Electrical Machines Notes
Chapter 1
Rotating electrical machines
Widely used for the purpose of converting energy from one form to another.
Most frequently used types:
1. Generators – converts mechanical energy to electrical energy.
2. Motors – converts electrical energy to mechanical energy.
Not used so often types:
Rotary converters- electrical energy of one form is changed into electrical energy of another.
Frequency converters- change AC electrical energy at one frequency into AC electrical energy at
another frequency.
Prime mover
Electric generator is driven or rotated by this mechanical machine.
May be ha steam turbine, gasoline engine, electric motor, or even hand operated crank.
Armature windings
Always wound on laminated steel cores of good magnetic permeability.
Current in the armature winding of all motors and generators, whether of direct or alternating,
is always alternating.
Alternating voltages are always generated in the windings of AC and DC generators.
Generator operation/action can take place when and only when there is relative motion between
conducting wires and magnetic lines of force.
When electric motor is in operation, it is supplied with electrical energy and develops torque, that is, a
tendency to produce rotation.
Two important parts of rotating electric generators.
1. Even set of electromagnets or permanent magnets.
2. Laminated Steel core containing current carrying copper wires (armature winding).
In DC generator:
Armature winding is mechanically rotated through the stationary magnetic fields created by the
electromagnets or permanent magnets.
In AC generator:
Electromagnets or the permanent magnets and their accompanying magnetic fields are rotated
with respect to the stationary armature winding.
In DC motor:
Current is sent into the armature winding which is being placed inside a set of radially supported
magnet poles.
In AC motor:
Current is set into the armature winding, which is usually placed in a stationary laminated iron
core; the rotating element may or may not be a set of magnet poles.
In AC generator:
Generated alternating EMF(Electro-motive Force) is transmitted directly to the load.
, In DC generator:
Generated alternating EMF is first rectified by a commutator and brushes, which is, changed to
direct current, before it is transmitted to its load.
In AC motor:
Receives its energy directly from AC source and without any change whatever in form, uses it as
alternating current in its winding to develop torque.
In DC motor:
Direct current is delivered to the brushes but flows as alternating current in the armature
winding after passing through the brushes and commutator.
Field poles
Each one consists of a laminated steel core of rectangular cross section, surrounded by one or
more copper coils.
Shoe
Spread out portion of the pole core, permits the magnetic flux to enter the armature core over a
wider area than would be possible without a core having straight sides.
Salient pole field
The electromagnets are bolted to a hub fastened to the shaft, so that they project radially
outward toward the stationary armature core.
Non-salient pole field
When the alternator is driven by a high speed turbine, the field winding is placed in a slotted
core.
Types of direct current generators
1. Shunt generators – The excitation is produced by a single winding connected to its own positive
and negative brushes
a). self-excited shunt generator
b). separately excited shunt generator
2. Compound generator
a). Shunt field
b). Series field- contains very few turns of heavy wire. Has an extremely low resistance and is
excited by the comparatively large load current.
Both shunt and series field windings are placed over one set of pole cores and act together to
create a common set of magnetic fluxes.
Alternating current generators:
Generally called alternators
Armature core and its windings are stationary while the field poles rotate.
The speed off the rotation must be kept absolutely constant.
Two reasons why AC machines can be built in large sizes and made to develop high voltages:
1. No commutator is required.
2. The armature winding can be placed in a stationary part of the machine, the stator, where it is
possible to provide good insulation strength for the high voltage winding.
Types and characteristics of DC motors:
1. Shunt motor – If a comparatively high resistance field winding of many turns of fine wires is
employed for this function, it is connected in parallel/shunt with the armature.
Chapter 1
Rotating electrical machines
Widely used for the purpose of converting energy from one form to another.
Most frequently used types:
1. Generators – converts mechanical energy to electrical energy.
2. Motors – converts electrical energy to mechanical energy.
Not used so often types:
Rotary converters- electrical energy of one form is changed into electrical energy of another.
Frequency converters- change AC electrical energy at one frequency into AC electrical energy at
another frequency.
Prime mover
Electric generator is driven or rotated by this mechanical machine.
May be ha steam turbine, gasoline engine, electric motor, or even hand operated crank.
Armature windings
Always wound on laminated steel cores of good magnetic permeability.
Current in the armature winding of all motors and generators, whether of direct or alternating,
is always alternating.
Alternating voltages are always generated in the windings of AC and DC generators.
Generator operation/action can take place when and only when there is relative motion between
conducting wires and magnetic lines of force.
When electric motor is in operation, it is supplied with electrical energy and develops torque, that is, a
tendency to produce rotation.
Two important parts of rotating electric generators.
1. Even set of electromagnets or permanent magnets.
2. Laminated Steel core containing current carrying copper wires (armature winding).
In DC generator:
Armature winding is mechanically rotated through the stationary magnetic fields created by the
electromagnets or permanent magnets.
In AC generator:
Electromagnets or the permanent magnets and their accompanying magnetic fields are rotated
with respect to the stationary armature winding.
In DC motor:
Current is sent into the armature winding which is being placed inside a set of radially supported
magnet poles.
In AC motor:
Current is set into the armature winding, which is usually placed in a stationary laminated iron
core; the rotating element may or may not be a set of magnet poles.
In AC generator:
Generated alternating EMF(Electro-motive Force) is transmitted directly to the load.
, In DC generator:
Generated alternating EMF is first rectified by a commutator and brushes, which is, changed to
direct current, before it is transmitted to its load.
In AC motor:
Receives its energy directly from AC source and without any change whatever in form, uses it as
alternating current in its winding to develop torque.
In DC motor:
Direct current is delivered to the brushes but flows as alternating current in the armature
winding after passing through the brushes and commutator.
Field poles
Each one consists of a laminated steel core of rectangular cross section, surrounded by one or
more copper coils.
Shoe
Spread out portion of the pole core, permits the magnetic flux to enter the armature core over a
wider area than would be possible without a core having straight sides.
Salient pole field
The electromagnets are bolted to a hub fastened to the shaft, so that they project radially
outward toward the stationary armature core.
Non-salient pole field
When the alternator is driven by a high speed turbine, the field winding is placed in a slotted
core.
Types of direct current generators
1. Shunt generators – The excitation is produced by a single winding connected to its own positive
and negative brushes
a). self-excited shunt generator
b). separately excited shunt generator
2. Compound generator
a). Shunt field
b). Series field- contains very few turns of heavy wire. Has an extremely low resistance and is
excited by the comparatively large load current.
Both shunt and series field windings are placed over one set of pole cores and act together to
create a common set of magnetic fluxes.
Alternating current generators:
Generally called alternators
Armature core and its windings are stationary while the field poles rotate.
The speed off the rotation must be kept absolutely constant.
Two reasons why AC machines can be built in large sizes and made to develop high voltages:
1. No commutator is required.
2. The armature winding can be placed in a stationary part of the machine, the stator, where it is
possible to provide good insulation strength for the high voltage winding.
Types and characteristics of DC motors:
1. Shunt motor – If a comparatively high resistance field winding of many turns of fine wires is
employed for this function, it is connected in parallel/shunt with the armature.
