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1

### GATE EE 2001

Subjective
A voltage commutated thyristor chopper circuit is shown in fig. The chopper is operated at $$500$$ $$Hz$$ with $$50\%$$ duty ratio. The load takes a constant current of $$20A$$

(a) Evaluate the circuit turn off time for the main thyristor $$T{h_1}$$
(b) Calculate the value of inductor $$L,$$ if the peak current through the main thyristor $$Th,$$ is limited to $$180$$% of the load current.
(c)Calculate the maximum instantaneous output voltage of the chopper (a) $$30$$ $$\mu \sec$$
(b) $$0.23$$ $$mH$$
(c) $$200$$ $$V$$
2

### GATE EE 2000

Subjective
A $$dc$$ motor with armature resistance $${R_a}$$ is fed from a step down chopper in the continuous mode, and operates at some known speed and known excitation current. The motor current rises from $${I_{\min }}\,\,$$ to $${I_{\max }}\,\,$$ in the $${T_{on}}$$ period Ton of the chopper; and drops from $${I_{\max }}\,\,$$ to $${I_{\min }}\,\,$$ in the $${T_{off}}$$ period Toff of the same circuit. Both the rise and fall of the current may be assumed to be approximately linear. What is the average power loss in the machine armature?

Average power loss $$= {\left( {{{{{\rm I}_{\max }} + {{\rm I}_{\min }}} \over 2}} \right)^2}\,\,{R_a}$$
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#### Questions Asked from Choppers and Commutation Techniques

On those following papers in Marks 5
Number in Brackets after Paper Indicates No. of Questions
GATE EE 2002 (1)
GATE EE 2001 (1)
GATE EE 2000 (1)

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