EE6703 SPECIAL ELECTRICAL MACHINES

UNIT – III SWITCHED RELUCTANCE MOTOR

PART – A

1. What is switched reluctance motor?

The switched reluctance motor is a doubly salient, singly excited motor. This means

that it has salient poles on both the rotor and the stator, but only one member (usually

the stator) carries windings. The rotor has no windings; magnet is or cage windings

but is built up from stacks of salient pole laminations.

2. What are the advantages of Switched Reluctance motor?(Dec 13, June 2016)

Rotor is simple and it tends to have a low inertia, The stator is simple to wind, In most

applications the bulk of the losses appear on the stator, which is relatively easy to cool,

Due to the absence of magnet the maximum permissible rotor temperature may be

higher than in PM motors, Under fault conditions the open circuit voltage and short

circuit current are zero or varying small, Extreme by high speeds are possible.

3. What is the difference between Switched Reluctance motor and variable

reluctance stepper motor?(May12)

Switched Reluctance motor Variable reluctance stepper motor

Conduction angle for phase current is

controlled and synchronized with the rotor

position, usually by means of a shaft

position sensor.

Stepper motor is usually fed with a

square wave of phase current without

rotor position feedback.

The SRM is designed for high speed. It is usually designed with a limited

speed.

Closed loop control is necessary Closed loop control is required for high

frequency operation

4. What are the applications of Switched Reluctance motor?(Dec 2016)

Precision position control system for Robotics and Low power servo motor.

5. Give basic features or characteristics of Switched Reluctance motor. (June 14,

May 15)

The switched reluctance motor is a doubly-salient, singly-excited motor. This

means that it has salient poles on both the rotor and the stator but only one

member (usually the stator) carries windings. The rotor has no windings, magnets,

or cage windings, but is built up from a stack of salient-pole laminations.

Low inertia and simple manufacturing, Losses appear only on the stator and easy

to cool, No magnets and so permissible rotor temperature is higher than in PM

motors, Torque is independent of the polarity of phase current. Reduction in no of

semiconductor devices in controller Open circuit voltage and Short circuit current

are zero or very small under faulty condition, Immune from shoot through failure,

High starting torque, extremely high speeds possible.

6. What are the disadvantages of a Switched Reluctance motor?

The absence of free PM excitation imposes the burden of excitation on the stator

windings and the controllers and increases the per unit copper losses, is limited,

torque/ampere is limited, Non uniform nature of the torque production which leads to

torque ripple and may contribute to acoustic noise.

7. Mention different modes of operation of SRM.(Dec 15)

Low speed mode, High Speed mode

8. Mention the application of micro stepping VR stepper motor? (Dec 14)

Application are Printing, photo type setting .V.R type stepper motor with µ stepping

provides very smooth low speed operation and high resolution.

9. Write the relations between the speed and fundamental switching frequency.

f=nNr=(r.p.m./60)Nr Hz , Nr=No. of rotor poles, If there are q phases there are qNr

steps per revolution and the step angle or stroke is given by ε=2π/(qNr) rad. The

number of stator poles usually exceeds the number of rotor poles.

10. What is co-energy?

In the ψ –i curve of a motor, the area between the curve and horizontal i axis is the

co-energy W’ and the other part is the stored field energy Wf.

11. Give the expression for torque of a Switched Reluctance motor.(May 12, June

13)

The torque is given by T = [∂W’/∂θ] i=const Or by T = [∂Wf / ∂θ] ψ =const.

With magnetic saturation negligible and with ψ–i curve straight line,ψ=Li, W’=Wf

=(½)Li2z,T = (½)i2 dL/dθ Nm where T is the torque, L is the inductance, Wf is the

stored field energy.

12.Why rotor position sensor is essential for the operation of Switched Reluctance

motor? (Dec 12, Dec 2016)

It is normally necessary to use a rotor position sensor for communication 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.

13. Define: Chopping mode of operation of Switched Reluctance motor?

In this mode, also called low – speed mode, each phase winding gets excited for

a period which is sufficiently long.

14. What are the types of power controllers used for Switched Reluctance motor?

(Dec’ 15)

Using two power semiconductors and two diodes per phase, (n + 1) power

switching devices and (n+ 1) diodes per phase, Phase windings using Bifilar wires

, Dump–C– converter, Split power supply converter.

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

Hysteresis type control, PWM type control.

16. What is the step angle of an 5 phase Switched Reluctance motor and

commutation frequency in each phase for the speed of 6000 rpm. SRM having 10

stator poles and 4 rotor poles.

Solution:

Step angle ()=(2π/qNr)=(360°/5*4)=18°.

Commutation frequency at each phase=(Nr*ω)/2π=(4*6000)/60=400Hz.[ω = 2πN].

17. What are the merits of Dump C – Converter?

This topology uses lower number of switching devices and has only one switch voltage

drop, the converter has full regenerative capability, and there is faster

demagnetization of phases during commutation.

18. What are the merits of split power supply Converter?

It requires lower number of switching devices, there is faster demagnetization of

phases during commutation.

19. What are the merits of classic converter or power controller in SRM? (May 12)

Control of each phase is completely independent of the other phases; the energy

from the off going phase is feedback to the source, which results in useful utilization

of the energy.

20. Why SR machines popular in adjustable speed drives?(Dec 12)

Rotor is simple and it tends to have a low inertia, The stator is simple to wind, In most

applications the bulk of the losses appear on the stator, which is relatively easy to cool,

Due to the absence of magnet the maximum permissible rotor temperature may be

higher than in PM motors, Under fault conditions the open circuit voltage and short

circuit current are zero or varying small, Extreme by high speeds are possible.

21. Mention some position sensors used in switched reluctance motor .(June 13)

Optical encoder, resolver, Speed sensors and Hall Effect sensor.

22. What is the significance of closed loop control in switched reluctance

motor.(Dec 13, June 2016)

Switched reluctance motor is always operated with closed loop control. Normally we

have to use a rotor position sensor for commutation and speed feedback. Here the

phase windings are energized by using power semiconductor circuit. He turning on and

off operation of the various semiconductor devices are influenced by signals obtained

from rotor position sensor. It is the main significance of closed loop control in SR motor.

23. Give the advantages of sensorless operation of switched reluctance

motor?(June 14)

Low cost, Reliable and it avoids additional cost size.

24. List out the advantages and disadvantages of the converter circuit with two

power semiconductor devices and two diodes per phase? (Dec 14)

Advantages

The converter has low number of switching devices which reduces the cost of

the converter.

The converter is able to freewheel during the chopping thus reducing the

switching frequency and losses.

Disadvantages

The common switch conducts for all phases and thus have higher switching

stress.

Disability to magnetize a phase while the off going phase is still demagnetizing

which results in high torque ripple during commutation.

25. What is Hysteresis current control?

This type of current controller maintains a more or less constant throughout the

conduction period in each phase. This controller is called hysteresis type controller.

PART B

1.Explain the construction and working of Switched Reluctance motor? (May 12,

Dec 13)

Construction of Switched Reluctance motor

working of Switched Reluctance motor

Construction:

The switched reluctance motor is a doubly salient, singly excited motor. This

means that it has salient poles on both the rotor and the stator, but only one member

(usually the stator) carries windings. The rotor has no windings; magnet is or cage

windings but is built up from stacks of salient pole laminations.

The stator is made up of silicon steel stampings with inward projecting poles. The

number of poles in the stator is either an even number or odd number. Most of the motors

have even number of stator poles. All these stator poles carry field coils.

The rotor is made up of silicon steel stampings with outward projecting poles.

Number of poles of rotor is different from the number of poles of stator. The rotor shaft

carries a position sensor.

Principle:

Faradays law of electromagnetic interaction. Whenever a current carrying

conductor is placed in a magnetic field, it experiences a force. The direction of force given

by Flemings left hand rule .The SRM develops an torque due to variable reluctance

2.With a block diagram explain the importance of closed loop control of Switched

Reluctance motor. (Dec 13,June 14,Dec 14)

Switched reluctance motor is always operated with closed loop control. Normally we

have to use a rotor position sensor for commutation and speed feedback. Here the

phase windings are energized by using power semiconductor circuit. He turning on

and off operation of the various semiconductor devices are influenced by signals

obtained from rotor position sensor. It is the main significance of closed loop control

in SR motor

3.Describe the Hysteresis type and PWM type current regulator for one phase of

Switched Reluctance motor.(Dec 14)

The control methods available for SRM are two types

1.hysteresis type current regulator

This type of current controller maintains a more or less constant throughout the

conduction period in each phase. This controller is called hysteresis type controller.

2.voltage PWM control or duty cycle control

The circuit consists of two transistors T1 and T2, two energy feedback diodes

D1 and D2, phase windings A and control circuit. The transistor T2 is turned on by

applying base signal from rotor position sensor. The operational amplifier compares

reference voltage signal and actual voltage signal. The output of the error signal is fed to

the monostable multivibrator.

4.Explain in detail about microprocessor based control of Switched Reluctance

motor.(May 12, Dec12,Dec 14, Dec 2016)

The input DC supply is fed to the power semiconductor switching circuits. The power

semiconductor devices are turned on and off by control circuit. The microprocessor or

computer functions can be general be categorized as follows 1) control of feed back

loops2) Optimal and adaptive control3) Estimation of feedback signals4)General

sequencing control5)Protection and fault overriding control

5.a.Describe the various power controller circuits to Switched Reluctance motor

and explain the operation of any one scheme with suitable circuit diagram. (June

2016, Dec 2016)

Classic Converter

Merits:

1. Control of each phase is independent of other phase.

2. The converter is able to freewheel during the chopping period.

3. The energy from the off going phase is feedback to the source.

Demerits:

1. Higher number of switches required in each phase which makes the converter

expensive.

b. State the advantages of sensorless operation. (June 2016)

The method works in a speed range from stand-still to rated speed (high-speed

sensorless operation is still under research).

For low-dynamic applications (estimation of commutation angles only), the method is

highly robust against heavy saturation of the machine.

The position information can be extracted from a large-amplitude voltage signal.

Depending on the bus bar voltage used, the measured signal can have an amplitude of

hundreds of Volts. Such a signal is less prone to disturbances compared to small-

amplitude voltage or current signals.

Due to the very short test pulses, no current flows in the idle phase winding. Therefore,

no torque disturbance is generated. The additional loss due to the voltage resonances

is negligible.

6.Describe the various operating modes of Switched Reluctance motor.(Dec 12,Dec

14)

There are two types of operating modes of SRM.

1. Single pulse mode

2. PWM mode

Single pulse mode:

Single pulse mode is also called as high speed mode, the current rise is

within limits during small time interval of each phase excitation.

PWM mode:

It is also called as low speed mode, each phase winding gets excited for a

period which is sufficiently long. The current build up during this long period of time may

be prohibitively high.

7.Derive the voltage and torque equation of Switched Reluctance motor and also

explain torque – speed characteristics of Switched Reluctance motor.(Dec 12,Dec

14, Dec 2016)

Voltage equation of SRM= iR+∂λ/∂t; V= iR+L(∂i/∂t)+iω(∂L/∂θ )

∂λ/∂t-slope of magnetic curve

∂L-incremental inductance

iR-resistive drop

L(∂i/∂t- emf due to incremental inductance

iω(∂L/∂θ- Self emf depends on current ,speed and rate of change of inductance with rotor

angle.

Torque equation

The torque is given by T = [∂ W’ / ∂θ] i=const Or by T = [∂ Wf / ∂θ] ψ =const With magnetic

saturation negligible and with ψ –i curve straight line ,

ψ = L i ,W’ = Wf = (½) L i2 ,T = (½) i2 dL /d θ Nm

b. Discuss the need of rotor position sensor in SRM.

CURRENT SENSING

CURRENT –SENSING METHODS

Current control could be inexpensively achieved both with sensors and without sensors.

A number of options emerge as indicated in the flow diagram shown in above Figure.

Direct measurement with galvanically isolated current transducer is still an expensive

solution. Non-inductive, low-ohmic resistors can be used to monitor the current by

measuring the voltage drop across them. They are not galvanically isolated, as in many

applications galvanic isolation is not a strict requirement. This method provides the most

accurate

current measurement with the least amount of investment in materials and processing

circuitry, but it does introduce an external resistor component in the circuit that consumes

power on the order of a watt or less. The indirect method, such as the one utilizing the

resistive nature of the drain-to-source conduction path of a MOSFET for monitoring the

current, is an attractive alternative. It also does not require any additional

components to serve as a transducer, as the phase power switches could be used for

monitoring indirectly 28 the currents. Special MOSFETs, called sense FETs, are available

with processed current signals that minimize additional control electronic circuitry to

transform the drain–source conduction voltage drop to the control level and keep it

isolated during its off state from the control circuit. Otherwise, the drain voltage at turn-off

will be at least equal to the dc source voltage, which is multiple times greater than the

control supply voltage and will damage the control circuitry.

CURRENT SENSING WITH RESISTORS

Two different implementations of resistor-based current measurement are possible. Only

one resistor could be used in the path from all the bottom switches to the negative dc bus

rail (say, for an asymmetric converter), as shown in Figure1. The phase-current

measurement is not available with this implementation during freewheeling of a phase.

More or less an average current can be obtained with this setup. If two phases are

simultaneously conducting, then it is not possible to separate each phase current in this

method. Therefore, it is useful when no overlapping conduction occurs in the machine

phases and average current control is acceptable. In the second arrangement, each

phase current is sensed independently with one resistor, as shown in below Figure.

Including the resistor in the freewheeling path allows the monitoring of phase currents

continuously except when the current is being turned off (commutated) once and for all

at the end

8.Describe the construction and working of rotary and linear switched reluctance

motors.(June 13,Dec 14, June 2016)

Since the torque in SRM drives is independent of the excitation current polarity, the SRM

drives require only one switch per phase winding. This is contrary to the ac motor drives

where at least two switches per phase are required for current control. Moreover, the

windings are not in series with the switches in ac motor drives, leading to irreparable

damage

in shoot-through faults. The SRM drives always have a phase winding in series with a

switch. In case of a shoot-through fault, the inductance of the winding limits the rate of

rise in

current and provides time to initiate protective relaying to isolate the faults. The phases

of the

SRM are independent and, in case of one winding failure, uninterrupted operation of the

motor drive operation is possible, although with reduced power output.

LINEAR SWITCHED RELUCTANCE MACHINES

INTRODUCTION

Linear motor drives are being increasingly considered for machine tool drives because

they reduce the need for mechanical subsystems of gears and rotary-to-linear motion

converters, such as lead screws. Positioning accuracy is improved by the absence of

gears that contribute to the backlashes in the linear motor drives. Linear machine drives

combined with electromagnetic levitation are strong candidates for conveyor applications

in semiconductor fabrication plants and possibly in low- and high speed transit

applications because of their ability to produce propulsion force on the rotating part,

known as the translator, without mechanical contact and friction. Linear switched

reluctance machines (LSRMs) are the counterparts of the rotating switched reluctance

machines. In fact, the linear switched reluctance machine is obtained from its rotary

counterpart by cutting, along the shaft over its radius, both the stator and rotor and then

rolling them out. In this section, various linear switched reluctance machine configurations

are introduced. Further, the ideal inductance profile is related to the stator and translator

lamination dimensions. A similar relationship for the rotary switched reluctance machine

that has been derived earlier is worth noting.

9.Discuss methods of rotor position sensing and sensorless operation. (June 13,

Dec 14)

CURRENT –SENSING METHODS

Current control could be inexpensively achieved both with sensors and without sensors.

A number of options emerge as indicated . Direct measurement with galvanically isolated

current transducer is still an expensive solution. Noninductive, low-ohmic resistors can be

used to monitor the current by measuring the voltage drop across them. They are not

galvanically isolated, as in many applications galvanic isolation is not a strict requirement.

This method provides the most accurate current measurement with the least amount of

investment in materials and processing circuitry, but it does introduce an external resistor

component in the circuit that consumes power on the order of a watt or less.

The indirect method, such as the one utilizing the resistive nature of the drain-to-source

conduction path of a MOSFET for monitoring the current, is an attractive alternative. It

also does not require any additional components to serve as a transducer, as the phase

power switches could be used for monitoring indirectly 28 the currents. Special

MOSFETs, called sense FETs, are available with processed current signals that minimize

additional control electronic circuitry to transform the drain–source conduction voltage

drop to the control level and keep it isolated during its off state from the control circuit.

Otherwise, the drain voltage at turn-off will be at least equal to the dc source voltage,

which is multiple times greater than the control supply voltage and will damage the control

circuitry.

SENSORLESS ROTOR POSITION ESTIMATION

Incremental Inductance Measurement

This method uses the fact that the current rise and fall times are proportional to the

incremental inductance, and under certain assumptions the rise and fall times reflect the

incremental inductance and hence the rotor position itself. Both these techniques are

described in detail and their merits and demerits are discussed. The only advantage of

this method is that it only needs to measure the current and its rise or fall time to predict

the rotor position and can use the active phase itself. Therefore, it can be implemented in

real time and online with very little or no extra cost other than some software codes

required in the controller.

10.Compare and contrast the performance of SR motor and VR stepper motor.(Dec

13)

Switched Reluctance motor Variable reluctance stepper motor

Conduction angle for phase current is

controlled and synchronized with the rotor

position, usually by means of a shaft

position sensor

Stepper motor is usually fed with a

square wave of phase current without

rotor position feedback.

The SRM is designed for efficient power

conversion at high speed

It is usually designed as a torque motor

with a limited speed.

11.Derive the expression for static torque in SRM(Dec 2016)