OTHER TYPES OF MOTORS

OTHER TYPES OF MOTORS

Two other types of motors- reluctance motors and hysteresis motors-are used in

certain special-purpose applications. These motors differ in rotor construction

from the ones previously described, but use the same stator design. Like induction

motors, they can be built with either single- or three-phase stators.

Reluctance Motors

A reluctance motor is a motor which depends on reluctance torque for its operation.

Reluctance torque is the torque induced in an iron object (such as a pin) in

the presence of an external magnetic field, which causes the object to line up with

the external magnetic field. This torque occurs because the external field induces

an internal magnetic field in the iron of the object, and a torque appears between

the two fields, twisting the object around to line up with the external field. In order

for a reluctance torque to be produced in an object, it must be elongated along

axes at angles corresponding to the angles between adjacent poles of the external

magnetic field.

A simple schematic of a two-pole reluctance motor is shown in Figure

1 . It can be shown that the torque applied to the rotor of this motor is proportional

to sin 20, where Ɵ is the electrical angle between the rotor and the stator

magnetic field s. Therefore, the reluctance torque of a motor is maximum when the

angle between the rotor and the stator magnetic fields is 45 °.

A simple reluctance motor of the sort shown in Figure 2 is a synchronous

motor, since the rotor will be locked into the stator magnetic fields as long

as the pullout torque of the motor is not exceeded. Like a normal synchronous motor, it has no starting torque and will not start by it self.

A self-starting reluctance motor that will operate at synchronous speed

until its maximum reluctance torque is exceeded can be built by modifying the

rotor of an induction motor as shown in Figure 2. In this figure, the rotor has

salient poles for steady-state operation as a reluctance motor and also has cage or

amortisseur windings for starting. The stator of such a motor may be either of

single- or three-phase construction. The torque- speed characteristic of this motor,

which is sometimes called a synchronous induction motor, is shown in Figure 3.

An interesting variation on the idea of the reluctance motor is the Syncrospeed

motor, which is manufactured in the United States by MagneTek, Inc.

The rotor of this motor is shown in Figure 4. It uses "flux guides" to increase

the coupling between adjacent pole faces and therefore to increase the maximum reluctance torque of the motor. With these flux guides, the maximum-reluctance

torque is increased to about 150 percent of the rated torque, as compared to just

over 100 percent of the rated torque for a conventional reluctance motor.

Hysteresis Motors

Another special-purpose motor employs the phenomenon of hysteresis to produce

a mechanical torque. The rotor of a hysteresis motor is a smooth cylinder of magnetic material with no teeth, protrusions, or windings. The stator of the motor can be either single- or three-phase; but if it is single-phase, a permanent capacitor should be used with an auxiliary winding to provide as smooth a magnetic field as

possible, since this greatly reduces the losses of the motor.