DC Motor is a type of electric motor that converts electrical energy to mechanical energy. This post will discuss in detail about DC Motor, classification of DC Motors, its working principle, applications, advantages and disadvantages.
What is DC Motor
DC Motor is a motor which converts DC power into rotary movement and so is the name Direct Current (DC) Motor. It works on the principle of energy conversion i.e. Electrical energy to Mechanical energy. Application of Voltage produces Torque which results in Motion.
Fig. 1 – Introduction to DC Motor
Internal view of this type of Motor is shown in the Fig. 2 below. It consists of four main components. They are:
The outside part of a motor is called as Stator. It is composed of two or more permanent magnet pole pieces.
The inner part of the motor which rotates is called as Rotor. It is composed of Armature Winding which is connected to the external circuit through Commutator. Similar to Stator, Rotor is also made up of Ferromagnetic materials.
Fig. 2 – Internal View of DC Motor
Winding consists of series or parallel connection of coils made of copper. They are of two types:
- Armature Winding
- Field Winding
Several coils are connected in series to form Armature Winding in which Voltage is induced.
Field Winding is a group of coils which produces main field Flux when current is passed through the Winding.
A Commutator acts as a switch that reverses the direction of current between the rotor and external circuit.
How DC Motor Works
DC Motor works on the principle that a current carrying conductor when placed in a magnetic field, the conductor experiences magnetic field. This direction of force is given by Fleming’s left hand rule, that when electric current passes through a coil in a magnetic field, this magnetic force produces a torque which is used to drive this type of Motor.
Fig. 3 – (a) Working Principle of DC Motor (b) Fleming’s Left Hand Rule
This is a multi-polar DC Motor as shown in the Fig. 3. Here the terminals of the motor are connected to an external DC supply.Thus the Armature conductors carry current and the Field Magnets are excited and develop alternating north and south poles. Here the the conductors in north pole carry current in one direction and the conductors in south pole carry current in opposite direction. A mechanical force acts on the current carrying armature conductor as it is placed in a magnetic field.
These forces add together to produce a driving Torque which results in Armature’s rotation. The conductor moves from one side of a Brush to the other, so that the current in that conductor is reversed but the direction of the force on the conductor remains the same. Thus by reversing current in each conductor when it passes from one pole to another, it develops a continuous and unidirectional Torque.
Classification of DC Motor
DC Machines are classified based on the electrical connections of the Armature Winding and the Field Winding. Different types of machines or motors are produced with different types of connections. There are three types of DC Motors. They are:
- Permanent Magnet DC Motors
- Separately Excited DC Motors
- Self Excited DC Motors
Permanent Magnet DC Motors
In Permanent Magnet Motor, the Armature Winding are placed in a magnetic field to produce field flux and it doesn’t contain the Field Winding. This type of Motor uses a Permanent Magnet to create the magnetic field required for its operation. The circuit diagram of Permanent Magnet Motor is shown in the Fig. 4, where:
Fig. 4 – Permanent magnet Motor
Separately Excited DC Motors
In this type of Motor, Field Winding is excited by a separate DC source. The Armature Winding and Field Winding are electrically separate from each other as shown in the Fig. 5
Fig. 5 – Separately Excited DC Motor
Self Excited DC Motors
In this type of machines, the Field Winding is connected across the main voltage terminals instead of a separate voltage source. Self Excited Motors are of three types. They are:
- Shunt Wound DC Motor
- Series Wound DC Motor
- Compound Wound DC Motor
Shunt Wound DC Motor
In a shunt wound Motor, the Field Winding is connected in parallel to the armature winding and is directly exposed to the entire terminal voltage. It is represented diagrammatically as shown in the Fig. 6.
Fig. 6 – Shunt Wound DC Motor
Series Wound DC Motor
In a Series Wound Motor the Armature current is connected in series with the field winding. The Series wound Motor has high torque and can work fine with high and low power drives and variable and fixed speed electric drives. Due to its simple construction and working mechanism it is used in applications such as electric traction, electric footing, cranes, lifts, air compressors etc. It is represented diagrammatically as shown in the Fig. 7.
Fig. 7 – Series Wound DC Motor
Compound Wound DC Motor
The Compound Wound Motor is a combination of both Series and Shunt type Motors and it has Field Windings connected both in series and in parallel to the Armature Winding. This type of Motor can be connected in two ways i.e.
- Long Shunt
- Short Shunt
In Long Shunt Compound Motor, the Shunt Field Winding is parallel with both Armature and Series Field Winding as shown in the Fig. 8 (a) where,
We know that, in a Shunt Motor, the Total Current (IL) is the sum of Armature Current and Shunt Current.
Likewise, in Series Motor Armature current and Series Current are same.
Therefore, the current equation of a Long Shunt Compound DC motor is:
In Short Shunt Compound DC Motor, the Shunt Field Winding is connected in parallel with the Armature Winding only as shown in the Fig. 8 (b) where,
In Series Motor, the Total Current (IL) and Series current are same.
In a Shunt Motor, the Total Current (IL) is the sum of Armature Current and Shunt Current.
Therefore, the Voltage equation of Short Shunt Compound DC Motor is:
Fig. 8 – Compound DC Motor
Applications of DC Motors
The applications include:
- DC Motors are used in weaving machines and spinning machines.
- Shunt Wound Motors are used in lathe machines and centrifuges.
- They are also used in lifts, fans and blowers.
- Compound Excited Motors are used in presses, electric shovels, conveyors and Rolling Mills.
- Permanent Magnet Motors are widely used in automobiles in the functioning of windshield wipers and washers, in blowers for Air conditioners and heaters and to operate windows.
- They are used in computer or laptop drives.
- They are also a part of cranes and compressors.
- They are used in food mixers, vacuum cleaners and electric toothbrushes and toys.
Advantages of DC Motors
DC Motors have their own significant advantages over the AC motors and some of them are listed below:
- It allows a wide range of controlling the speed of the rated speed in DC Shunt type motor by using the Armature control method. This technique is used in paper mills where fine speed applications are used.
- DC Series Motors have high starting torque which is used for driving heavy loads in starting conditions such as electric trains and cranes.
- It can maintain a constant Motor Shaft Torque over a given speed range.
- It can start, stop, reverse or accelerate quickly.
- It has lesser reactive power consumption than AC Motors.
Disadvantages of DC motors
There are also some disadvantages which are listed below:
- The initial set up cost of this Motor is higher than the AC Motors.
- The Commutator and the Brush gear present in this Motor increase the operational and maintenance cost.
- They cannot be employed in an explosive environment as there is always a risk of spark from the Brush gear.