Design of Switched Reluctance Motor

Prof. A N Patel

Electrical Engineering Department

Institute of Technology

Nirma University.

Construction

The SRM is a doubly-salient, singly-

excited machine with independent

windings of the stator.

It consists of a stator with excitation

windings.

There is no winding or permanent

magnet on rotor

Stator windings on diametrically

opposite poles are connected in

series or parallel to form one phase

of the motor.

Several combinations of stator and

rotor poles are possible, such as 6/4

(6 stator poles and 4 rotor poles),

8/6, 10/6, 12/8 etc.

Construction (contd.)

SRM Configurations Depends on:

Number of stator/rotor poles

Number of phases

Number of repetitions

Connections of the stator windings

Common Configurations:

6/4 (6 stator poles/4 rotor poles), 3

phases

8/6, 4 phases

Principle of Operation

Switching of current from phase1 to phase2 results inrotation in anticlockwise direction.

Aligned condition and Unaligned condition

Alligned condition and Unalligned condition

Inductance Profile

Aligned inductance is inductance offered when stator poles and rotor polesare in aligned condition. It is the maximum inductance offered by phasebecause reluctance is minimum when poles are aligned.

Minimum inductance is unaligned inductance when stator and rotor poles areunaligned. It is the minimum inductance offered by phase because reluctanceis maximum.

Current pulse must coincide with rising inductance period for motoringaction.

Equivalent Circuit

Induced emf,

Constant of emf,

Equivalent circuit is derived from voltage and

emf equations.

Equivalent Magnetic Circuit

Pole base and arc shaping of the

SRM

Magnetic equivalent circuit assuming

half symmetry

Rrp = Rotor pole reluctance

Rry = Rotor core reluctance per side

Rsy = Stator back iron reluctance per side

Aligned Condition

Equivalent Magnetic Circuit ( contd.)

MMF ,

Aligned Flux ,

Aligned Inductance,

Aligned Condition

Equivalent Magnetic Circuit (contd.)

Unaligned Condition

MMF ,

Unaligned

Inductance,

Flux linkages vs. stator current (λ Vs i )

The area enclosed by OABCO denotes the output mechanical energy

of the motor for one stroke.

Torque Equation

• When current is passed through the phase winding rotortends to align with stator poles to achieve minimumreluctance position.

• Produced torque is known as reluctance torque.

Elementary reluctance motor

Torque Equation (contd.)

• Instantaneous Torque,

Field energy and Co-energy

• W’ is co-energy,

Ψ= Li if saturation is neglected

Torque Equation( contd.)

T=1/2 i2 dL/dɵ N.m.

The torque is proportional to the square of the phase current. As aresult, the torque depends only on the magnitude of the phasecurrents and not on the polarity. Thus, the currents supplied can beunidirectional i.e. bidirectional currents are not required.

This unidirectional current requirement has a distinct advantage inthat only one power switch is required for control of current in thephase winding. Such a feature greatly reduces the number ofpower switches in the converter and thereby makes the driveeconomical.

The direction of rotation can be reversed by changing thesequence of stator excitation which is a simple operation.

A generating action is made possible with unipolar current due toits operation on the negative slope of the inductance profile. Dueto these features, this machine is suitable for four-quadrantoperation with converter.

Torque Equation ( contd.)

Since the torque is proportional to the square of the current, thismachine resembles a dc series motor. Hence it has a good startingtorque.

Torque and speed control is achieved with converter control. Thismachine requires a controllable converter for its operation andcannot be operated directly from a 3-phase supply. Hence forconstant speed applications, this motor drive is expensive incomparison to induction and synchronous motors.

Because of its dependence, on a power a power converter for itsfunctioning, this motor drive is an inherently variable-speed motordrive system.

These torque zeros can be seen to occur at rotor positions where allthe stator phases are simultaneously at a position of eithermaximum or minimum inductance. Since the torque depends onthe derivative of inductance with respect to angular position, thissimultaneous alignment of maximum and minimum inductancepoints necessarily results in zero net torque.

Torque Equation ( contd.)

Figure shows a 6/4 SRMfrom which we see that afundamental feature of the 6/4machine is that no suchsimultaneous alignment ofphase inductances is possible.As a result, this machine doesnot have any zero torquepositions.

This is a significant pointbecause it eliminates thepossibility that the rotor mightget stuck in one of thesepositions at stand still,requiring that it bemechanically moved to a newposition before it can bestarted.

Torque Equation ( contd.)

In case of SRM, with Ps stator poles and Pr rotor poles, if the

ratio

Ps / Pr or Pr / Ps

is an integer, there will be zero-torque positions.

For example for 6/4 machine, the ratio is 1.5 and hence there will

be no zero-torque positions. However, the ratio is 2.0 for 6/3

machine and there will be zero torque positions .

Design of SRM

Design Considerations :

Cost

Size

Durability

Performance

Design of SRM

Main Steps :

Main Dimensions calculation

Calculation of winding details

Pole numbers and selection of pole arcs

Performance estimation

Derivation of Output Equation

The output equation correlates the stator bore diameter, length,

speed, magnetic and electric loadings to the output of a machine.

In general, the machines are designed starting from the output

equation.

A similar derivation of the output equation for SRM will make its

design systematic.

Moreover, the experience of the machine designers can be

effectively used in the design of these new machines.

A 3-phase , 6/4 SRM with major dimensional variables

Calculation of Main Dimensions