Modern machinery has an important role in the usage of an electric motor. Because every machine requires the operation of a motor. Most of the equipment in today’s life has the usage of a motor. An Electric motor is a machine that converts electrical energy to mechanical energy. There are two types of electric motors. They are the AC motor and the DC motor. It uses a battery to operate, it carries the unidirectional current from the battery and operates to convert the EE to ME. In this article, we shall discuss what is motor, construction, working, types, speed control, and applications.
What is DC Motor?
Definition: It is a device that is used for the conversion of mechanical energy to electrical energy. It uses DC supply for its operation.
Construction of DC Motor
The Frame is used as a mechanical support for the parts to be housed. It is also used as a protection device for the inner parts against environmental hazards. The assembled parts of the motor are shown in the figure below.
The armature core consists of armature winding, commutator, and brushes. The core is formed by the collection of steel laminated sheets. These sheets are laminated to reduce eddy current losses. The laminations are housed together to form a core. The armature consists of slots on which armature winding is enclosed. The armature winding is connected to the commutator segments and then to the brush.
The commutator segments are connected to the armature winding used for the collection of current from it. It is also used for the conversion of the current direction. The commutator segments are insulated with mica in order to avoid circulating currents with the shaft.
The brushes are made of either carbon or graphite depending on the size of the motor. The brushes are used to collect the current from the commutator.
Bearings are used for the smoother operation of the rotor. A fan is used for cooling the inner peripherals of the motor. It removes the excess heat that is generated in the motor. A shaft is used to deliver the output as mechanical energy. The whole armature is assembled on the shaft.
It has its armature winding wound on the armature core while the field winding is wound on the stator. The armature is the rotating part of the machine and the stator is the stationary part of the machine.
The small motor uses permanent magnets whereas in the case of large type electromagnets are used. Permanent magnets lose their magnetism during the wear and tear. The electromagnets are those which get energize when current is passed through the winding mounted on the core.
It works on the principle of the Lorentz force equation. It states that whenever a current-carrying conductor is placed in a magnetic field, it experiences a force. It also works on the principle of faraday’s law of electromagnetic induction. Because an EMF is induced in the coil when this coil is rotated in the magnetic field.
A DC supply is given to the armature winding such that the winding carries current. Due to the current in the winding, the coil is exerted by some force. This force is according to the Lorentz force equation, as the current-carrying coil is under the influence of the magnetic field.
The typical working diagram of the machine is shown in the figure below.
The coil in the figure is marked on one side as AB and the other side as the CD. The current in the side AB is inwards and the current in the side CD is outwards. As this coil is under the magnetic lines of forces, it experiences some force. This force direction is found by using Fleming’s Left-hand rule.
Fleming’s Left-hand rule states that the three fingers forefinger, middle finger, and the thumb are pointed perpendicularly. The thumb shows the force direction, forefinger gives the field direction and the middle finger should be pointed along the current direction. Fleming’s Left-hand rule explanation is shown in the figure below.
By applying fleming left hand-rule in the coil side AB we can observe that the force is acting upwards and in the side CD it is acting downwards. So, it creates a motion for the coil to rotate in the clockwise direction. Now, as the coil rotates, the coil changes its side I,e the side AB is under N pole influence and the side CD is under the S pole influence. Now the current in coil side AB is outwards and in coil side CD it is outwards. But, the force acts in the clockwise direction. This is achieved by using the commutator segments. Without commutator segments there would unidirectional current in the coil of the motor net force acting would be zero and no torque would be developed.
Force acting when the coil is perpendicular to the magnetic field is shown in the figure below. At this position, the force in the coil side AB will act downwards.
As the coil is rotated the coil side also changes their positions which are shown in the figure below. At this position, the force exerted on the coil side AB is upwards.
So, we can understand that when the coil is parallel to the field the net force will be opposite to that of the position of the coil when it is perpendicular. To avoid this, multiple coils are used such that a unidirectional torque is developed which allows the rotor to rotate continuously.
There are different types of these machines based on the excitation and the placement of winding. They are Separately excited and self-excited motors.
Based on the winding placement classified as follows
- Series Motor
- Shunt Motor
- Compound Motor
The classification of these motors is shown in the figure below.
Seperately Excited DC Motors :
It requires a DC supply for its operation. DC supply is given to the rotor winding to exert a force on the conductors and a DC supply is also given to the stator winding to energize the field winding. So, it requires two separate DC sources for performing the operation. This process is known as separate excitation. But, these motors are not used mostly.
Self-Excited Motor :
The self-excited motor requires only one DC source which is used for both the stator as well as the rotor winding. These are the motors that are most commonly used for their advantage.
DC Series Motor :
In the DC series motor, the field winding is connected in series with the armature winding. The resistance of the field winding is kept low such that high current flows through the winding. For this purpose, the stator winding is wound thicker with less number of turns. It produces high starting torque and decreases gradually.
DC Shunt Motor :
In the DC shunt motor, the field winding is connected in parallel with the armature winding. For this purpose, the stator winding is wound thinner with more number of turns. It produces low starting torque and maintains constant speed characteristics.
DC Compound Motor :
In the DC compound motor, the field winding is connected in series with the armature winding and also in parallel with the armature winding. This motor uses the advantages of both the series as well as the shunt motor. For this purpose, the stator winding is wound thicker with less number of turns and also wound thinner with more number of turns.
The figure which shows the DC shunt, series and compound motor is shown in the figure below.
Speed Control of DC Motors
Speed control of these DC motors is achieved by the following methods.
By varying the armature resistance, flux, and terminal voltage.
The speed of these motors is denoted as
N = (Vt – IaRa)/Ф
- By decreasing the terminal voltage, the overall speed of the motor can be decreased.
- By increasing the flux, speed can be controlled, and
- By increasing the resistance of armature also, we can decrease the speed of the motor.
The detailed information of the speed control of DC motors is explained in the next article.
- Used in shavers, fans, blowers.
Thus, in this article, we had an overview of what is a motor. It is a machine that changes the form of the input mechanical energy to electrical energy with an input DC supply. Apart from this, we had also discussed the construction, working, types, speed control, applications, advantages, and disadvantages. Here is a question for the readers, what are the advantages and disadvantages of DC motors?