CHAPTER 3 DC MOTOR Electrical Machines 1 INTRODUCTION

CHAPTER 3 DC MOTOR Electrical Machines 1

INTRODUCTION A motor is a machine which converts electrical energy into mechanical energy. A DC motor finds application in rolling mills, in traction system and overhead cranes. The principle of operation of DC motor is that when a current carrying conductor is kept in a magnetic field, a force is produced in that conductor. Electrical Machines 2

BACK EMF Electrical Machines 3

TORQUE EQUATION Electrical Machines 4

ARMATURE REACTION If the magnetic field windings of a DC machine are connected to the power source and the rotor is turned by an external means, a voltage will be induced in the conductors of the rotor. Electrical Machines 5

ARMATURE REACTION This voltage is rectified and can be supplied to external loads. However, if a load is connected, a current will flow through the armature winding. This current produces its own magnetic field that distorts the original magnetic field from the machine’s poles. This distortion of the machine’s flux as the load increases is called armature reaction. Electrical Machines 6

TYPES OF DC MOTOR i) Permanent magnet DC motor ii) Separately excited DC motor iii) Series DC motor iv) Shunt DC motor v) Compound DC motor Electrical Machines 7

Permanent Magnet DC Motor In this type of DC motor, the field is made up of permanent magnet and hence, it requires no field winding. Electrical Machines 8

Separately Excited DC Motor Both the field winding and the armature winding are fed from two different sources, Electrical Machines 9

Series DC Motor The field winding is connected in series with the armature winding. Electrical Machines 10

Shunt DC Motor The field winding is connected in parallel with the armature winding. Electrical Machines 11

Compound DC Motor A compound DC motor has both series and shunt field winding. It can be a short-shunt DC compound motor or a long-shunt DC compound motor. Electrical Machines 12

Speed of DC Motor Speed of a DC motor depends on the i) Supply voltage ii) Armature current iii) Armature resistance iv) Field flux. Electrical Machines 13

Characteristics of a DC Shunt Motor Speed versus armature current characteristics Electrical Machines 14

Characteristics of a DC Shunt Motor Torque versus armature current characteristics Electrical Machines 15

Characteristics of a DC Shunt Motor Speed–Torque characteristics Electrical Machines 16

Characteristics of a DC Series Motor Speed versus armature current characteristics Electrical Machines 17

Characteristics of a DC Series Motor Torque versus armature current characteristics Electrical Machines 18

Characteristics of a DC Series Motor Speed–Torque characteristics Electrical Machines 19

Characteristics of a DC Compound Motor Speed versus armature current characteristics Electrical Machines 20

Characteristics of a DC Compound Motor Torque versus armature current characteristics Electrical Machines 21

Characteristics of a DC compound Motor Speed Torque characteristics Electrical Machines 22

DC MOTOR STARTER Since the armature resistance is small, generally less than 1Ω, the current through the armature at the time of starting will be very high. This large current may damage the armature winding, brushes and commutator segments Electrical Machines 23

DC MOTOR STARTER Due to the high inrush of current at the time of starting the motor, it becomes necessary to limit this current to a safe value, which is generally obtained by connecting an extra resistance in series with the armature winding. Electrical Machines 24

Three-point Starter for DC Shunt Motor Electrical Machines 25

Four-point Starter for DC Shunt Motor Electrical Machines 26

SPEED CONTROL OF DC SHUNT MOTOR The different methods of speed control of DC shunt motor are as follows: i) Shunt field control or flux control method. ii) Armature resistance control or armature control method. iii) Voltage control method. Electrical Machines 27

Shunt Field Control Electrical Machines 28

Armature Resistance Control Electrical Machines 29

SPEED CONTROL OF DC SERIES MOTOR There are three methods of speed control for DC series motor, namely, i) Flux control method ii) Armature control method iii) Series-parallel control. Electrical Machines 30

ELECTRIC BRAKING �Braking is used to stop the motor quickly. There are three types of electric braking. i) Rheostatic or dynamic braking ii) Braking by plugging iii) Regenerative braking Electrical Machines 31