Construction of DC Motor

A DC motor or a DC machine consists of two windings namely field winding and armature winding. The field winding is stationary and armature winding can rotate.
 
The field winding produces a magnetic flux in the air gap between the armature and field windings and the armature is placed in this magnetic field.
 
The construction of a DC machine is shown in Figure.
 

construction of dc motor

 
The main parts used in the construction of a DC motor are the yoke, poles, field winding, commutator, carbon brushes bearings etc. A brief description of the various parts is as follows:
 

Yoke

 
The yoke acts as the outer cover of a DC motor and it is also known as the frame. The yoke is an iron body, made up of low reluctance magnetic material such as cast iron, silicon steel, rolled steel etc.
 
Yoke serve two purposes, firstly it provides mechanical protection to the outer parts of the machine secondly it provides low reluctance path for the magnetic flux.
 

Poles and Pole Shoe

 
The pole and pole shoe are fixed on the yoke by bolts. These are made of thin cast steel or wrought iron laminations which are riveted together. Poles produce the magnetic flux when the field winding is excited.
 
Pole shoe is an extended part of a pole. Due to its shape, the pole area is enlarged and more flux can pass through the air gap to the armature.
 

Field Winding

 
The coils around the poles are known as field (or exciting) coils and are connected in series to form the field winding. Copper wire is used for the construction of field coils. When the DC current is passed through the field windings, it magnetizes poles which produce magnetic flux.
 

Armature Core

 
It is a cylindrical drum and keyed to the rotating shaft. A large number of slots are made all over its periphery, which accommodates the armature winding. Low reluctance, high permeability material such as cast iron and cast steel are used for armature core.
 
The laminated construction is used to produce the armature core to minimize the eddy current losses. The air holes are also provided on the armature core for the air circulation which helps in cooling the motor.
 

construction of a dc motor image

 

Armature Winding

 
The armature winding plays very important role in the construction of a DC motor because the conversion of power takes place in armature winding. On the basis of connections, there are two types of armature windings named:

  • Lap winding
  • Wave Winding

 
Lap Winding
 
In lap winding the armature conductors are divided into P groups.
 
All the conductors in a group are connected in series and all such groups are connected in parallel. For a lap winding the number of parallel paths (A) is equal to the number of poles (P).
 
Due to the existence of a large number of parallel paths, the lap wound armature is capable of supplying larger load currents, therefore, lap winding is used for low voltage high current DC motors.

 
Wave Winding
 
In wave winding, all the conductors are connected in series to form a single closed circuit. This closed circuit is tapped at various points. The two carbon brushes touch two diametrically opposite tappings.
 
For wave winding the number of parallel paths is equal to two irrespective of the number of poles. The wave winding is useful for high voltage low current motors.
 

Commutator

 
It is a mounted on the shaft. It is made up of a large number of wedge-shaped segments of hard drawn copper, insulated from each other by a thin layer of mica.
 
The commutator connects the rotating armature conductor to the stationary external circuit through carbon brushes. It converts alternating torque into unidirectional torque produced in the armature.
 

Carbon Brushes

 

construction of a dc motor yourelectricalguide.com

 
The current is conducted from voltage source to armature by the carbon brushes which are held against the surface of commutator by springs. They are made of high-grade carbon steel and are rectangular in shape.

 

Bearings

 
The ball or roller bearings are fitted in the end housings. The friction between stationary and rotating parts of the motor is reduced by bearing. Mostly high carbon steel is used for making the bearings as it is very hard material.
 

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DC Motor MCQ

 

1. The counter e.m.f. of a d.c. motor

2. The normal value of the armature resistance of a d.c. motor is

3. The mechanical power developed by the armature of a d.c. motor is equal to

4. The induced e.m.f. in the armature conductors of a d.c. motor is

5. A d.c. motor can be looked upon as d.c. generator with the power flow

6. In a d.c. motor, the mechanical output power actually comes from

7. The maximum torque of d.c. motors is limited by

8. Which of the following quantity maintains the same direction whether a d.c. machine runs as a generator or as a motor ?

9. Under constant load conditions, the speed of a d.c. motor is affected by

10. It is possible to increase the field flux and, at the same time, increase the speed of a d.c. motor provided its .......... is held constant.

11. The current drawn by a 120 - V d.c. motor of armature resistance 0.5 Ω and back e.m.f. 110 V is .......... ampere.

12. The shaft torque of a d.c. motor is less than its armature torque because of .......... losses.

13. A d.c. motor develops a torque of 200 N-m at 25 rps. At 20 rps it will develop a torque of .......... N-m.

14. Neglecting saturation, if current taken by a series motor is increased from 10 A to 12 A, the percentage increase in its torque is ........ percent.

15. If load on a d.c. shunt motor is increased, its speed is decreased due primarily to

16. If the load current and flux of a d.c. motor are held constant and voltage applied across its armature is increased by 10 per cent, its speed will

17. If the pole flux of a d.c. motor approaches zero, its speed will

18. If the field circuit of a loaded shunt motor is suddenly opened

19. Which of the following d.c. motor would be suitable for drives requiring high starting torque but only fairly constant speed such as crushers ?

20. A d.c. shunt motor is found suitable to drive fans because they require

21. As the load is increased, the speed of a d.c. shunt motor

22. Between no-load and full-load, .......... motor develops the least torque

23. As compared to shunt and compound motors, series motor has the highest torque because of its comparatively .......... at the start.

24. When load is removed, .......... motor will run at the highest speed.

25. A series motor is best suited for driving

26. A 220 V shunt motor develops a torque of 54 N-m at armature current of 10 A. The torque produced when the armature current is 20 A, is

27. The d.c. series motor should never be switched on at no load because

28. A shunt d.c. motor works on a.c. mains

29. A 200 V, 10 A motor could be rewound for 100 V, 20 A by using .......... as many turns per coil of wire, having .......... the cross-sectional area.

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