Lecture 7 DC Machines

Machine
Lecture 7 & 8
DC Machines ‫بـرنامج هـندسـة الـقوى واالالت الكـهربيـة‬
Third Year Mechanical Engineering
Dr. Mahmoud Gamal Hemeida

Assistant professor power and machines
Introduction
A DC Machine is an electro-mechanical energy conversion device
There are two types of DC machines DC generator, DC motor

DC generator: converts mechanical power (ωT) into DC electrical power (EI). It
may be used in electroplating and electrolysis when the required power is small,
batteries are preferred)
DC motor. Converts DC electrical power into mechanical power. DC motors have

many practical applications (because it is easy to control their speed and torque),
such as electric traction system, automobile and aircraft.
Introduction
Difference between AC and DC machines
- Most DC machines are similar to AC machines
- They both have AC voltages and current within them
- DC machines have DC outputs because they have a commutator

mechanism that converting AC voltages to DC voltages at their
terminals so, DC machines are also called commutating machines
Basic Structure of Electrical Machines
• The rotating electrical or DC machine has mainly two parts;
- Stator
- Rotor.
• The stator and rotor are separated from each other by an air gap.
• The stator is the outer frame of the machine and is static.
• The rotor is free to move and is the inner part of the machine.
• Both the stator and the rotor are made of ferromagnetic materials.
Basic Structure of Electrical Machines
• Slots are cut on the inner periphery of the stator and the outer periphery of
the rotor.
• Conductors are placed in the slots of the stator or rotor. They are
interconnected to form windings.
• The windings in which voltage is induced is called the Armature windings.
• The winding through which a current is passed to produce the main flux is
called the Field windings.
• To provide main flux in some of the machine permanent magnets is also used.
DC Machine – Construction
• A DC generator can be used as a DC motor without any constructional
changes and vice versa is also possible. Thus, a DC generator or a DC
motor can be broadly termed as a DC machine.
• These basic constructional details are also valid for the construction
of a DC motor.
DC Machine – Construction
As shown in the following figure, DC
machines have the following parts
Stator
1. Yoke
2. Pole Core
3. Pole Shoes
4. Field Coils
5. Interpoles
Rotor
1. Shaft
2. Armature Core
3. Armature Windings
4. Commutator
5. Brushes
Yoke
• Hollow cylinder made of cast steel or roll steel
• Act as outer protective cover of machine.
• Provides mechanical support for poles.
• Carries flux produced by poles
Pole Core and Pole Shoes

• Field windings are wound on pole core and supported
by pole shoes.
• Are made of number of sheet steels stacked and riveted
together.
• made of Silicon steel – to reduce hysteresis loss
• laminated – to minimize eddy currents
• Pole cores are then bolted to yoke.
• Pole shoes serve two functions
1. support field coils
2. spread out flux in airgap
Field Windings
• Coil made of copper wire
• Field coils are wound over the core.
• When energized with DC, magnetize the poles producing the working flux.
• All coils are connected in series such that as current flows, alternate N and
S poles are produced.
Inter Poles
• Fitted to yoke between main poles.
• Windings are made of copper and are connected in series with field
windings.
• Flux produced by inter poles provides spark less commutation.
Shaft
• Provides mechanical strength to armature and commutator.
• Rotor assembly is free to rotate by two bearing fixed between shaft and two end covers.
• Transfers mechanical energy to and from machine.
Armature Core and Windings
• Laminated cylinder mount on shaft made of number of sheet steels stacked and riveted together.
• made of Silicon steel – to reduce hysteresis loss
• laminated – to minimize eddy currents
• Has slots on outer periphery to accommodate armature windings
Commutator
• Ends of armature coils terminated at commutator segments
• Made of copper segments insulated by mica
Functions
• Collect current from armature conductors.
• The e.m.f. generated in the armature conductors is alternating
e.m.f. The commutator helps in converting this alternating e.m.f.
into a direct one.
Brushes
• Made of graphite or carbon
• Held by brush holders
Desirable Properties
• Good electrical conductivity
• Less co-efficient of friction.
Functions
• The brushes collect the armature current from the commutator
segments and supply it to the load (in the case of generator) or
feed the current into the commutator segments (in the case of
motor).
Principle of Operation of a D.C. Generator
• According to Faraday’s laws of electromagnetic induction, whenever a
conductor is placed in a varying magnetic field (OR a conductor is moved in a
magnetic field), an emf (electromotive force) gets induced in the conductor.
• In a DC generator, field coils produce an electromagnetic field and the
armature conductors are rotated into the field. Thus, an electromagnetically
induced emf is generated in the armature conductors.
• The direction of induced current is given by Fleming’s right hand rule.
Principle of Operation of a D.C. Generator
Need of a Split ring commutator:
• According to Fleming’s right hand rule, the direction of induced current
changes whenever the direction of motion of the conductor changes.
• Let’s consider an armature rotating clockwise and a conductor at the left is

moving upward. When the armature completes a half rotation, the direction of
motion of that particular conductor will be reversed to downward. Hence, the
direction of current in every armature conductor will be alternating.
• with a split ring (commutator), connections of the armature conductors also

gets reversed when the current reversal occurs. And therefore,
we get unidirectional current at the terminals.
Need of a Split ring (commutator)
Single Turn Generator
• Consider a single turn coil rotating about its own axis in a magnetic field. The two
ends of the coil are connected to split rings(commutator)
• When the plane of coil is at right angles to the direction of field, flux linked with coil is
maximum, but rate of change of flux is minimum. Hence induced EMF is minimum.
• When the plane of coil is at parallel to the direction of field, flux linked with coil is
minimum, but rate of change of flux is maximum. Hence induced EMF is maximum.
Two Coil Generator
By winding more coils on the rotor, and bringing the ends of each coil
out to separate pair of segments on the commutator that are opposite
each other, the pulsations or ripple in the output e.m.f. is reduced

Lecture 7 DC Machines

Uploaded by

Copyright:

Available Formats

Lecture 7 DC Machines

Uploaded by

Document Information

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Lecture 7 DC Machines

Uploaded by

Copyright:

Available Formats

Machine

Dr. Mahmoud Gamal Hemeida

There are two types of DC machines DC generator, DC motor

DC motor. Converts DC electrical power into mechanical power. DC motors have

- They both have AC voltages and current within them

- DC machines have DC outputs because they have a commutator

• The stator is the outer frame of the machine and is static.

• The windings in which voltage is induced is called the Armature windings.

Pole Core and Pole Shoes

• Let’s consider an armature rotating clockwise and a conductor at the left is

• with a split ring (commutator), connections of the armature conductors also

You might also like