Types of Motors - 3

Reluctance Motor-An Overview

A reluctance motor works on the principle of reduced reluctance.  Reluctance motors have high power density.  However, their downsides are low efficiency, low torque and low pullout torque.  However, they are ideal for small power applications such as hard disk drive motor, Analog electric meters, etc.

The reluctance motor works on the principle that a piece of iron placed in a magnetic field will rearrange itself such that the reluctance of the magnetic path is minimal.

The Reluctance motor consists of a wound stator.  The rotor is made of laminated material in which poles are cut so that a salient pole rotor is produced.  The number of rotor poles is made less than the number of stator poles.

When a three phase supply is connected to the stator, a rotating magnetic field is set up. The rotor tries to align itself in a minimum reluctance path with reference to the magnetic field of the stator.  As the stator magnetic field keeps rotating, the rotor moves along with it.

Reluctance Motors are classified into

Switched Reluctance Motors and

Synchronous Reluctance Motors

Fractional Horse Power motors

Fractional HP Motors are motors whose power rating is less than one horse power i.e. 746 watts.  Fractional Motors range from an output of 1/20th horsepower to 1 horse power.  Motors less than 1/20th horse power are called sub-fractional horsepower motors. 

Fractional motors find wide application in automobiles for rolling up windows, windshield wipers, etc.  Induction motors, synchronous motors and dc motors can be used as Fractional HP motors. 

Fractional HP Motors also find wide application in household appliances.   Fractional Horse Power motors used in household application such as exhaust fans, blowers etc are usually single phase.  They are generally of the split phase or the capacitor run type. 

Extremely low speeds can be obtained using Fractional HP motors by means of suitable drives.

Stepper motors and servo motors are also types of Fractional HP motors.  Fractional HP motors are also available as geared motors.

Frameless Motors

Frameless motors as the name suggests, are motors which do not have a frame.  The stator and the rotor are delivered separately and are assembled to the load.  Thus, the motor does not have bearings.  The rotor is directly coupled to the load while the stator is coupled to the frame of the machine.  These motors are custom designed to meet the speed requirement.

Thus this arrangement eliminates the need for the coupling.  This increases the stiffness of the power train of the machine.  It also eliminates torsional application.  The size of the motor is also considerably reduced to around one seventh. This reduced weight for a given power output reduces inertial in the machine and enables quick movements and direction reversals. 

A hand tool with Frameless Motor
Frameless motors are widely used in Hand tools, Medical devices, in satellite technology and in semiconductor equipment.

Vibration motor used in cellphones

Vibration motors are motors which deliberately generate vibration.  These motors are used in mobile phones to create vibration alerting the user to a call or a message.

They are also used widely in the industries such as in construction industry to vibrate the concrete so that air pockets are not formed which the concrete is solidifying.  They are also used in mixers to prevent material from being left behind after the mixing process is over.

Vibration motors are also used in flour mills, pharmaceutical, food industries, etc to facilitate the smooth flow of materials in conveyors, hoppers and other mixing equipment. 

These motors are constructed just like normal motors.  However, they have a mechanically unbalanced weights attached to the output shaft.  These unbalanced weights create vibration.  The frequency and the magnitude of the vibration can be changed by modifying the shape and weight of the counterweights.

Split Phase Motors

A single phase induction motor is not self starting.  This is because the ac supply creates a pulsating magnetic field in the stator core and not a rotating magnetic field which is required for an induction motor to be self starting.

Many methods are used to start a single phase motor.  One such method is the split phase method.  The split phase motor has a main winding in the stator.  In addition to this, it has a starting winding wound in the stator.  The starting winding is connected parallel to the main winding.  When the supply to the windings.  The starting winding is displaced 90 degrees from the main winding.

This is because the starting winding has a higher resistance and occupies less space in the stator.  It is wound over a few slots in the stator and is usually placed above the main winding in the slots. The main winding has many turns and thus has a higher reactance.  Thus it lags behind the starting winding. 

Split phase motors are used where low to moderate starting torque is sufficient.  They are designed up to 1/3 hp. 

The starting winding is only used when the motor is starting.  The winding is connected through a centrifugal switch.  When sufficient rpm is reached, the centrifugal switch operates and isolates the winding.