EE 2403 SPECIAL ELECTRICAL MACHINES 2 MARKS QUESTION AND QUESTION BANK

EE 2403 SPECIAL ELECTRICAL MACHINES
2 MARKS
UNIT-I - SYNCHRONOUS RELUCTANCE MOTOR

1. What is synchronous reluctance motor?
A reluctance motor is a type of synchronous electric motor which induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance.

The stator consists of multiple salient (ie. projecting) electromagnet poles, similar to a wound field brushed DC motor. The rotor consists of soft magnetic material, such as laminated silicon steel, which has multiple projections acting as salient magnetic poles through magnetic reluctance.

The number of rotor poles is typically less than the number of stator poles, which minimizes torque ripple and prevents the poles from all aligning simultaneously -- a position which can not generate torque.

2. Define the characteristics of synchronous reluctance motor.

The synchronous reluctance motor is not self starting without the squirrel cage. During run up it behaves as an induction motor but as it approaches synchronous speed, the reluctance torque takes over and the motor locks into synchronous speed.

3. Write the applications of syrm.

Used where regulated speed control is required in applications suc as metering pumps and industrial process equipment.

4. What are the classification of syrm


Axially laminated

Radially laminated


5. What are the primary design consideration of syrm?


High o/p power capability

Ability of the rotor to withstand high speed.

High reliability

Low cost

High efficiency


6. Define power factor of syrm
PF max=(Ld/Lq-1)/(Ld/Lq+1)
Higher Ld/Lq ratos yield higher power factors,which corresponds to reduced I^2R losses and reduce volt ampere ratings of the inverter driving the machine.

7. What are the applications of the torque – speed characteristics of syrm?

Comparable power density but better efficiency than induction motor

Slightly lower power factor

Sensorless control is much easier due to motor saliency.

8. What are advantages of syrm over pm machine?

More reliable than PM machine

There need not be any excitation field as torque is zero,thus eleiminating electro magnetic spinning losses.

9. What are applications of syrm?

Synthetic fiber manufacturing equipment

Wrapping and folding machine

Auxiliary time mechanism

Synchronized conveyors

Metering pumps

10. What is vernier motor?

It is an unexcited reluctance type sync.motor.the peculiar feature of this motor is that a small displacement of the rotor produces a large displacement of the axis of maximum and minimum permeance.

11. What are the advantages of syrm ?

a. Freedom from pm

b. Ability to maintain full load torque at zero speed

c. A wide speed range at constant power.

12. What are the classifications of SYRM?


Rotor configuration

i)cage rotor for line start
ii)cageless-rotors for variable speed

Stator windings

Stator current controlled mode


13. What are the rotor configurations of SYRM?

Rotor configuration i)cage rotor for line start ii)cageless-rotors for variable speed

14. What is meant by Slow-speed synchronous timing motors

Representative are low-torque synchronous motors with a multi-pole hollow cylindrical magnet (internal poles) surrounding the stator structure. An aluminum cup supports the magnet. The stator has one coil, coaxial with the shaft. At each end of the coil are a pair of circular plates with rectangular teeth on their edges, formed so they are parallel with the shaft. They are the stator poles. One of the pair of discs distributes the coil's flux directly, while the other receives flux that has passed through a common shading coil. The poles are rather narrow, and between the poles leading from one end of the coil are an identical set leading from the other end. In all, this creates a repeating sequence of four poles, unshaded alternating with shaded, that creates a circumferential traveling field to which the rotor's magnetic poles rapidly synchronize. Some stepping motors have a similar structure.

15. What is meant by Watthour-meter motors?

These are essentially two-phase induction motors with permanent magnets that retard rotor speed, so their speed is quite accurately proportional to wattage of the power passing through the meter. The rotor is an aluminum-alloy disc, and currents induced into it react with the field from the stator. One phase of the stator is a coil with many turns and a high inductance, which causes its magnetic field to lag almost 90 degrees with respect to the applied (line/mains) voltage. The other phase of the stator is a pair of coils with very few turns of heavy-gauge wire, hence quite-low inductance. These coils are in series with the load.

16. How does the Watthour-meter motors look like?

The core structure, seen face-on, is akin to a cartoon mouth with one tooth above and two below. Surrounding the poles ("teeth") is the common flux return path. The upper pole (high-inductance winding) is centered, and the lower ones equidistant. Because the lower coils are wound in opposition, the three poles cooperate to create a "sidewise" traveling flux. The disc is between the upper and lower poles, but with its shaft definitely in front of the field, so the tangential flux movement makes it rotate.

17. Electronically commutated motors?

Such motors have an external rotor with a cup-shaped housing and a radially magnetized permanent magnet connected in the cup-shaped housing. An interior stator is positioned in the cup-shaped housing. The interior stator has a laminated core having grooves. Windings are provided within the grooves. The windings have first end turns proximal to a bottom of the cup-shaped housing and second end turns positioned distal to the bottom. The first and second end turns electrically connect the windings to one another. The permanent magnet has an end face rom the bottom of the cup-shaped housing. At least one galvano-magnetic rotor position sensor is arranged opposite the end face of the permanent magnet so as to be located within a magnetic leakage of the permanent magnet and within a magnetic leakage of the interior stator. The at least one rotor position sensor is designed to control current within at least a portion of the windings. A magnetic leakage flux concentrator is arranged at the interior stator at the second end turns at a side of the second end turns facing away from the laminated core and positioned at least within an angular area of the interior stator in which the at least one rotor position sensor is located

18. what is meant by repulsion motor?

Repulsion motors are wound-rotor single-phase AC motors that are similar to universal motors. In a repulsion motor, the armature brushes are shorted together rather than connected in series with the field. By transformer action ,the stator induces currents in the rotor, which create torque by repulsion instead of attraction as in other motors. Several types of repulsion motors have been manufactured, but the repulsion-start induction-run (RS-IR) motor has been used most frequently. The RS-IR motor has a centrifugal switch that shorts all segments of the commutator so that the motor operates as an induction motor once it has been accelerated to full speed. Some of these motors also lift the brushes out of contact with the commutator once the commutator is shorted. RS-IR motors have been used to provide high starting torque per ampere under conditions of cold operating temperatures and poor source voltage regulation

19. Define Slip.

If the rotor of a squirrel runs at high speed, the flux in the rotor at any given place on the rotor would not change, and no current would be created in the squirrel cage. For this reason, ordinary squirrel-cage motors run at some tens of rpm slower than synchronous speed, even at no load. Because the rotating field (or equivalent pulsating field) actually or effectively rotates faster than the rotor, it could be said to slip past the surface of the rotor. The difference between synchronous speed and actual speed is called slip, and loading the motor increases the amount of slip as the motor slows down slightly.

20. Write the formula for the speed of the AC motor.

The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation: Ns = 120F / p Where Ns = Synchronous speed, in revolutions per minute F = AC power frequency p = Number of poles per phase winding

16 Mark Questions
1. Explain the constructions and working principle of synchronous reluctance motor. (16)
2. Explain in detail about classification of synchronous reluctance motor. (16)
3. Draw the phasor diagram of synchronous reluctance motor. (16)
4. Derive the torque equation of synchronous reluctance motor. (16)
5. Draw and explain the characteristics of synchronous reluctance motor. (16)
6. Explain in detail about vernier motor. (16)


UNIT – II - STEPPER MOTORS
1. What is stepper motor?

A stepper motor is a digital actuator whose input is in the form of programmed energization of the stator windings and whose output is in the form of discrete angular rotation.

2. Define step angle.

Step angle is definrd as the angle through which the motor rotates for each command pulse.it is denoted as β. β=(Ns-Nr/Ns.Nr)360 (or)360/(mNr) OM SAKTHI DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING 4th YEAR/ 7th SEMESTER

3. Define slewing

The stepper motor operates at very high speed is called slew angle.i,e (25000 steps per sec).

4. Define resolution

It is defined as the no.of steps needed to complete one revolution of the shaft.
Resolution = no . of steps /revolution

5. Mention some applications of stepper motor

I.floppy disc drives
ii. qurtz watch
iii. camera shutter operation
iv. dot matrix and line printers
v. small tool application
vi. robotics

6. What are the advantages and disadvantages of stepper motor?

Adv:
1. it can be driven in open loop without feedback
2. it is mechanically simple
3. it requires little or no maintenance.
Disadv:
1.low efficiency
2.fixed step angle
3.limited power output

7. Define holding torque.

Holding torque is the maximum load torque which the energized stepper motor can withstand without slipping from equilibrium position

8. Define detent torque

Detent torque is the maximum torque which the unenergised stepper motor can withstand without slipping.it is also known as cogging torque.

9. What is meant by full step operation?

Full step operation or single phase on mode is the one in which at a time only one phase winding is energized, due to which one stator winding is energized and causes the rotor to rotate some angle.

10. What is meant by two phase mode of operation?
Two phase on mode is the one in which two phase windings are energized at a time, due to which two stator windings are energized and causes the rotor to rotate through some angle.

11. Define pull in torque.

It is the maximum torque the stepper motor can develop in start – stop mode at a given stepping rate Fs (step/sec) without losing synchronism.

12. Define pull out torque.

It is the maximum torque the stepper motor can develop in slewing mode at a given stepping rate Fs (step/sec) without losing synchronism.

13. What is synchronism in stepper motor?

It is the one to one correspondence between the number the number of pulses applied to the stepper motor and the number of steps through which the motor has actually moved.

14. Define mid frequency resonance in stepper motor.

The phenomenon at which the motor torque drops to a low value at certain input pulse frequencies.

15. Define static stiffness.

It is a measure of ability of the actuator to resist disturbing torques and forces and thereby to maintain position.it is defined as S=torque / rad

16. Give the types of driver circuits.


Resistance or L/R drive

Dual voltage or bilevel drive

Chopper drive


17. What is multi stack VR motor

Multi stack VR motor is the one in which the stepper motor has three separate magnetically nisolated sections or stacks.here the rotor and stator teeths are equal.


18. What is meant by micro stepping in stepper motor.

The methods of modulating currents through stator windings so as to obtain rotation of stator magnetic field through a small angle to obtain micro stepping action is known as micro stepping.

19. What are the advantages of micro stepping?


Improvement in resolution.

Dc motor like performance

Elimination of mid frequency resonance

Rapid motion at micro stepping rate.


20. Define bandwidth in stepper motor.

It is a measure of the frequencies upto which the actuator or servo motor system can respond.

16 Mark Questions
1. Explain the construction and various modes of excitation of VR stepper
motor. (16)
2. Explain the construction and various modes of excitation of PM stepper
motor. (16)
3. Explain the construction and working principle of Hybrid Stepper motor.
(16)
4. State and explain the static and dynamic characteristics of a stepper
motor. (16)
5. Explain in detail about different types of power drive circuits for stepper
motor. (16)
6. Explain the mechanism of torque production in VR stepper motor. (16)
7. Draw any two drive circuits for stepper motor. (16)

Unit-III - Switched reluctance motor

1. What is srm?

It is a doubly salient , single excited motor.this means that it has salient poles on both rotor and the stator.but only one member carries winding.the rotor has no windings,magnets or case windings.

2. What are the advantages od SRM?


Construction is very simple

Rotor carries no winding

No brushes and requires less maintenance


3. What are the disadvantages of SRM?


It requires a position sensor

Stator phase winding shold be capable of carrying magnetizing currents


4. Why rotor position sensor is essential for the operation of switched reluctance motor?

It is necessary to use a rotor position sensor for commutation and speed feedback. The turning on and off operation of the various devices of power semiconductor switching circuit are influenced by signals obtained from rotor position sensor.

5. What are the different power controllers used for the control of SRM?

Using two power semi conductors and two diodes per phase

Phase windings and bifilar wires

Dump – C converter

Split power supply converter


6. What are the applications of SRM?


Washing machines

Fans

Robotic control applications

Vacuum cleaner

Future auto mobile applications


7. What are the two types of current control techniques?


Hysteresis type control

PWM type control



8. What is meant by energy ratio?

Energy ratio = Wm/(Wm+R)=0.45 Wm=mechanical energy transformed This energy cannot be called as efficiency. As the stored energy R is not wasted as a loss but it is feedback to the source through feedback diodes.

9. Write the torque equation of SRM?

T=1/2(i^2 dL/dθ)

10. What is phae winding?

Ststor poles carrying field coils.the field coils of opposite poles are connected in series such that mmf „s are additive and they are called „phase winding‟‟ of SRM.

11. Write the characteristics of SRM.


Lowest construction complexity, many stamped metal elements

Like a BLDC or stepper without the magnets

High reliability (no brush wear), failsafe for Inverter but...acoustically noisy

High efficiency



12. Write the voltage,power range of SRM.

Industrial
Voltage
Motor Power
Speed Range
100 - 240 Vac
50W - 10'sKW
0 - 60,000 RPM

Automotive

Voltage
Motor Power
Speed Range
12 - 42Vdc
50W -1kW
0 - 20,000 RPM


13. Define the control system of SRM.
The control system is responsible for giving the required sequential pulses to the power circuitry in order to activate the phases as required. There are two options for producing the sequence including a microcontroller to produce the signal or a timer circuit which could also produce the desired signal

14. Define the timer circuit of SRM.

The use of a timer circuit would be very effective in producing the necessary signal in which to control the circuit. As the required signal is very simple it could easily be implemented by digital timer, such as the 555 timer. A digital timer is more precise than any other form of timer, such as a mechanical timer. With the widespread use of digital logic within integrated circuits the cost of these timers has reduced considerably. The latest controllers in use incorporate programmable logic controllers (PLCs) rather than electromechanical components in its implementation. Within PLCs, the timers are normally simulated by the software incorporated in the controller; the timer is therefore controlled by the software. There are obvious advantages to this system, although the control of a soft start could be hard to implement in this way.

15. Write the soft starters of SRM.
Mechanical – come in the form of torque limiters utilizing clutches and various couplings,
Electrical – these soft starters alter the power supply to the motor to reducing the torque and current demand. This is normally performed either by reducing the supply voltage, or controlling the frequency of excitation. Since switched reluctance motors are driven by a controlled pulsed supply, frequency control is an obvious choice in this case.

16. What are the goals to contro, soft starting?

Fixed start-up time - the start up will be controlled to achieve full speed within a fixed time
Current limit - the motor current can be monitored and the start up controlled to keep it below a specified limit
Torque limit - an intelligent starter can calculate the motor torque based on the current and voltage demand and control the start up to provide a constant starting torque

17. What are the major advantages of frequency control of SRM?

This has a major advantage of being easily controlled and changed at any point by simply altering the programming. By using this method the development time is reduced and the number of modules to implement is also reduced.

18. Define the isolation of SRM.

The electrical isolation of the control and power circuitry modules is very important and is used so that the control electronics are protected from any voltage fluctuations in the power circuitry. The major method of isolation used today are optoisolators, these isolators use short optical transmission paths to transfer a signal from one part of a circuit to another. The isolator incorporates a transmitter and a receiver, the signal therefore converts from electrical to optical before converting back to electrical thereby breaking any electrical connection between input and output.

19. Define the power circuitry of SRM.


The most common approach to the powering of a switched reluctance motor is to use an asymmetric bridge converter.

There are 3 phases in this in an asymmetric bridge converter corresponding to the phases of the switched reluctance motor. If both of the power switches either side of the phase are turned on, then that corresponding phase shall be actuated. Once the current has risen above the set value, the switch shall turn off. The energy now stored within the motor winding shall now maintain the current in the same direction until that energy is depleted.

N+1 Switch And Diode

This basic circuitry may be altered so that fewer components are required although the circuit shall perform the same action. This efficient circuit is known as the (n+1) switch and diode configuration.

A capacitor can be added to either configuration, and is used to address noise issues by ensuring that the switching of the power switches shall not cause fluctuations in the supply voltage.


20. What are the current control schemes?


Hysteresis type current regulator

PWM type current regulator

PART – B
1. Explain the construction and working principle of switched reluctance motor. (16)
2. Describe the various power controller circuits applicable to switched reluctance motor and explain the operation of any one scheme with suitable circuit diagram. (16)
3. Draw a schematic diagram and explain the operation of a „C dump converter used for the control of SRM. (16)
4. Derive the torque equation of SRM. (16)
5. Draw and explain the general torque-speed characteristics of SRM and discuss the type of control strategy used for different regions of the curve. Sketch the typical phase current waveforms of low speed operation. (16)
6. Describe the hysterisis type and PWM type current regulator for one phase of a SRM. (16)

UNIT -4 - PERMANENT MAGNETS AND BRUSHLESS DC MOTORS

1. what are the advantages of brushless dc motors drives?


Regenerative braking is possible

Speed can be easily controllable

2. what are the disadvantages of brushless dc motors drives?


It requires a rotor position sensor

It requires a power semiconductor switching circuits.


3. Define mechanical commutators?

Its arrangement is located in the rotor
No of commutators segments are very high .

4.Define electronic commutators?

Its arrangement is located in the stator

No of switching devices limited to six

5.mention some applications of PMBL DC motor?

Power alternators

Automotive applications

Computer and Robotics applications

Textile and Glass industries

6.what are conventional Dc motor?

Field magents on the stator

Maintenance is high

7.what are PMBL DC motor?

Field magents on the rotor

Low maintanace

8.why is the PMBLDC motor called electronically commulated motor?
The PMBL DC motor is also called electronically commutated motor because the phase windings of PLMBL DC motor is energized by using power semiconductor switching circuits.here the power semiconductor switching circuits act as a commutator.

9.what are the classification of BLPM DC motor?

BLPM square wave motor

BLPM sine wave motor

10.what are the two types of BLPM SQW DC motor?

180”polerarc BLPM SQW motor

120”polearc BLPM SQW motor

11.what are the two types of rotor position sensors?

Optical position sensor

Hall effect position sensor
12.what are the materials used for making Hall IC pallet?

Indiem-antinomy

Gallium-arsenide

13.what are applications of stator?

Automotive applications

Veticular electric drive motors

14.what are the classification of BLPM dc motor?

One phase winding and one pulse BLPM dc motor

One phase winding and two pulse BLPM dc motor

Two phase winding and two pulse BLPM dc motor

Three phase winding and three pulse BLPM dc motor

Three phase windings and six pulse circuits

15.what are the features of one phase winding and one pulse BLPM dc motor?

It is inertia should be high,such that rotor rotates continuously

Utilization of transistor and windings are less

16.what are the features of one phase winding and two pulse BLPM dc motor?

In this case winding utilization is better,however transistor utilization is less.

Torque developed is more uniform

17.what are the features of two phase winding and two pulse BLPM dc motor?

Winding utilization is only 50%which is less

It provide better torque waveforms

18.what are the features of three phase windings and 6 pluse circuits?

Utilization factor of winding will be better

Torque pulse and ripple frequency components are less

21.what is meant by self control?

Self control ensures that for all opearating points the armature and rotor fields move exactly at the same speed.

22.what is meant by vector control?

PMSm are employed for variable speed applications. The process of controlling voltage and frequency to get the desired speed and torque is known as vector control of PMSM

PART – B

1. Sketch the structure of controller for PMBLDC motor and explain the
functions of various blocks. (16)
2. Explain the closed loop control scheme of a permanent magnet brushless
dc motor drive with a suitable schematic diagram. (16)
3. Drive the expressions for the emf and torque of a PMBLDC motor. (16)
4. Draw the diagram of electronic Commutator. Explain the operation of
electronic Commutator. (16)
5. Discuss the use of Hall sensors for position sensing in PMBLDC motor.
(16)
6. Sketch the torque-speed characteristics of a PMBLDC motor. (16)

UNIT -5  PERMANENT MAGNETS AND SYNCHRONOUS MOTORS
1.Define stator?

Stator is made up of silicon steel stampings.stator slots carry a balanced 3phase armature winding, wound for a specified even number of poles.The ends of the armature windings are connected to the terminals of the motor.
2.Define rotor?

Rotor is made up of forged steel with outward projected poles.The number of rotor poles must be same as that of stator.These rotor poles carry field coils.They aare suitably connected to form a field winding.The ends of the field windings are connected to the two slip rings which are also mounted on to the same shaft.

3.what are merits of 3phase BLPM synchronous motor?

It runs at a constant speed.

No sliding contacts so it requires less maintenance.

4.what are the demerits of 3 phase BLMP synchronous motor?

Power factor of operation cannot be controlled as field current cant be controlled.

5.what are the rotor configurations?

Peripheral

Interior

Claw-pole or Lundell

6. what are the advantages of load commutation?

It does not require commutation circuits

Frequency of operation can be Transverse higher

7. what are the applications of load commutation?

Some prominent applications of this drive are high speed and high power drives for compressors,blowers,conveyers,steelrolling.

8. what are advantages of synchronous motor?

Four quadrant operation with regenerative braking is possible

High power ratings(up to 100MW)and run at high speeds(6000rpm)

9. what are the applications of synchronous drive?

High speed and high power drives for compressors,blowers,fans,pumps,aircraft test facilities.

10.what are the features of permanent magnet synchronous motor?

Robust, compact and less weight

High efficiency

11. what are the advantages of load commutation?

It does not require commutation circuits

Frequency of operation can be higher

12. what are the applications of PMSM?

Used as a direct drive traction motor

Used as high speed and high power drives for compressors,blowers,conveyors

13.what are features of closed-loop speed control of load commutated inverter fed synchronous motor drive?

High efficiency

Four auadrant operation with regeneration braking is possible

14. what are the merits of PMSM?

It runs at constant speed

No field winding,no field loss,better efficiency

15.what are the demerits of PMSM?

Power factor of operation cannot be controlled as field winding cannot be controlled It leads to losses and decreases efficiency

16.what are assumptions made in derivation of emf equation for PMSM?

Flux density distribution in the air gap is sinusoidal

Armature winding consists of full pitched,concentrated similarly located coils of equal number of turns

17. Why PMSM operating in self controlled mode is known commutatorless dc motor?

Load side controller performs some what similar function as commutator in a dc machine.The load side converter and synchronous motor combination function similar to a dc machine.
First,it is fed from a dc supply and secondly like a dc machine. The stator and rotor field remain stationary with respect to each other at all speeds.Consequently,the drive consisting of load side converter and synchronous motor is known as “Commutator less dc motor”.

18.what is”pulsed mode”?
For speeds below 10%of base speed,the commutation of load side converter thyristors Is done by forcing the current through the conducting thyristors to zero
This is realized by making source side converter to work as inverter each time load side converter thyristors are to be turned off Since the frequency of operating of load side converter is very low compared to the source frequency.Such an operation can be realized.The operation of inverter is termed as”Pulsed mode”

19. What is load commutation?
Commutation of thyristors by induced voltages of load is known as”Load commutation”.Here,frequency of operation is higher and it does not require commutation circuits.

20. What is meant bysynchronous reactance?
It is the sum of armature leakage reactance and fictitious reactance.
Xs=Xt+Xa

PART – B
1. Explain the construction and operation of PMSM. (16)
2. Explain the principle of operation of a sine wave PM synchronous machine
in detail. Draw its phasor diagram and derive its torque equation. (16)
3. Derive the emf equation of PMSM. (16)
4. Write about Self control of PMSM. (16)
5. Derive the expressions for power input and torque of a PMSM. Explain
how its torque speed characteristics are obtained. (16)
6. Explain in detail the vector control of permanent magnet synchronous
motor. (16)

EE 2403 SPECIAL ELECTRICAL MACHINES 2 MARKS QUESTION AND QUESTION BANK Reviewed by Rejin Paul on 7:08 AM Rating: 5

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