3. Reluctance motor:
Reluctance motor operates on the principle that when a magnetic material is placed in the magnetic field, a force of magnetic pull exerts on the magnetic material tending to bring the piece of magnetic material in the dynest portion of the magnetic field. Thus magnetic material align itself at the place where the reluctance is minimum.
STEPPER MOTOR:
It is an electromechanical device which actuates a train of step angular (or linear) movements in response to a train of input pulses on one to one basis i.e. one step actuation for each pulse input.
Depending upon the construction, there are 2 types of stepper motor.
(i) Variable reluctance Motor:
A 3 φ motor with 6 stator teeth and 4 rotor tooth are shown here. The 3 phases are excited in succession.
At the time when first phase is excited, the magnetic flux axis is shown. When 2nd phase is excited, the magnetic flux axis shifts and rotor rotates. Hence by changing the magnetic flux axis, rotor rotates in steps.
(ii) Permanent magnet stepper motor:
A four phase permanent magnet motor is shown here. The rotor is a cylindrical permanent magnet and coils are wound on stator tooth
When phases are excited in sequence, rotor rotates clockwise. Since it is a 4-phase motor, the step angle is 45°. For small step angles, no. of magnetic poles and stator tooth are increased.
Universal Motor:
They can be operated either on ac or dc supply at approximately the same speed and output. The armature is of the same type as ordinary series motor. These are of 2 types :
1. Concentrated-pole. non-compensated type (Low power rating).
2. Distributed field. compensated type (high power rating).
The power factor is improved by reducing the inductance of field winding and hence no. of turns. It decreases the magnetic native force in air-flux gap for a given current. The armature turns are increased to get the same torque but the reactance of motor increases. It is neutralized by using compensating winding in which current is proportion to armature current but 180° out of phase.
The voltage induced by transformer action in the coils during its commutation does not produce serious commutation problem High resistance brushes are used to aid the commutation.
These motors are used in sewing machines, vacuum cleaners, kitchen appliances, drills hair dryers etc.
The speed torque characteristics for dc and ac input supply is shown below :
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