Electric Drives MCQ Quiz in தமிழ் - Objective Question with Answer for Electric Drives - இலவச PDF ஐப் பதிவிறக்கவும்

• In this method, motor is reconnected to the supply in such a way that it has to develop a torque in opposite direction to the movement of the rotor, it can be done by reversing the connections of armature
• Motor will decelerate until speed is zero and then accelerates in opposite direction; Immediately, it is necessary to disconnect the motor from the supply as soon as system comes to rest
• The kinetic energy of the rotating parts of the motor is wasted and an additional amount of energy from the supply is required to develop the torque in reverse direction, i.e. the motor should be connected to the supply during braking
• It can be applied to both DC and AC motors

The speed control of three-phase induction (AC drive) are classified as:

• V/f control or frequency control
• Changing the number of stator poles
• Controlling supply voltage
• Adding rheostat in the stator circuit
• Adding external resistance on the rotor side
• Cascade control method
• Injecting slip frequency emf into rotor side

The speed control method of DC drives are

• Field control method
• Armature control method 
• Ward-Leonard method 

Concept:

The average output dc voltage of a 1ϕ semi-converter is given by:

\(V_{o(avg)} = {V_m \over \pi}(1+cosα)\)

where, V_m = Maximum value of the source voltage 

α = Firing angle

Calculation:

Given, Vs = 230 V

V_m = √2 (220) V

α = 30° 

\(V_{o(avg)} = {\sqrt {2} \times 220\over \pi}(1+cos30)\)

\(V_{o(avg)} = 193.2 \space V\)

Fundamental torque Equation:

The fundamental torque equation is given as:

\(T_M - T_L = \dfrac{d}{dt}(jω) = J \dfrac{d ω}{dt} + ω \dfrac{dJ}{dt}\)

where, 

J = Polar moment of inertia of motor load system referred to the motor shaft in kg-m2

ω = Instantaneous angular velocity of the motor shaft in rad/sec.

TM = Instantaneous value of developed motor torque in N-m

TL = Instantaneous value of load torque referred to the motor shaft in N-m

When the drive is of constant inertia,

⇒ \(\dfrac{dJ}{dt} = 0\)

∴ \(T_M = T_L + \dfrac{Jd\omega}{dt}\)

Now various cases are shown as:

Case I: 

When TM > TL

In this case \(\dfrac{d\omega}{dt} > 0\), so the drive accelerates.

Case II: 

When TM < TL

In this case \(\dfrac{d\omega}{dt} < 0\), so the drive decelerates.

Case III:

When T_M = TL

In this case \(\dfrac{d\omega_m}{dt} = 0\), so the motor runs at the same speed.

Type A chopper (first quadrant chopper):

• When the chopper is ON, V0 = Vs, and current flows in the direction of the load (as shown in fig).
• When the chopper is off, V0 = 0 but I0 continues to flow in the same direction through a freewheeling diode, thus V0 and I0 are always positive.

​Type B chopper (Second quadrant chopper):

• In type B chopper, the load must always contain DC sources.
• When the chopper is ON, V0 = 0 but load E drives the current through the inductor and the chopper, and thus inductor stores energy during the time TON of the chopper.
• When the chopper is off, \({V_0} = E + L\frac{{di}}{{dt}}\) thus V0 will be more than Vs and thus diode DB will be forward biased and begins conducting and power starts flowing to the source.

Note: No matter the chopper is ON or Off, current I0 will be flowing out of the load (opposite direction to the given circuit shown) and treated as negative.

Since V0 is +ve and I0 is -ve, Type B chopper is IInd quadrant chopper

Type C chopper (two quadrant type A chopper):

• Type – C chopper is obtained by connecting type A chopper and type B chopper in parallel.
• In this chopper, V0 is always +ve as F.D. is connected across it.
• In this type of chopper average value of current may be +ve or -ve.
• For regenerative braking and motoring, this type of chopper configuration is used.
  ​

Type D chopper (Two quadrant Type B chopper):

• When the two choppers are ON, V0 = Vs
• Average output voltage V0 will be +ve when the chopper turns ON time ‘TON’ will be more than their turn off time ‘Toff’, otherwise it will be negative.
• As the diodes and the choppers conduct current only in one direction, the direction of load current will always be positive.

Type E chopper (four-quadrant chopper):

Only in type E chopper, both current and voltage remain negative i.e. when CH3-CH2 is ON and CH2-D4 conducts (In IIIrd quadrant)

Concept:

Cycloconverter is a device that converts the AC power at one frequency input to an AC power at different frequency output.

Cycloconverters are the direct type converters used in high power applications for driving induction and synchronous motors.

Explanation:

• Cycloconverters are usually phase-controlled devices. Variable frequency drives are mostly used for controlling either the torque or the speed of the AC motor.
• Applications such as pumps, centrifugal use the technique of variable frequency to achieve variable speed and variable torque.
• These are primarily used where precise control is required like machine tool drive.
• Variable frequency in AC drives can be achieved by inverter fed or cycloconverter fed drives.
• The advantage of cycloconverter over inverter fed drive is its single-stage conversion.

Variable speed drive:

A Variable Speed Drive (VSD) is a type of motor drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage.

Advantages of VSD application for fans:

• Due to the more precise control of motor output and soft starting capabilities resulting in good performance of the system.
• Many fixed-speed motor load applications that are supplied direct from AC line power can save energy when they are operated at variable speed by means of VSD.
• Such energy cost savings are especially pronounced in variable-torque centrifugal fan and pump applications, where the load torque and power vary with the square and cube, respectively, of the speed.
• This change gives a large power reduction compared to fixed-speed operation for a relatively small reduction in speed.
• For this reason, a 20 percent reduction in fan speed provides nearly 50 percent savings in fan power consumption.
• Due to the inclusion of power electronics drives instead of electromechanical drives, the absence of rotational equipment leads to a reduction in losses which will improve the efficiency of the overall system.

• Fully controlled bridge converters are two quadrant converters having unidirectional current with both positive and negative voltage polarity.
• Half controlled bridge converter is used for one quadrant-operation.
• Dual converters are used for the four-quadrant operation. It has two converters which are connected back to back. One of the bridges works as rectifier (AC to DC) and other bridge works as inverter (DC to AC)
• Step down chopper is used for first quadrant operation (motoring mode) and step up chopper is used for the second quadrant operation (regenerative breaking mode).
• Chopper is also used for two quadrant and four quadrant operation.

Concept:
In the fourth quadrant of operation of electrical drives, the nature of operation of a DC motor is "Reverse regeneration".

Four quadrants of operation:
The four quadrants of operation are divided based on the direction of torque and direction of speed (or rotation). Here's a brief overview of each quadrant:

• First Quadrant (Forward Motoring): Motor speed and torque are positive, meaning the motor is rotating in the forward direction and the load is driving the motor.
• Second Quadrant (Forward Braking or Regeneration): Motor speed is positive, but torque is negative. This means the motor is rotating in the forward direction, but the load is opposing the direction of the motor, so it's slowing down.
• Third Quadrant (Reverse Motoring): Both motor speed and torque are negative, which means the motor is rotating in the reverse direction and the load is driving the motor.
• Fourth Quadrant (Reverse Braking or Regeneration): Motor speed is negative, but torque is positive. This means the motor is rotating in the reverse direction, but the load is opposing the direction of the motor, causing it to slow down.
• So in the fourth quadrant, the motor is operating in the forward regeneration mode. It's slowing down, but still turning in the forward direction. The motor acts as a generator, converting mechanical power back into electrical power and feeding it back into the supply.

The correct answer is option 4):(DC servo drives)

Concept:

• DC servo drives consist of a high-performance permanent magnet motor, often with an integral tachogenerator, and a chopper-type power amplifier module in D. C. motor.
• The drive amplifier for DC servo drives typically requires a separately regulated DC power supply to ensure stable and precise operation.
• This regulated power supply is crucial for maintaining consistent voltage and current levels, which are necessary for the accurate control and performance of the servo drive system. 
• DC servo drives are used for applications which need rapid acceleration and retartdation
• The servo motors are used in closed-loop position control applications

Additional InformationServomechanism is a feedback control system required to control position or a derivative of position.

A servo system primarily consists of three basic components:

A controlled device

Output sensor

Feedback system.

A typical block diagram of the servo system is shown below: