1. CHAPTER 3
DC MOTOR
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2. 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.
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3. BACK EMF
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4. TORQUE EQUATION
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5. 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.
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6. 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.
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7. 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
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8. 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.
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9. Separately Excited DC Motor
Both the field winding and the armature winding are fed from two different sources,
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10. Series DC Motor
The field winding is connected in series with the armature winding.
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11. Shunt DC Motor
The field winding is connected in parallel with the armature winding.
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12. 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.
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13. Speed of DC Motor
Speed of a DC motor depends on the i) Supply voltage ii) Armature current iii) Armature resistance iv) Field flux.
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14. Characteristics of a DC Shunt Motor
Speed versus armature current characteristics
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15. Characteristics of a DC Shunt Motor
Torque versus armature current characteristics
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16. Characteristics of a DC Shunt Motor
Speed–Torque characteristics
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17. Characteristics of a DC Series Motor
Speed versus armature current characteristics
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18. Characteristics of a DC Series Motor
Torque versus armature current characteristics
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19. Characteristics of a DC Series Motor
Speed–Torque characteristics
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20. Characteristics of a DC Compound Motor
Speed versus armature current characteristics
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21. Characteristics of a DC Compound Motor
Torque versus armature current characteristics
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22. Characteristics of a DC compound Motor
Speed Torque characteristics
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23. 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
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24. 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.
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25. Three-point Starter for DC Shunt Motor
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26. Four-point Starter for DC Shunt Motor
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27. 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.
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28. Shunt Field Control
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29. Armature Resistance Control
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30. 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.
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31. 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
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