A universal motor is a modified form of a DC series motor and is widely used in home appliances like mixers and machine tools. In this article, I am discussing the universal motor working principle.

The construction of universal motor is very similar to a two-pole DC series motor. It consists of two field coils wound on a laminated core connected in series with an armature. The iron structure of the 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.

Universal Motor Working Principle

 If an AC single-phase supply is connected to a DC series motor, the motor gives circular rotation. During a 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 the 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 φ_sl_a

 Where φ_s is the series field winding flux and l_a is the armature current.

 Also, T α (-φ_s)(-l_a) for the lower half cycle.

 Or T α φ_sl_a

 So a DC series motor, if connected to AC single-phase supply produces a torque in the same direction.

Since the field winding and the armature winding, are connected in series, the same current passes through them when the motor is connected to either AC or DC supply.

 The magnetic fluxes of the 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 speeds at no load. That is why they are always directly coupled to the device they derive.

Characteristics of Universal Motor

Speed-torque characteristics of the universal motor are similar to that of the DC series motor. The motor runs at very high speed at no-load and the speed decreases on loading the motor. Usually, gears trains are used to adjust the speed according to the load.

• Typical speed-torque characteristics of the universal motor have been shown in the figure. The torque varies as the square of the 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, machine tools, etc.

• The efficiency will not be as good as that of the corresponding DC machine because of greater eddy current loss and effects of p.f.

Applications of Universal Motor

Universal motors are used for high-speed and high-torque applications. Some most common applications of universal motors are as follows:

• They are used in kitchen appliances like mixers and grinders.
• They are used in household appliances like vacuum cleaners.
• They are used in portable machine tools such as drill machines, electric hammers, and cutters.

 Advantages of Universal Motor 

• The universal motor may be used with both DC and AC power sources, unlike other motors, as its name suggests. 
• The universal motor is more affordable and cost-effective due to its simple design. 
• Since they are a modified version of DC Series Motors, they can run at a very high speed ranging from 3.6K rpm to 25K rpm. This makes them suitable to be used in applications that call for very high RPMs.

• The compact size and ability to deliver strong torque of the universal motor make it ideal for use in portable drill machines.

Disadvantages of Universal Motor 

• Eddy current losses and hysteresis losses decrease the efficiency of the motor.
• The power factor of the motor is poor.
• At the brushes, there is excessive sparking.
• They produce more noise as compared to other motors. Moreover, with an increase in the motor’s speed, noise levels increase likewise rise.
• Even a small unbalance of the armature may cause strong vibrations that could be harmful to the motor or the surrounding area. Therefore, careful balancing of the rotor is necessary for its proper performance.

Thanks for reading about the working principle of universal motor.