EE6351 Electrical Drives and Control B.E MECH Question Bank

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Anna University Chennai
Mechanical Engineering
QUESTION BANK
Subject Code: EE6351
Subject : Electrical Drives and Control
Semester : III
Regulation : 2013

UNIT I
INTRODUCTION

1. Explain the factors governing the selection of motors.

The limits of speed range
The efficiency
The braking
Starting Requirements
Power factor
Load factor
Availability of supply
Effects of supply variations
Economical aspects
Reliability of operation
Environmental effects

2. Discuss in detail the determination of power rating of motors.

Continuous Duty, variable Load
Method on average losses
Method of equivalent current
Method of equivalent Power
Method of equivalent Torque
Short time Duty
Overloading Factor

3. Draw the typical temperature rise-time curve and derive the equation for temperature rise in an
electric drive.

Heating Curve
Heating Time constant
Cooling Curve
Cooling Time constant

4. Explain the loading of an electric motor and its duty cycle with a simple diagram.

Continuous duty
Short time duty
Intermittent periodic duty
Intermittent periodic duty with starting
Intermittent periodic duty with starting and braking
Continuous duty with intermittent periodic loading
Continuous duty with starting and braking
Continuous duty with periodic speed changes

5. Explain in detail about the various types of electric drives.

Group Drive
Individual Drive
Multimotor Drive
Advantages and Disadvantages

UNIT II
DRIVE MOTOR CHARACTERISTICS

1. Draw and explain the torque-speed characteristics of three phase induction motor with necessary equation.

Speed-Torque characteristics
Methods of modifying characteristics
Change in rotor resistance
Change in applied voltage
Change in the supply frequency
Change in the stator impedance

2. Explain the characteristics of DC shunt motor during Regenerative Braking (b) Dynamic

braking (c) plugging.
Regenerative braking
Motor speed is greater than no load speed
Eb > V
Used in lowering the cage of a hoist or down grade motion of an electric train
Dynamic Braking
Armature is disconnected from supply
Field remains connected to supply
Tb α N
Plugging
Armature connection is reversed
Tb α K3 2N +K2

3. Explain the characteristics of DC shunt motor and derive the corresponding equations:

Torque vs Armature current
Tb α Ia (flux is constant)
Speed vs Armature current
N α V-IaRa
Speed vs torque
Effect of change in Ra
Effect of change in filed resistance
Effect of change in Voltage

4. Explain the characteristics of DC series motor and derive the corresponding equations:

Torque vs Armature current
Tb α Ia2 (flux is proportional to Ia)
Speed vs Armature current
N α (V-IaRa)/Ia
Speed vs torque.
Effect of change in field current
Effect of change in Voltage

5. Explain the characteristics of DC series motor during (a) Regenerative braking (b) Dynamic

Braking (c) plugging.
Regenerative braking
Motor speed is greater than no load speed
Eb > V
Used in lowering the cage of a hoist or down grade motion of an electric train
Dynamic Braking
Armature is disconnected from supply
Field remains connected to supply
Tb α N
Plugging
Armature connection is reversed
Tb α K3 2N +K2

6. Explain the braking of an induction motor

Dynamic Braking: In this type R,Y or B is disconnected from the supply.
Two lead connection: The disconnected line is kept open
Three lead connection: The disconnected line is connected directly to the other line
of the machine.
Plugging : In this motor can be quickly stopped by interchanging any two stator leads
D.C Dynamic braking: Stopping of motor can be achieved by connecting stator terminals
to a dc supply.
Regenerative Braking: Rotor speed is increased greater than synchronous speed.

7. Explain three modes of operation of induction motor.

Motoring mode: 0<S<1 Where S is slip
Generating mode: S<0
Braking mode: S>1

UNIT III
STARTING METHODS

1. Draw a neat schematic diagram of a three point starter and explain its working.

Three point starter is used to start Dc shunt motor. It has 3 points.
L- Line terminal to be connected to positive of supply
A-To be connected to the armature
F-to be connected to the field winding
Overload release(OLR)
No volt coil

2. Draw a neat schematic diagram of a four point starter and explain its working.

The basic difference between 3 and 4 point starter is the connection of NVC.
In 3 point starter, NVC is in series with field winging while in four point starter NVC is connected independently across the supply.

3. Explain the different starting methods of three phase squirrel cage induction motors with neat
sketches.

Stator resistance starter:
To reduce the applied reduced voltage to the stator, three resistances are added to the stator winding.
Due to this voltage gets dropped across the resistance
Autotransformer starter:
A three phase star connected autotransformer can be used to reduce the voltage applied to stator.
Star-delta starter:
In this method, the switch connects the stator winding in star in start. Hence per phase voltage gets reduced by the factor 1/√3.
When the switch is thrown on other side , the winding gets connected in delta across the supply
Direct online starter:
Motors having less than 5 h.p, the starting current is not very high and it can withstand starting current without starter.
Such a motor use a type of starter which is used to connect stator directly to the supply lines without any reduction in voltage.
Hence starter is known as direct on line starter.

UNIT IV
CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C DRIVES

1. Explain with neat sketch the chopper control method of speed control of DC Motors.

Chopper fed d.c motor circuit diagram
Output Waveforms
Chopper output voltage equation Va = αVs
Eb=Kb N
N= Eb/ Kb
N=V-Ia(Ra+Rf)/Kb
Speed torque characteristics

2. Explain with neat sketches about the DC Shunt Motor speed control by using single phase fully
controlled bridge converter.

Converter fed dc motor circuit diagram
Output Waveforms
Converter output Equation
Speed Equation
Speed torque characteristics

3. Explain how the speed of a DC Shunt Motor can be varied both above and below the speed at
which it runs with full field current.

Armature control
Field kept constant. Armature voltage is varied
Field control
Armature is kept constant. Field current is varied

4. Explain the different methods of speed control employed in DC Shunt Motor.

Flux control
Armature voltage control or Rheostatic control
Applied voltage control

UNIT V
CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A.C DRIVES

1. Explain in detail about Slip power recovery scheme.

In this method, a voltage is injected in the rotor circuit. The frequency of rotor circuit is a slip frequency and hence the voltage to be injected must be at a slip frequency.
Two types
Kramer system
Scherbius system

2. Explain the different methods of speed control used in three phase induction motors.

Supply frequency control or v/f control
Supply voltage control
Adding rheostats in stator circuit
Controlling number of stator poles to control Ns
Injecting slip frequency voltage into the rotor circuit

3. Explain the static Kramer method and static Scherbius method of speed control of three phase
induction motor. OR Explain the various methods of speed control of a three phase induction motor when fed through