Classification of DC motor

Classification of DC motor:

Motor is used to convert mechanical energy into electrical energy. As shown in construction of DC motor, there are two winding is used. One is field winding and second is armature winding. According to connection of this winding, DC motor is classify into main two parts; 

1. Separately excited 
2. Self excited

Self excited is further classify into three types;

1. Series motor
2. Shunt motor
3. Compound motor

1) Separately excited: In separately excited motor, field winding and armature winding is not physically connected but this winding is magnetically connected. Here, the field coil is energized from a separate DC voltage source and the armature coil is also energized from another source. Armature voltage source may be variable but, independent constant DC voltage is used for energizing the field coil. So, those coils are electrically isolated from each other, and this connection is the specialty of this type of DC motor.

Application: Separately excited dc motors have industrial applications. This type of motors is used in trains and for automatic traction purposes. They are often used as actuators. 

2) Self Excited DC motor: In self excited DC motor, field winding and armature winding is physically connected. The current in the winding is supplied by the machine or motor itself. Self-excited DC Motor is further divided into three parts according to connection of both winding and this is known as shunt wound, and series wound motor. They are explained below in detail.

• Series DC motor: In  this case, the armature winding and field winding are connected in series so, the entire armature current flows through the field winding. The series wound self excited dc motor is diagrammatically represented as below figure for better understanding. In a series dc motor, the speed varies with load and operation wise this is its main difference from a shunt wound dc motor.

Application of DC series motor:

1. This type of motor has good speed regulation even as the load varies.
2. It has high starting torque so it has wide application in conveyors,trains,compressors etc.
3. It is typically used were adjustable speed is required.

• Shunt DC motor: In case of shunt wound DC motor, the armature winding and field winding are connected in parallel. So, the field winding are exposed to the entire terminal voltage. 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. 

Application of DC shunt motor:

1. A DC shunt motor is constant speed motor(as for a given field current the drop in speed from no load to full load is less than 6 to 7 % ) so it is used where constant speed is required.But SCIM is replacing it in industry.
2. When the driven load needs a wide range of speed variation both below and above base speed, this motor is used.(e.g:lathe)
3. DC shunt motor is easy to control using VFD(variable frequency drive) so it is used in the places where variable speed is required

• Compound 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.

1. Cumulative Compound DC Motor (Long Shunt): In which the shunt field flux assists the main field flux, produced by the main field connected in series to the armature winding. 
2. Differential compound dc motor (Short shunt): 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.

Application of compound DC motor:

Cumulative compound wound motors are virtually suitable for almost all applications like business machines, machine tools, agitators and mixers etc. Compound motors are used to drive loads such as shears, presses and reciprocating machines


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