Upload
nina-gregory
View
343
Download
45
Tags:
Embed Size (px)
DESCRIPTION
Induction Motor (Asynchronous Motor). ELECTRICAL MACHINES Compiled by Prof Mitali Ray. Learning Outcomes. At the end of the lecture, student should to: Understand the principle and the nature of 3 phase induction machines. - PowerPoint PPT Presentation
Citation preview
1
Induction MotorInduction Motor(Asynchronous Motor)(Asynchronous Motor)
ELECTRICAL MACHINESELECTRICAL MACHINES
Compiled byCompiled by
Prof Mitali RayProf Mitali Ray
2
Learning Outcomes
• At the end of the lecture, student should to:– Understand the principle and the nature of 3 phase principle and the nature of 3 phase
induction machines.induction machines.
– Perform an analysis on induction machines which is the most rugged and the most widely used machine most widely used machine in industry.in industry.
3
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
4
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.
5
Introduction
General aspectsGeneral 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
7
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.
Squirrel Cage Rotor
MZS FKEE, UMP
9
Construction (Stator construction)
• The stator is the stationary electrical part of the motor.stationary electrical part of the motor.• The stator core of a National Electrical Manufacturers Association
(NEMA) motor is made up of several hundred thin laminationsseveral hundred thin laminations.• Stator laminations are stacked togetherstacked together forming a hollow cylinderhollow cylinder.
Coils of insulated wireCoils of insulated wire are inserted into slots of the stator core. are inserted into slots of the stator core.• Electromagnetism is the principle behind motor operation. Each Each
grouping of coilsgrouping of coils, together with the steel core it surrounds, form an electromagnet. The stator windings are connected directly to the power source.
10
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.
11
• Induction motor types:
Squirrel cage type: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: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)
MZS FKEE, UMP
12
Construction (Rotor construction)
Wound Rotor
Squirrel-Cage Rotor
/rotor winding/rotor winding
Short circuits allShort circuits allrotor bars.rotor bars.
13
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 rotorThe rotor fits inside fits inside the statorthe stator with a with a slight slight air gapair gap separating it from the stator. There is separating it from the stator. There is NONO direct physical connection between the rotor and the direct physical connection between the rotor and the stator.stator.
Stator
Rotor
Air gap
• The enclosure also The enclosure also protectsprotects the electrical the electrical and operating parts of the motor and operating parts of the motor from from harmful effects of the environmentharmful effects of the environment in which in which the motor operates. the motor operates. Bearings, mounted on Bearings, mounted on the shaftthe shaft, support the rotor and allow it to , support the rotor and allow it to turnturn. . A fanA fan, also mounted on the shaft, is , also mounted on the shaft, is used on the motor shown below used on the motor shown below for coolingfor cooling..
14
Construction (Enclosure)
15
Nameplate
16
Rotating Magnetic Field
• When a 3 phase stator winding is connectedconnected to a 3 phase voltage supply, 3 phase current will flow in the windingsflow in the windings, which also will inducedinduced 3 phase flux in the stator.
• These flux will rotate at a speed called a Synchronous Speed, Synchronous Speed, nnss. 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
p
fns
120
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 () in degrees
FFa Fc
Fb
t = t0= t4
t = t1t = t2 t = t3
t = t0= t4
RMF(Rotating Magnetic Field)
AC Machine Stator
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
1 Cycle
Amp
timet0t1 t2 t3 t4
t01 t12
Currents in different phases of AC Machine
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.
MZS FKEE, UMP
22
Slip and Rotor Speed1.1. Slip Slip ss
– 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 negligibles is not negligible
)1( snnORn
nns sr
s
rs
23
Slip and Rotor Speed
• Rotor SpeedRotor 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 = slip
f = supply frequency
sf
pnnf rsr
)(
fsfiii
iipnn
f
nnRotorAt
ipn
f
nstatorAt
Note
r
rsr
pf
rs
s
pf
s
.:)()(
).....(120
)(
:
).....(120
:
:
120
120
24
Principle of Operation
• Torque producing mechanismTorque producing mechanismWhen a 3 phase stator winding is connectedconnected to a 3
phase voltage supply, 3 phase current will flow in the flow in the windingswindings, 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.
25
Direction of Rotor Rotates
• Q: How to change the direction of
• rotation?
• • A: Change the phase sequence of the
• power supply.
26
• 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 always bases on the Y connection regardless of the actual connection of the motorregardless 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
27
• Step1 Rotor winding is openStep1 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
28
Equivalent Circuit of Induction Machines
29
Equivalent Circuit of Induction Machines
• Step2 Rotor winding is shortedStep2 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 rotatingrotor is rotating.
f fr
30
• Step3 EliminateStep3 Eliminate ff22
Keep the rotor current same:
Equivalent Circuit of Induction Machines
MZS FKEE, UMP
31
• Step 4 Referred to the stator sideStep 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
MZS FKEE, UMP
32
• IEEE recommended equivalent circuitIEEE recommended equivalent circuit
• Note:– Rc is omitted. The core loss is lumped with the
rotational loss.
Equivalent Circuit of Induction Machines
MZS FKEE, UMP
33
• IEEE recommended equivalent circuitIEEE recommended equivalent circuit
Note: can be separated into 2 PARTSNote: can be separated into 2 PARTS
• Purpose : Purpose : – to obtain the developed mechanicalto obtain the developed mechanical
Equivalent Circuit of Induction Machines
I1 1R1X
mX
'2X
'2R
s
sR
1'21V
s
R2
s
sRR
s
R )1(22
2
34
Analysis of Induction Machines
• For simplicity, let assume
IIss=I=I1 1 , I, IRR=I=I22
(s=stator, R=rotor)
RmsTotal
sss
cmm
cmcm
RR
R
ZZZZ
jXRZ
neglectedRjXZ
neglectedRjXRZ
jXs
RZ
//
;
;
;//
;''
ZZRRZZmm
ZZss
VVs1s1
IIs1s1 IIm1m1 IIR1R1
T
s
s Z
VI
1
1
35
Analysis of Induction Machines
m
RMm
R
RMR
sT
mRRM
Z
VI
Z
VIHence
VZ
ZZV
RulesDividingVoltage
11
11
11
,
//
,
ZZRRZZmm
ZZss
VVs1s1
IIs1s1 IIm1m1 IIR1R1
11
11
,
sRm
mR
sRm
Rm
IZZ
ZI
IZZ
ZI
RulesDividingCurrent
OR
Note : 1hp =746WattNote : 1hp =746Watt
36
Power Flow Diagram
PPinin (Motor) (Motor)
PPinin (Stator) (Stator)
PPcore losscore loss
(P(Pcc))
PPair Gapair Gap
(P(Pagag))
PPdevelopeddeveloped
PPmechanicalmechanical
PPconvertedconverted
(P(Pmm))
PPout, out, PPoo
PPstator copper stator copper
loss, loss, (P(Pscuscu))PProtor copper rotor copper
lossloss (P (Prcurcu))PPwindage, friction, etcwindage, friction, etc
(P (P - -
Given)Given)
cos3 ss IV
s
RI RR
''3 2
2
3
c
RM
R
V ''3 2RR RI
s
sRI RR
1''3 2
Whp 7461
ss RI23
PPinin (Rotor) (Rotor)
37
Power Flow Diagram• Ratio:Ratio:
Pag Prcu Pm
s
RI RR
''3 2 ''3 2
RR RI
s
sRI RR
1''3 2
s
11
1s
1
1 s s1
Ratio makes the analysis simpler to find the value of the particular power if we haveanother particular power. For example:
s
s
P
P
m
rcu
1
38
Efficiency
WattxWhpxP
IVP
otherwise
PPP
PPP
givenarePif
P
P
out
ssin
mo
lossesino
losses
in
out
746746
cos3
,
,
%100
39
Torque-Equation
• Torque, can be derived from power equation in term of mechanical power or electrical power.
n
PTHence
sradn
whereTPPower
2
60,
)/(60
2,,
r
oo
r
mm
n
PTTorqueOutput
n
PTTorqueMechanical
Thus
2
60,
2
60,
,
40
Torque-Equation
• Note that, Mechanical torque can written in terms of circuit Note that, Mechanical torque can written in terms of circuit parameters. This is determined by using parameters. This is determined by using approximation approximation methodmethod
...
...
...
)1('
'3
)1('
'3
2
2
r
RR
r
mm
mrmR
Rm
ssR
IPT
TPandss
RIP
22
2
)'()'(
'
2
)(3
RR
R
s
RMm sXR
sR
n
VT
Hence, Plot Tm vs s
Tm
ns
ssmaxmax is the slip for T is the slip for Tmaxmax to occur to occur
s=1
Tst
Tmax
smax
41
Torque-Equation
22
2
)'()'(
'
602
)(3
1,
RsRs
R
s
sst XXRR
Rn
VT
sTorqueStarting
22
2
max
22max
)'()(
1
6022
)(3
)'()(
'
Rssss
s
Rs
R
XXRRn
VT
XR
Rs
42
Speed Control
• There are 3 types of speed control of 3 phase induction machines
i.i. Varying rotor resistanceVarying rotor resistance
ii.ii. Varying supply voltageVarying supply voltage
iii.iii. Varying supply voltage and supply frequencyVarying supply voltage and supply frequency
43
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
44
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
ns~nNL
T
nr1nr2nr3 n
nr1> nr2 > nr3
V1
V2
V3
V1> V2 > V3
V decreasing
45
• The best method since supply voltage and supply frequency is varied to keep VV//ff 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
f decreasing
nNL2nNL3