Single Phase Commutator Motor
2. Universal Motor
There are small capacity series motors which can be operated on d.c. supply or single phase alternating supply of same voltage with same characteristics, called universal motors. The general construction of such motor is similar to that of a.c. series motor as discussed in last article. It is manufactured in two types.
i) Non compensated, low h.p ii) Compensated type, high h.p.
Non compensated type pole has 2 poles, having entire magnetic path as laminated. Armature is wound type similar to the normal d.c. motor. Such non compensated construction is shown in the Fig. 1.
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| Fig. 1 Cross-section of non compensated universal motor |
While in compensated type, the motor has distributed field winding consisting of main field and compensating winding. This is some what similar to the stator of split phase single phase induction motor type construction. This also has a wound armature similar to the normal d.c. motor. Fig.2 shows the connection diagrams for both the types of universal motor.
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| Fig. 2 Connection diagrams for a universal motor |
Speed torque characteristics : The speed - torque characteristics for both the types of universal motor are shown in the Fig.3.
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| Fig. 3 Speed-torque characteristic of universal motor |
Compensated type universal motor has better speed - torque characteristics i.e. the characteristics are same for the operation of motor on a.c. or d.c. supply. The motors are generally designed for full load operation speeds ranging between 3000 to 20000 r.p.m.
Applications : Though compensated type characteristics are better, the non compensated type are more preferred for low h.p. applications. While compensated type of universal motors are preferred for h.p. applications. High starting torque is the important feature of universal motors.
The universal motors are used for domestic applications like vacuum cleaners, food processor and mixers, hair driers, coffee grinders, electric shavers etc. Their other applications are blowers, portable tools like drilling machines and small drivers.
2.1 Reversal of Rotation of Universal Motors
By reversing the flow of current through the armature or field windings, the direction of rotation can be reversed for salient pole non compensated type universal motor. This is possible by interchanging the terminals on brush holders as shown in the Fig. 4.
In case of compensated type, the armature or field loads are interchanged and brushes are shifted against the direction of rotation of motor, to achieve the reversal of direction.
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| Fig. 4 Reversal of rotation of universal motors |
2.2 Speed Control of Universal Motors
The speed control of universal motor is achieved by three methods, as shown in the Fig.5.
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| Fig. 5 Speed control of universal motor |