be control by the user.
-use concept ON and OFF signal Duty cycle= t ON /(t ON +t OFF ) x 100% Example: 1) t ON +t OFF =100 % duty cycle=10% means total ON=10% 2) duty cycle=90% means total ON 90% Note: To control speed, when %ON increase, the motor faster
Speed control circuit 1) When Q1 is ON,Ia will go through La and motor will start to move.At this time La will start to charge and magnetic field exist. 2) When Q1 is OFF after ON, energy in La need to discharge. This is done by discharge through D1. This kind of circuit is for safety so Q1 is not damage by very high voltage or current. -Please refer to Figure 6.9 for full bridge circuit for reverse and forward movement of motor.
Chopper the relatives ON-OFF thyristor determine the average of the motor. -when thyristor is ON,Ia will flow through L and motor will start to move. -when motor is move until around 75% speed of motor,commutating switch will closed. This will caused thyristor off. -when thyristor off after time t1,armature current decline through freewheeling diode as energy stored in a circuit inductance is applied to the armature.
Chopper -motor will slow down(since thyristor OFF),then commutating switch will be open and thyristor ON again. -diode serve a path to maintain the armature current which no longer flow through external circuit. -as the ratio ON/OFF increased,the average motor Vm rises. -This process will be repeated.
A 100 hp series motor rated 180A is operating in a chopper circuit from a
500V dc source ( Vsupply=500V ). The armature and field inductance is 0.06H(L=0.06H). At the minimum ratio t1/(t1 + t2) of 0.20 .
Find the pulse frequency to limit the amplitude of the armature current excursion to 10A.(Ia max=10A)
1)Vave arm=Vsupply x (t1/(t1+t2) = 500x0.2 =100V Note: Vave arm is an average voltage that cause motor to move.InPWM usage,the voltage value to move the motor is lower than Vsupply.This is power saving and security. 2) Voltage time area apply to inductor =(Vsupply-Vaverage)xT ON =(500-100)t1 =400t1