Introduction to electric motor
Let’s understand the Classification of electric motor, DC motor, Construction of DC motor. Working of DC Motor, Advantages of DC motor, Disadvantages of DC motor.
What is an electric motor?
An electric motor converts electrical energy into mechanical energy, which people then use for mechanical work. So, an Electric motor is a converting device that is useful for mechanical work.
Ex. A famous example of work done using an electric motor is in our home, which pulls water from a tank. Whenever we hear the word motor, this is the first example that comes to mind. It is a general-purpose application where the input supply given is electricity, and pulling water from the tank is mechanical work that will be done using an electric motor.
In short, an electric motor is a machine that acts as a medium that converts one form of energy to another, which is used to perform the required mechanical work.
Classification of electric motor
Motors are classified into two types according to the type of input supply used.
- The motor which has an input supply is known as an AC motor,
- Whereas the motor having a DC supply is known as a DC motor
These are mainly the primary classification of motors. Obviously, these are not only ways to classify the motors but also the primary causes of classification.
Apart from this, each category is then subdivided into different types,
DC motors are two types: one is a brushless DC motor, which means there are no brushes placed at the rotor, which provides commutation action, and another one is a brushed DC motor, which means brushes are present on the rotor to provide commutation. Brushed DC motors are further divided into four types:
- Series wound
- Shunt-wound
- Compound wound
- Permanent magnet DC motor
Similarly, AC motors are also divided into subcategories, like synchronous motors, which have this name because they always run at synchronous speed. In detail, synchronous speed is the speed at which flux links with the rotor. In synchronous motors, the case speed of the stator and rotor is the same, which means it synchronises with the stator’s magnetic field.
The other type is an induction motor, which signifies that the speed of the rotor is less than that of the stator. There will always be a difference between the stator and rotor speeds, and they cannot be synchronised. Further, this induction motor is classified into two types according to the number of phases used to provide supply: single-phase and three-phase.
DC motor
A DC motor is a machine that converts electric energy to mechanical energy in the form of rotation, using a DC supply system as a primary electrical input.
Construction of DC motor
DC motors contain different parts, mainly the stator, known as the field coil, and the rotor, known as the armature, armature core, commutator, and brushes. These are the main parts of DC motor construction, and some supplementing constructions are yoke, pole, and pole shoe.
We will discuss it part by part.
Yoke
The yoke is the outer frame of a DC motor that holds all the internal parts inside a hollow cylinder made of cast steel or rolled steel.
The yoke serves two purposes:
It acts as a protective cover for the machine and also supports a field pole core.
It also provides a closed-loop structure for magnetic flux produced by field winding, providing a path for magnetic flux to link and flow.
Pole core
Pole is the structure around which field winding is wounded and acts as an electromagnet whenever field winding is energised.
Cast steel or cast iron are the materials used for the construction of pole cores. Annealed steel laminations can also reduce power drops due to eddy currents.
Pole shoe
The pole shoe works as an extensive part of a pole, which increases the area of the pole core. It provides an enlarged area to the pole, causing an increase in flux linkage due to the spread-out area. and then it reduces the air gap flux and provides a more concentric and symmetric path for magnetic flux to link. It is mainly made of cast iron; otherwise, it is cast steel.
Field winding
Winding is the main reason for producing a magnetic field, and it is called field winding. The field winding is wounded on the pole core, and when supply is given, it produces an electromagnetic field with the help of the pole; it will be explained in the later part of the blog.
Armature core:
The material used to construct the Armature core has high permeability and low reluctance materials like cast iron or cast steel.
The armature core consists of slots where the armature coil is placed. Generally, the armature core and winding are rotating parts of the machine. The armature core provides a low-reluctance path to the flux generated by the field winding.
Armature winding
Armature winding, or armature coil is formed by interconnecting the number of armature conductors together.
When the prime mover is connected to the armature, it induces voltage and current in it. Copper is the material used to make the Armature winding.
Commutator
The primary function of the commutator is to collect the current from the armature conductor as well as supply the current to the load using brushes,
Also, the commutator provides unidirectional torque for the DC motor even if the supply to the motor is DC, but the magnetic field produced is of an alternating nature. The commutator acts as a rectifier, converting this alternating current to DC, and hence, it helps in producing unidirectional torque.
Brushes
The simple working function of Brushes is to collect current from the commutator segment and supply it to the load connected. Another function is to stop the wear off of the commutator while collecting the current. As brushes are made of carbon and graphite, they can frequently wear off and can be easily replaced rather than a commutator. Also, they are less costly.
Working of DC Motor
The DC motor is used to convert electric energy in the form of Direct current into mechanical work.
It works on the principle of Lorentz law, which states that whenever a current-carrying conductor is placed in a magnetic field, it experiences a force.
The armature winding wound on the armature core is supplied through a DC source, which causes current in the coil and makes it a current-carrying conductor.
Permanent magnets provide the magnetic field in small DC motors, but in large-scale DC motors, an electric coil wound on a field pole-type assembly produces an electromagnetic field. This assembly is also connected to the same DC supply.
So, whenever this current-carrying conductor of the armature is placed under the magnetic field created by field poles, it experiences a force. At least one of these arrangements should be rotating, so in the case of a DC motor, the current-carrying conductor of the armature is the rotating part.
Even if the supply is DC, it produces alternating flux in the coil. This is where the commutator comes into the picture, which converts this AC into DC, working as a rectifier and producing a unidirectional torque in the motor.
Advantages of DC electric motor
The advantages of DC motor are:
- Easy speed control
- High starting torque
- Simple construction
- High efficiency
- Linear speed torque characteristics
- Regenerative breaking
Disadvantages of DC electric motor
Disadvantages of DC motor are:
- Brushes wear off quickly and cause high maintenance
- The sparking caused by brushes can generate electromagnetic interference
- DC motor has limited speed control range
- It provides low reliability at high speed
- Requires external speed control device
