Types of D.C Machine
The magnetic flux in a d.c machine is produced by field coils carrying current. The production of magnetic flux in the device by circulating current in the field winding is called excitation.
There are two types of excitation in D.C machine. Separate excitation, and self-excitation. In self-excitation, the current flowing through the field winding is supplied by the machine itself, and in separate excitation, the field coils are energized by a separate D.C. Source.
The principal types of D.C machine are:
1. Separately excited D.C. machine:
When a separate D.C. source is used to energize the field coils it is called as separately excited D.C. machine. The connections showing the separately excited D.C. Machines are given in the figure.
Figure: (a) Separately excited D.C. Generator, (b) Separately excited D.C. Motor.
2. Shunt wound D.C. Machine:
Shunt wound D.C. Machines is the machine in which field coils are connected in parallel with the armature. Since the shunt field receives the full output voltage of a generator or the supply voltage of a motor, it is generally made of a large number of turns of fine wire carrying a small field current.
3. Series wound D.C. Machine:
Series wound D.C. Machines is the machine in which the field coils are connected in series with the armature. The series field winding carries the armature current, and the armature current is large, that is why series field winding consists of few turns of wire of large cross-sectional area.
Figure: (a) D.C. series generator (b) D.C. series motor.
4. Compound wound D.C. machine:
A Compound machine is a machine which has both shunt and series fields. Two windings are carried out by each pole of the machine. The series winding has few turns of large cross-sectional area, and the shunt windings have many turns of fine wire.
It can be connected in two ways. If the shunt field is connected in parallel with the armature alone, the machine is called the short-shunt compound machine and if the shunt field in parallel with both the armature and series field, the machine is called the long-shunt compound machine.
Figure: (a) Short-shunt compound D.C. generator (b) Short-shunt compound D.C. motor.
Figure: (a) Long-shunt D.C. generator (b) Long-shunt D.C. motor.
EMF and Torque Equation:
A voltage is generated in the coils when the armature rotates. In case of a generator, the e.m.f. of rotation is called the generated e.m.f and Er = Eg.
In the case of the motor, the e.m.f. of rotation is known as back e.m.f., and Er = Eb. The expression for both the conditions of operation is same.
The clockwise current produces downward field, and anticlockwise current generates the upward field.
P = Number of poles.
Let us consider conductor in moving from position P to Q under the pole pitch τPand required to reach from P to Q in t.
Time taken to complete N revolution = 1 min = 60 sec.
Time is taken to complete one revolution =
During one revolution of the armature in a P pole generator, each armature conductor cuts the magnetic flux P times. So, the flux cut by one conductor in one revolution = фP
Average E.M.F induced in one conductor
E.M.F generated in each parallel path which is connected across a pair of carbon brushes.
E.M.F in each path
Mechanical power input = ωT .....................equation 1
On equating equation 1 and 2
The above equation is the torque equation for the D.C. machine.