3.SWITCHED-RELUCTANCE MOTORS

3.1 Introduction

Like the stepper motor, the switched-reluctance motor (SRM) produces torque through

the magnetic attraction which occurs between stator electromagnets (formed by winding coils on

salient poles) and a corresponding set of salient poles formed on a simple rotor made only of

electrical steel (or other ferromagnetic material). The stepper and switched-reluctance motor

share the same basic principle of energy conversion, and both are members of the family of

variable-reluctance motors.

In the Switched Reluctance Motor, motion is produced due to the variation of reluctance

in the air gap between the stator and rotor. When a stator winding is energized, producing a

single magnetic field, reluctance torque is produced by the tendency of the rotor align itself with

minimum reluctance position. When a rotor pole is aligned with a stator pole, as shown in Fig.

3.1, there is no torque because field lines are orthogonal to the surfaces (considering a small

gab). In this position, the inductance is maximum since reluctance is minimum. If the rotor is

displaced its position, there will be torque production that will tend to bring back the rotor

towards the aligned position. While two rotor poles are aligned to the two stator poles, another

set of rotor poles is out of alignment with respect to a different set of stator poles. Then, this set

of stator poles is excited to bring the rotor poles into alignment. Likewise, by sequentially

switching the currents into the stator windings, the rotor is rotated. Both the stator and rotor have

salient poles, hence the machine is referred to as a doubly salient machine.

FIGURE 3.1 Switched reluctance motor configurations. (a) One tooth per pole. (b) Two teethper pole (12/10 poles).

Fig 3.2 Aligned position. Fig 3.3 Unaligned position.

3.2 Switched Reluctance Motor Construction

Stator

Stator is made up of silicon steel stampings with projected poles. The number of poles of the stator can be either in even number or odd number. Most of the motors have even number of stator poles. All these poles carry concentric windings. The coils on opposite poles are connected in series, such that mmf,s are additive and they are called phase windings. The overlapping of coils is avoided to minimize the mutual inductance between the phases. Such a typical machine is shown in Figure 2.1a, and a modified version with two teeth per pole is shown in Figure 2.1b.

Rotor

The rotor contains no winding or permanent magnet. The rotor is also made up of silicon steel stampings with outward projected poles. Numbers of poles of the rotor are different from the number of stator poles. The rotor position which is mounted on a shaft of the SRM, provides signal to the controller about the position of the rotor with respect to the reference axis. Controller collects this information and also reference signal and suitably turn on and off the concern power semiconductor devices of switching circuit, such that the desired phase winding is connected to the DC supply. The current signal is also feedbacks to the controller circuit is monitoring and control the motor current within permissible limits.

3.3 Principle of working

Figure 3.4 3-Phase 6 / 4 SR Motor

Fig: 3.5 Phase A excited

Consider an SRM with six stator poles and four rotor teeth.it has three phases. A-A’, B-B’, C-C’.These phases can be excited by DC supply through semiconductor switches. Let phase A-A’ be energized for a significant time so that the rotor rests in the equilibrium position. The stator poles A-A’ and rotor poles a-a’ are in alignment, they are in minimum reluctance position. In this

condition dLAdθ

=0.At the position inductance ‘B’ winding is neither maximum nor minimum.

Fig: 3.5 Phase B excited

Now there exists dLBdθ

.the stator phase B is excited at the unaligned position (the position where

the stator pole and the rotor tooth have the maximum air gap length).The magnetization of the excited pole will magnetically polarize the closest rotor teeth b-b’ and produce a force of

attraction. The tangential component of this force produces an electromagnetic torque( 12iB

2 dLBdθ )

in the direction which reduces the airgap length between the stator pole and the rotor tooth. As the rotor tooth approaches aligned position (the position where the stator pole and the rotor tooth have the minimum air gap length), the tangential component of the attractive force decreases and becomes zero.

Fig: 3.6 Phase C excited

Now there exists dLCdθ

.the stator phase C is excited at the unaligned position (the position where

the stator pole and the rotor tooth have the maximum air gap length).The magnetization of the excited pole will magnetically polarize the closest rotor teeth a-a’ and produce a force of

attraction. The tangential component of this force produces an electromagnetic torque( 12iC

2 dLCdθ )

in the direction which reduces the airgap length between the stator pole and the rotor tooth. As the rotor tooth approaches aligned position (the position where the stator pole and the rotor tooth have the minimum air gap length), the tangential component of the attractive force decreases and becomes zero.

3.4 The advantages of an SRM:

1. Machine construction is simple and low-cost because of the absence of rotor winding and

permanent magnets.

2. There are no shoot-through faults between the DC buses in the SRM drive converter because

each stator winding is connected in series with converter switching elements.

3. Bidirectional currents are not necessary, which facilitates the reduction of the number of

power switches in certain applications.

4. The bulk of the losses appear in the stator, which is relatively easier to cool.

5. The torque-speed characteristics of the motor can be modified to the application requirement

more easily during the design stage than in the case of induction and PM machines.

6. The starting torque can be very high without the problem of excessive in-rush current due to

its higher self-inductance.

7. The open-circuit voltage and short-circuit current at faults are zero or very small.

8. The maximum permissible rotor temperature is higher, since there are no permanent magnets.

9. There is low rotor inertia and a high torque/inertia ratio.

10. Extremely high speeds with a wide constant power region are possible.

11. There are independent stator phases, which do not prevent drive operation in the case of loss

of one or more phases.

3.5 The disadvantages of an SRM:

1. SRM torque ripple and acoustic noise are the most critical.

2. The higher torque ripple also causes the ripple current in the DC supply to be quite large,

necessitating a large filter capacitor.

3. The doubly salient structure of the SRM also causes higher acoustic noise compared with

other machines.

4. The absence of permanent magnets imposes the burden of excitation on the stator windings

and converter, which increases the converter KVA requirement.

5. Compared with PM brushless machines, the per unit stator copper losses will be higher,

reducing the efficiency and torque per ampere.

6. The maximum speed at constant power is not limited by the fixed magnet flux as in the PM

machine, and, hence, an extended constant power region of operation is possible in SRMs.

3.6 Applications of SRM:

a) General purpose industrial drives;

b) Application-specific drives: compressors, fans, pumps, centrifuges;

c) Domestic drives: food processors, washing machines, vacuum cleaners;

d) Electric vehicle application;

e) Aircraft applications;

f) Servo-drive.