commutation circuits

TECHNICAL MANUAL FOR

COMMUTATION CIRCUITS

Manufactured by :

PREMIER TRADING CORPORATION (An ISO 9001:2000 Certified Company)

212/1, Mansarover Civil Lines, MEERUT.

Phone : 0121-2645457, 2654068

INDEX NO. : M-121

PREMIER TRADING CORPORATION

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COMMUTATION CIRCUITS ACCESSORIES

1. Demonstration Board with following facilities :-

(a) Isolated DC 230 V.

(b) Isolated DC 30 V.

(c) Pulse Frequency Generator

(d) External Load (Lamp Load)

(e) 10:1 Resistive Attenuator for observation of CRO (Two Sets).

(f) 3A Fuse for short circuit protection.

2. 15 Watt 250 V Lamp – 2 Nos.

3. Demonstration Board Cover

4. Patch Cords – 16 Nos.

INTRODUCTION Commutation means to turn off the SCR. The SCR will turn off when its anode

current goes less than holding current. In conversion from AC to DC the SCR anode current

goes less than the holding current naturally at its negative cycle of the AC. This type of

commutation is called natural commutation. Here no need of extra turn off circuit.

To off the SCR at DC circuits, there is no any condition that its anode current goes

below the holding current so the time at which the SCR turn off is wanted, the anode current

reduces less than the holding current by force with extra circuit. That extra circuit is called

commutation circuit. Here we will discuss 6 type of different commutation circuits.

The fundamental of commutation is move around the benefit of negative cycle of

RLC Circuit. Here R is the load resistance. The Negative current of hunting of RLC circuit

should be more than the anode current of the SCR.

THEORY

These types of commutation circuits is given below in detail -:

1. Resonant Commutation type A

2. Resonant Commutation type B

3. Auxiliary Commutation

4. Self Commutation

5. Complementary Commutation

6. External Pulse Commutation


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1. Resonant Commutation type A

FIG-‘1’ Here two SCRs are used, One for charging of capacitor and another for discharging of

the capacitor C. When the firing pulse will appear at gate point PF1 the SCR will fire and

TURN OFF after charging the capacitor C. The next function the firing pulse 2 will appear

another SCR at its gate, this SCR become fire and discharge the capacitor. Now the previous

SCR is ready for next TURN ON. The charging and discharging of the capacitor time should

be less than the time of firing pulse appearing at other SCR. Otherwise both the SCR will turn

on at the same time and our supply will short circuited.

2. Resonant Commutation type B

FIG-‘2’

Here one SCR is used for charging the capacitor at appearing of firing pulse FP1 at its

gate. After completion of charging the anode current of SCR will become below the holding

current and SCR will TURN OFF. The capacitor C will discharge through load L1. The SCR

is ready to fire at next firing pulse.


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3. Auxiliary Commutation

FIG-‘3’

Here two SCR are used, one is main SCR to draw the load current and other is

auxiliary SCR to turn of the main SCR. First firing pulse will appear at auxiliary SCR and

capacitor C will charge. Now the firing pulse will appear at main SCR. The load L1 is

connected to the supply. But at the same time capacitor C will discharge through main SCR,

Diode and Inductance L. Here due to capacitor C and L the forward hunting current will flow.

Diode will prevent to flow the negative hunting current. At the end of current the capacitor C

will be negatively charged. Now again firing pulse appear at auxiliary SCR. Due to negative

charge at capacitor C the main SCR is reverse biased and turn OFF. Now the capacitor C

negative charge will discharge and it will positively charged through auxiliary SCR and

process will continue.

4. Self Commutation

FIG-‘4’

First the capacitor will charged through Inductor L and load-1 before applying the

firing pulse at PF1. As the firing pulse appearing at PF1 the SCR will fire and load L1 is

connected with supply but at the same time LC current will flow. At the negative cycle of

hunting of LC circuit the SCR anode current will become less than the holding current and

SCR will turn OFF. Now the capacitor C will fully charged before appearing the next firing

pulse at SCR.


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5. Complementary Commutation

FIG-‘5’ Here the firing pulse 1 and firing pulse 2 will appear at its gate one by one. The load-1

is connected, SCR 1 will fire and load-1 current will flow through SCR-1. Capacitor C will

charge through load-2. Now its load-2 side will positive and load-1 side will negative. As

firing pulse-2 will apply at SCR-2, load-2 current will flow but at the same time capacitor C

will discharge through SCR-2 (forward direction) and through SCR-1 (reverse direction). As

the negative current flows through SCR-1, it will commute. After commute SCR-1 the

capacitor C will now charge in opposite direction through load-1 and same phenomenon will

to commute the SCR-2.

6. External Pulse Commutation

FIG-‘6’

Here three SCRs and three firing pulses are used. First FP3 will appear and all the

capacitors will charge through inductor. Due to inductor the charged voltage of capacitor will

be more than the charging voltage of supply. After charging all the capacitor SCR-3 will

commute. Now FP-1 will appear and SCR-1 will fire, load current will flow. When

commutation of SCR-1 is desired, FP-2 will appear and the charged voltage at capacitor C

will reaches at cathode of SCR-1, SCR-1 will reverse biased and commute.


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ASSOCIATE CIRCUITS

Here two circuits are associating the operation of Chopper Circuit study.

1. Isolated 250V DC Power Supply Circuit.

2. Firing Pulse circuit.

ISOLATED 250V DC POWER SUPPLY CIRCUIT

The complete power supply circuit diagram is given in fig ‘3’. First the applied 250V

AC is isolated by Transformer TR1 and converted 170 V AC. The diode D1, D2, D3 and D4

are for rectification, resistance R1 for slow charging of Capacitor C1, C2 and C3. Resistance

R2 is discharging the capacitor after the use.

An 18V DC is developed through transformer TR2, diode D5, D6, D7 and D8. When

18V DC voltages will develop relay RL1 will energize. The purpose of RL1 is to start the

timer. Diode D9 is to prevent reverse flow of current due to residual charge of capacitor C4.

The C4 is for smoothing the 18V DC. IC1 is a regulator IC and gives 12 V DC output LED1

and resistance R3 for indication of power in board. Resistance R4 and DZ1 is to developed

+5V. These 12 & 5 Volts will apply to timer circuit.

The timer has the time of 20 Sec. Just applying the power to it the relay RL2 will

operate after 20 Sec. This timer is placed here for slow charging of capacitor C1, C2 and C3.

The capacitor will fully charged before the 20 sec and we are such a position to attach the

capacitor C1, C2 and C3 directly with rectified DC and through these capacitor the DC will

become total smooth. The contact RL2/2 and RL2/3 will carry the smooth 250V DC to DC

terminal.

If any down flicker comes in AC supply, the RL1 will de-energize and pin No. 2 of

IC2 will ground through its contact RL1/1 and 20 sec timer will restart. That prevent for

surge charging of capacitor C.

In the OFF condition of ON-OFF switch SW1 will connect the R2 through its contact

SW1/2.


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FIRING PULSE CIRCUIT

The firing pulse circuit diagram is given in fig ‘8’. To operate firing pulse circuit +

5V DC is developed through diode D10, D11, D12, D13, C8, IC3 and C9.

IC4 is astable multivibrator VR1 to control its frequency. It will give the output from

Pin No. 3 and fed to IC5 that is JK flip-flop and its output will come through pin No. 14 &

15. The frequency and 14 and 15 will half the astable multivibrator frequency. The output

from Pin No. 15 will fed to pulse circuit using IC6. Here IC6 is a monostable multivibrator

and gives a pulse to pulse transformer and that pulse transformer will isolate and also sharp

the given pulse and sharp pulse. Is ready at its point PF2.

Output from JK flip-flop pin No. 14 is fed to another two monostable multivibrators

using IC7 & IC8 and function of IC7 and IC8 will same as IC6 and finally a pulse will appear

at PF1 and PF3 at the same time. The pulses PF2 and PF1, PF3 will appear one by one.

EXPERIMENTAL PROCEDURE Experiment No. 1 Observation of firing pulses

1. Switch ON the demonstration board.

2. Keep maximum frequency.

3. Connect the CRO at the point G & K.

4. Observed the firing pulses as given in fig-‘9’.

5. Keep the CRO at dual trace mode.

6. Connect the channel 1 with the point G1 & K1.

7. Connect the channel 2 with the point G2 & K2.

8. Observe that the firing pulses are as given in fig-‘10’.

Experiment No. 2 To Study Resonant Commutation A

1. Connect the main lead and switch on the board.

2. Consider the Fig ‘11’.

3. Connect the Lamp Load in circuit.

4. Connect FP-1.


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5. Connect the 30 V DC with Resonant Commutation A.

6. As the DC will connect observe that the capacitor C will charge through load-3.

7. Connect FP-2.

8. Increase the frequency and observe the load waveform as given in fig-‘12’.

9. Observe the waveform across SCR-1.

10. Observe the waveform across SCR-2.

11. Observe the waveform across capacitor C.

Experiment No. 3 To Study Resonant Commutation B

1. Connect the main lead and switch on the board. The indicator indicates the DC

is available.


3. Connect capacitor C.

4. Connect the FP-1.

5. Connect the 250 V DC with Resonant Commutation B.

6. As the DC will connect observe the capacitor C will charge through lamp load

and the lamp will glow between the capacitor charging period.

7. Ensure that capacitor is charged and keep the frequency minimum.

8. Connect the Load-1 in circuit.

9. Observe that the SCR is firing and commuting consciously. So load-1 is

blinking.

10. Increase the frequency and observe that blinking is increased accordingly.

11. Switch OFF the demonstration board connects 10:1 Resistive Attenuator

across the load.

12. Connect the CRO with 10:1 resistive attenuator.

13. Switch ON the demonstration board and observe the wave form as given

in Fig-‘14’.

14. Observe the waveform across the SCR.

Experiment No. 4 To Study Auxiliary Commutation

1. Connect the main lead and switch on the board. The indicator indicates the DC is

available.


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4. Connect the auxiliary SCR pulses FP2.

5. Connect the 250 V DC with Auxiliary Commutation Circuit.

6. As the DC will connect observe that the capacitor C will charge through lamp load

and the lamp will glow between the capacitor charging period.

7. Ensure that capacitor is charged and keep the frequency minimum then connect

the firing pulse cords with main SCR FP1.

8. Observe that lamp load is firing and commuting continuously.


10. Switch OFF the demonstration board connects 10:1 Resistive Attenuator across

the load.


12. Switch ON the demonstration board and observe the wave form as given in

Fig-‘16’.

13. Like the load wave form, the wave form across the SCR1 & SCR2 can also

observed and ensure that these are same as given in Fig-‘16’.

14. Observe the waveform across the capacitor C.

Experiment No. 5 To Study Self Commutation


is available.



4. Connect the 250 V DC with Morgan Chopper circuit.

5. As the DC will connect observe that the capacitor C will charge through lamp

load and the lamp will glow between the capacitor charging period.

6. Ensure that capacitor is charged and keep the frequency minimum then

connect the firing pulse cords.

7. Observe that the SCR is firing and commuting continuously, so the lamp is

blinking.



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across the load.


11. Switch ON the demonstration board and observe the wave form as given

in Fig-‘18’.

12. Like the load wave form, the wave form across the SCR can also observed and

ensure that it is same as given in Fig-‘18’.

Experiment No. 6 To Study Complimentary Commutation


is available.

2. Consider the Fig-‘19’

3. Connect the load-1.

4. Connect the FP1 & FP2.

5. Connect 250 V DC.


7. As load-2 is connected, both the SCRs start firing and commuting one by one.

8. Both the loads blinking one by one.

9. Increase the firing pulses frequency and observe that the blinking is increased

accordingly.


across the load.


12. Switch ON the demonstration board and observe the wave form of load-1

as given in Fig-‘20’.

13. Observe that load-2 waveform is like the load-1 wave form and ensure that it

is same as given in Fig-‘20’.

Experiment No. 7 To Study External Pulse Commutation

1. Connect the main lead and switch on the board. The second indicator indicates

the DC is available.

2. Consider the Fig-‘21’


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3. Connect the FP1, FP2 & FP3.

4. Connect 250 V DC at both the points.


6. As load-1 is connected, the SCRs start firing and commuting.

7. The load-1 is blinking.

8. Increase the firing pulses frequency and observe that the blinking is increased

accordingly.


across the load.


11. Switch ON the demonstration board and observe the wave form of load-1


12. Observe that the waveform across the main SCR-1 and ensure that it is same


MERITS OF THE CIRCUIT

1. These are the good circuits for DC Power control.

2. Power losses are minimum in comparison with Transistorize Circuits.

3. Most suitable circuit for DC-to-DC converter, Chopper Circuits etc.

DEMERITS OF THE CIRCUIT

1. Starting complicacy because first the capacitor should charge.

2. The circuit is respectively too costly.

PRECAUTIONS

1. Ensure that capacitor should charge before firing the main SCR.

2. Lamp load should not be more then 25watt 250V.

3. Better to select coloured Lamp load.


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commutation circuits

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