DC

Machine
s
ADAMA UNIVERSITY Solomon G.G.
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SOLOMON G.G.

INTRODUCTION
DC machine is an electrical machine that can be used either in generator or motor mode. DC
generator is a machine which converts mechanical energy to electrical energy, while DC motor
converts electrical energy to mechanical energy. The energy conversion is based on the
principle of motional induced emf.

 When armature conductors move in a magnetic field produced by the current in stator field
winding, voltage is induced in the armature conductors.
 When current carrying armature conductors are placed in a magnetic field produced by the
current in stator field winding, the armature conductors experience a mechanical force.

Every DC machine has two essential parts

 Magnetic field
 Conductors which so move as to cut the flux

CONSTRUCTION OF DC MACHINES
The essential parts of DC machines are:

i. Yoke
 It is the outer frame of the machine which serves as:
 Mechanical supporting and protecting cover
 It carries the magnetic flux produced by the poles.

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ii. Pole
Salient pole on the stator
Two types of poles
a. Main pole :
 Have poles cores and pole shoes.
 It carries pole coils in which main magnetic field is produced. These coils are
called main field winding. Main field winding can be series field winding or
shunt field winding
 Contains compensating winding to reduce armature reaction effect.
 Pole shoes serves in
 In spreading out the flux in the air gap so as to reduce the reluctance
 Support exciting (main field winding).

b. Commutating pole
 Placed at the half way between main poles ( a place where the main
magnetic flux is zero)
 Caries commutating winding which is used to reduce the effect of armature
reaction
iii. Armature
 It’s the rotating part of the machine.
 It has core which

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 Houses armature coils

 Provide the path of very low reluctance to the flux through the armature
from N pole to S pole
 It is cylindrical or drum shaped built up of circular sheet steel discs or
laminations.
 Armature conductors are arranged in such a way that the distance between
two legs of coil is 180 electrical degrees.
𝚹e=P/2*𝚹m where P is number of poles, 𝚹e is electrical degree,
𝚹m is mechanical degree.
iv. Commutator
 It’s located on the rotor
 It’s made of wedge shaped segments of copper and it’s cylindrical in
structure.
 The segments are insulated from each other by thin layers of mica. The
number of segments is equal to the number of armature coils.
 Each Commutator segments are connected to armature conductors by
means of copper strip or risers.
 It’s function is to facilitate the collection of current from armature
conductors
 It serve as rectifier ( converts AC to DC power)

v. Brushes
 Its function is to collect the current from the Commutator
 It’s made of carbon or graphite.
 It’s placed in the neutral zone (where main magnetic flux is zero) to reduce
arcing

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PRINCIPLE OF OPERATION OF DC MACHINES

a. Generator mode
The armature of the machine is driven by a prime mover in the CW direction and the
stator field is excited by DC source so as to produce the stator poles. When conductors
of the armature cuts the magnetic field produced by field circuits voltage is induced in
them. Then a current starts to flow in the armature winding. When the current carrying
conductor moves in a magnetic field it experiences a force. Then the rotor experiences a
torque (electromagnetic torque) which opposes the driving torque from prime mover.
Then the prime mover produces extra torque in order to cancel out the effect of
electromagnetic torque. The voltage induced in the armature winding is collected by
the help of Commutator and brushes.
b. Motor mode
The field circuit of the machine is excited by DC source so as to produce main field in the
machine. When a DC source is connected to the armature winding, a current starts to
flow in the armature conductors. When a current carrying conductor is moving in a
magnetic field it experiences a force and the torque that rotates the machine is
produced. As the machine is rotating, a magnetic field is produced due to rotor current.
This magnetic field induces voltage (back emf) in armature winding which direction is
opposite to applied voltage to the armature. The applied voltage has to over come back
emf. It is this electrical work done in overcoming opposition due to back emf is
converted to mechanical energy. The armature current is given by

Where Ia is armature current, V is the applied voltage, V b is back emf. Back emf depends
on armature speed . if speed is high back emf is large and armature current will be
small. The power developed by the armature is proportional back emf.
Power developed by the armature= VbIa

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Power Division in DC Machines

Arm. copper loss

Ia2Ra+brush contact loss

Input from Elec-magnetic Arm. terminal Output power

DC Generator prime-mover power = Vta Ia
Power =EaIa = Vt IL

No-load rotational loss (friction Series field loss IL2Rs

+windage+core)+stray load loss
+shunt field loss If2Rf

Arm. copper loss

Ia2Ra+brush contact loss

Input power from Arm. terminal Elec-magnetic Output available

DC Motor mains =Vt IL power = Vta Ia Power =EaIa at the shaft

Series field loss IL2Rs No-load rotational loss (friction

+shunt field loss If2Rf +windage+core)+stray load loss

TYPES OF DC MACHINES
Both the armature and field circuits carry direct current in the case of a DC machine.
Types:
 Self-excited DC machine: when a machine supplies its own excitation of the field
windings. In this machine, residual magnetism must be present in the
ferromagnetic circuit of the machine in order to start the self-excitation process.

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IL
+ +
If IL
+ Ra
Ra
Rf
DC Supply Rf
+ Vt Vt

- Ea Ea Ia
- -
Separetely excited Self Excited
 Separately-excited DC machine: The field windings may be separately excited
from an eternal DC source.
 Shunt Machine: armature and field circuits are connected in parallel. Shunt
generator can be separately-excited or self-excited.
 Series Machine: armature and field circuits are connected in series.

DC Generator
Equivalent circuit
The magnetic field produced by the stator poles
induces a voltage in the rotor (or armature)
coils when the generator is rotated.
This induced voltage is represented by a
voltage source.
The stator coil has resistance, which is
Vbrush
Rfconnected in series. Ra Load
The pole max flux is produced by the DC
I excitation/field current, Iwhich
ag is magnetically
Vf f Eag Vdc
coupled
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The field circuit has resistance and a source

 power out
by a power in
battery
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Equivalent circuit of a separately excited

dc generator.
When the generator is loaded, the load
current produces a voltage drop on the rotor
winding resistance.
In addition, there is a more or less constant
1–3 V voltageEag =Vdrop
+I R on
+V the brushes.
dc ag a brush
DC Motor Equivalent
These two voltage drops reduce the terminal
voltage of the generator. The terminal
circuit
voltage is;
Vbrush Electrical
Rf Ra power in
max
DC Power
V f If Iam Vdc
Eam supply

Mechanical
power out

• Armature voltage equation

V =E +I R +V
dc am amTECHNOLOGY
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