Apr 27 • Engineering Notes • 3383 Views • 2 Comments on Classification of Self Excited DC Machines Motor or Generator
CLASSIFICATION OF SELF EXCITED DC MACHINE
“Energy can neither be created nor destroyed, but can be transformed from one form to the other” is the fundamental law of universe. A DC machine transforms electrical energy to mechanical(motor) or vice-versa (generator). The working principle in both the cases remains the same. The DC motor finds its applications in fields of engineering and technology, ranging from an electric shaver to parts of automobiles, in all small or medium sized motoring.
DC MOTOR CLASSIFICATION
1. SEPARATELY EXCITED DC MOTOR
In this case the supply is given separately to the field and armature winding. The main distinguishing fact in these types of dc motor is that, the armature current does not flow through the field winding, as the field winding is excited from an external source of dc current as shown in the figure alongside.
2. PERMANENT MAGNET DC MOTOR
It has the usual armature winding like the other motors, but the field winding are not necessarily present. In this radially magnetized permanent magnets are mounted on the inner periphery of the stator core to produce the required field flux.The rotor however has a conventional dc armature with commutator segments and brushes. The diagrammatic representation of a permanent magnet dc motor is given along.
3. SELF-EXCITED DC MOTOR
In this the shunt winding is connected partly or completely in series or parallel, to the armature winding and so it can thus be subdivided as:
a. SHUNT WOUND SELF EXCITED DC MOTOR
In this case , the field winding are exposed to the entire terminal voltage as they are connected in parallel to the armature winding as shown in the figure.The shunt wound dc motor is a constant speed motor, as the speed does not vary here with the variation of mechanical load on the output. It comes under the category of Self excited DC Motor.
b. SERIES WOUND SELF EXCITED DC MOTOR
In this case, the entire armature current flows through the field winding as its connected in series to the armature winding. The series wound self excited dc motor is diagrammatically represented for better understanding. In a series wound dc motor, the speed varies with load. And operation wise this is its main difference from a shunt wound dc motor.
c. COMPOUND WOUND SELF EXCITED DC MOTOR
The compound excitation characteristic in a dc motor is obtained by combining the operational characteristic of both the shunt and series excited dc motor. The compound wound self excited dc motor or simply compound wound dc motor essentially contains the field winding connected both in series and in parallel to the armature winding. The excitation of compound wound dc motor can be of two types depending on the nature of compounding.
Cumulative Compound DC Motor, in which the shunt field flux assists the main field flux, produced by the main field connected in series to the armature winding. φtotal = φseries + φshunt.
Differential compound dc motor, in which the arrangement of shunt and series winding is such that the field flux produced by the shunt field winding diminishes the effect of flux by the main series field winding.The net flux produced in this case is lesser than the original flux and hence does not find much of a practical application. φtotal = φseries – φshunt
Both the cumulative compound and differential compound dc motor can either be of short shunt or long shunt type depending on the nature of arrangement.
Short Shunt DC Motor
If the shunt field winding is only parallel to the armature winding and not the series field winding then its known as short shunt dc motor or specifically short shunt type compound wound dc motor. Shunt is basically an arm that is connected in parallel, so it is c a Short Shunt DC Motor. DC Shunt motor is used in devices where Speed control is essential. Its major application is in centrifugal pumps as they produces constant flux and
Long Shunt DC Motor
If the shunt field winding is parallel to both the armature winding and the series field winding then it’s known as long shunt type compounded wound dc motor or simply long shunt dc motor.
Q1. What is self excitation in case of a dc machine?
Ans. The process of generating a magnetic field by means of an electric current is called excitation. Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils.
Q2. What is the main operational difference between a shunt wound and series wound self excited dc machine?
Ans. The Shunt wound motor is a constant speed motor in which the speed doesn’t depend on the mechanical output, but in case of a series wound machine, the speed varies with the output load attached.
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