Universal Motor - Construction, Working & Application

     Universal Motor is a special type motor because either it can be run on a.c. or d.c. supply. The main advantage of this type of motor is they can be developed in ratings of some fractional kilowatt. Hence they are used where there is a need of small motors with low ratings like electric shavers, hair driers etc.

Construction :

     Modifications are done to the d.c. series motor to work as an a.c. series motor. Similarly small ratings ( less than 1 kW ) of such motors are known as universal motors. This motors modified again so that it can operate on both a.c. and d.c. supply.

     Generally fields of this type of motors produces low flux density. This motors can run upto the speed of around 10,000 rpm. As this speed ratings are vary high, speed control methods are adopted to control the speed of the motor. 

     The field winding of a universal motor consists of less number of turns in it which provides low reluctance magnetic path with laminated armature and field circuits. Mostly the construction is similar to the construction of a.c. or d.c. series motor.  

Working :

     The working principle of universal motor is same as that of a.c. series motor.

Types of Universal Motor :

      Generally, universal motors are manufactured in two types 

1.  Non compensated type (concentrated field), low power rating.  

2.  Compensated type (distributed field), high power rating.    

     The construction of non-compensated type of motor is same as normal d.c. motor. Generally it consists of 2 Poles with laminated magnetic path.

      In compensated type of motor it consists of two compensating winding and main field winding, has same split phase induction motor. The armature is of wound type. The characteristics of this type of motors are better than the non-compensated type of motor, but cost of this motor are more.

Characteristic :

     Compensated type universal motor has better speed torque characteristics. Very high torque at low speeds can, therefore, be achieved. No-load speeds of small universal motors may be much higher than their full-load speeds.

Universal Motor

     Full load speed of universal motor may be as high as 10.000 r.p.m When such high speeds are not required. gear trains are used to reduce the speed. No load speeds of small universal motors may be much higher than their full load speeds 

    Universal motors are easily recognized because they use a commutator and brushes.

Speed Control of Universal Motor :

     The following methods are used for speed control purposes,

1. Resistance Method

2. Tapped Field Method and

3. Governor Mechanism Method 

1. Resistance Method :

     By varying the resistance 'R' connected in series with the motor, the speed can be varied. It is used for sewing machines, the variation of resistance is affected by means of a foot-pedal.

Universal Motor

2. Tapped Field Method :

     In this method tappings are made on the field winding of the motor. Speed can be controlled by changing the tappings of the winding so that flux produced by the winding is changed, hence it changes speed of the motor.  

Universal Motor

3. Governer Mechanism ( Centrifugal Mechanism ):

     We can see in home mixers or grinders that a switch is provided on the motor to control the speed at different levels this can be done by mechanism of governer.

      If the motor speed rises above that set by the lever, the centrifugal device opens two contacts and inserts resistance 'R 'in the circuit to reduce the current and so reduce the speed. When motor runs slow, the two contacts close and short circuit the resistance so that the motor speed increases. This process is repeated so rapidly until the variations in speed occurrences.

     A capacitor 'C' is used to reduce the sparking produced due to the opening and closing of the governer contacts.

Universal Motor

Applications :

     The more common applications of universal motors are the following 

1. Vacuum cleaners

2. Electric shavers

3. Portable drives

4. Mixers or grinders

5. Hair dryers etc.

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