Test: Inverters - 1 - Electrical Engineering (EE) MCQ

In low-power electronic circuits oscillators are used for converting dc power into ac power. These oscillator use transistors for converting dc voltage into sinusoidal ac voltage. Since transistor is used in active region, therefore there is substantial loss of power which decreases efficiency. In high power applications inverters are used instead of transistors and the inverters operate in saturation region or cut-off region. Thus, assertion is true but reason is false.

• In CSI at light load, commutation time is considerably increased, which can restrict the highest frequency. Hence, commutation is load dependent process in CSI. Thus, statement-1 is correct.
• A VSl requires an additional line commutated converter for reverse power flow while it is hot so for a CSI i.e. a CSI has inherent four quadrant operation. Thus, statement-2 is correct.
• In VSl, dynamic braking is applicable during ac line failure. Thus, statement-3 is correct.
• MOSFET and transistor are more suitable for VSl not for CSI because in CSI large transient voltage is produced during commutation. Hence, statement-4 is not correct.

Output is a square wave with an amplitude of 100V.
RMS value of a square wave is equal to its peak value.
So, rms value of output voltage is

The n^th harmonic-component of output voltage is

The lowest harmonic is third harmonic

The fundamental output voltage for a modulation index of M is

A CSl can be either load commutated or force commutated.

Voltage control of an inverter

The waveform of the output voltage obtained from a single-phase inverter is rectangular in nature with an amplitude approximately equal to the input dc voltage.

However, in many applications, the output voltage of the inverter needs to be controlled due to the following reasons:

• The voltage required by ac loads may be constant or adjustable. 
• In motor control applications, inverters handle the control of circuit voltage along with frequency so that the saturation of motor magnetic circuits is avoided.
• Voltage control of inverters is employed in order to compensate for changes in input dc voltage.

Basically, there are three techniques by which the voltage can be controlled in an inverter. They are:

1. External Control of AC Output Voltage

• This method is also known as the series-inverter control method.
• In this method of control, an ac voltage controller is connected to the output of the inverter to obtain the required (controlled) output ac voltage. 
• The voltage control is primarily achieved by varying the firing angle of the ac voltage controller that feeds the ac load.

2. External Control of DC Input Voltage

• The external control of dc input voltage is a technique that is adapted to control the dc voltage at the input side of the inverter itself to get the desired ac output voltage at the load side.

3. Internal Control of Inverter

• The output voltage of an inverter can be adjusted by employing the control technique within the inverter itself.
• This control technique can be accomplished by the following two control methods: Series Inverter Control, and Pulse Width Modulation Control.