Inverters Questions and Answers

Answer: D

A chopper cannot be used to obtain a variable frequency supply for an induction motor. A chopper is a DC to DC converter and gives a variable DC voltage, but it does not produce an alternating output.

A cycloconverter changes AC at one frequency to AC at a lower frequency directly, and voltage source and current source inverters change DC to AC of variable frequency. So the fourth option is the answer.

Answer: D

The output line voltage is free from the 3rd harmonic.

In square wave (180° conduction) mode, each phase voltage of a three-phase inverter contains the odd harmonics. The triplen harmonics (3rd, 9th, 15th and so on) are in phase in all three phase voltages, so they cancel when two phase voltages are subtracted to form a line voltage.

The line voltage therefore contains only harmonics of order 6k ± 1: the 5th, 7th, 11th, 13th and so on. So the 7th, 11th and 13th harmonics are present, and only the 3rd is absent.

Answer: D

The correct matching is a-iv, b-iii, c-ii, d-i.

• A single phase full bridge inverter feeding a suitably tuned (leading power factor) load can be commutated by the load itself (iv).
• The IGBT combines a MOSFET input (voltage-controlled insulated gate) with a bipolar output stage (iii).
• A free wheeling diode carries the inductive load current while the SCR is off (ii).
• A step up (boost) chopper converts a low dc voltage to a higher dc voltage (i).

The other codes give the IGBT role to a diode or the boost function to the inverter, mixing up devices with circuits.

Answer: B

Square wave inverters provide fixed AC with appreciable low frequency harmonics.

A square wave inverter switches the DC supply alternately in each direction, giving an AC output of fixed amplitude set by the DC voltage. The Fourier series of a square wave contains all the odd harmonics, with amplitudes falling as 1/n, so the 3rd, 5th and 7th harmonics, which are close to the fundamental, are large.

The output is AC, not DC, which rules out two options. Its amplitude is not variable unless the DC input is varied, and the problem harmonics are low order, not high; high-frequency harmonics are what PWM inverters produce, because they are easy to filter.

Answer: C

Statements (2), (3) and (4) are correct, so the answer is (2), (3) and (4) are correct.

In the McMurray inverter each thyristor has a diode in inverse parallel. During commutation the LC circuit drives a current pulse larger than the load current through the outgoing thyristor; the excess flows through its antiparallel diode (2). The forward drop of that conducting diode is what reverse biases the outgoing thyristor so that it can turn off (3). With an inductive load the same diodes return the reactive (load) current to the DC source (4).

Statement (1) is wrong: because each diode is connected in antiparallel with a thyristor, the reverse voltage across it can never exceed the thyristor's on-state drop, so it never has to withstand a large reverse voltage. That rules out every option containing (1).