## 05/03/2020

### Inverter Interview Question Answer - 2

How many pulses per half cycle in the single PWM?

One

How the gate signal is generated in the single PWM technique?

The gate signal is generated by comparing rectangular reference signal and triangular carrier signal.

Explain the term : amplitude modulation index and frequency modulation index

Amplitude modulation index

It is defined as the ratio of the reference voltage to the carrier voltage.

M = VR / VC

M = Modulation index

VR = Reference voltage

VC = Carrier voltage

Frequency modulation index

It is defined as the ratio of the carrier frequency to the output frequency.

mf = fc / fo

mf  = Frequency modulation index

fc = Carrier frequency

fo = Output frequency

Describe the effect of harmonic as the output voltage decreases in the single PWM?

As the output voltage decreases the harmonic increases whereas the harmonics decreases as the output voltage increases.

How the gate signals are generated in the multiple PWM?

The gate signals in the MPWM are generated by comparing rectangular reference signal with triangular carrier signal.

Which signal determines numbers of pulses per half cycle in the MPW?

Triangular carrier signal

Why the MPWM is also called as symmetrical PWM?

Symmetrical PWM

The width of the all pulses and number of pulses are same in the MPWM therefore it is also called as symmetrical PWM.

Describe the number of pulses per half cycle on the harmonics in the MPWM?

Effect of number of pulses on the harmonics

As the number of pulses increases, lower order harmonics decreases whereas the higher order harmonics increases. However the higher order harmonics is completely eliminates by using filter. As the number of pulses increases, switching of the semiconductor also increases therefore the switching losses increases.

In which method of PWM, the width of the all pulses is not same?

Sinusoidal PWM

What do you mean by term ‘series’ inverter?

Series inverter

The commutating components L and C are connected in series with the load so it is called as series inverter.

Whether the output waveform of the series inverter is sinusoidal?

The output waveform of the series inverter is approximately sinusoidal particularly at very high frequency.

Write the condition for the under damped circuit in the series inverter.

The condition for the under damped circuit in the series inverter is

R2 < √ ( L / C)

Why the series inverter is called as load commutated inverter or self commutated inverter?

Self commutated inverter

As the anode current becomes zero itself in this inverter, the SCR turns off automatically. Therefore the series inverter is called as self commutated or load commutated inverter.

Describe the effect of load current on the capacitance voltage in the series inverter.

Effect of anode current on the capacitance voltage in the series inverter

As the anode current reaches up to maximum, the capacitor charges up to supply voltage. As the anode current starts to decreases zero from its peak value, the voltage across capacitor still increases up to twice the supply voltage.  The capacitor voltage remains constant during zero anode current. ( The voltage across capacitor remains maximum during switching off of the both thyristors )

Why there should be enough time delay between turn off of the one thyrisor and turn on of the other thyrisor in the series inverter?

In order to avoid short circuit of the input supply

What is meaning of the dead zone of the series inverter?

It is time duration between turn off of the one thyrisor and turn on of the other thyrisor. This time duration is called as dead zone of the series inverter. The dead zone time must be more than the turn off time of the thyristors.

What should be rating of the capacitor in the series inverter?

Rating of capacitor

The rating of capacitor should be greater than the twice supply voltage.

Describe the limitations of the series inverter.

Limitations of the series inverter

The load current flows from supply source only during positive half cycle. The current drawn from the supply source is not continuous. The SCR T2 turns on only when the SCRT1 turns off otherwise, the DC supply source is short circuited. As the load current passes through commutating components, the rating of the commutating components should be large. The load waveform gets distorted if the dead zone time or SCR turn off time is higher. The maximum output frequency of the inverter is less than the ringing frequency. If the inverter output frequency is greater than the ringing frequency, the supply source is short circuited. The peak amplitude of the load current during each half cycle depends upon the load parameters, it will results in poor output regulation.

State the applications of the series inverter.

Applications

High frequency heating

Ultrasonic generators

Fluorescent lighting

Sonar trans meter

Describe the difference between commutating circuit in the modified series inverter and series inverter.

The mutual coupled inductance L1 and L2 is used in place of inductor L in the modified series inverter.