Construction and Working Principle of DC Motor

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A DC motor is an electromechanical energy conversion device, which converts electrical energy input into the mechanical energy output.

The operation of the DC motor is based on the principle that when a current carrying conductor is placed in a magnetic field, a mechanical force acts on the conductor.

The magnitude of this force is given by,

$$\mathrm{F\:=\:BIl\:Newtons}$$

Where,

• $\mathit{B}$ is the magnetic flux density,
• $\mathit{I}$ is the current flowing in the conductor or coil, and
• $\mathit{l}$ is length of the conductor.

The direction of this is given by the Fleming's left hand rule.

Construction of a DC Motor

Here is the schematic diagram of a DC Motor

A DC motor consists of six main parts, which are as follows

Yoke

The outer frame of a DC motor is a hollow cylinder made up of cast steel or rolled steel is known as yoke. The yoke serves following two purposes

• It supports the field pole core and acts as a protecting cover to the machine.
• It provides a path for the magnetic flux produced by the field winding.

Magnetic Field System

The magnetic field system of a DC motor is the stationary part of the machine. It produces the main magnetic flux in the motor. It consists of an even number of pole cores bolted to the yoke and field winding wound around the pole core. The field system of DC motor has salient poles i.e. the poles project inwards and each pole core has a pole shoe having a curved surface. The pole shoe serves two purposes

• It provides support to the field coils.
• It reduces the reluctance of magnetic circuit by increasing the cross-sectional area of it.

The pole cores are made of thin laminations of sheet steel which are insulated from each other to reduce the eddy current loss. The field coils are connected in series with one another such that when the current flows through the coils, alternate north and south poles are produced.

Armature Core

The armature core of DC motor is mounted on the shaft and rotates between the field poles. It has slots on its outer surface and the armature conductors are put in these slots. The armature core is a made up of soft steel laminations which are insulated from each other and tightly clamped together. In small machines, the laminations are keyed directly to the shaft, whereas in large machines, they are mounted on a spider. The laminated armature core is used to reduce the eddy current loss.

Armature Winding

The insulated conductors are put into the slots of the armature core. The conductors are suitably connected. This connected arrangement of conductors is known as armature winding. There are two types of armature windings are used – wave winding and lap winding.

Commutator

A commutator is a mechanical rectifier which converts the direct current input to the motor from the DC source into alternating current in the armature winding. The commutator is made of wedge-shaped copper segments insulated from each other and from the shaft by mica sheets. Each segment of commutator is connected to the ends of the armature coils.

Brushes

The brushes are mounted on the commutator and are used to inject the current from the DC source into the armature windings. The brushes are made of carbon and is supported by a metal box called brush holder. The pressure exerted by the brushes on the commutator is adjusted and maintained at constant value by means of springs. The current flows from the external DC source to the armature winding through the carbon brushes and commutator.

Working Principle of DC Motor

In order to understand the working principle of dc motor, consider a two pole DC motor as shown in Figure-1.

When terminals of this DC motor are connected to an external source of DC supply, the following two phenomenon happen inside the machine −

• The field electromagnets are excited developing alternate N and S poles.
• The armature conductors carry electric currents. Where, conductors under N-pole carry currents in one direction (say inside of the plane of the paper), while conductors under S-pole carry currents in the opposite direction (say outward of the plane of the paper).

Since, in this case, each conductor is carrying a current and is placed in a magnetic field. Due to the interaction between the current and magnetic field, a mechanical force acts on the conductor.

By applying Flemings left hand rule, it is clear that the mechanical force on each conductor is tending to move the conductor in the anticlockwise direction. The mechanical forces on all the conductors add together to produce a driving torque that sets the armature rotating.

When the conductor moves from one pole side to the other, the current in that conductor is reversed due to commutation action, and at the same time, it comes under the influence of the next pole of opposite polarity. As a result, the direction of the force on the conductor remains the same. In this way, the armature of a DC motor rotates continuously in one direction.

Armature Torque of DC Motor

The armature of the dc motor rotates about its axis. Thus, the mechanical force acting on the armature is known as armature torque. It is defined as the turning moment of a force acting on the armature conductors, and is given by,

$$\mathrm{\mathit{\tau _{a}}/conductor\:=\:\mathit{F\times r}}$$

Where, F is the force on each conductor and r is the average radius of the armature.

If Z is the number of conductors in the armature, then the total armature torque is given by,

$$\mathrm{\therefore \mathit{\tau _{a}}\:=\:\mathit{ZF\times r}\:=\:\mathit{ZBIL\times r}}$$

Since,

$$\mathrm{\mathit{B}\:=\:\frac{\mathit{\phi }}{\mathit{a}};\:\mathit{I\:=\:\frac{I_{a}}{A}};\mathit{a\:=\:\frac{\mathrm{2}\pi rl}{P}}}$$

Where, $\phi$ is flux per pole,$\mathit{I_{a}}$ is armature current,l is the effective length of each armature conductor, A is the number of parallel paths, and P is the number of poles. Then,

$$\mathrm{\mathit{\tau _{a}}\:=\:\frac{\mathit{Z\phi I_{a}}P}{\mathrm{2}\pi A}}$$

Since for a given dc motor, Z, P and A are fixed.

$$\mathrm{\therefore \mathit{\tau _{a}}\propto \mathit{\phi I_{a}}}$$

Hence, the torque in a DC motor is directly proportional to flux per pole and armature current.