Induction Motor (Asynchronous Motor) · 2014. 3. 13. · • Induction motor types: Squirrel cage...
Transcript of Induction Motor (Asynchronous Motor) · 2014. 3. 13. · • Induction motor types: Squirrel cage...
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Induction Motor(Asynchronous Motor)
ELECTRICAL MACHINES
Compiled byProf Mitali Ray
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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 isthe most rugged and the most widely used machinein industry.
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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
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Overview of Three-Phase Induction Motor
• Induction motors are used worldwide in many residential, commercial, industrial, and utility applications.
• Induction Motors transform electrical energy intomechanical energy.
• It can be part of a pump or fan, or connected to someother form of mechanical equipment such as a winder,conveyor, or mixer.
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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
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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.
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Squirrel Cage Rotor
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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.
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• 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 ringsSimple, 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.
Construction (Rotor construction)
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Construction (Rotor construction)
Wound Rotor
Squirrel-Cage Rotor
/rotor winding
Short circuits allrotor bars.
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Construction (Enclosure)
• The enclosure consists of a frame (or yoke) and two endbrackets (or bearing housings). The stator is mountedinside the frame. The rotor fits inside the stator with aslight air gap separating it from the stator. There is NOdirect physical connection between the rotor and thestator.
Stator
Rotor
Air gap
• The enclosure also protects the electricaland operating parts of the motor fromharmful effects of the environment in whichthe motor operates. Bearings, mounted onthe shaft, support the rotor and allow it toturn. A fan, also mounted on the shaft, isused on the motor shown below for cooling.
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Construction (Enclosure)
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Nameplate
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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
• Where; p = is the number of poles, and f = the frequency of supply
pfns
120=
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a Fc
-93 10 113 216-1.5
-1
-0.5
0
0.5
1
1.5
a’
c’ b’
b c
a
a’
c’ b’
b c
a
a’
c’ b’
b c
a
a’
c’ b’
b c
Fb
Fa F
FbFc
F
Fa
F
Fb
Fc Fc Fb
F
Space angle (θ) in degrees
FFa Fc
Fb
t = t0= t4
t = t1t = t2 t = t3
t = t0= t4
RMF(Rotating Magnetic Field)
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AC Machine Stator
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Axis of phase a
a’a’
-90 -40 10 60 110 160 210 260-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Fa
Space angle (theta) in degrees
t0
t01
t12
t2
a
MMF Due to ‘a’ phase current
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1 Cycle
Amp
timet0t1 t2 t3 t4
t01 t12Currents in different phases of AC Machine
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Slip Ring Rotor
•The rotor contains windings similar to stator.
•The connections from rotor are brought out using slip rings thatare rotating with the rotor and carbon brushes that are static.
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Slip and Rotor Speed1. 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.
– 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
)1( snnORn
nns srs
rs −=−
=
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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:
• Where; s = slipf = supply frequency
sfpnnf rsr
=−= )(
fsfiii
iipnnf
nnRotorAt
ipnf
nstatorAtNote
r
rsr
pf
rs
s
pf
s
.:)()(
).....(120
)(
:
).....(120
::
120
120
=÷
−=∴
=−
=∴
=
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Principle of Operation
• Torque producing mechanismWhen a 3 phase stator winding is connected to a 3
phase voltage supply, 3 phase current will flow in thewindings, hence the stator is energized.
A rotating flux Φ is produced in the air gap. The flux Φinduces a voltage Ea in the rotor winding (like atransformer).
The induced voltage produces rotor current, if rotorcircuit is closed.
The rotor current interacts with the flux Φ, producingtorque. The rotor rotates in the direction of the rotatingflux.
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Direction of Rotor Rotates
• Q: How to change the direction of• rotation?• • A: Change the phase sequence of the• power supply.
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• Conventional equivalent circuit Note:
● Never use three-phase equivalent circuit. Always use per-phase 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
Equivalent Circuit of Induction Machines
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• Step1 Rotor winding is open(The rotor will not rotate)
• Note: – the frequency of E2 is the same as that of E1 since the rotor is at
standstill. At standstill s=1.
Equivalent Circuit of Induction Machines
f f
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Equivalent Circuit of Induction Machines
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Equivalent Circuit of Induction Machines
• Step2 Rotor winding is shorted(Under normal operating conditions, the rotor winding is shorted. The slip is s)
• Note: – the frequency of E2 is fr=sf because rotor is rotating.
f fr
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• Step3 Eliminate f2
Keep the rotor current same:
Equivalent Circuit of Induction Machines
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• 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.
Equivalent Circuit of Induction Machines
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• IEEE recommended equivalent circuit
• Note:– Rc is omitted. The core loss is lumped with the
rotational loss.
Equivalent Circuit of Induction Machines
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• IEEE recommended equivalent circuit
Note: can be separated into 2 PARTS
• Purpose : – to obtain the developed mechanical
Equivalent Circuit of Induction Machines
I1 1R1X
mX
'2X '
2R
ssR −1'
21V
sR2
ssRR
sR )1(2
22 −
+=
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Analysis of Induction Machines
• For simplicity, let assume
Is=I1 , IR=I2
(s=stator, R=rotor)
[ ]RmsTotal
sss
cmm
cmcm
RR
R
ZZZZjXRZ
neglectedRjXZneglectedRjXRZ
jXs
RZ
//;
;;//
;''
+=+=
==≠=
+=
ZRZm
Zs
Vs1φ
Is1φ Im1φ IR1φ
T
ss Z
VI φ
φ1
1 =
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Analysis of Induction Machines
=
=
=
m
RMm
R
RMR
sT
mRRM
ZV
I
ZV
IHence
VZ
ZZV
RulesDividingVoltage
φφ
φφ
φφ
11
11
11
,
//
,
ZRZm
Zs
Vs1φ
Is1φ Im1φ IR1φ
φφ
φφ
11
11
,
sRm
mR
sRm
Rm
IZZ
ZI
IZZ
ZI
RulesDividingCurrent
+
=
+
=
OR
Note : 1hp =746Watt
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Power Flow Diagram
Pin (Motor)
Pin (Stator)
Pcore loss(Pc)
Pair Gap(Pag)
PdevelopedPmechanicalPconverted
(Pm)
Pout, Po
Pstator copper
loss, (Pscu)Protor copper
loss (Prcu)Pwindage, friction,
etc(Pµ - Given)
θcos3 ss IV
sRI R
R''3 2
2
3
c
RM
RV ''3 2
RR RI
−
ssRI RR
1''3 2
Whp 7461 =
ss RI 23
Pin (Rotor)
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Power Flow Diagram• Ratio:
Pag Prcu Pm
sRI R
R''3 2 ''3 2
RR RI
−
ssRI RR
1''3 2
s1 11
−s
1
1 s s−1
Ratio makes the analysis simpler to find the value of the particular power if we haveanother particular power. For example:
ss
PP
m
rcu
−=
1
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Efficiency
WattxWhpxPIVP
otherwisePPPPPP
givenarePifPP
out
ssin
mo
lossesino
losses
in
out
746746cos3
,
,
%100
=×==
−=−=
×=
θ
η
µ
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Torque-Equation
• Torque, can be derived from power equation in term of mechanical power or electrical power.
nPTHence
sradnwhereTPPower
π
πωω
260,
)/(60
2,,
=
==
r
oo
r
mm
nPTTorqueOutput
nPTTorqueMechanical
Thus
π
π
260,
260,
,
=
=
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Torque-Equation• Note that, Mechanical torque can written in terms of circuit
parameters. This is determined by using approximation method
...
...
...
)1(''3
)1(''3
2
2
−==∴
=−=
r
RR
r
mm
mrmR
Rm
ss
RIPT
TPandss
RIP
ωω
ω
+
=∴ 22
2
)'()'('
2)(3
RR
R
s
RMm sXR
sRn
VT
πφ
Hence, Plot Tm vs s
Tm
ns
smax is the slip for Tmax to occur
s=1
Tst
Tmax
smax
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Torque-Equation
+++
=∴
=
22
2
)'()'('
602
)(3
1,
RsRs
R
s
sst XXRR
Rn
VT
sTorqueStarting
π
φ
+++
=
+±=
22
2
max
22max
)'()(1
6022
)(3
)'()('
Rssss
s
Rs
R
XXRRnV
T
XRRs
π
φ
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Speed Control
• There are 3 types of speed control of 3 phase induction machines
i. Varying rotor resistanceii. Varying supply voltageiii. Varying supply voltage and supply frequency
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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
ns~nNL
T
nr1nr2nr3 n
nr1< nr2< nr3R1R2R3
R1< R2< R3
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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 electronicscircuit for voltage controller
• Suitable for fan type load• Disadvantages :
– Large speed regulation since~ ns
T
ns~nNL
T
nr1nr2nr3n
nr1> nr2 > nr3
V1
V2
V3
V1> V2 > V3
V decreasing
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• The best method since supply voltage and supply frequency is varied to keep V/f constant
• Maintain speed regulation• uses power electronics
circuit for frequency and voltage controller
• Constant maximum torque
Varying supply voltage and supply frequency
T
nNL1
T
nr1nr2nr3 n
fdecreasing
nNL2nNL3