First time reluctance motors is mostly used by Davidson as a traction
drive for an electric locomotive in 1838. But the “Re-Invention” has
been possibly due to the region of inexpensive and by the help of high-
power switching devices used in the motor. The term became popular
from the 1980s onwards, through the efforts of the first commercial
exploiters of the technology
Reluctance motors are the special type of motor, where the output
mechanical power and the overall performance is quite good as
compared to the maintenance and operating cost. In fact, there are
many timing devices where small motors having constant speed
characteristic are very advantageous. These motors operate from a
single phase ac supply.
They do not require dc source of power supply for excitation nor they
do they use permanent magnets. The most commonly used types of
single phase synchronous motors are reluctance motor and hysteresis
motor. The output of reluctance motors lies within a few kilowatts
and they are used in several applications, especially in various
industries. The theory of reluctance motor is slightly different from
the conventional motoring theory. That is why they are treated as
special motor. In this chapter we are going to discuss about the
construction and working principle of the reluctance motor.
CONSTRUCTION OF RELUCTANCE MOTOR
Reluctance motor is actually a split phase induction motor with salient poles. The motor
essentially consists of a stator and a rotor. In general stator of a single phase reluctance
motor is similar to that of any one of the single-phase induction motors. Except for the
permanent split capacitor (PSC) motor, all motors (1phase IM) are provided with a
starting switch. The stator have two different windings – the main (having more number
of turns) and auxiliary winding. This helps in developing a synchronously rotating
magnetic field. The starting switch mentioned earlier is connected in series with the
auxiliary winding. The rotor of a reluctance motor is made up with soft magnetic
material and is almost similar to that of a squirrel cage motor except some rotor teeth
are removed at appropriate places to produce salient poles. If we remove teeth from four
locations then we produce a 4 pole salient pole structure. Each slot of rotor is provided
with rotor bar conductors made of aluminum or copper which are short circuited by a
pair of end rings as in case of squirrel cage induction motor.
OPERATING PRINCIPLE OF RELUCTANCE
MOTOR
When the stator of the reluctance motor is supplied with a single phase ac supply the motor
starts as an induction motor (single phase). The starting / centrifugal switch disconnects the
axillary winding of the motor at a speed of about 75% of synchronous speed. Now the motor
operates with its main winding in operation. Gradually it accelerates and attains speed very
close to synchronous speed. When the reluctance motor starts to run at a speed close to
synchronous speed, a reluctance torque is produced. The rotor aligns itself in minimum
reluctance position. The rotor pulls into synchronism. After pulling into synchronism, the
induction torque disappears but the rotor remains in synchronism due to synchronous
reluctance torque alone. The motor adjusts its torque angle for change in load as in 3-ph
synchronous motor. If load is excessive motor may not pull into synchronism and if already
running it may pull out of synchronism.
SPEED TORQUE CHARACTERISTICS OF
RELUCTANCE MOTOR
The starting torque of the reluctance motor is dependent upon position of rotor because the
rotor is a salient pole one. The value of starting torque lies between 300 to 400 percent of its
full load torque. The motor operates at a constant speed up to a little over 200% of its full load
torque. We have already discussed that at about 75 percent of the synchronous speed, a
centrifugal switch disconnects the auxiliary winding and motor runs with main winding only.
So when it attains speed close to synchronous speed, the reluctance torque is produced that
pulls rotor into synchronism and rotor rotates at synchronous speed. The motor may face some
problem in proper synchronism when loading is increased beyond the value of pull out torque.
Reluctance motors are subjected to cogging at the time of starting. Cogging is the locking
tendency of the rotor which can be minimized by skewing the rotor bars and not making the
number of rotor slots equal to an exact multiple of the number of poles.
TYPES OF RELUCTANCE MOTORS
SWITCHED RELUCTANCE MOTOR.
VARIABLE RELUCTANCE MOTOR.
SWITCHED RELUCTANCE MOTOR
CONSTRUCTION
• The stator and rotor of a Switched Reluctance motor have salient poles. This doubly
salient arrangement is very effective for electromagnetic energy conversion.
• In the stator part carries coils on each pole, the coils on opposite poles being connected
in series. The rotor does not have magnets or coils attached with the rotor slots. It is a
solid salient-pole rotor made of soft magnetic material with a laminated-steel.
• The six stator coils shown in Figure are grouped to form three phases which are
independently energized from a three-phase converter. Therefore cheap to manufacture
and extremely robust.
WORKING PRINCIPLE
• Usual arrangement is to energize stator coils sequentially with a single pulse of
current at high speed. However, at starting and low speed, a current-chopper type
control is used to limit the coil current.
• The motor rotates in the anticlockwise direction when the stator phases are
energized in the sequence 1, 2, 3 and in clockwise direction when energized in the
sequence 1, 3, 2. When the stator coils are energized, the nearest pair of rotor poles
is pulled into alignment with the appropriate stator poles by reluctance torque.
Closed-loop control is essential to optimize the switching angles of the
applied coil voltages. The stator phases are switched by signals derived
from a shaft-mounted rotor position detectors such as Phototransistor
sensors. This causes the behavior of the Switched Reluctance motor to
resemble that of a dc motor. The closed loop control of a switched
reluctance motor Phototransistor sensors is based on the photoelectric
principle.
ADVANTAGES OF SWITCHED RELUCTANCE
MOTOR
Construction of the motor is simple.
Brushes, commentator’s , permanent magnets are absent
Starting torque is quite good
Accurate speed control is possible
Cost effective and easy maintenance.
Higher efficiency
More power per unit weight and volume
Has no windings or slip rings in the rotor
Can run at very high speed (upto 30,000 rpm) in hazardous atmospheres.
Four-quadrant operation is possible with appropriate drive circuitry
DISADVANTAGE OF SWITCHED RELUCTANCE
MOTOR
Noisy in the operation
This type of motor not well-suited for smooth torque production.
Flux linkage and Non-linear function of stator currents as well as
rotor position control of the motors a tough challenge.
VARIABLE RELUCTANCE MOTOR.
• An electronic driven variable reluctance motor resembles a brushless DC
motor without a permanent magnet rotor. This makes the motor simple and
inexpensive. However, this is offset by the cost of the electronic control,
which is not nearly as simple as that for a brushless DC motor. Electronic
control makes it practical to drive the motor well above and below the power
line frequency. A variable reluctance motor driven by a servo, an electronic
feedback system, controls torque and speed, minimizing ripple torque
This is the opposite of the high ripple torque desired in stepper motors. Rather than
a stepper, a variable reluctance motor is optimized for continuous high speed
rotation with minimum ripple torque. It is necessary to measure the rotor position
with a rotary position sensor like an optical or magnetic encoder, or derive this from
monitoring the stator back EMF.
ADVANTAGES
Simple construction- no brushes, commutator, or permanent magnets, no
Cu or Al in the rotor.
High efficiency and reliability compared to conventional AC or DC
motors.
High starting torque.
Cost effective compared to brushless DC motor in high volumes.
Adaptable to very high ambient temperature.
Low cost accurate speed control possible if volume is high enough.
DISADVANTAGES
Current versus torque is highly nonlinear
Phase switching must be precise to minimize ripple torque
Phase current must be controlled to minimize ripple torque
Acoustic and electrical noise
Not applicable to low volumes due to complex control
issues
APPLICATIONS
WASHING MACHINES
WEAVING MACHINERY.
CENTRIFUGAL PUMPS, COMPRESSORS, DOOR
OPENERS
ANALOG ELECTRONIC METERS.
MICROCONTROLLER BASED OPERATION CONTROL
CIRCUITS
