REVIEW QUESTIONS

1) In the circuit of Figure Q.1, both voltage sources and the diodes can be treated as ideal, and the load is a resistor. (Note: this question is specifically aimed at reinforcing the understanding of how diodes behave: it is not representative of any practical circuit.)

Sketch the voltage across the load under the following conditions:

(a) V1 is a sinusoid of amplitude 20 V, and V2 is a constant voltage of ‏10V

(b) V1 is a sinusoid of amplitude 20 V, and V2 is a constant voltage of _10V

(c) The same as (a), except that the diode D1 is placed below, rather above, V1.

2) What is the maximum d.c. voltage available from a fully controlled bridge converter supplying a motor and operating from lowimpedance 230 V mains?

3) Estimate the Wring angle required to produce a mean output voltage of 300 V from a fully controlled 3-phase converter supplied from stiff 415 V, 50 Hz mains. Assume that the load current is continuous.

How would the Wring angle have to change if the supply frequency was 60 Hz rather than 50 Hz?

4) Sketch the waveform of voltage across one of the thyristors in a fully controlled single-phase converter with a Wring angle delay of 60⁰. Assume that the d.c. load current is continuous. Figure 1.9 may prove helpful.

5) Sketch the current waveform in the a.c. supply when a single-phase fully controlled converter with a ¼ 45_ is supplying a highly inductive load which draws a smooth current of 25 A. If the a.c. supply is 240 V, 50 Hz, and losses in the devices are neglected, calculate the peak and average supply power per cycle.

6) A d.c. chopper circuit is often said to be like an a.c. transformer.

Explain what this means by considering the input and output power relationships for a chopper-fed inductive motor load supplied with an average voltage of 20 V from a 100 V battery. Assume that the motor current remains constant throughout at 5 A.

7) A 5 kHz step-down transistor chopper operating from a 150 V battery supplies an R/L load which draws an almost-constant current of 5 A. The resistance of the load is 8V.

Treating all devices as ideal, estimate:

(a) the mark:space ratio of the chopper; (b) the average power in the load; and (c) the average power from the source.

8) This question relates to the switching circuit of Figure Q.8, and in particular to the function of the diode.

Some possible answers to the question ‘what is the purpose of the diode’ are given below:

(a) to prevent reverse current in the switch;

(b) to protect the inductor from high voltages;

(c) to limit the rate of change of current in the supply;

(d) to limit the voltage across the MOSFET;

(e) to dissipate the stored energy in the inductance;

Discuss these answers and identify which one(s) are correct.

 

9) In the circuit of Figure Q.8, assume that the supply voltage is 100 V, and that the forward volt-drop across the diode is 0.7 V. Some common answers to the question ‘when the current is freewheeling, what is the voltage across the MOSFET’ are given below:

(a) 99.3 V;

(b) 0.7 V;

(c) 0 V;

(d) depends on the inductance;

(e) 100.7 V.

Discuss these answers and identify which one is correct.