Table of Contents
Introduction
A universal motor is a special type of motor which is designed to run on either DC or single
phase AC supply. These motors are generally series wound (armature and field winding are in series), and
hence produce high starting torque (See characteristics of DC motors here). That is why, universal
motors generally come built into the device they are meant to drive.
. Most of the universal motors are designed to operate at higher speeds, exceeding 3500 RPM. They run at lower speed on AC supply than they run on DC supply of same voltage, due to the reactance voltage drop which is present in AC and not in DC.
There are two basic types of universal motor
(i) Compensated type and
(ii)Uncompensated type
Compensated Type Universal Motor
A compensated type universal motor is a special kind of electric motor that can operate on either AC or DC supply. It is an enhanced version of the basic universal motor, designed to reduce sparking and improve performance, especially under AC operation.
Universal motors are known for their high speed, compact size, and high starting torque, making them ideal for use in portable tools, domestic appliances (like mixers, vacuum cleaners), and power tools.
In the compensated type, additional windings known as compensating windings are introduced. These are embedded in the stator and connected in series with the armature. Their main role is to counteract the reactance voltage (inductive effects) and reduce armature reaction, especially when running on AC supply. This leads to:
- Better commutation (less sparking at brushes)
- Improved efficiency and torque characteristics
- Enhanced durability
Two main types of compensation
Conductively Compensated Motor –
Compensating winding is placed in slots in the stator pole face.
Inductively Compensated Motor –
Uses a short-circuited winding placed in the pole face like a squirrel cage.
Uncompensated Type Universal Motor
An uncompensated universal motor is the most basic form of a universal motor—designed to operate on both AC and DC power supplies without any additional compensation for armature reaction or reactance voltage. Structurally and functionally, it’s similar to a DC series motor, with the field winding and armature connected in series, but built with laminated cores to minimize eddy current losses when running on AC.
These motors are known for their high-speed capability, lightweight construction, and strong starting torque, which makes them ideal for compact, handheld devices. However, when operated on AC, the absence of compensating windings leads to significant performance drawbacks. The alternating nature of the supply causes inductive reactance in the armature circuit, resulting in:
- Poor commutation ( increased sparking at the brushes)
- Lower efficiency due to higher iron and copper losses
- Audible noise and electromagnetic interference
Construction of Universal Motor
Construction of a universal motor is very similar to the construction of a DC machine It consists
of a stator on which field poles are mounted. Field coils are wound on the field poles.
However, the whole magnetic path (stator field circuit and also armature) is laminated. Lamination is
necessary to minimize the eddy currents which induce while operating on AC.
The rotary armature is of wound type having straight or skewed slots and commutator with brushes
resting on it. The commutation on AC is poorer than that for DC. because of the current induced in the
armature coils. For that reason brushes used are having high resistance.
Main Components:
- Stator (Field Winding):
The stator carries the field winding. In a universal motor, the field winding is laminated to reduce eddy current losses caused by alternating current. - Rotor (Armature):
The rotor is a laminated core with slots that hold the armature winding. It’s connected to a commutator, just like in a DC motor. - Commutator and Brushes:
The commutator ensures current direction in the armature winding is always correct, allowing unidirectional torque. Brushes (usually carbon) maintain contact with the commutator segments. - Fan:
A small fan is usually mounted on the shaft to provide cooling.
Working of Universal Motor
A universal motor works on either DC or single phase AC supply. When the universal motor is
fed with a DC supply, it works as a DC series motor. (see working of a DC series motor here). When
current flows in the field winding, it produces an electromagnetic field. The same current also flows from
the armature conductors. When a current carrying conductor is placed in an electromagnetic field, it
experiences a mechanical force. Due to this mechanical force, or torque, the rotor starts to rotate. The
direction of this force is given by Fleming’s left hand rule.
When fed with AC supply, it still produces unidirectional torque. Because, armature
winding and field winding are connected in series, they are in same phase. Hence, as polarity of AC
changes periodically, the direction of current in armature and field winding reverses at the same time.
Thus, direction of magnetic field and the direction of armature current reverses in such a way that the
direction of force experienced by armature conductors remains same. Thus, regardless of AC or DC
supply, universal motor works on the same principle that DC series motor works.
Speed/Load Characteristics
A speed/load characteristic of a universal motor is similar to that of DC series motor. The speed
of a universal motor is low at full load and very high at no load. Usually, gears trains are used to get the
required speed on required load. The speed/load characteristics are shown for both AC and DC supplies in the figure.
Speed-Torque Characteristics:
- The universal motor has high starting torque.
- It exhibits a steep speed-torque curve, meaning speed drops rapidly with an increase in load.
- At no-load, it can reach dangerously high speeds, which is why it’s usually operated under load.
Speed Control:
- Speed can be controlled by varying the supply voltage or using electronic speed controllers (common in drills or mixers).
Efficiency and Noise:
- Universal motors are less efficient than AC induction motors due to higher losses (especially at high speeds).
- They tend to be noisier, partly due to the commutator and brushes.
Applications of Universal Motor
- Universal motors find their use in various home appliances like vacuum cleaners, drink and food mixers, domestic sewing machine etc
- The higher rating universal motors are used in portable drills, blenders etc
- Portable power tools (drills, grinders, saws)
- Kitchen appliances (mixers, blenders)
- Hair dryers
- Vacuum cleaners
- Sewing machines
Advantages
Operates on both AC and DC.
High starting torque.
Compact and lightweight.
High-speed operation.
Disadvantages
- High maintenance due to brushes and commutator.
- Noisy operation.
- Shorter lifespan compared to induction motors.
- Not suitable for continuous operation under heavy load.
Conclusion
The universal motor is a versatile and powerful motor type, especially suited for applications demanding high speed and portability. Despite its drawbacks—like noise, wear and tear on brushes, and lower efficiency—its ability to operate on both AC and DC, along with its high torque and speed, make it ideal for many small appliance and tool applications.