Machines the dc motor

ROTATING MACHINES
DC MOTOR
The manufacture and construction of a DC motor is the same as the DC generator. The only
difference is that the generated emf is greater than the terminal voltage in a
generator, whereas in a motor it is less than the terminal voltage.
Main Construction
The main assembly parts of any dc machine are shown below,
Stator Steel ring called a Yoke to which the magnetic poles are
fixed.
Poles Fitted to the yoke and house the field windings.
Field Windings Comprising many turns of conductor to form an
electromagnet.
Armature Rotating part mounted in bearings.
Armature Core Laminated cylinder of iron with slots cut to house the
winding.
Armature Winding Multiple turns of conductor to form electromagnet.
Commutator Copper segments used to connect armature windings
to brushes.
machines-thedcmotor-130517155707-phpapp02.doc Page
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ARMATURE
PARTS
Teeth
Conductors
Commutator
STATOR PARTS
Pole Piece
Field Winding
Pole lip
Yoke

Winding Connections
There are three main styles of winding connections used in a dc machine’
(i) Shunt wound machines have the field winding connected in parallel with the
armature circuit.
(ii) Series wound machines have the field winding connected in series with the
armature.
(iii) Compound wound machines use both series and shunt windings.
(i) (ii) (iii)
There are two styles of armature windings;
i) Wave, providing two parallel paths between brushes irrespective of number of
poles.
ii) Lap, as many parrallel paths as the machine has poles.
Wave wound generators produce high voltage low current output.
Lap wound generators produce low voltage high current output.
2

3
Internal Connections of a Compound-Wound Motor
Internal and External Connections to give counter-clockwise and clockwise
rotation

LOAD CHARACTERISTICS
Shunt Wound Motor
In the shunt wound motor the field winding is connected in parallel with the armature therefore
the field flux is constant so the speed is constant.
Armature reaction weakens the field slightly when under heavy load due to heavy armature
current which gives an upward slope to the current / torque curve.
Since nE Φ∝ , if the flux is constant then En ∝ , therefore as armature current and volt drop
increase, the speed will reduce, thus the speed / torque curve droops.
Separately excited shunt motors are used where a steady speed is required.
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torque T
speed N
and
current I
0
N/T
I/T
Shunt Field
Armature Circuit
DC SUPPLY

Series Wound Motor
In the series wound motor the field winding is connected in series with the armature across the
supply therefore the field current is also the armature current. On no-load the current is small
therefore the machine speed will be high. As the load increases the current and flux will
increase and so the speed will reduce.
This characteristic provides a high torque at low speeds and is therefore ideal for electric
vehicles, trains and starter motors. Series motors are also used to drive fixed loads such as
fans.
The load must not be removed as the load current and hence flux will reduce causing the
speed to increase to a very high level. This will cause permanent damage to the motor due to
large centrifugal forces on the armature windings.
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torque T
speed N
and
current I
0
N/T
I/T
DC Supply
Armature Circuit

Compound Wound Motor
The compound wound motor has both series and shunt field windings. The shunt winding is
present to restrict the no-load speed to a safe value, however by varying the number of turns
on both the shunt and series windings a combination of characteristics to suite almost any
application.
There are two types of compound wound motor;
Cummulative compound, in which the series winding is so connected that its
field assists that of the shunt field.
Differential compound, in which the series winding is so connected that its
field opposes that of the shunt field.
Compound wound motors can be referred to as either ‘long shunt’ or ‘short shunt’ .
Compound wound motors are used for heavy industrial duties particularly where sudden heavy
load changes can occur such as lifts, pumps, presses conveyors, hoists etc.
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Shunt Field
DC Supply
Armature Circuit
‘long shunt’
Shunt Field
DC Supply
Armature Circuit
‘short shunt’
torque T
speed N
and
current I
0
N/T
I/T

BACK EMF (E.M.F. Generated in armature)
When the armature in a DC motor rotates its coils cut the field flux generating an emf that
opposes the supply, known as back emf. The back emf is equal to the supply voltage V minus
the volt drop in the armature circuit.
Back emf,
Armature circuit volt drop,
Supply voltage,
Back emf,
KEY TASK A – BACK EMF
A DC motor operates from a 220V supply. If the armature resistance is 0.1Ω and armature
current is 40A determine the back emf produced.
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A
Ra
E
DC Supply
Voltage
V
Ia
Shunt
Field
RIEV
RI
RIVE
a
aa
aa
+=
×=
−=
)( aa RIVE ×−=

ARMATURE REACTION
Armature reaction is the effect that the magnetic field produced by the armature current has on
the magnetic field produced by the field system.
In a generator armature reaction results in a reduction of output voltage and in a motor
armature reaction results in increased speed. Fitting compensating windings into the pole
faces can overcome this effect.
Brush Position
The effect of armature reaction is poor commutation resulting in sparking at the brushes. To
overcome this effect the brush position is set against the direction of rotation in a motor and
with the direction of rotation in a generator.
The natural position of the brushes is at 90° to the main field and is known as the “magnetic
neutral” axis. At this position there is no emf induced into the conductor immediately beneath
the brushes.
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Effect of Armature Reaction on Brush Position

Interpoles
Armature current depends on the load therefore the amount of distortion due to armature
reaction is variable. Since it is not possible to alter the brush position as the load changes a
set of poles are placed between the main poles called INTERPOLES and connected in series
with the armature.
The magnetic effect of the interpoles is to reduced the distortion of the field and improve
commutation.
KEY TASK B – COMPOUND WOUND MOTOR
A 500V compound wound motor has a shunt field winding resistance of 300Ω, an armature
resistance of 0.2 ohm and a series resistance of 0.6 ohm. If the current taken from the supply is
30A calculate the back emf produced by the armature.
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a. Magnetic field due to Poles.
b. Magnetic field due to Armature.
c. Resulting magnetic field.

NOTE:
If the back emf equalled the supply voltage then no current would flow in the armature and the
motor would not start.
BRUSHES
Brushes are used to provide an electrical connection to the rotating part of the motor,
Armature in a DC machine or Wound Rotor in an AC machine.
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The brushes must,
1. Maintain an uninterruptable contact with the commutator or slip rings.
2. Carry the full load current.
3. Have a wear rate that does not wear the commutator or slip rings.
Brush Materials
There are many grades of brush but they all fall into the categories listed below,
HARD CARBON
Having a high co-efficient of friction this material has a tendency to wear the mica insulation
and copper commutator. They are mechanically robust and long lasting but their low electrical
and thermal conductivity restricts their use to moderate speed low current machines. Some
incorporate a proportion of graphite to assist in lubrication.
NATURAL GRAPHITE
A soft material having good natural lubricating properties and a co-efficient of friction less than
carbon. Brushes made of this material are silent running have long life and are suitable for
high speed machines.
ELECTRO-GRAPHITE
Electro-Graphite is formed from carbon by heating in an electrical furnace producing a brush
which is soft but very tough. Having a low co-efficient of friction and high current carrying
capabilty this material is suitable for severe operating conditions, ie high running speed, heavy
overloads and mechanical shock.
METAL-GRAPHITE
Metal-Graphite is a combination of copper and graphite in varrying proportions. The graphite
reduces the rate of wear and the copper providing increased mechanical strength. Having a
low co-efficient of friction and low electrical resistance this material is used for low voltage high
current DC machines, ie engine starter motors, vehicle electric motors and switch gear
contacts.
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Machines the dc motor

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