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 DC motor or machine is shown in Figure.
Parts Used in 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:
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.
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.
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.
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
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.
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.
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.
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.
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.
Thanks for reading about construction of dc motor. If you have any questions then you can ask me in the comment section given below.
DC Motors — 1 | Objective Type Question Answers
#1 Starters are used with D.C. motors because
to restrict armature current as there is no back e.m.f. while starting
#2 For starting a D.C. motor a starter is required because
it limits the starting current to a safe value
#3 The-type of D.C. motor used for shears and punches is
cumulative compound D.C. motor
#4 If D.C. motor is connected across the A.C. supply it will
burn due to heat produced in the field winding by eddy currents
#5 To get the speed of D.C. motor below the normal without wastage of electrical energy is…………. used.
Ward Leonard control
#6 When two D.C. series motors are connected in parallel, the resultant speed is
#7 The speed of a D.C. shunt motor more than its full-load speed can be obtained by
decreasing the field current
#8 In a D.C. shunt motor, speed is
independent of armature current
#9 If the speed of a D.C. shunt motor is increased the back e.m.f. of the motor will
#10 What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?
The armature may burn
#11 In case of D.C. shunt motors the speed is dependent on back e.m.f. only because
flux is practically constant in D.C. shunt motors
#12 In a D.C. shunt motor, under conditions of maximum power, the current in the armature will be
more than full-load current
#13 Which D.C. motor will have least percentage increase of input current for the same percentage increase in torque?
Cumulative compound motor
#14 Which of the following load normally needs starting torque more than the rated torque?
#15 The starting resistance of a D.C. motor is generally
#16 The speed of a D.C. series motor is
inversely proportional to the armature current
#17 In a D.C. series motor if the armature current is reduced by 50%, the torque of the motor will be equal to
25% of the previous value
#18 The current drawn by the armature of D.C. motor is directly proportional
the torque required
#19 The power mentioned on the name plate of an electric motor indicates
the output power available at the shaft
#20 In Ward Leonard method of speed control of a D.C. motor, change in speed of motor is obtained by the
change in armature voltage of D.C. motor
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- Series motor speed control
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- DC Motors | Wikipedia
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