What is the DC Motor and Generator?
An electrical machine which converts mechanical energy into electrical energy is called Electric Generator. Whereas an electrical machine that converts electrical energy into mechanical energy is called Electric Motor. Such electrical machines may be related to the electrical energy of the AC type or DC type. The DC type machines are classified as DC generators or DC motors. The construction of a DC machine remains the same whether it is a generator or motor.
Construction of DC Motor and DC Generator:
As stated, whether a DC type machine is a generator or motor, the construction of both remains the same. The DC machine consists of the following parts:
It serves the purpose of the outermost cover of the DC machine. So that the insulating material gets protected from the harmful atmospheric elements. It provides mechanical support to the poles. It forms a part of the magnetic circuit which provides the path for the low reluctance for magnetic flux as it is necessary to avoid wastage of power to provide the same magnetic flux. For the necessity of the low reluctance path, it must be made of some magnetic material that is made by using cast iron and it is used for smaller machines. For larger machines, it must be made up of rolled steel, cast steel & silicon steel is used which provides good mechanical strength and low reluctance path.
Each pole is divided into two parts named “Pole core” and “Pole shoe”.
Pole core basically carries a field winding which is necessary to produce magnetic flux. And it directs the flux produced through the air gap to armature core, to the next pole. Pole shoe enlarges the area of armature core to come across the flux, which is necessary to produce larger induced emf and it is given a particular shape to achieve this process. It is made up of magnetic material like cast iron or cast steel and It is made in the form laminations stamped together and bolted with the yoke because of achieving the particular shape.
. Field Winding:
It is wound on the pole core with a definite direction. To carry current due to which pole core, on which field winding is placed behaves as an electromagnet producing necessary flux. It has to carry current due to which the field coils must be made up of copper, as copper is a good conducting material and easy to bend.
It is further divided into two parts named “Armature core” & “Armature winding”.
Armature core is a cylindrical shape mounted on the shaft. It consists of the slots on its periphery and air ducts to permit the airflow through armature which serves the cooling purpose. It provides a house for the armature core. And it also provides the path of low reluctance to magnetic flux produced by the field winding.
Armature winding is nothing but the interconnection of the armature conductors placed in the slots provided on the periphery of the armature core. It generates emf in the case of generators. It carries current supplied in case of motors. Armature winding is of two types named as lap winding and wave winding.
We know that the emf induced in the armature windings is alternating. To convert it into the DC nature known as the rectification process, a device is needed in the case of DC generators called Commutator. It facilitates the collection of current from the armature winding. It produces unidirectional torque in the case of DC motors.
. Brushes and Brush Gear:
Brushes are stationary and resting on the surface of the commutator. They are used to collect the current from the commutator and make it available to the external stationary circuit.
Bearings are used usually as they are more reliable. For heavy-duty machines, roller bearings are preferred.
Principle of Operation of DC Generator:
All generators work on the principle of dynamically induced emf. This principle is nothing but Faraday’s law of electromagnetic induction which states that whenever the magnetic lines of forces change in the coils linking with the magnetic flux, an electromotive force is set up in those coils.
The change in flux in the coils is due to the relative motion between the coils and the flux and this can be achieved by either rotating the coils or the flux due to which induced emf gets generated as long as the relative motion withstands. The coils are rotated with the help of the device called Prime mover. The direction of the induced emf can be determined by the Fleming’s Right Hand Rule which states that if three fingers of the right hand named the thumb, the middle finger and the index finger are outstretched so that they are at the right angle to each other; the index finger will point the direction of lines of force, thumb in the direction of relative motion of conductor with respect to flux, then the index finger will point in the direction of induced emf.
EMF Equation of DC Generator:
P = Number of poles of the generator
Ф = Flux produced by each pole
N = Speed of armature in rpm
Z = Total number of armature conductors
A = number of parallel paths in which ‘Z’ are divided
A = P for lap type winding
A = 2 for wave type winding
Now, emf gets induced in the armature conductors according to the Faraday’s Law of Electromagnetic Induction which is written as
E = ФPNZ/60A
This is nothing but the emf equation of the DC generator.
Principle of Operation of DC Motor:
The principle of operation of the DC motor can be stated as to when a current-carrying conductor is placed in the magnetic field; it experiences a mechanical force.
The magnitude of the force experienced by the armature conductors in the motor is given by
F = BlI
B = Flux density produced due to the flux produced by the field winding
l = Active length of the armature conductors
I = Magnitude of the current passing through the armature conductors
The direction of the motion of this force is determined by Fleming’s Left-Hand Rule which states that outstretch your left-hand thumb, first finger, and middle finger so that they are right angle to each other and the first finger will point in the direction of the magnetic field, the middle finger in the direction of current, then the thumb will point the direction of the force experienced by the armature conductors.
Voltage Equation DC Motor:
In the case of DC motor, supply voltage V has to overcome the back emf Eb which is opposing V and also various drops as armature resistance IaRa and brush dop, etc. Hence, the voltage equation of the DC motor can be written as,
V = Eb + IaRa + brush drop
Neglecting the brush drop, the voltage equation can be written as
V = Eb + IaRa
And the armature current can be written as
Ia = (V – Eb)/Ra