Control of DC Motors by Choppers Dr. D G Padhan PSD 1
DC Chopper a static power electronic device that converts fixed dc input voltage to a variable dc output voltage considered as dc equivalent of an ac transformer since they behave in an identical manner used all over the world for rapid transit systems used in trolley cars, marine hoist, forklift trucks and mine haulers offer smooth control, high efficiency, faster response and regeneration facility The power semiconductor devices used for a chopper circuit can be force commutated thyristor, power BJT, MOSFET and IGBT Dr. D G Padhan PSD 2
GTO based chopper are also used These devices are generally represented by a switch. When the switch is off, no current can flow. Current flows through the load when switch is on. The power semiconductor devices have on-state voltage drop of 0.5V to 2.5V across them. For the sake of simplicity, this voltage drop across these devices is generally neglected Dr. D G Padhan PSD 3
PRINCIPLE OF CHOPPER OPERATION A chopper is a high speed on" or off semiconductor switch It connects source to load and disconnect the load from source at a fast speed. In this manner, a chopped load voltage is obtained from a dc supply of constant magnitude Dr. D G Padhan PSD 4
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During the period T on, chopper is on and load voltage is equal to source voltage Vs. During the period T off, chopper is off, load voltage is zero. In this manner, a chopped dc voltage is produced at the load terminals Dr. D G Padhan PSD 6
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CONTROL STRATEGIES The average value of output voltage Vo can be controlled through duty cycle by opening and closing the semiconductor switch periodically. The various control strategies for varying duty cycle are as following: 1. Time ratio Control (TRC) 2. Current-Limit Control. Dr. D G Padhan PSD 8
Time ratio Control (TRC) Time ratio Ton/T(duty ratio) is varied. 1. CONSTANT FREQUENCY SYSTEM on-time is varied but chopping frequency f is kept constant. Variation of Ton means adjustment of pulse width, as such this scheme is also called pulse-width-modulation scheme. 2. VARIABLE FREQUENCY SYSTEM the chopping frequency f is varied and either (i) on-time Ton is kept constant or (ii) off-time Toff is kept constant. This method of controlling duty ratio is also called Frequency-modulation scheme. Dr. D G Padhan PSD 9
CURRENT- LIMIT CONTROL the on and off of chopper circuit is decided by the previous set value of load current. The two set values are maximum load current and minimum load current. When the load current reaches the upper limit, chopper is switched off. When the load current falls below lower limit, the chopper is switched on. Switching frequency of chopper can be controlled by setting maximum and minimum level of current. Dr. D G Padhan PSD 10
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Types of choppers Dr. D G Padhan PSD 12
Step down chopper When S is ON, e 0 is equal to E dc. When S is OFF, e o is equal to zero. Dr. D G Padhan PSD 13
Step up chopper Dr. D G Padhan PSD 14
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The choppers are also classified based on their regions of operation Single Quadrant chopper 1. Type A Chopper 2. Type B Chopper Two Quadrant Chopper 1. Type C Chopper 2. Type D Chopper Four Quadrant Chopper 1. Type E Chopper Dr. D G Padhan PSD 18
First quadrant chopper or Type A chopper Dr. D G Padhan PSD 19
The equivalent circuit is shown below Dr. D G Padhan PSD 20
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Second quadrant or type-b chopper Dr. D G Padhan PSD 27
If the chopper (or switch S) is turned ON and turned OFF at regular intervals the average voltage E 0 is +ve and I 0 is ve. The stored energy in the rotor is converted into electrical energy and fed back into the system. This is equivalent to regenerative braking Dr. D G Padhan PSD 28
During the period T ON the switch S ON and hence e 0 =0. The equivalent circuit is shown in Fig.4.6(a) Dr. D G Padhan PSD 29
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Two-quadrant chopper or Type C chopper V a + T1 D1 V dc i a Q2 Q1 T2 + D2 V a I a - T1 conducts v a = V dc Dr. D G Padhan PSD 34
Two-quadrant converter V a + T1 D1 V dc i a Q2 Q1 T2 D2 + V a I a - D2 conducts v a = 0 T1 conducts v a = V dc Quadrant 1 The average voltage is made larger than the back emf Dr. D G Padhan PSD 35
Two-quadrant converter V a + T1 D1 V dc i a Q2 Q1 T2 D2 + V a I a - T2 conducts v a = 0 Quadrant 2 The average voltage is made smaller than the back emf, thus forcing the current to flow in the reverse direction Dr. D G Padhan PSD 36
Two-quadrant converter V a + T1 D1 V dc i a Q2 Q1 T2 D2 + V a I a - D1 conducts v a = V dc Dr. D G Padhan PSD 37
Two quadrant or Type C Chopper Dr. D G Padhan PSD 38
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Two quadrant chopper- Type D Chopper Dr. D G Padhan PSD 46
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Chopper fed DC drive Four-quadrant Chopper + Q1 D1 + V a D3 Q3 V a V dc Q2 D2 D4 Q4 I a Forward Motoring Q1 & Q4 is ON Current Flow : V + dc _ Q 1 _Motor_Q 4 _V - dc Current I a & V a are positive Operates in First Quadrant 59
Chopper fed DC drive Four-quadrant Chopper + Q1 D1 + V a D3 Q3 V dc Q2 D2 D4 Q4 Q1 is OFF & Q4 is ON. Inductor current has to flow in the same Direction. Diode D2 is FB Inductor Current freewheels through D 2 & Q4 Output Voltage is Zero 60
Chopper fed DC drive Four-quadrant Chopper + Q1 D1 + V a D3 Q3 V dc Q2 D2 D4 Q4 Q4 is OFF. Q2 is ON. Load is not connected with the source. Back Emf drives the current through Q2 & D4 61
Chopper fed DC drive Four-quadrant Chopper + Q1 D1 + V a D3 Q3 V a V dc Q2 D2 D4 Q4 I a Forward Braking Q2 is OFF. Diode D1 is FB Current flows through D4 & D1 Current Ia is negative & Va is positive. Operates in second quadrant. 62
Chopper fed DC drive Four-quadrant Chopper + Q1 D1 - V a + D3 Q3 V dc Q2 D2 D4 Q4 Q3 & Q2 is ON Current Flow : V dc + _ Q 3 _Motor_Q 2 _V dc - Current I a & V a are negative Operates in third Quadrant 63
Chopper fed DC drive Four-quadrant Chopper + Q1 D1 - V a + D3 Q3 V dc Q2 D2 D4 Q4 Q3 is OFF. Q2 is ON. Current has to be continuous. Diode D4 is FB Current flows through Q2,D4 & (Eb,La,Ra) 64
Chopper fed DC drive Four-quadrant Chopper + Q1 D1 - V a + D3 Q3 V dc Q2 D2 D4 Q4 Q4 Is ON & Q2 is OFF. Back emf Drives the current through Q4 D2 - MOTOR 65
Chopper fed DC drive Four-quadrant Chopper + Q1 D1 - V a + D3 Q3 V dc Q2 D2 D4 Q4 Q4 IS Turned Off, D3 is FB Current Flows through Va + - D 3 D 2 Va - Va is negative. But current Ia is positive Operates in fourth quadrant 66
Four Quadrant Chopper or Type E Chopper 67
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