Working principle of DC motor is very simple but we need to know about DC motor and its parts first. We will then discuss the working principle of DC motor in detail.
What is DC Motor?
DC motors as the name indicates runs on DC. These are used where the speed control is needed. Its rated power can be in watts or mega-watts. For variable speed it is preferred as its cost is lower than other motors and its controlling is simple. We can see DC motors in toys which consume a few watts while rolling mills require such dc motors which is rated at several mega-watts. These motors are used in textile mills, excavators, printing presses. robotics etc. Today, AC motors are widely used but speed and torque control is still the ground of DC motors. The reason behind this is good speed regulation and frequent starting.
Parts of DC Motor:
Poles:
Poles are present on the stator. The field windings are placed on poles. So it serves as the home of magnetic field. In AC motors the speed of motor depends a lot on the number of poles but in DC motor poles have very little impact on the speed because there are other factors on which the speed depends more than the number of poles. To save the cost there are two of four poles even in the large motors as well. In two pole motors there will be two brushes while in four pole motors there will be four brushes.
Windings:
There are maximum two windings present in a dc motor. The two windings are,
- Field Winding
- Armature Winding
Field Winding:
Field winding produces the main magnetic flux which is necessary to run motor. It is placed on the poles of stator. Windings on the poles are connected in series with each other. Field Winding is connected with a DC source. When current passes through the coil, it produces magnetic field. Since the source is DC therefore the magnetic poles formed do not change with time. For instance, the North will remain North and the South will remain South.
The purpose of field winding is used to produce the magnetic flux. This can also be achieved by using permanent magnet. In small motors permanent magnet is used. The reason to avoid permanent magnets in large motors is that the magnets which give good performance are very expensive and difficult to manufacture while low performance Ferrite magnets are inexpensive and easy to manufacture which are suitable for small motors.
Armature Winding:
Armature winding consists of set of coils. This is the main source of power. These coils are present on the rotor slots. Since the rotor will revolve therefore there should be some mechanism to connect the external supply to rotor. For this purpose, commutator is placed on the rotor shaft. The coils of armature winding are connected to Commutator segments and these segments make contact with the carbon brushes. Carbon brushes are further connected to the external source.
What changes we have to make if we want to power a 12V motor with a 24V volt supply to run at the same speed?
To apply 24V source we need to double the number of turns of Armature winding. Now the windings will carry half of the current. It means at 12V, if the motor was drawing 2A current, now at 24V it will draw 1A only. So, it is better to reduce the cross-sectional area of the coil as well as it is carrying half of the current. There will be no effect at motor speed and input and output power of the motor.
Carbon Brushes:
The Carbon Brushes are connected with the external source. These are attached with the commutator segments. From here the commutator collects the current. These brushes are in sliding contact with the commutator so there should be no friction. These should be checked and replaced after certain time. There are two brushes in small motors but there can be more than two. If there are more than two brushes, then they are connected in parallel in such a way that half of them will serve as entering path and the other halves will serve as the leaving path of current. If there is any kind of dust or particles present between the brushes and commutator, then it can be the cause of sparks or flashovers. To ensure the proper contact between commutator and Brushes there are springs attached to the bushes.
Voltage drop occurs at the brushes which is unavoidable, if the motor operated at low voltage, then the higher fraction of the voltage drops at brushes. For instance, we have a 2V motor and 1V drops at brushes which means half of the total voltage is dropped at brushes. In contrast if we have a 6V motor then the fractional voltage drop will be much lower than the previous case. This is the reason DC motor are usually rated above 6V.
Split Ring Commutator:
The commutator plays an important role in dc motors. It collects the current coming from external source via Carbon Brushes. The commutator segments are connected to the coils of armature winding which are present at the slots of rotor. The Carbon Brushes are in contact with more than one segment at a time. At best both the pairs should cover all segments and none of the segment should be left. Due to this half of the coils will serve as entrance path of the current and half of them will serve as the leaving path. Due to commutator, the pattern of flow of current remains same. Consider the following picture,
The purpose of commutator is to provide path for the flow of current in armature winding even when the rotor is moving. The flow of direction of flow of current always remains the same. For instance, in the figure, current will always enter form segment connected to the left brush and it will leave from the segments connected to left brush until the direction of current in the Armature is changed. In figure half of the segments are connected to positive terminal and the other half are connected to negative terminal. If the rotor rotates in Clockwise direction, then the lower black segment will move towards the positive brush and the upper red segment will move towards the negative brush.
The commutator segments are insulated therefore; they are not in contact with each other. These insulation can bear a specific voltage. At very high voltages the insulation cost increases. So commonly we find motors rated below 700V.
Working Principle of DC Motor:
The working principle of DC motor is very simple. We know that a current carrying coil in magnetic field starts to rotate due to the interaction of current carrying conductor and radial magnetic flux.
In DC motor the magnetic field is either by a permanent magnet or produced by Field Winding. The current carrying conductor are the coils of Armature Winding.
In a permanent magnet DC motor there is no need of external supply to produce magnetic flux but in case of Field Winding motor, we provide DC voltage to the winding. The North and South poles are formed after applying the DC voltage.
After establishing the magnetic field out task is to direct the current from a source to Armature Winding. We give supply to the terminals which are connected with the Carbon Brushes. From the Carbon Brushes the current flows to the connected segments. The current enters in the Armature via segments and leaves from other segments. Now there is a current carrying coil in the magnetic field, therefore, the coil will start to rotate. The Torque produced will be directly proportional to the current in armature coil and the flux produced.
Here, I is the current in armature, ф is the magnetic flux produced by the field and KT is the motor constant.
Direction of Torque: Working Principle of DC Motor
The commutator is arranged in such a way that the current entering coils will be always under certain pole and the current leaving coils will be under other pole. This is necessary to rotate the rotor in particular direction. This is the reason due to which the four pole motors are provided with four brushes and two pole motors are provided with two brushes.
The direction of torque can be determined by Fleming’s Left Hand Rule. According to the rule if we point the direction of magnetic field (i.e. from North to South) with the forefinger of our left hand and point out the direction of current with the middle finger of our left hand, then our thumb will point the direction of force.
In the figure, the current entering coil (Red) under the North pole will feel the downward force and the current leaving coil (Black) under the South will experience upward force, this will rotate the rotor in Counter Clock Wise Direction.
How we can change the direction of torque in DC motors?
The direction of the torque can be reversed by reversing the magnetic field or reversing the flow of current in armature winding. It means we have only one control in permanent magnet motor i.e. to change the Armature Current.
If we reverse the poles (as compared to the last figure), then the direction of torque will be changed, which can be determined by Fleming’s Left Hand Rule.
In the figure, the current entering coil (Red) under the North pole will feel the downward force and the current leaving coil (Black) under the South will experience upward force, this will rotate the rotor in Clock Wise Direction.
Here, we have changed the direction by reversing the poles, the direction of torque can be changed by changing the direction of current in Armature Coils.
How we can control speed and torque in DC motors?
The speed can be controlled by controlling the magnitude of current in Armature of Field Winding or both. We have both controlling options in Field Winding motor but we can change the armature current only in permanent magnet motor.
Changes in Torque:
It is desirable to achieve a constant torque throughout the operation but in actual this is not possible. Let’s study the reason. Consider the following pictures,
In the first picture there are five coils under the magnetic field, while in the second picture there are four coils under the magnetic field. Due to this the torque will change. If we reduce the area which is not under the pole, the change in torque can be reduced but we cannot do this. The reason is that the coils show inductive behavior and in inductors the current change is not frequent, therefore the coils will require some time to change the phase, this time is their commutation time. Otherwise there will be sparks at commutator. The solution to the problem is to increase the number of slots. The increase in number of slots will increase the number of coils. This arrangement will greatly reduce the changes in torque as the rotor will be identical at every position.
A Wrong Concept:
You may have heard that the Commutator converts AC to DC but this is a wrong concept. To convert AC to DC there are Power-Electronic derives which converts AC to DC. The purpose of commutator is to ensure the direction of current i.e. if the current enter from the left segments then it will be same while the rotor is rotating. The pattern of flow of current remains same as discussed under the heading of commutator.
