synchronous machines

Slovak University of Technology
Faculty of Material Science and Technology in Trnava
ELECTRICAL ENGINEERING
AND ELECTRONICS
Unit 7 - Synchronous Machines and
Direct Current Motors

SYNCHRONOUS MACHINES
Round Rotor Machine
• The stator is a ring
shaped laminated ironcore
with slots.
• Three phase windings
are placed in the slots.
• Round solid iron rotor
with slots.
• A single winding is
placed in the slots. Dc
current is supplied
through slip rings.
Concept (two poles)
Rotor with dc
winding
B
A
Stator with
laminated iron-core
C
-
A
-
+ C
N
B
+
B
S
A
-
+
C
Slots with
winding

Synchronous Machines
Salient Rotor Machine
• The stator has a laminated ironcore
with slots and three phase
windings placed in the slots.
• The rotor has salient poles
excited by dc current.
• DC current is supplied to the
rotor through slip-rings and
brushes.
• Concept (two poles)
B - C +
N
A -
A +
S
B +
C -
• The number of poles varies
between 2 - 128.

Synchronous Machines
Round Rotor Generator
Generator
Exciter
View of a two-pole round rotor generator and exciter.

Synchronous Machines
A
Stator with
laminated iron core
C -
Slots with
phase
winding
B
Rotor with
dc winding
A +
+
+
N
B -
C +
-
+
-
-
+
-
+
S
-
AḆ+
C
Major components of a round rotor two-pole generator

SYNCHRONOUS MACHINES
Housing ,cooling ducts
Stator
Rotor
Shaft
Bearing
Stator winding
Rotor winding
Connections

SYNCHRONOUS MACHINES
Construction
• Construction
• The picture shows the laminated
iron core and the slots (empty
and with winding).
• The winding consists of copper
bars insulated with mica and
epoxy resin.
• The conductors are secured by
steel wedges.
• In large machines, the stator is
liquid cooled.
• The iron core is supported by a
steel housing.
empty slots
insulated
winding

SYNCHRONOUS MACHINES
Stator
• Laminated iron core
with slots
• Steel Housing

SYNCHRONOUS MACHINES
Iron core
Stator details
• Coils are placed in slots
Coil
• Coil end windings are
bent to form the
armature winding.
Slots
End winding

SYNCHRONOUS MACHINES
Round rotor
• The round rotor is used
for large high speed
(3600rpm) machines.
• A forged iron core (not
laminated,DC) is
installed on the shaft.
• Slots are milled in the
iron and insulated
copper bars are placed
in the slots.
• The slots are closed by
wedges and re-enforced
with steel rings.
Round rotor

SYNCHRONOUS MACHINES
Rotor Details

SYNCHRONOUS MACHINES
Round rotor
Shaft
Steel ring
DC current terminals
Wedges

Synchronous Machines
Salient pole generator
Rotor with
dc winding
B +
C -
N
- +
- +
A -
- +
A + - +
- +
S
C +
B -
Stator with
laminated iron core
Slots with
phase
winding
Two-pole salient pole generator concept.

Synchronous Machines
B +
A -
C +
C -
N
- +
- +
A +
S
S
B -
C +
-
- +
+
+
+
N
- +
-
+
-
-
C -
B -
A +
A -
B +
Four-pole salient pole generator concept.

Synchronous Machines
Stator of a large salient pole hydro generator; inset shows the
insulated conductors and spacers.

Synchronous Machines
Large hydro generator rotor with view of the vertical poles.

Synchronous Machines
Slip
rings
Pole
Fan
DC excitation
winding
Rotor of a four-pole salient pole generator.

SYNCHRONOUS MACHINES
Salient pole rotor construction
• The poles are bolted to the shaft.
• Each pole has a DC winding.
• The DC winding is connected to the slip-rings (not shown).
• A DC source supplies the winding with DC through brushes
pressed into the slip ring.
• A fan is installed on the shaft to assure air circulation and
effective cooling.

SYNCHRONOUS MACHINES

SYNCHRONOUS MACHINES
Construction
• Low speed, large hydrogenerators
may have more
than one hundred poles.
• These generators are
frequently mounted vertically.
• The picture shows a large,
horizontally arranged
machine.

DIRECT CURRENT MACHINES
DC machine Construction
Dc motor construction
• The major advantages of DC
machines are: easy speed and
torque regulation.
• The stator of the DC motor has
poles, which are excited by DC
current to produce magnetic fields.
• The rotor has a ring-shaped
laminated iron-core with slots.
• Coils with several turns are placed in
the slots. The distance between the
two legs of the coil is about 180
electric degrees.
N
Field
Brush
S
Rotor
Stator with
with poles

DIRECT CURRENT MACHINES
DC machine Construction
Concept of the commutator
• The coils are connected in series.
• The junction points of the coils are
connected to a commutator.
I_
• The commutator consists of
insulated copper segments
mounted in a cylinder.
• Two brushes are pressed to the
commutator to permit current flow.
• The brushes are placed in the
neutral zone (magnetic field is
close to zero) to reduce arcing.
I +
coil
Insulator
Copper

DIRECT CURRENT MACHINES
DC machine Construction
Dc motor stator construction
• The picture shows the stator of a
large DC machine with several
poles.
• Note the interpoles between the
main poles. These poles reduce the
field in the neutral zone and
eliminate arcing of the commutator.
• A compensation winding is placed
on the main poles to increase field
during high load.
• The iron core is supported by a cast
iron frame.
Field poles
Inter pole
Compensation
winding

DIRECT CURRENT MACHINES
DC machine Construction
• The adjoining picture shows the rotor
of a DC machine.
• The rotor iron core is mounted on the
shaft.
• Coils are placed in the slots.
• The end of the coils are bent and
tied together to assure mechanical
strength.
• Note the commutator mounted on
the shaft. It consists of several
copper segments, separated by
insulation.
DC motor rotor construction
Poles
Fan
Brushes
Rotor winding
Bearing
Commutator

DIRECT CURRENT MACHINES
DC Machine Construction
Ring
Insulator
Flag
• The adjoining picture shows the
commutator of a large DC machine.
• The segments are made out of
copper and mica insulation is placed
between the segments.
• The end of each segment has a flag
attached. The coil endings are
welded to these flags.
• An insulated ring is placed on the
coil ends to assure proper
mechanical strength.
Copper

DIRECT CURRENT MACHINES
Operation principles
DC motor operation concept
Stationary
Brushes
• The poles are supplied by DC
current producing a DC magnetic
field.
• The poles are shaped in such a way
the field distribution along the pole is
more or less sinusoidal.
• When the rotor coil is rotated, the
flux linkage changes during the
rotation. It is maximum when the coil
is in vertical position and zero when
it is in horizontal position.
N
S
Rotating Coil Sides

DIRECT CURRENT MACHINES
Concept of commutation
• The current direction changes
as the conductor passes
through the neutral zone.
• The direction of magnetic field
also changes as the conductor
passes through the neutral zone.
Neutral Zone
B
F
B
F
I
N
S
I
N
S
Magnetic field

DIRECT CURRENT MACHINES
Induced voltage and torque calculation.
• The magnetic field is generated by the field current I f .
• The flux Φ f is proportional to the field current.
Φ f = K f I f .
• The K f factor is calculated from the magnetic circuit using Amperes
Law.
• The flux change is proportional to the motor speed ω m .

DIRECT CURRENT MACHINES
Induced voltage and torque calculation.
• The induced voltage after rectification is:
E a = K Φ f ω m = K K f I f ω m = K m I f ω m
• The out put power if the losses are neglected is:
P dc = I a E a = T m ω m .
• The torque is:
T m = I a E a / ω m = K Φ f I a = K m
I f I a

DIRECT CURRENT MACHINES
Equivalent circuit.
• The DC machine can be represented by a voltage source and a
resistance connected in series. The armature winding has a resistance,
R a .
• The field circuit is represented by a winding that generates the magnetic
field and a resistance connected in series. The field winding has
resistance R f .
R a
R f
I f
Φ f
I a
E a
V a
V f

DIRECT CURRENT MACHINES
Equivalent circuit.
The equations are:
Φ f = K f I f
R a
E a = K Φ f w m = K m I f ω m
E a = V a + I a
R a
R f
Φ
f
E a
I a
V a
V f = I f R f
V f
I f
T m = K Φ f I a = K m I f I a

DIRECT CURRENT MACHINES
Type of DC machines.
• Separately excited machine.
The main winding supplies the load.
The field winding is supplied by a separate DC source whose voltage is
variable.
Good speed control.
Φ f
V a
V f

DIRECT CURRENT MACHINES
Type of DC machines.
• Shunt DC machine.
The armature and field windings are connected in parallel.
Constant speed operation.
Φ f
V a

DIRECT CURRENT MACHINES
Major Types of DC machines.
• Series DC machine.
The armature and field winding are connected in series.
High starting torque.
V a
Φ s

DIRECT CURRENT MACHINES
Major Types of DC machines.
• Compound DC machine.
The machine has two field windings: One connected in series; the other
in parallel.
The series winding provides additional, load dependent excitation.
Reduced voltage drop at high load.
Φ s
Φ f
V a