Chapter 6: Introduction to Rotating Machines
1)
2)
3)
4)
5)
6)
7)
Energy conversion
Electromagnetic principles
Electromagnetic principles
Motors
AC motors
DC motors
Motor selection Generators
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 1
Chapter 6: Introduction to Rotating Machines
Electromechanical Energy Conversion
Electrical System
y
Losses
Rotating Machines:
•Motors •Generators
Coupling field Losses
Mechanical System
y
Losses
Generator
Motor
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 2
Chapter 6: Introduction to Rotating Machines
Electromagnetic principles
Electromagnetic principles
1) Electromotive force (EMF) produces current in a conductor.
2) An electric current constitutes magnetomotive force (MMF).
3) Motor action: A magnetic field exerts a force on a current‐carrying conductor. 4) Generator action: A conductor moving through a magnetic field induces an EMF.
G
i
A
d
i
h
h
i fi ld i d
EMF
Right Hand Rules
g
Lorentz Force Law:

 
F  q (v  B )
  
F  I B
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 3
Chapter 6: Introduction to Rotating Machines
Electromagnetic principles
Electromagnetic principles

 
F  q (v  B )
  
F  I B
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 4
Chapter 6: Introduction to Rotating Machines
C t ti
Construction and Components
dC
t
• The stator is a stationary part containing magnetized poles.
containing magnetized poles. • The rotor consists of a rotating shaft supported by bearings
shaft supported by bearings.
An air gap is between the rotor and th t t
the stator. Rotor/Stator Components: • Core (ferromagnetic material). • Windings or coil • Insulation Winding Currents:
Winding Currents: • Load current • Magnetizing (exciting) current
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 5
Chapter 6: Introduction to Rotating Machines
C t ti
Construction and Components
dC
t
• Armature windings refer to a windings which carry ac currents (load current)
(load current).
• Field windings carry dc current (magnetizing current) producing the main magnetic flux. • Primary windings
Pi
i di
carry both load and magnetizing currents. b th l d d
ti i
t
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 6
Chapter 6: Introduction to Rotating Machines
Types of Rotating Machines
f
h
1) Synchronous machines •Speed of machines is proportional to frequency of its armature current
•Speed of machines is proportional to frequency of its armature current.
•Synchronous generators are always used in power generation units. •They require external excitation for generating magnetic flux. 2) Induction (asynchronous) machines
• Induction generators seldom use. • 1φ induction motors are used in household applications. • Many industries use 3φ induction motors.
3) DC machines
• DC machines provide rapid acceleration or deceleration • Both motors and generators are used. Both motors and generators are used.
• DC motors give speed over wide range and constant torque.
• DC generators are always used in small units of power generating, e.g., wind turbine
turbine.
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 7
Chapter 6: Introduction to Rotating Machines
Motors
oto s
DC motors
•
•
•
•
Permanent Magnet
Series Wound
Shunt Wound
Compound Wound
Si l Ph
Single Phase
Squirrel Cage
•
•
•
•
•
Split Phase Capacitor Start
Permanent Split Capacitor
Shaded Pole h d d l
Two‐Value Capacitor
I d ti
Induction
Wound Rotor
Synchronous
AC motors
Poly Phase
Induction
Synchronous
•
•
•
•
• Repulsion
• Repulsion Start
• Repulsion Induction
Shaded Pole Hysteresis Reluctance
Permanent Magnet
Squirrel Cage
W
Wound Rotor
dR t
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 8
Chapter 6: Introduction to Rotating Machines
IInformation from Nameplate f
ti f
N
l t
• Device type
D i
• Manufacturer
• Rated voltage and frequency
Rated voltage and frequency
• Rated current and VA
• Rated speed and HP
(1HP = 746W)
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 9
Chapter 6: Introduction to Rotating Machines
DC Machines
DC Machines
1) Main magnetic flux is generated from the stator which can be permanent magnetic material or field windings.
p
g
g
2) Its rotor acts as the armature winding connected with the rotating comumutator. 3) The commutator contact with stationary brushes. 4) For DC motor, the commutator converts DC to AC (current in the armature) ensuring the direction of rotation. 5) For DC generator, the commutator
F DC
h
converts AC (induced AC (i d d
current in the armature) to approximating DC at the brushes. Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
Brush
page 10
Chapter 6: Introduction to Rotating Machines
DC Machines
DC Machines
DC Motors DC Generators Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 11
Chapter 6: Introduction to Rotating Machines
DC Machines
DC Machines
• Developed Torque Tdev  K a  d I A
where K
e e a is a machine constant, Φ
s a ac e co s a , Φa is s
the total magnetic flux per pole and Ia
is the armature current. • Speed Voltage (voltage between brushes)
Speed Voltage (voltage between brushes)
E A  K a  d m
where ωm is rotational speed (rad/s).
where ω
is rotational speed (rad/s)
• Developed Power
Pdev  E A I A  mTdev
Loss in DC machines
•
•
Pcu Copper losses (in windings)
C
l
(i i di )
Prot Rotational losses (friction, core losses)
Note: Units of rotational speed can be either rad/s or rpm.
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 12
Chapter 6: Introduction to Rotating Machines
DC Machines
Equivalent circuit of DC Machine
Motor
Generator : Same as motor but armature current is opposite direction
Copper Losses Armature Current
Pcu  I 2f R f  I a2 Ra
IA 
VT  VA
Ra
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 13
Chapter 6: Introduction to Rotating Machines
DC Machines
Power Flow
Pelec
• Motor: Pdev
Pmech
Pin  Pelec ; Pout  Pmech
• Generator: Pcu
Prot
Pin  Pmech ; Pout  Pelec
Efficiency
Pout

 100%
Pin
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 14
Chapter 6: Introduction to Rotating Machines
DC Machines
Example: A permanent‐magnet DC machine has armature resistance 3 ohms. When it operates as a generator, it generates the voltage of 100 volts at speed of 1000 rpm. If its armature is supplied by a source of 150 volts operating as a motor compute the following
armature is supplied by a source of 150 volts operating as a motor, compute the following quantities:
1) The starting current
2) The induced voltage (back emf) when it runs at 1200 rpm and at 1400 rpm
2) The induced voltage (back emf) when it runs at 1200 rpm and at 1400 rpm. 3) The armature current and developed torque at speed 1200 rpm and 1400 rpm. Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 15
Chapter 6: Introduction to Rotating Machines
DC Machines
Classification of DC Machines
1) Separately Excited
• A DC Source is supplied to the field winding.
• In generator, the field current is a small fraction of the rated armature current.
• Separately excited motors are used in control systems.
• For small motor, permanent magnet is used instead of the field windings. K
Keywords: d
‐Starting torque (stall torque) Motor torque‐speed curve ‐No‐load speed
N l d
d
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 16
Chapter 6: Introduction to Rotating Machines
DC Machines
Cl ifi i
Classification of DC Machines
f DC M hi
2) Self‐Excited Shunt DC machines
• The field winding is connected in parallel with the armature. • For generators, induced voltage from g
,
g
armature is applied to the field. • For
For motors, a voltage source is applied to motors a voltage source is applied to
both the field and the armature. •A
A rheostat is connect in series with field rheostat is connect in series with field
winding for voltage control (in generators) or speed control (in motors).
• Shunt motors give nearly constant speed over a wide range of loads. Motor torque‐speed curve Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 17
Chapter 6: Introduction to Rotating Machines
DC Machines
Cl ifi i
Classification of DC Machines
f DC M hi
3) Self‐Excited Series DC machines • Armature current and field current are identical. identical
• Series generator gives poor voltage regulation. • Series motors give very high starting torque and high speed when the load is small. Hence, they should not operate under unload condition (ran‐away).
• Series motors can be supplied by both DC and AC. It is also called universal motor.
Motor torque‐speed curve Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 18
Chapter 6: Introduction to Rotating Machines
DC Machines
Classification of DC Machines
4) Self‐Excited Compound DC machines
•A
A compound motor has a series field and a compound motor has a series field and a
shunt field. • Wh
When a compound motor runs at no‐load, d
t
t
l d
armature current and mmf of the series filed are small. It behaves like a shunt motor.
• Keywords: short‐shunt connection, long‐
shunt connection • Keywords: cummulative compounding, differential compounding.
p
g
Motor torque‐speed curve Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 19
Chapter 6: Introduction to Rotating Machines
DC Machines
Characteristics of DC Machines
Generator%
Motor
oo
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 20
Chapter 6: Introduction to Rotating Machines
DC Machines
Example: A shunt DC motor operating at 1200 rpm is supplied by a 120 volts. The line current is 50 A. The shunt‐field resistance and armature resistance are 100 ohms and 0.2 ohms, respectively Compute
respectively. Compute 1) The field current 2) The armature current 3) Induced voltage (back emf)
3) Induced voltage (back emf) 4) The mechanical power (in hp) Further interesting topics
•Starting DC machines
•Stopping DC machines
S
i DC
hi
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 21
Chapter 6: Introduction to Rotating Machines
AC Machines
AC Generators
AC Motor
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 22
Chapter 6: Introduction to Rotating Machines
AC Machines: Synchronous Machines
AC Machines: Synchronous Machines
• Mechanical speed is proportional to the frequency of the current in
to the frequency of the current in the armature. • The
The field winding produces a single field winding produces a single
pair of magnetic poles.
• Armature winding is on the stator
A
t
i di i
th t t
• Field winding is on the rotor and is excited by dc. Two poles, single phase, synchronous generator Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 23
Chapter 6: Introduction to Rotating Machines
AC Machines: Synchronous Machines
AC Machines: Synchronous Machines
 poles 
Electrical frequency; f e  n
 Hz
 120 
n is mechanical speed in revolutions per minute (rpm)
Four poles, single phase synchronous
phase, synchronous generator  poles 
or e  
m
 2 
The relation is known as synchronous speed. y
p
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 24
Chapter 6: Introduction to Rotating Machines
AC Machines: Synchronous Machines
AC Machines: Synchronous Machines
Salient poles
IInstallation for hydroelectric generator t ll ti f h d l t i
t
which operate at low speeds.
Four poles, single phase
p
, g p
Non‐Salient poles or Cylindrical‐rotor.
Installation for steam (or gas) turbine generators which operate at high speeds.
Two poles, single phase Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 25
Chapter 6: Introduction to Rotating Machines
AC Machines: Synchronous Machines
AC Machines: Synchronous Machines
Salient poles
IInstallation for hydroelectric generator t ll ti f h d l t i
t
which operate at low speeds.
Four poles, single phase
p
, g p
Non‐Salient poles or Cylindrical‐rotor.
Installation for steam (or gas) turbine generators which operate at high speeds.
Two poles, single phase Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 26
Chapter 6: Introduction to Rotating Machines
AC
AC Machines: Synchronous Machines
Machines: Synchronous Machines
Four poles, three‐phase Y‐connection generator Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 27
Chapter 6: Introduction to Rotating Machines
AC Machines: Induction Motors
AC Machines: Induction Motors
• AC appears in both stator and rotor windings.
• The stator winding is the same as a synchronous machine.
• An external AC source is supplied to the stator windings. • The rotor winding is short‐circuited and has not external connections.
• Currents in rotor are induced by transformer action. • The rotor does not rotate synchronously.
• The rotational speed of the rotor is less than synchronous speed.
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 28
Chapter 6: Introduction to Rotating Machines
AC Machines: Induction Motors
AC Machines: Induction Motors
Advantage
• Induction motors are rugged, inexpensive and easy to maintain.
• Induction motors have many size from a few watts up to 10,000 hp.
• Induction motors give nearly constant speed.
Disadvantage
• It is difficult to control speed.
• Starting current is peak, five to eight times the full‐load current.
• The lagging power factor is low when the motor drives a lightly load. Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 29
Chapter 6: Introduction to Rotating Machines
AC Machines: Induction Motors
AC Machines: Induction Motors
Rotor Types
• A
A wound rotor
wound rotor carries a poly phase carries a poly phase
winding having the same number of poles as the stator. The terminals of th
the rotor are connected to insulated t
t dt i l t d
slip rings mounted on the shaft providing external connection. • A
A squirrel‐cage rotor is made from squirrel‐cage rotor is made from
conducting bar embedded in slots in the rotor. Hence this type of motor is rugged. t i
d
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 30
Chapter 6: Introduction to Rotating Machines
AC Machines: Induction Motors
AC Machines: Induction Motors
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 31
Chapter 6: Introduction to Rotating Machines
AC Machines: Induction Motors
AC Machines: Induction Motors
nslip
Slip speed is the difference between the motor speed n and the synchronous speedns
speed and the synchronous speed .
nslip  ns  n
Recall that
f
ns  120 (rpm)
P
It can be verified that
ca be e ed a
nslip  ns  n  120
f slip
P
(rpm)
f slip
li  sf
Slip is defined as normalized slip speed. ns  n

s
ns
ns
nslip
nslip  sns
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 32
Chapter 6: Introduction to Rotating Machines
Example: A 3Φ, 460 V, 100 hp, 60 Hz four‐pole induction motor delivers rated output power a slip 5%. Compute 1) Synchronous speed
Synchronous speed
2) Motor speed 3) Frequency of the rotor circuit 4) Slip speed Slip speed
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 33
Chapter 6: Introduction to Rotating Machines
AC Machines: Induction Motors
AC Machines: Induction Motors
Warit Silpsrikul ‐‐ 11‐205 Fundamental of Electrical Engineering‐‐ North Chiang Mai University
page 34