DC Motor Working Principle and Significance of Back E.M.F

What is DC motor?

DC motor converts D.C. electrical energy into mechanical energy. The DC motor works on, or the principle of operation of DC motor is that when a current-carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. Fleming gives the direction of this force left-hand rule. DC motor working principle is almost the same as the DC generator. We will discuss the DC motor working principle and the significance of Back E.M.F, and others. So read this blog to get a better idea about the working principle of DC motors, and all about DC motors.

What is DC motor back E.M.F.?

When voltage is applied to the motor, it starts rotating, and an E.M.F. is induced in the armature conductor called a back E.M.F. is back E.M.F. is opposed to the applied voltage by Lenz’s law. Back E.M.F. is always less than the applied voltage when D.C. voltage is applied across the motor terminal. The field magnets are excited, and armature conductors are supplied with current there for driving torque to act on the armature. Who begins to rotate is the armature, and back E.M.F. is induced, which tries to oppose the applied voltage.

Significance of back emf in DC motor

Therefore energy conversion in a D.C. motor is only possible due to the production of back E.M.F. If the DC motor speed becomes high, the back E.M.F. will be increased, and hence the motor will draw less armature current and vice versa. The presence of back E.M.F. makes the D.C. motor a self-starting machine. For this reason, the significance of back EMF in DC motors is immeasurable. The significance of back emf in DC motor is given below:

  • If the motor is suddenly loaded, the first effect is because the armature slows down. Therefore the speed of the D.C. motor the armature conductor moves through the magnetic field is reduced, hence the back E.M.F. falls.
  • Suppose the load on the motor is increased. In that case, the driving torque is momentarily more than the requirement so that the armature is accelerated as the armature is speed increases. The back E.M.F. also increases and causes the armature current to decrease the motor will stop accelerating when the armature current is sufficient to produce the load’s reduced torque requirement.

What is the DC Motor Working Principle?

A DC motor consists of a stator, rotor, armature, and commutator with brushes. DC motors are the simplest type of engine and are widely used in household appliances, such as electric razors, shavers, and electric windows in cars. A DC motor is equipped with magnets, permanent magnets, or electromagnetic windings.

When current passes through the armature of the DC motor, also known as the coil of wire, which is placed between the north and south poles of the magnet, the armature’s field interacts with the magnetic field and applies torque. In a D.C. motor, the magnetic field forms the stator, the armature is placed on the rotor, and a commutator switches the current flow from one coil to another.

The commutator connects with the stationary power source to the armature through the use of brushes. Furthermore, DC motors operate at a constant speed for a constant voltage, and there is no slip.

Stator of dc machine

Every electric motor has two crucial parts one static and another rotating. The stationary part is the stator. Though configurations vary, the stator is often a permanent magnet or row of magnets lining the motor casing edge, which is usually a round plastic drum. The rotating part of the motor is called a rotor.

Rotor of DC machine

The rotor of the DC machine is inserted into the stator. Usually, the rotor consists of a copper wire wound into a coil around an axle. When an electric current flows through the ring, the resulting magnetic field pushes against the stator’s field and makes the axle spin.


A commutator essential component electric motor has another, which is placed at one end of the coil. The commutator is a metal ring separated into two halves. It reverses the electrical current in the loop each time the coil rotates half a turn. The commutator periodically reverses the current direction between the rotor and the external circuit. It ensures that the coils’ ends do not move in opposite directions and that the axle spins in one direction. The commutator is necessary because the spinning rotor gets its motion from magnetic attraction and repulsion between the rotor and the stator.

Why is a DC motor used?

D.C. motors are being used all around in our daily lives. They are used in many ways and take on many other forms. Examples from your home include air conditioners, refrigerators, and water heaters. At work, D.C. motors will likely be used in your office’s audiovisual projectors or inside the ATMs operated by banks.

The engines’ use extends well beyond these examples, encompassing home appliances, especially air ventilation systems, cars, and medical devices. D.C. motors are produced in vast volumes of noise and as a means of achieving energy efficiency, and they play a significant role in determining living conditions and the global environment.

What is the classification of a DC motor?

DC motor is an indispensable motor for industrial applications. DC motor converts electrical energy into mechanical energy based on its working principle. DC motors are widely used in a lot of applications. There are four significant types of D.C. motors. Such as

  • Series
  • Shunt
  • Compound
  • Permanent Magnet

In a permanent magnet DC motor, a permanent magnet coil is used to create field flux. Each of them has a unique torque characteristic or speed. DC motor is an important electrical machine and it is widely used in industrial applications due to its greater features. The D.C. motor may be subdivided into some major types based on the type of construction and electrical connection.

  • Permanent Magnet DC Motor
  • Shunt Motor
  • Series Motor
  • Compound Motor
  • Short Shunt Motor
  • Long Shunt Motor
  • Differentially Compound
  • Cumulative Compound

Permanent Magnet DC Motor

In these types of D.C. motors, the permanent magnet D.C. motor is used to create a magnetic field. A permanent magnet D.C. motor is used in automobile starters, wipers, air conditioners, etc.

D.C. Shunt Motor

Here, the D.C. Shunt motor field is connected with the armature windings in parallel or a shunt motor. The shunt field may be separately excited from the armature windings. It may be used for greater speed regulation and may also offer very simplified reverse control.

D.C. Series Motor

Here, the D.C. Series motor has a large wire carrying the full armature current that winds the field with few turns. This kind of engine generates a large amount of starting torque, but it cannot regulate the speed. If they run with no load, then it might be damaged.

D.C. Compound Motor

DC Compound Motor has a shunt field, which is separately excited. It has a good starting torque but might face problems in variable-speed applications.

Short Shunt DC Motor

Short Shunt DC Motor has the shunt field winding. It is only connected with the armature winding and connected in parallel. The temporary shunt coil field is connected in series that is entirely exposed to current before being split up into the armature.

Long Shunt DC Motor

Long Shunt DC Motor has the shunt field winding connected in parallel with both series field coil and armature, which are again associated with each other in series.

Differentially Compound DC Motor

Differentially compound D.C. motors produce flux due to the shunt field windings reducing the main series windings’ effect.

Cumulative Compound DC Motor

Cumulative Compound DC Motor produced flux by the shunt field windings enhances the main field flux effect produced by series winding.

What are the Features of brush full D.C. motor?

Brush-full DC motors are the most commonly used because they are easy to miniaturize and provide a sound control system of rotation and high efficiency. Brushless DC motors benefit from prolonged life, ease of maintenance, and low noise because they do away with the brushes and commutator, which are the downsides of brushed D.C. motors.

What are the advantages and disadvantages of D.C. motors?


  • No need for a drive circuit when the motor running at a constant speed
  • High-efficiency 
  • Able to operate at high speeds
  • Very high starting torque
  • Responsive and easy to control to use as speed and torque.


  • Motor life is shortened for brushes and a commutator.
  • The meetings make both electrical and acoustic noise.

What is the brushless motor characteristics?


  • Lack of brushes means long motor life.
  • High-efficiency design
  • Stability of speed control
  • Able to operate at high speeds
  • High startup torque


  • A drive circuit is required.

What is a DC series motor?

A large wire of the DC series motor is carrying the full armature current with few turns. This kind of engine generates a large number of starting torque, 

What is a compound motor?

The compound motor has a separately excited shunt field. They have a good starting torque but face problems in variable-speed applications.

What are the types of compound-excited D.C. motors?

The compound excited D.C. motor types are Short Shunt Motor, Long Shunt Motor, Differential Compound, and Cumulative Compound.

What are the types of self-excited D.C. motors?

There are three types of self-excited D.C. motors are given below

  • Shunt excited,
  • Compound excited
  • Series excited D.C. motor.

What happens when the speed of a D.C. motor increases?

When a D.C. motor increases, back emf increases, but there will be a fall in the line currents. Because speed is proportional to the back emf, the armature current will be equal to the load current if there is a series motor.

How do determine the direction of the motor?

Fleming’s left-hand rule determines the direction of rotation of the motor. Fleming’s law states that when the current-carrying capacity of the conductor is placed in the magnetic field. On the other side of the motor, the conductor is forcefully kept under a magnetic field, and there will be an induced current in the conductor.

What is the main difference between the A.C. motor and the D.C. motor?

While both A.C. and D.C. motors perform the same function of converting electrical energy into mechanical energy, they are powered, constructed, and controlled differently. The most fundamental difference is the power source. A.C. motors provide alternating current (A.C.) while D.C. motors provide direct current (D.C.), such as

  • Batteries supply D.C. power or an AC-to-DC power converter. D.C. field motors are made with brushes and a commutator.
  • A.C. induction motors do not use brushes.
  • The speed of a D.C. motor is controlled by varying the armature winding’s current. In contrast, an A.C. motor’s rate is controlled by varying the frequency, commonly done with an adjustable frequency drive control.

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