EE 328 _lect_03

Electrical Power and Machines
Lecture 3
<Dr Ahmed El-Shenawy>
<Dr Hadi El Helw>
DC Machines
The direct current (dc) machine can be used as a motor or as a generator.
DC Machine is most often used for a motor.
The major advantages of dc machines are the easy speed and torque
regulation.
However, their application is limited to mills, mines and trains. As examples,
trolleys and underground subway cars may use dc motors.
Automobiles are equipped with dc dynamos to charge their batteries.
Most DC machines are similar to AC machines: i.e. they have AC voltages and
current within them. DC machines have DC outputs just because they have a
mechanism converting AC voltages to DC voltages at their terminals. This
mechanism is called a commutator; therefore, DC machines are also called
commutating machines.
DC Machines Construction
DC Machines Construction
The construction is similar for both d.c generators and d.c motors
1. Stator
• Provides mechanical support for the machine
• consists of the yoke and poles
 Yoke provides high permeable path for
magnetic flux (thin laminated steel sheets)
 Poles are designed to accommodate field
windings
DC Machines Construction
Stator
DC Machines Construction
2.
Field Winding
The coils are wound in such a way that the poles
alternate in their polarity
a)Shunt Field Winding: many turns of fine wire
b)Series Field Winding: few turns of heavy wire,
which are placed in series with armature winding
3.
Armature
o The rotating part of the dc machine
o Has Circular Cross section and is made of thin
laminations that have axial slots to house the
armature coils
DC Machines Construction
Armature
DC Machines Construction
Armature
DC Machines Construction
Armature
DC Machines Construction
Armature Windings - Terms and Definitions
• electrical degrees θed
• mechanical degrees θmd
• p number of poles
pole pitch = distance between centers of two adjacent poles =180o
coil pitch = distance between two sides of a coil
full-pitch: coil pitch = pole pitch
short-pitch: coil pitch < pole pitch (mainly in ac-machines)
DC Machines Construction
Armature
DC Machines Construction
Armature Windings
Lap Winding
1/p of the total coils are connected in series
suitable for high-current low voltage
number of parallel paths = number of poles
= number of brushes
Wave Winding
p/2 coil connected in series between two
adjacent commutator bars
suitable for high voltage low current
number of parallel paths = 2
number of brushes positions = 2 or more
DC Machines Construction
DC Generator
In a generator, moving a conductor through a stationary magnetic
field generates voltage. If a coil is rotated through a magnetic field as
shown, an alternating voltage will be produced.
DC Generator
For the external circuit to produce DC voltage, it is necessary to reverse the
polarity of the external leads at the same time the voltage in the coil is reversed.
This is accomplished by segmenting a slip ring to form what is called a
commutator
DC Generator
DC Generator
Improving the DC output waveform:
To improve the DC signal 4 coils and 4 commutators are introduced. The
voltage between the bruches is more uniform.