Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
Three point and four point starters are used for d.c. shunt motors. In case of series motors, field and armature are in series and hence starting resistance is inserted in series with the field and armature. Such a starter used to limit the starting current in case of d.c. series motors is called two point starter.
Working Principle, Disadvantage | D.C. Motors
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
• All the d.c. motors are basically self starting motors. Whenever the armature and the field winding of a d.c. motor receives supply, motoring action takes place. So d.c. motors do not require any additional device to start it. The device to be used as a starter conveys a wrong meaning.
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
Instead of just stating the applications, the behaviour of the various characteristics like speed, starting torque etc., which makes the motor more suitable for the applications, is also stated in the Table 4.16.1.
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
• Compound motor characteristics basically depends on the fact whether the motor is cumulatively compound or differential compound. All the characteristics of the compound motor are the combination of the shunt and series characteristic.
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
• It is seen earlier that motor armature current is decided by the load. On light load or no load, the armature current drawn by the motor is very small. In case of a d.c. series motor, ϕ ∝Ia and on no load as I is small hence flux produced is also very small.
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
• The equation represents a straight line, passing through the origin, as shown in the Fig. 4.13.1. Torque increases linearly with armature current. It is seen earlier that armature current is decided by the load. So as load increases, armature current increases, increasing the torque developed linearly.
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
• The performance of a d.c. motor under various conditions can be judged by the following characteristics.
Speed Regulation | DC Motors
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
Before analyzing the various characteristics of motors, let us revise the torque and speed equations as applied to various types of motors.
Long Shunt, Short Shunt
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
• The compound motor consists of part of the field winding connected in series and part of the field winding connected in parallel with armature. It is further classified as long shunt compound and short shunt compound motor.
Voltage and Current Relationship, Solved Example Problems
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
Let Rse be the resistance of the series field winding. The value of Rse is very small and it is made of small number of turns having large cross-sectional area.
Voltage and Current Relationship, Solved Example Problems
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
The value of Ra is very small while Rsh is quite large. Hence shunt field winding has more number of turns with less cross-sectional area.
Subject and UNIT: Electrical Machines: Unit III: a. D.C. Motors
The d.c. motors are classified depending upon the way of connecting the field winding with the armature winding. The different types of d.c. motors are shunt motors, series motors and compound motors.