23053-SPECIAL ELECTRICAL MACHINES ONE MARK QUESTIONS AND ANSWERS

KUMARAN

POLYTECHNIC COLLEGE

DIPLOMA IN ELECTRICAL AND ELECTRONICS ENGINEERING

23053-SPECIAL ELECTRICAL MACHINES

ONE MARK QUESTIONS AND ANSWERS

STAFF-IN-CHARGE: N.SURESH KUMAR B.E.,

UNIT- 1

PERMANENT MAGNET SYNCHRONOUS MOTOR

1. State the difference between permanent magnet synchronous motor and

conventional synchronous motor.

S.no	Conventional synchronous motor	PM synchronous motor
1. 2.	The rotor has field winding which is supplied from a DC source through slip rings and brushes Maintenance requirement is high because of the presence of slip rings and brushes	The rotor has permanent magnets Low maintenance

2. Write the torque equation of permanent magnet synchronous motor.

Torque equation=ÂB̂ π r l sin B N-M

3. What type of supply is given to the permanent magnet synchronous motor?

3phase supply (3ϕ) AC motor supply

4. List any two advantages of synchronous reluctance motor.

Ø The motor has simple and rugged construction

Ø It has high efficiency

Ø It can be used in multi motor drives because of its simplicity

5. State any two properties of synchronous reluctance motor.

ü It has high speed capability

ü It can operate at very high temperature

6. Write the EMF equation of permanent magnet synchronous motor.

EMF Equation: Eph=4.44fϕm Tph kw volts

7. Mention any two applications of pure motor.

v Used in Fiber spinning mills

v It can be used in recording instrument and timing devices it used in regulators

8. State any one applications of permanent magnet synchronous motor.

·        In industries, they used in small machine drives

·        Used in fiber spinning mill

9. What is permanent magnet synchronous motor?

Brushes less ac motors other name is permanent magnet synchronous motor

UNIT-2

SWITCHED RELUCTANCE MOTOR

1. What is the principle of switched reluctance motor?

Ø Switched reluctance motor is electromagnetic and electro dynamic equipment that converts electrical energy into mechanical energy

Ø The term switched reluctance does not mean that the reluctance itself is switched, but it refers to the switching of phase current.

2. Give any two applications of SRM.

ü It can be used in space craft

ü It is used in robotic controllers

3. State any two advantages of SRM.

v Self starting

v Also works in high speed

v high efficiency

4. What do you meant by switched reluctance motor?

In this type of motors worked on the principle of switching of phase currents, Hence it is called switched reluctance motor

5. Draw the speed torque characteristics of SRM.

6. Write the expression of torque in SRM.

Torque T=1/2 i2 ƏL/Əθ

7. State the names of power controller circuits used.

·        SRM: split-link circuit used with even-phase number

·        (n+1) power switching devices for n-phase motor

·        c- Dump circuit

·        Two power semiconductor switching device per phase circuit or classic converter circuit

·        Phase winding using bifilar wires or in a nutshell it is called as bifilar windings

8. Give the relationship between fundamental switching frequency and speed of

SRM.

Switching frequency of SRM=number of rotor poles*speed in rpm/16

UNIT-3

STEPPER MOTORS

1. State the principle of stepper motor.

Stepper motor is a digital electro-mechanical device, which rotates through fixed angular steps for each input current pulse received by its controller

2. Different types of stepper motor.

Ø Variable reluctance(VR)stepper motor

Ø Permanent magnet(pm)stepper motor

Ø Hybrid stepper motor

3. List any two or few applications of stepper motor.

ü It is used in computer control system

ü It is used in Tape drive

ü It is used in Robotic

ü It is used in Type writer

ü It is used in Integrated circuits

4. Define step angle as applied to stepper motor.

v It is the angular displacement of the rotor of a stepper motor for every pulse of excitation given to the stator windings of the motor

v Formula: Step angle (β) =360o/M Nr  Where: M=No of stator phases Nr=number of rotor poles

5. What is permanent magnet stepper motor?

A stepper motor using a permanent magnet in the rotor is called as permanent magnet(PM) stepper motor

6. Define pull in torque.

It is the maximum torque of the stepper motor can develop in start stop mode at a given stepping rate, without losing synchronism

7. Define pull out torque.

It is the maximum torque of the stepper motor can develop at a given stepping rate in the slewing mode without losing synchronism

8. Write the expression for reluctance torque in stepper motor.

Reluctance Torque (T) =1/2 i2 ƏL/Əθ

9. What is hybrid stepper motor?

Hybrid stepper motor combines the features of both PM and VR stepping motors

10. Draw the dynamic characteristics of stepper motor.

11. Define holding torque as applied to static characteristics of stepper motor.

It is the maximum load torque which the energized stepper motor can withstand

without slipping from equilibrium position. It is denoted as TH.

12. Classify the characteristics of stepper motor.

       Stepper motor characteristics are classified into two types

1. Static characteristics

2. Dynamic characteristics

13. Define detent torque.

It is the maximum load torque up to which the unenergized stepper motor can withstand without slipping from equilibrium position. It is denoted as TD.

UNIT- 4

PERMENANT MAGNET BRUSHLESS DC MOTORS

1. Name the materials used for permanent magnet.

Ø ALNICO =best quality

Ø Cobalt samarium=energy density is more

Ø Barium and strontium ferrites= applicable in high temperature

Ø Neodymium-iron-boron=more energy density

2. Draw the demagnetization characteristics curve of permanent magnet.

3. Write the expression for the emf equation of PMBL DC motor.

Eph (EMF)=2Bgrl wm Tph volts

Where:

·        Eph=resultant emf when all nc coils are connected in series

·        Nc Tc=Tph, No.of turns per phase

4. Write the expression for the EMF torque equation of PMBL DC motor.

Torque equation EMF (T) =4TphBgrl INM

5. Mention any two applications of PMBL DC motor.

v Turn table drives of record players

v Tape drive for video recorders

v Hard disk drives for computer

6. Draw the BH curve of permanent magnet.

7. What are the types of permanent magnet brushless DC motor?

·        PMBL square wave DC Motor

·        PMBL sine wave DC Motor

8. What are the salient features of brushless DC motor?

Ø Long life

Ø More reliability

Ø Low friction and inertia

9. What are the disadvantages of brushless DC Motor as compare to PMDC commutator motor?

S.no	PMBL DC Motor	PM synchronous motor
1.	Known as brushless DC motor	Known as brushless A.C. motors
2.	Used as low power drives	Used as high power drives

10.Compare PMBL DC Motor with conventional DC motor.

S.no	PMBL DC Motor	Conventional motor
1.	The rotor has permanent magnets	Electro magnets are located in the stator
2.	Size of motor is small	Motor size is large

UNIT- 5

OTHER SPECIAL MACHINES

1. Which type of supply is used in hysteresis motor?

1ϕ AC supply is used in hysteresis motor

2. Where is linear motor used?

Ø It is used in conveyors, travelling cranes, high speed rail traction etc.,

Ø It is used for trolley cars for internal transport in workshop

3. Mention the applications of AC series motor.

Because of high starting torque it is used in electric train, hoists, locomotives etc.

4. State the principle of permanent magnet DC motor.

Interaction between the flux produced by the permanent magnet and current carried by the armature, a torque is produced and hence the motor rotates.

5. What is the type of supply used in permanent magnet A.C. motor?

Poly phase alternating supply gives to Ac motor

6. What is linear motor?

Linear induction motor works on the same principle as that of normal induction motor but the stator and rotor of the induction motor are made flat then it forms the linear induction motor.

7. Give an application of permanent magnet DC motor.

v Small 12v PMDC motors are used for driving automobile heater, air conditioner, blower, windshield wiper, windows fan and radio antenna etc.,

v They are also used for electrical fuels, pumps, marine engine starters, wheel chairs, cordless power tools etc.,

8. State the principle of hysteresis motor.

Hysteresis motor is a single phase synchronous motor which does not require any DC excitation to the rotor and it uses non projected poles.

9. State the applications of hysteresis motor.

Ø Due to noiseless operation it is used in sound recording instruments,. Sound producing equipments

Ø Tape recorders, electric clocks etc.,

10. State the difference between the linear induction motor and conventional

induction motor.

S.no	Conventional induction motor	Linear induction motor
1.	Rotational movement	Linear movement
2.	Rotor and stator are in cylindrical shape	Rotor and stator are in flat position

11. State the applications of permanent magnetic DC motor.

v Applicable in constant speed require places such as synthetic fiber drawing

v These motors are also used for tape drives