Principle of operation
Electric motors are the interface of electronic boards to almost every mechatronic and mechanical product including robots, drones, home appliances, etc.
A DC motor is an electrical machine devised to convert electrical energy into mechanical energy.
The main principle behind working a DC motor is the electromagnetic law according to which a current carrying conductor placed in magnetic field experiences a force, and the direction of the force is given by Fleming’s left-hand rule. In order to understand the basic principles, we need to understand the basic constructional features of a DC motor.
Parts of a DC Motor
Every DC motor has 6 parts. Axle, Rotor, Commutator, Field magnets, and Brushes.
The basic component of a DC motor is a current carrying armature which is connected to the supply end through the commutator segment and brushes. The armature is placed between two permanent magnets which produce a magnetic field.
The applied direct current converts the electrical energy into the mechanical energy because of the interaction of two magnetic fields. One field is produced by the permanent magnet and the other is produced by the electric current flowing through the armature winding. Because of the interaction of these two fields, the armature experiences a force which tends to rotate the rotor. This is how a DC motor Works.
Types of DC Motors
On the basis of the field excitation method, DC motors can be classified as the following.
- Separately excited
- Permanent Magnet
- Series Wound
- Shunt wound
- Compound Wound
- Long Shunt
- Short shunt
Classification of DC motors
1. Separately Excited DC Motors (SEDC)
In every DC motors, there is a stator that refers to the static part consisting of the field winding, and the rotor which is the moving part consisting of the armature winding.
In a Separately Excited DC Motor, separate power is given to the field and the armature windings.
Classification of DC motors
The field windings are energized by a separate DC source as the armature current does not flow through the field windings. So these coils are separately isolated from each other and this is the special feature of this motor.
There are different methods for controlling the speed of this kind of motor
- Field control method
- Field rheostat method
- Armature control method
- In order to achieve a speed control, the armature can be connected to variable DC source.
- The motor can run easily in reverse direction by reversing the polarity of the field supply.
- They are limited to amount of load they can drive
- Torque is limited to 150% of the rated torque
- Used as actuators.
- Used in trains.
- Used for automatic Traction Purpose.
2. Permanent Magnet DC Motors (PMDC)
In permanent Magnet DC Motors, the magnetic field is produced by the permanent magnet. It consists of an armature winding and stator but does not contain the field winding. In this kind of DC motor, the field flux is produced by a radially magnetized permanent magnet which is placed on the inner periphery of the stator core.
Permanent magnet DC Motor
The field poles of these motors are made up of permanent magnet DC motor as shown at the bottom.
Structure of PMDC
Magnets are mounted on the inner periphery of the stator which is in a cylindrical shape. These magnets are arranged in such a way that N-pole and S-pole of these magnets are arranged alternatively which is faced towards the armature.
- Their construction is easy and simple.
- Input power does not affect the efficiency of a PMDC Motor.
- For small kW rated applications, it is very cheaper and more cost-effective.
- Since there is no field coil, the space used for placing field coils can be saved hence the overall size of the motor get reduced.
- To reduce the armature reaction extra ampere-turn cannot be added
- The filed between the air gaps cannot be controlled externally.
- Due to the excessive armature during the different condition of the motor, there is a chance for the poles to get permanently demagnetized.
- Windshield Wiper
- Automobiles as a starter motor
- Wheel Chair
3. Shunt Wound DC Motor (SWDM)
In this motor, field winding is connected in parallel to the armature winding. So a full voltage is applied across field winding. It is made up of a large number of winding and having high resistance. So a very small amount of current will intake and start the motor.
Shunt Exited DC Motor
- Wide control Range
- Smooth running
- Low speed
- We can’t use the DC shunt motor for variable speed drives
- It requires commutator which leads to sparking at brushes
- Lathe Machines
- Centrifugal Pumps
- Boring mills
- Weaving Machines
4. Series Wound DC Motor (SWDM)
In this type of DC motor, field winding is connected internally in series to the armature winding. It consists of a stator housing the field winding and a rotor carrying the armature conductor.
Series wound field winding
The field winding of this DC motor has fewer turns and relatively thicker dimensions which provides the minimum resistance to the full armature current.
- Used in applications where high torque is required
- Draws less current and power from the source
- Provides lowest starting current for the given torque
- Speed regulation is very poor.
- As speed decreases, torque also decreases.
- It always requires being loaded before starting the motor.
- Trolley cars
5. Compound Wound DC Motor (CWDM)
Compound Wound DC motor comes under the category of self-excited DC motors. In this motor, there are two sets of the field winding. One is connected in series with the armature winding and other is connected in parallel to the armature winding.
It is basically a combination of Shunt Wound DC Motor and Series Wound DC Motor which provides better properties of both these type of motors.
Compound DC Motor can be again divided as a Long shunt and short shunt.
Compound DC motor
Long shunt: Field winding connected in parallel across the combination of both the armature and series field coil.
Long shunt compound wound DC Motor
Short Shunt: Shunt field winding connected in parallel across the armature winding only.
Short Shunt Compound Wound DC Motor
- Respond Better to heavy loads
- High Torque and speed
- High Maintenance
- Not suitable for cleaning environment
- Air compressor