What is an electric motor?

An electric motor can be described as an electrochemical machine that transforms electrical energy into mechanical energy. This mechanical energy is utilized to revolve the pump, fan, and impeller, operate compressors and lift bodies. The motors employed in industries operate on 70% electrical load.

Overview of electrical motor

As described earlier, an electric motor is a machine that transforms electrical energy into mechanical energy, and most of the work is done by the cooperation of the motor’s magnetic field and the current flowing in the wires of winding of the motor. Thus it generates a force in the form of torque provided to the shaft. DC sources such as batteries serve the purpose of power source for the motors, and AC generators can also serve the purpose of power source to them. There are many structural similarities between an electric motor and an electric generator; it is just the reverse flow of power that differentiates the two, such that, the electric generator transforms mechanical energy into valuable electrical energy.

Electric motors can be categorized on the basis of various factors, such as power source, construction, and the type of output movement they produce from electrical energy supplied to them.

• On the basis of the power source from which they get the necessary electrical energy, the electric motor can be categorized as AC motors and DC motors.
• On the basis of construction materials employed, electric motors can be classified as brushed or brushless motors.
• On the basis of the type of movement, that is, linear or rotational, they produce from the input electrical energy, electric motors are categorized as linear and rotational (torque giving) motors.

What are the important components of an electrical motor?

There are the following important components of an electric motor:

• Rotor
• Bearings
• Stator
• Air gap
• Winding
• Brush assembly
• Commutator

Rotor

A rotor is the most essential part of an electric motor. The rotor is the turntable component of an electric motor, which is attached to the shaft and turns the shaft by conveying the mechanical energy produced from the input electrical energy. Usually, the rotor consists of a conductor inside it, which conveys current and interacts with the magnetic field of the stator. In some instances, the rotor has a permanent magnet and stator with the conductor.

Bearings

Bearings are the essential components that are attached to the rotor, and it causes the rotor to turn on its own axis, which is further supported on the motor housing. The rotor is attached to the shaft that persists overhung outside the principal frame of the electric motor at which all the mechanical energy generated is shifted.

Stator

A stator is the fixed component of an electric motor that can contain windings in some states and a permanent magnet in other states. Fundamentally, the stator consists of a laminated core built up of thin metal sheets to decrease the losses that could have been there if a solid core was practiced.

Air gap

The air gap is the passage between the stator and rotor of an electric motor, which has a very significant role to play in estimating the performance of a motor. As the gap rises, the value of magnetizing current rises which acquires large losses. This is a necessary gap between the stator and rotor but it is retained minute otherwise it will have a disadvantageous impact on the efficiency of the electric motor.

Windings

Winding consists of wires that create the coil, and it winds throughout the soft iron magnetic core that creates the magnetic poles when the current moves into that coil.

Brush assembly

Brush assembly conducts electricity to the several sections of an electric motor by turning and keeping in contact with all brushes.

Commutator

The commutator assures that the current continuously moves in a particular direction within the rotor windings. The commutator is built up of slip discs that are insulated from each other and the shaft. The commutators are generally utilized for switching of input to DC motor. The commutator assists in providing unidirectional torque.

Working of an electrical motor

An electric current in a magnetic field will encounter a force. If the current-carrying wire is turned into a loop, which is at right angles to the magnetic field, it will encounter forces in the reverse direction. A couple of forces produce a rotating torque or moment to turn the coil.  The effective motors have various loops on an armature to give more consistent torque, and the magnetic field is generated by an electromagnetic system designated by the field coils.

Main classification of electrical motor

There are the following two main types of electric motor:

• Alternating current motors (AC motor)
• Direct current motors (DC motor)

AC motor

An electric motor that transforms the alternating current into mechanical energy by employing an electromagnetic induction aspect is termed an AC motor. Alternating current is used for actuating the AC motors. The two most essential components of the AC motors are the rotor and stator. The AC motors are further classified into two categories:

• Synchronous motor
• Induction motor

The induction motor is further classified into two categories:

• Single phase induction motor
• Three phase induction motor

DC motor

The DC motor can be described as the device that changes electrical power into mechanical power in the form of rotation. The movement of the DC motor is generated by the natural behavior of electromagnetism. DC motor has inductors inside that generate the magnetic field, which is utilized to produce the movement of the shaft. The DC motors are further classified into two categories:

• Separately excited motors
• Self excited motors

The self-excited motors are further classified into three categories:

• Series wound motors
• Shunt wound motors
• Compound wound motors

Applications of electric motor

• Electric motors are used to turn pumps and compressors.
• It is used to facilitate the circulation of air, refrigerants, water, and other fluids.
• It is used in microwave ovens.
• It is used in computers in CD players
• It is used in drills, saws, and forklifts.
• It is also used in electric cars.
• It is used in hairdryers and can openers.
• It is used in turbines, paper mills, rolling mills, and pumps.

Efficiency of electric motor

The efficiency of any electric motor is the parameter that determines how efficiently a motor can operate or give output. Efficiency is the factor that foretells the performance of any motor or how thoroughly the electrical power will be changed to mechanical power. It is always determined by taking the ratio of output power to input power. The efficiency of any motor can not be 100% because there is always some class of losses in the output of a device, such as iron losses, copper losses, and friction losses. Electric motors are typically more effective at higher voltage and less effective at a lower voltage.

The expression for the efficiency of the electric motor is as follows:

$\eta =\frac{P_m}{P_e}$

Here, P_m indicates the mechanical output power and P_e indicates the electrical input power.

The expression for the mechanical output power is as follows:

$P_m=\tau \omega$

Here, $\tau$ indicates the torque and $\omega$ indicates the angular velocity.

From the above equation, it is clear that the mechanical output power is equal to the product of the torque and angular velocity, and the value of the torque is directly proportional to the value of the power. It means more torque equals more mechanical output power.

The expression for the electrical input power is as follows:

$P_e=IV$

Here, I represents the electric current and V represents the electric voltage.

So, the electrical input power is equal to the product of the electrical current and electrical voltage. The value of the electrical voltage is directly proportional to the value of the electrical input power. It means more voltage is equal to more electrical input power.

Common Mistakes

Students get confused between the electric motor and the electric generator. An electric motor transforms electrical power into mechanical power, while an electric generator transforms mechanical power into electrical power.

Students get confused between the synchronous motor and induction motor. Synchronous motors are not self-starting motors, while induction motors are self-starting motors.

Context and Applications

The topic of the electrical motor is very much significant in the several professional exams and courses for undergraduate, diploma level, graduate, and postgraduate. For example:

• Bachelor Technology in Electrical Engineering
• Master of Technology in Electrical Engineering
• Bachelor of Technology in Mechanical Engineering

Related Concepts

• Electrical generator
• Electromagnetic induction
• Regenerative brake
• Motor capacitor
• Traction motor

Practice Problems

Q1: Among the four options, which of the following machine needs large beginning torque?

(a) Locomotive

(b) Hairdryer

(c) Milling machine

(d) Centrifugal pump

Correct option: (a)

Explanation: The torque is directly proportional to the power. Among the locomotive, hairdryer, milling machine, and centrifugal pump, the locomotive required more power to run. Therefore, the locomotive also needs large beginning torque.

Q2: Which DC motor is fancied for machine tools?

(a) Cumulative compound motor

(b) Series motor

(c) Shunt motor

(d) None of these

Correct option: (c)

Explanation: A direct current (DC) shunt motor is described as a constant speed motor, and the torque produced by the motor is a function of the current supplied. It is used in lathe machines, centrifugal pumps, and various machine tools.

Q3: Which DC motor will be fancied for conveyors?

(a) Differential compound motor

(b) Cumulative compound motor

(c) Series motor

(d) None of these

Correct option: (b)

Explanation: A direct current (DC) cumulative compound motor has high starting torque, and its speed can vary within certain limits. It is used in compressors, blowers, elevators, conveyors, etc.

Q4: A magnet may repel or attract by:

(a) Resistor

(b) Dynamo

(c) Electric current

(d) Another magnet

Correct option: (d)

Explanation: When two same poles of different magnets bring nearby, they repel each other. In contrast, when opposite poles of magnets bring nearby, they attract each other.

Q5: Many commonly used machines are powered by:

(a) Dynamos

(b) Sound energy

(c) Electric motors

(d) Thermal energy

Correct option: (c)

Explanation: An electric motor transforms the electrical form of energy into rotational energy. The electric motor is used in various common machines, like blowers, hairdryers, drilling machines, etc.