24-01-2013, 10:43 AM
STUDY OF FORCED COMMUTATION CIRCUITS
STUDY OF FORCED.docx (Size: 13.04 KB / Downloads: 50)
AIM:
To Verify the different types of forced commutation circuits by connecting
a resistive load.
APPARATUS:
S.No EQUIPMENT Qty
1. Forced commutation Kit 1
2. Regulated Power Supply 1
3. Rheostat 2
4. CRO 1
5. Patch cards
THEORY:
Commutation is the process of turning off the SCR and it normally causes the
transfer of current flow to other parts of circuit. Commutation can be divided into
a) Natural commutation
b) Forced commutation
a) Natural commutation: If the source voltage AC the SCR current goes through a natural zero
and reverse voltage appears across the SCR. The device is automatically turns off due to the
natural behavior of the source voltage. This is known as natural commutation or line
commutation.
b) Forced commutation: In some SCR circuits the input voltage is DC and the forward current
of the SCR is DC and the forward current of the SCR is forced to zero by external or
additional circuitry called as commutation circuitry to turn off SCR. This Technique is called
forced commutation and normally applied in DC to DC converters .
PROCEDURE:
CLASS-A COMMUTATION:
1. Connect the circuit as shown in the circuit.
2. Connect Trigger output T1 to gate and cathode of SCR T1
3. Switch on the DC supply to the power circuit and observe the voltage waveform across
load by varying the frequency potentiometer.
4. Repeat the same for different values of L,C and R.
CLASS-B COMMUTATION:
1. Connect the circuit as shown in the circuit.
2. Connect Trigger output T1 to gate and cathode of SCR T1
3. Switch on the DC supply to the power circuit and observe the voltage waveform across
load by varying the frequency potentiometer.
4. Repeat the same for different values of L,C and R.
Note: Same procedure for Class-A and Class-B Commutation.
CLASS-C COMMUTATION:
1. Connect the circuit as shown in the circuit.
2. Connect T1 and T2 from firing circuit to gate and cathode of Thyristor T1 and T2.
3. Observe the waveforms across R1,R2 and C by varying frequency and also duty cycle
potentiometer.
4. Repeat the same for different values of C and R.