If AC single phase supply is connected to a DC series motor, the motor gives circular rotation. During one-half cycle of current, it would pass through series winding as well as armature winding in a certain direction. Its magnitude, of course, would vary from zero to maximum and then back to zero. Electromagnetic torque will be produced throughout this half cycle. This torque will act to produce rotation of the armature in the same direction throughout all of the half cycles.
Now during the next half cycle, the current through both the series winding as well as armature winding is reversed. Since current in both armature and field have been reversed, the torque developed in this half cycle will tend to produce rotation of armature in the same direction as throughout the preceding half cycle. It means the direction of the armature is a circular rotation type.
Torque in DC series motor, T α φsla
where φs is the series field winding flux and la is the armature current.
Also, T α (-φs)(-la) for lower half cycle.
Or T α φsla
So a DC series motor, if connected to AC single phase supply produces a torque in same direction.
Construction of Universal Motor
The construction of universal motor is very similar to a two pole DC series motor. It consists two field coils wound on laminated core connected in series with armature. The iron structure of field, yoke and armature are laminated to reduce the eddy current losses. Such a motor is so designed that it can be operated either on AC or DC supply. For this reason, it is called a universal motor.
Working of Universal Motor
Since the field winding and the armature winding, both are connected is series, the same current passes through them when the motor is connected to either AC or DC supply.
The magnetic fluxes of series field and armature produced by this current react with each other and, hence produce rotation.
Because the series field magnetic flux and armature current reverse at the same time, the torque always acts in the same direction.
Since universal motors are series wound, they have high starting torque. They run at dangerously high speed at no load. That is why they are always directly coupled to the device they derive.
Characteristics of Universal Motor
- Typical speed-torque characteristics of the universal motor have been shown in the figure. The torque varies as the square of current and speed varies inversely as the current approximately.
- Universal motors have high speed and large starting torque. They can, therefore, be used to derive high-speed vacuum cleaners, drills and machine tools etc.
- The efficiency will not be as good as that of corresponding DC machine because of greater eddy current loss and effects of p.f.