What is a DC Motor and Its Working Principle?

A Direct Current motor, a DC motor, is an electrical instrument that is operated by direct current and transforms electrical energy into mechanical energy. In this article, components, working principles, types, and applications of DC motors have been discussed in detail.

Components of a DC Motor

1. Rotor:

• The rotor of a DC Motor is a cylinder of magnetic laminations insulated with each other.
• It is vertical to the axis of the cylinder.
• It is detached from the field coil by an air gap and rolled on its axis.

2. Stator:

• The field coil of a DC motor is a static portion where winding is wound to generate a magnetic field.
• An electromagnet holds a cylindrical cavity in the middle of the poles.

3. Commutator and Brushes:

• The commutator is a cylindrical configuration in a DC Motor. It is composed of copper fractions that are piled together.
• Brushes are carbon and graphite configurations.
• Brushes carry electric current from the outer field to the moving commutator.
• The commutator and brush unit transfer power from the stationary electrical circuit to the moving rotor.

Working Principle of a DC Motor

The operation of a direct current (DC) motor is that if a current-carrying conductor is set up in a magnetic area, the conductor faces a mechanical force. The direction of force can be found from Fleming’s left-hand rule, and the magnitude of this force can be mentioned as:

F= B I L Newton

Where B is a magnetic field, I is a current and L is the length of the conductor.

If thumb, forefinger and middle finger of the left-hand are vertical with each other followed by Fleming’s Left Hand Rule, thumb will move to the magnetic force, forefinger will be to the magnetic field and middle finger will be into the flow of current.

A magnetic field is generated through the air gap if the DC motor is stimulated. The magnetic field is generated through the direction of the radii of the armature. The magnetic field penetrates into the armature from the north pole of the field coil and leaves from the South Pole.

The conductors on the other pole experience a force with the equal strength in the reverse direction. These two forces generate a torque that causes the motor armature for rotation. When the flow of current in the wire becomes opposite, the direction of rotation also becomes opposite. If magnetic field and electric field interact with each other, they generate a mechanical force that causes the rotation of the armature.

Types of DC Motor

Self- Excited DC Motor

In a self-excited DC motor, field winding is linked through the series or parallel connection with the armature winding.  It can be of three types, such as:

i) Shunt Wound DC Motor
ii) Series Wound DC Motor
iii) Compound Wound DC Motor

Shunt Wound DC Motor

The field winding is linked through the parallel connection with the armature in a shunt wound motor.  Below is the figure of a shunt wound DC motor:

Series Wound DC Motor

In a series wound DC motor, the field winding is coupled through the series connection with the armature winding in a series wound DC motor. The figure is the following:

Compound Wound DC Motor

Compound DC motor has both shunt and series field winding. The compound motor is two types, such as:

a) Cumulative Compound Motor: If the magnetic flux generated by both windings in the same direction, then it is called as cumulative compound motor

b) Differential Compound Motor: If the flux generated by the series field windings is opposite to the flux generated by the shunt field winding, then it is called as a differential compound motor.

2) Separately Excited DC Motor

In a separately excited DC motor, the field coils are energized from an outside origin of the DC supply. The figure is shown below:

Characteristics

Torque, speed and efficiency are the important parameters to determine the performance of a DC motor.

Back EMF

Back EMF is an important parameter for the characteristics of DC motor. If a conductor is set up in a varying magnetic field based on Faraday’s Law of electromagnetic induction, it generates a current in the conductor.

So, when the motor begins rotating, the armature cancels the magnetic field of the stator and generates an EMF. This is known as a counter EMF or a back EMF. The direction of this induced EMF is provided by Lenz’s Law and it resists the armature current.

If Back EMF is expressed by E_b, then

E_b  = Φ. N.Z.P / 60.A

Φ is the magnetic flux per pole

N is the speed of the armature in RPM

Z is the total conductors in the armature

P is the number of poles in the motor

A is the number of parallel paths through the armature

Torque

Torque is the twisting force from the motor that is measured in Newton-meter (Nm). The torque developed from a DC motor can be expressed from the following equation:

T = 0.159 * (Φ. N. Z. P / A) * I_a

Where,

I_a is armature current and unit is Nm

Φ is the magnetic flux per pole

N is the speed of the armature in RPM

Z is the number of total conductors in the armature

P is the number of poles in the motor

A is the number of parallel paths through the armature

The total torque produced at the armature is not available at the shaft for executing the work. Some of the torque can be lost in frictional losses. The torque that is used for doing useful work is called shaft torque. Shaft torque can be expressed from the following equation:

T_SH = 9.55 * Output power / N

Where,

T_SH is the shaft torque and expressed in Nm

Output power in watts

Speed

Speed is known as how fast the armature of the DC motor spins in revolutions per minute. Speed relies on some parameters, such as input voltage, field current and armature resistance.

The speed of the DC motor can be found from the following equation:

N = K * E_b / Φ

Where,

N is the speed in rpm

K is the proportionality constant

Efficiency

Efficiency is defined from the ratio of the output power to the input power of a DC motor. The efficiency of a DC motor can be found from the following equation:

h  = P_out / P_in

Applications of DC Motors

DC series motor: Cranes Lifts and elevators, Power tools

DC shunt motor: Drills, Fans, Centrifugal pump

Compound DC motor: Compressors, Rolling mills, Presses

Permanent magnet DC motors: Toys, Starter motors, Wheelchairs

Brushless DC motor: Heating and ventilation, Cooling systems, Power tools

Conclusion

The article highlights the significance of DC motors in various applications, underscoring their advantages in precision speed control and high torque.

Assun Motor, recognizing these benefits, offers an extensive line of products tailored for precise motion engineering, including brushed, brushless, and servo motors.

These motors are designed for a wide range of applications, from home and industrial uses to robotics, demonstrating Assun Motor’s commitment to providing high-quality, versatile solutions for motion control needs.