 Summer 2019 Q.1 Solutions - Grad Plus

# Summer 2019 Q.1 Solutions

Q1) a) Explain with neat circuit diagram operation of R-C triggering circuit of Thyristor [5 M]

Ans.

• The circuit diagram for R-C full wave triggering is shown below. :-
• It consists of four diodes D1, D2, D3, D4 which convert Ac into DC.
• This pulsating DC is supplied to SCR.
• When SCR is off, the capacitor start charging from 0v. When it is charged to Vgt, SCR triggers and rectified voltage Vs appears across the load as Vo.
• The load impedance can have various values , therefore for paticular value of R, the instant of firing is determied by load impedance.
• For fixed load impedance , low values of R causes quick charging and early firing whereas high value of R causes delayed charging and delayed firing .

The waveforms are shown below.

• The merits are –

i) It is very simple and economical.

ii) The firing angle is controlled in the wide range from 0o to 180o.

• The demerits are –

i) Control circuit is directly connected to high voltage power circuit.

ii) The performance is different for different SCRs.

The value of RC is

RC=\frac{157}{W}
• At the instant of triggering , if capacitor voltage is constant at Vgt and supplying a current to gate , the triggering gate current Igt has to be supplied by Vs through R.

Vs > R. Igt + Vgt

R \leq \frac{V_{s}-V_{gt}}{I_{gt}}

Q.1. b) Write short note on Class E Chopper. [05 M]

Ans.

Four-quadrant chopper is nothing but class-E chopper, which can be formed by combining two class C choppers.

• First quadrant Operation

When chopper S1 and S4 are ON, input dc source voltage Vs appears across load and dc source takes load current iL. Thus both VL and iL are positive . So chopper is said to be operated as rectifier and power flows from dc source to load. When S1is turned off, the load inductor voltage reverses itself and drives iL in same direction through S4 and D2. The load voltage VL becomes zero.

• Second quadrant operation

When D1 and D4 are on, load inductance voltage VL drives current through D1, Vs and D4. Now the load voltage VL is positive which is equal to Vs but the load current iL is negative. So the power is fed back by load inductor to the source . Therefore the chopper acts as an inverter.

When S2 is fired , D1 is turned off and negative load current iL is maintained through S2, D4 and load . Now VL becomes zero.

• Third Quadrant operation

When S2 , S3 are on, VL=-Vs and negative load current flows, the power is applied by the source to the load so it is operated as rectifier. If S3 is off, the load current flows through S2, D4 and load. Now VL becomes zero.

• Fourth Quadrant operation

When diodes D2 and D3 are on, the energy stored in load inductor is given out and positive load current iL is maintai9ned. The load voltage VL=-Vs. The power is fed back to source by load inductor hence it is operated as inverter.

When S4 is fired , D3 is oFF and iL starts flowing through S4, D4 and load. The load voltage becomes zero.

Scroll to Top