Transcript Induction Motor

Induction Motor
(Asynchronous Motor)
ELECTRICAL MACHINES
Compiled by
Prof Mitali Ray
1
Learning Outcomes
• At the end of the lecture, student should to:
– Understand the principle and the nature of 3 phase
induction machines.
– Perform an analysis on induction machines which is
the most rugged and the most widely used machine
in industry.
2
Contents
–
–
–
–
Overview of Three-Phase Induction Motor
Construction
Principle of Operation
Equivalent Circuit
•
•
Power Flow, Losses and Efficiency
Torque-Speed Characteristics
– Speed Control
– Overview of Single-Phase Induction Motor
3
Overview of Three-Phase Induction
Motor
•
•
•
Induction motors are used worldwide in many
residential, commercial, industrial, and utility
applications.
Induction Motors transform electrical energy into
mechanical energy.
It can be part of a pump or fan, or connected to some
other form of mechanical equipment such as a winder,
conveyor, or mixer.
4
Introduction
General aspects
• A induction machine can be used as either a induction
generator or a induction motor.
• Induction motors are popularly used in the industry
• Focus on three-phase induction motor
• Main features: cheap and low maintenance
• Main disadvantages: speed control is not easy
5
Construction
• The three basic parts of an AC motor are the rotor, stator,
and enclosure.
• The stator and the rotor are electrical circuits that perform as
electromagnets.
7
Squirrel Cage Rotor
Construction (Stator construction)
• The stator is the stationary electrical part of the motor.
• The stator core of a National Electrical Manufacturers Association
(NEMA) motor is made up of several hundred thin laminations.
• Stator laminations are stacked together forming a hollow cylinder.
Coils of insulated wire are inserted into slots of the stator core.
• Electromagnetism is the principle behind motor operation. Each
grouping of coils, together with the steel core it surrounds, form an
electromagnet. The stator windings are connected directly to the
power source.
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Construction (Rotor construction)
• The rotor is the rotating part of the electromagnetic
circuit.
• It can be found in two types:
– Squirrel cage
– Wound rotor
• However, the most common type of rotor is the
“squirrel cage” rotor.
10
Construction (Rotor construction)
• Induction motor types:
 Squirrel cage type:
Rotor winding is composed of copper bars embedded in
the rotor slots and shorted at both end by end rings
Simple, low cost, robust, low maintenance
 Wound rotor type:
Rotor winding is wound by wires. The winding terminals
can be connected to external circuits through slip rings
and brushes.
Easy to control speed, more expensive.
11
Construction (Rotor construction)
Wound Rotor
Squirrel-Cage Rotor
Short circuits all
rotor bars.
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/rotor winding
12
Construction (Enclosure)
• The enclosure consists of a frame (or yoke) and two end
brackets (or bearing housings). The stator is mounted
inside the frame. The rotor fits inside the stator with a
slight air gap separating it from the stator. There is NO
direct physical connection between the rotor and the
stator.
• The enclosure also protects the electrical
and operating parts of the motor from
harmful effects of the environment in which
the motor operates. Bearings, mounted on
the shaft, support the rotor and allow it to
turn. A fan, also mounted on the shaft, is
used on the motor shown below for cooling.
Stator
Rotor
Air gap
13
Construction (Enclosure)
14
Nameplate
15
Rotating Magnetic Field
• When a 3 phase stator winding is connected to a 3 phase
voltage supply, 3 phase current will flow in the windings,
which also will induced 3 phase flux in the stator.
• These flux will rotate at a speed called a Synchronous
Speed, ns. The flux is called as Rotating magnetic Field
• Synchronous speed: speed of rotating flux
120 f
ns 
p
• Where;
p = is the number of poles, and
f = the frequency of supply
16
a
c’
Fc
b
b’
Fa
c
RMF(Rotating Magnetic Field)
1.5
F
1
Fa
0.5
Fb
a’
0
F
c’
a’
t = t1
10
113
216
Space angle () in degrees
Fc
b’
c
b
Fc
-1
-1.5
-93
Fb a
t = t 0 = t4
Fb
-0.5
t = t0= t4
F
Fb a
c’
Fa
F b
Fc a’
t = t2
b’
c
a
b’
b
Fc a’
t = t3
F
c
c’
Fb
AC Machine Stator
MMF Due to ‘a’ phase current
1
Axis of phase a
0.8
t0
0.6
0.4
t01
0.2
Fa
0
a’
a
a’
t12
-0.2
-0.4
-0.6
t2
-0.8
-1
-90
-40
10
60
110
160
Space angle (theta) in degrees
210
260
Currents int different
phases
of
AC
Machine
t
01
12
Amp
t0
t1
t2
1 Cycle
t3
t4
time
Slip Ring Rotor
•The rotor contains windings similar to stator.
•The connections from rotor are brought out using slip rings that
are rotating with the rotor and carbon brushes that are static.
Slip and Rotor Speed
1. Slip s
–
The rotor speed of an Induction machine is different from the
speed of Rotating magnetic field. The % difference of the speed
is called slip.
ns  nr
s
ns
OR nr  ns (1  s)
– Where;
ns = synchronous speed (rpm)
nr = mechanical speed of rotor (rpm)
– under normal operating conditions, s= 0.01 ~ 0.05, which is
very small and the actual speed is very close to synchronous
speed.
– Note that : s is not negligible
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Slip and Rotor Speed
•
Rotor Speed
–
When the rotor move at rotor speed, nr (rps), the stator flux will
circulate the rotor conductor at a speed of (ns-nr) per second.
Hence, the frequency of the rotor is written as:
f r  (ns  nr ) p
 sf
•
Where;
s = slip
f = supply frequency
Note :
At stator : ns  120p f
ns p
120
At Rotor: ns  nr  120p f
f 
 fr 
(ii )  (i ) :
f r  s. f
.....(i )
(ns  nr ) p
120
.....(ii )
23
Principle of Operation
• Torque producing mechanism
 When a 3 phase stator winding is connected to a 3
phase voltage supply, 3 phase current will flow in the
windings, hence the stator is energized.
 A rotating flux Φ is produced in the air gap. The flux Φ
induces a voltage Ea in the rotor winding (like a
transformer).
 The induced voltage produces rotor current, if rotor
circuit is closed.
 The rotor current interacts with the flux Φ, producing
torque. The rotor rotates in the direction of the rotating
flux.
24
Direction of Rotor Rotates
•
•
•
•
Q: How to change the direction of
rotation?
• A: Change the phase sequence of the
power supply.
25
Equivalent Circuit of Induction
Machines
• Conventional equivalent circuit
 Note:
● Never use three-phase equivalent circuit. Always use perphase equivalent circuit.
● The equivalent circuit always bases on the Y connection
regardless of the actual connection of the motor.
● Induction machine equivalent circuit is very similar to the
single-phase equivalent circuit of transformer. It is
composed of stator circuit and rotor circuit
26
Equivalent Circuit of Induction
Machines
• Step1 Rotor winding is open
(The rotor will not rotate)
f
f
• Note:
– the frequency of E2 is the same as that of E1 since the rotor is at
standstill. At standstill s=1.
27
Equivalent Circuit of Induction
Machines
28
Equivalent Circuit of Induction
Machines
• Step2 Rotor winding is shorted
(Under normal operating conditions, the rotor winding is shorted. The slip is s)
f
fr
• Note:
– the frequency of E2 is fr=sf because rotor is rotating.
29
Equivalent Circuit of Induction
Machines
• Step3 Eliminate f2
Keep the rotor current same:
30
Equivalent Circuit of Induction
Machines
• Step 4 Referred to the stator side
• Note:
–
X’2 and R’2 will be given or measured. In practice, we do not
have to calculate them from above equations.
– Always refer the rotor side parameters to stator side.
– Rc represents core loss, which is the core loss of stator side.
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Equivalent Circuit of Induction
Machines
• IEEE recommended equivalent circuit
• Note:
– Rc is omitted. The core loss is lumped with the
rotational loss.
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Equivalent Circuit of Induction
Machines
• IEEE recommended equivalent circuit
I1
V1
X1
R1
X 2'
R2'
Xm
R2'
1 s
s
Note: R2 can be separated into 2 PARTS
s
R2
R2 (1  s )
 R2 
s
s
• Purpose :
– to obtain the developed mechanical
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Analysis of Induction Machines
• For simplicity, let assume
Is=I1 , IR=I2
(s=stator, R=rotor)
RR '
 jX R ' ;
s
Z m  Rc // jX m ; Rc  neglected
ZR 
Z m  jX m
; Rc  neglected
Z s  Rs  jX s
Is1
Vs1
Zs
Im1
IR1
Zm
ZR
;
ZTotal  Z s  Z m // Z R 
I s1 
Vs 1
ZT
34
Analysis of Induction Machines
Current Dividing Rules,
Is1
Zs
Im1
IR1
I m1
I R1
Vs1
Zm
ZR
 ZR 

 I s1
 Zm  ZR 
 Zm 

 I s1
 Zm  ZR 
OR
VoltageDividing Rules,
Note : 1hp =746Watt
 Z // Z m 
VRM 1   R
Vs1
 ZT 
VRM 1 
Hence, I R1  

Z
 R 
VRM 1 
I m1  

 Zm 
35
Power Flow Diagram
3Vs I s cos
1hp  746W
Pin (Motor)
Pin (Stator)
Pin (Rotor)
Pair Gap
(Pag)
RR '
3I R '
s
2
Pstator copper
loss, (Pscu)
2
3I s Rs
Pcore loss
(Pc)
V
3 RM
 Rc



2
Pdeveloped
Pmechanical
Pconverted
(Pm)
Pout, Po
1 s 
3I R '2 RR ' 

 s 
Protor copper
loss (Prcu)
Pwindage, friction,
3I R '2 RR '
(P - Given)
etc
36
Power Flow Diagram
•
Ratio:
Pag
Prcu
Pm
3I R ' RR '
1 s 
3I R ' RR ' 

 s 
1
s
1
1
1
s
1
s
1 s
RR '
3I R '
s
2
2
2
Ratio makes the analysis simpler to find the value of the particular power if we have
another particular power. For example:
Prcu
s

Pm 1  s
37
Efficiency
Pout

100%
Pin
if Plosses are given,
Po  Pin  Plosses
Po  Pm  P
otherwise,
Pin  3 Vs I s cos
Pout  x hp 746W  746x Watt
38
Torque-Equation
• Torque, can be derived from power equation in term of
mechanical power or electrical power.
Power, P  T , where  
Hence, T 
2 n
(rad / s )
60
60P
2 n
Thus,
60Pm
MechanicalTorque, Tm 
2nr
60Po
OutputTorque, To 
2nr
39
Torque-Equation
•
Note that, Mechanical torque can written in terms of circuit
parameters. This is determined by using approximation
method
Hence, Plot Tm vs s
2 RR '
Pm  3I R '
(1  s ) and Pm  rTm
s
Tmax
T
m


2 RR '
3
I
'
(
1

s
)

Pm  R s
Tm 


r 
r



...
Tst
...
...
 3(VRM ) 2  

sRR '
Tm  

2
2
2

n
(
R
'
)

(
sX
'
)

  R
s
R

s=1
smax
smax is the slip for Tmax to occur
ns
40
Torque-Equation
Starting Torque, s  1


2
 3(V )  

RR '
s


Tst 

2
2
 2  ns    ( Rs  RR ' )  ( X s  X R ' ) 
  60  
smax


RR '

 
2
2
 ( R s )  ( X R ' ) 
Tmax


 3(V ) 2  

1
s




2
2
   ns    Rs  ( Rs )  ( X s  X R ' ) 

 2 2  60   
    
41
Speed Control
•
There are 3 types of speed control of 3 phase
induction machines
i. Varying rotor resistance
ii. Varying supply voltage
iii. Varying supply voltage and supply frequency
42
Varying rotor resistance
• For wound rotor only
• Speed is decreasing
• Constant
maximum
torque
• The speed at which max
torque occurs changes
• Disadvantages:
– large speed regulation
– Power loss in Rext
reduce the efficiency
–
T
R3
R2
R1
R1< R2< R3
nr1< nr2< nr3
T
nr3 nr2 nr1 n ~n n
s
NL
43
Varying supply voltage
• Maximum torque changes
• The speed which at max
torque occurs is constant
(at max torque, XR=RR/s
• Relatively simple method –
uses power electronics
circuit for voltage controller
• Suitable for fan type load
• Disadvantages :
– Large speed regulation since
~ ns
T
V1
V2
V3
V
decreasing
V1> V2 > V3
nr1> nr2 > nr3
T
nr3 nr2 nr1 n ~n n
s
NL
44
Varying supply voltage and supply
frequency
• The best method since
supply voltage and supply
frequency is varied to keep
V/ constant
f
• Maintain speed regulation
• uses power electronics
circuit for frequency and
voltage controller
• Constant maximum torque
f
decreasing
T
T
nr3
n
nr2
nr1 nNL1
nNL3 nNL2
45