Before discussing the advantages and disadvantages of universal motor need to understand the basis of the universal motor.
An electric motor is a machine, which converts one form of energy (Electrical) to other forms of energy (mechanical). Electrical energy or voltage can be divide into two major types i.e. AC voltage and DC voltage. Usually, electric motors operate on one of these two electrical voltage supplies and also given the name of DC motor and AC motor depending upon its electrical input supply type. DC motors can only operate using DC supply and cannot operate using an AC voltage source. Similarly, AC motors only work properly when been connected with the AC voltage source and do not work with DC source.
There is a special type of electric motor known as the universal motor has the capability to operate at both AC and DC supply voltage type. Unlike DC motor, the universal motor does not have a permanent magnet Stator, but it has a series circuit of field winding and armature winding. Due to the series path, the same current flows through each winding causing the same kind of magnetic field in both windings. Interaction of those fields produces unidirectional torque irrespective of a supply voltage of either DC or AC.
Due to this property of operation at both types of voltage sources and compactness, the universal motor has many advantages as compared to AC and DC sources. However, as in each device and electric machine, there are many disadvantages also associated with the universal motors. Detail of the advantages and disadvantages of universal motor is being presented below.
Advantages of Universal Motor
- Unlike other motors, as the name suggests, the universal motor can be operated from both DC and AC voltage sources. On the other hand, DC motors can only operate properly when the input source is a DC source. Similarly, AC motors operate properly when the input source connected to it is an AC source.
- The universal motor is less costly as compared to other motors. Cost-effectiveness is possible due to its simple construction and no requirement of permanent magnets stator winding around the rotating rotor.
- Universal motors have the capability to provide good torque at low speed, which enables its use in many daily life handheld tools operated directly through AC main supply.
- Due to the series construction of field winding and armature winding, just like series DC motor, they can rotate at very high speed due to larger current ranging from 3.6K r.p.m. to 25K r.p.m. Which enables their use in applications requiring very higher speed rotations.
- Due to the small size and less weight of the universal motor, its use is preferred in many portable tools requiring compact size.
- Universal motors can have very high power ratings ranging from 5W to 500W. Due to a higher rating and compact size, the universal motor is been used in portable drill machines and blenders.
- Normal DC motors have eddy current losses due to the heating of magnetic components of the motor when AC is applied. Due to this eddy current, the overall efficiency of the motor decreases. However, in the case of universal motors, due to the use of electromagnetic field winding instead of permanent magnet and ability to run at both supplies, eddy current and loss of efficiency due to magnetic component heating is been avoided. Due to this property, they are been used in railway 15KV AC electrification traction motors.
- Universal motors with low input power ratings ranging from 5W to 10W are also available. These low input power universal motors are been used in portable applications like hairdryers, fans, electric shaver and motion projectors.
- The universal motor also has the capability to run at adjustable speeds, making it useful for many modern tools demanding speed adjustment. The speed of the universal motor can be adjusted by using a simple governor or by using modern PWM techniques. Due to this, the universal motor is used in house applications such as food mixers, vacuum cleaner, kitchen appliances, and sewing machines.
Disadvantages of Universal Motor
- Universal motors have relatively higher noise associated with them as compared to other AC and DC motors such as induction motors. The sound of noise also increases with the increase in the speed of the universal motor.
- Slight unbalancing can cause severe vibrations possibly damaging motor or surroundings. Therefore, careful balancing of the universal motor is required for the proper operation of the universal motors.
- Unlike DC motors, it is difficult to reverse the universal motor to run in the opposite direction.
- As construction of universal motor is similar to the series DC motor. So similar to series DC motor, at no load very high current will flow causing the motor to attain dangerously high speed. Therefore, the universal motor should have some load at starting in order to provide proper speed control. For this purpose, universal motors usually have a fan mounted on a rotor shaft to add some load to it, resulting in avidness of no-load condition preventing the runaway condition.
- Universal motors require frequent service and maintenance. As carbon brushes and commutators have limited life and they wear out with the time. They are responsible to provide a link of current to the armature winding. With time an intensive use, they start to wear out and finally a stage reaches when the connection between field winding and the armature is braked requiring replacement of brushes. In universal motors, due to intense usage of brushes and commutators, they wear out very quickly, requiring frequent maintenance.
- During wear out of commutators and brushes, sparking in the brushes is been observed which can cause electromagnetic television and radio interference. Due to these issues, the universal motor is been used in appliances, which do not have intense use and require high starting torque such as food mixers and other kitchen electric appliances.
- Another disadvantage of the universal motor is it’s overheating and usually require an environment with fresh air to avoid heating and cooling down. Few universal motors are made with self-cooling fans, however, they are not that practically feasible due to increased power requirements, increased size and weight eliminating its key features of compactness, lightweight and low cost.
From the above discussion, it is clear that universal motor has many applications due to its compactness, cost-effectiveness and torque, and power ratings. However, it also has many limitations such as frequent maintenance requirements and overheating. So the selection of universal motor for any application can be made on its advantages and disadvantages.