Important Short Questions and Answers: Conventional and Solid State Speed Control of A.C Drives

Conventional & Solid State Speed Control of AC Drives

1.What are the speed control methods available for speed control of induction motor on stator side?

Speed control methods available for speed control from stator side are: 

i. By controlling supply voltage

ii. By pole changing

iii. Changing supply frequency

What are the speed control methods available for speed control of induction motor on rotor side?

The speed control methods available for sped control of IM on rotor side are; 

i. Rotor resistance control

ii.  Cascade speed control

iii. By injected voltage method.

What are the three possible speeds that can be obtained in cascaded operation of induction motor?

What are the disadvantages of inserting resistance in the rotor circuit in slip ring induction motor?

Disadvantages:

Losses (I²R) is increasing and efficiency of the motor is decreased.

Since speed is dependent on both resistance and load. We can change speed for short periods only.

under what condition, the slip in an induction motor is

Negative

Greater than one

Slip of an induction motor is negative when the induction motor is operating in generating mode.

Slip of an induction motor is greater than one when the induction motor is operating in the braking mode (its direction is opposite to the direction of rotating magnetic field).

How the speed is controlled by changing the supply voltage?

Since,

Torque, T µ V ²

By changing the supply voltage, torque is proportional to the square of the supply voltage. Hence, example if voltage is reduced by half (1/2), the torque will be reduce d by (1/4) quarter.

How the speed is changed by changing the supply frequency?

Since

By changing the supply frequency directly, the speed is reduced directly.

What is „slip‟ in an induction motor?

The induction motor speed is always less than the speed of synchronous speed of revolving flux. The difference in speed between synchronously revolving flux and the rotor speed is called slip speed.

The ratio between the slip speed and synchronous speed of induction motor is called slip. It is denoted by„s‟.

How the speed is controlled by changing the supply voltage?

Speed is inversely proportional to number of poled, the speed decreases with increase in number of poles.

However, torque capability of motor increases with increase in number of poles.

How speed control can be achieved by inserting resistance in the rotor circuit of slip ring induction motor?

By inserting external resistance in the rotor circuit of slip ring induction motor, the total rotor circuit resistance is increased. For a particular motor speed, the induced emf in the rotor is constant. Hence, with increase in the rotor circuit resistance, rotor winding current wills decreases.

Since I₂ is decreased torque is decreased and the motor is peed starts to fall.

When motor speed is reduced, the term SE₂ will be becomes increased since slip S is increased. Hence with decrease in motor speed, rotor circuit current I₂ will increase and hence the torque will goes on increasing.

The motor will attains a speed at which I ₂ = I₁ condition achieved ie the motor is able to handle the load torque.

What is slip power?

It is the power wasted in the rotor circuit resistance. IN case of slip ring induction motor, the speeded control is achieved by adding external resistance in the rotor circuit. But, with adding external resistance in the rotor circuit, (I²R) losses in the external resistances will be increased. Hence, slip power is wasted in rotor resistance.

Without wasting this slip power, the slip power can be usefully utilized in slip power recovery scheme.

What is slip power recovery scheme?

The power wasted in the external resistance in the rotor circuit in case of slip ring induction motor, is called slip power. This wastage slip power can be usefully fed back to the source by used of slip power recovery scheme.

Two types of slip power recovery scheme available are;

Static scherbius drive

Static Kramer‟s drive

What is meant by inverter?

A device that converts dc power into ac power at desired output voltage and frequency is called an inverter

What are the applications of inverters?(UQ)

Adjustable speed ac drives

Induction heating

Stand by air-craft power supplier

UPS

HVDC transmission

What is the main classification of inverters?

Voltage source inverter(VSI)

Current source inverter(CSI)

What is meant by VSI?

A VSI, is one in which the dc source has small or negligible impedence. In other words a VSI has stiff dc voltage source at its input terminals.

What is meant by CSI?

A current fed inverter or CSI is fed with adjustable current from a dc source of high impedance is from a stiff dc current source.

Give two advantages & applications of CSI?

Advantages

CSI does not require any feedback diodes.

Communication circuit is simple as it contains only capacitors.

Applications

Induction heating

Lagging VAR compensation

Speed control of AC motors

Synchronous motor sharing

How the thyristor inverters are classified?

According to the method of commutation

Line commutated inverter

Forced commutated inverter

According to the connection

Series inverter

Parallel inverter

Bridge inverter

What is meant by series inverter write its applications?

Series inverter means commutating elements L and C connected in series with load.

The thyristorised series inverter produces an approximately sinusoidal waveform at a high output

frequency, ranging from 200HZ to 100KHZ.It is commonly used for fixed output applications such as

Ultrasonic generators

Induction heating

Sonar transmitter

Fluorescent lighting

Compare VSI and CSI?

Voltagesource inverter(VSI)

In voltage source inverter, input voltage is maintained constant.

In VSI,the output voltages does not depend on the load

The magnitude of output current and its waveform depends upon thenature of the load impedance

It requires feed back diodes

Commutation circuit complex

Current source inverter(CSI)

In current source inverter, input currents constant but adjustable

In CSI,the amplitude of output currents constant but adjustable

The magnitude of output voltage and its waveform depends upon the nature of the load impedance

It does not require any feedback diode

Commutation circuit is simple i.e., it contains only capacitors.