Upload
vijayneetayadav
View
264
Download
0
Embed Size (px)
Citation preview
8/9/2019 Drive Ac Basics 1
1/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
1
Induction Motor Drive Why induction motor (IM)?
Robust; No brushes. No contacts on rotor shaft High Power/Weight, Lower Cost/Power ratios
Easy to manufacture
Almost maintenance-free, except for bearing and otherexternal mechanical parts
Disadvantages
Essentially a fixed-speed machine Speed is determined by the supply frequency
To vary its speed need a variable frequency supply
Motivation for variable-speed AC drives Inverter configuration improved
Fast switching, high power switches
Sophisticated control strategy Microprocessor/DSP implementation
Applications Conveyer line (belt) drives, Roller table, Paper mills,
Traction, Electric vehicles, Elevators, pulleys, Air-conditioning and any industrial process that requiresvariable-speed operation.
The state-of-the-art in IM drives is such that most of theDC drives will be replaced with IM in very near future.
8/9/2019 Drive Ac Basics 1
2/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
2
Torque production (1)
Only squirrel-cage IM (SCIM) is considered inthis module
Neglecting all harmonics, the stator establishes aspatially distributed magnetic flux density in theair-gap that rotate at asynchronous speed, 1 :
where
e : supply frequency (in Hz)
p: pole pairs (p=1for 2 pole motor, p=2for 4 pole motor etc)
If the rotor is initially stationary, its conductor issubjected to a sweeping magnetic field, inducingrotor current at synchronous speed.
If the rotor is rotating at synchronous speed (i.e.equals tof1), then the rotor experience no induction.No current is induced in the rotor.
pe =1
8/9/2019 Drive Ac Basics 1
3/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
3
Torque production (2)
At any other rotor speed, say wm, the speeddifferential i-2 creates slip. Per-unit slip is
defined as:
pairpole:
frequencyrotor:
frequencysupply::where
; 11
1
p
ps
m
e
em
=
=
Slip frequency is defined as: 2=1-m.
When rotor is rotating at m., rotor current at slipfrequency will be induced.
The interaction between rotor current and air-gapflux produces torque.
8/9/2019 Drive Ac Basics 1
4/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
4
Single-phase Equivalent Circuit
(SPEC)
voltage)(phasetageSupply vol:
inductancegMagnetisin:
inductanceleakageRotor:
resistanceRotor:
inductanceleakageStator:
resistanceStator:
1
2
2
1
1
v
L
L
R
L
R
m
R1 L1
V1 Rm Lm VmV2 = nSVm
L2
R2
STATOR SIDE
1:nS
8/9/2019 Drive Ac Basics 1
5/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
5
SPEC, referred to stator
From previous diagram, SPEC is a dual frequency
circuit. On the stator is 1 and on the rotor m
Difficult to do calculations.
We can make the circuit a single frequency type, byreferring the quantities to the stator
I1 R1 L1
V1 Rm Lm
L2
R2
S
I2
8/9/2019 Drive Ac Basics 1
6/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
6
Rotor current
.alterationresistancerotorhebut with tstator,the toreferrednowarequantitiesthat theNote
phase,rotorin thecurrenttheHence
0)0(
,1wheni.espeed,ssynchronouAt
1
,0wheni.e,standstillAt
:whereEbewillratioturnseffectivesamethewithphaserotorequivalentaninEMFbackthe
thenphase,statorin theEMFbacktheisEIf
22
1
22
1
22
22
12
m
1112
m
12
2
1
jXs
R
EjsXR
sE
jsXR
EI
EE
EEsEE
sEE
+
=
+=
+=
==
=
===
=
=
8/9/2019 Drive Ac Basics 1
7/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
7
Performance calculation using SPEC
22
2
22
2
21
12
1
11
11
3:losscopperRotor
3:gap-airtheacrossPower
3:lossCore
3:losscopperStator
currentandvoltagephasebemustand:Note
cos3:PowerInput
RIP
PPPs
RIP
R
VP
RIP
IV
IVP
lrlclsin
g
mlc
ls
in
=
=
=
=
=
=
I1 R1 L1
V1 Rm Lm
L2
R2
S
I2
8/9/2019 Drive Ac Basics 1
8/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
8
Performance calculation (2)
ee
ee
e
mm
mm
oe
FWFWosh
g
lrgo
s
RpIT
p
s
RIT
ss
s
sRIPT
PPPP
sPs
sRII
s
RI
PPP
22
2
1
1
22
2
11
1
22
2
22
222
22
2
3
Then,
frequency.supplytheis;But
3
,)1(
Since
)1(3
:torquegnetic)(electromaDeveloped
loss.windageandfriction:;:shaftat thePower
)1()1(3
33
:poweroutputGross
=
=
=
=
=
==
=
=
==
=
8/9/2019 Drive Ac Basics 1
9/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
9
Example calculation
A single phase equivalent circuit of a 6-pole SCIM
that operates from a 220V line voltage at 60Hz is
given below. Calculate the stator current, output
power, torque and efficiency at a slip of 2.5%. The
fixed winding and friction losses is 350W. Neglect
the core loss.
I1 R1 X1
V1 Xm
X2
R2
I2
0.2 0.5
20 0.1
0.2
1V 220V line-to line 3
220V127V
3
2.5% 0.025
=
= =
= =
8/9/2019 Drive Ac Basics 1
10/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
10
Calculation (solution)
mN
sp
PPT
WPPP
WsPP
W
PPP
WRIP
W
IVP
A
Z
VI
jj
j
jj
jXs
RjXjXRZ
XXX
e
o
m
oe
FWosh
go
lsing
ls
oin
o
oin
o
min
m
.4.78
)025.01)(3/60(2
9611
)1()(
TorqueneticElectromag
%3.8910758
9611
powerInput
powerOutputEfficiency
611,9350961,9
shaftat thePower
961,9)025.01(216,10)1(
powerGross
216,10540758,10
loss)coreg(neglectinrotortosferredPower tran
540)2.0)(30(33
758,10
)20)(cos30)(3220(3cos3
200.30
202.4
3220
202.4
202.0025.0
1.0
2.0025.0
1.0
205.02.0
//)(
20,2.0,5.0
1
21
21
11
11
22
11
21
=
=
==
===
===
===
==
=
===
=
==
=
==
=
++
+
++=
+++=
===
8/9/2019 Drive Ac Basics 1
11/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
11
Starting current
For the previous example, Calculate the stating currentwhen motor is first switched on to rated applied
voltage.
motors.inductionstartingtaken whenbeshouldemotors.CarinductionforcommonisThis
current.loadfull
thantimes5aboutiscurrentstartingthat theNote
16776.0
322076.0
202.01.02.01.0205.02.0
1,standstillAt:Solution
11 A
Z
VI
jjjjjZ
s
in
in
===
=
+++++==
=
8/9/2019 Drive Ac Basics 1
12/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
12
( )
( )
++
+
==
=
++
+
=
221
21
22
11
2
12
1
222
221
21
22
1
12
m
33
torque,)(developedneticElectromag
phase),(perrotorat thePower
drawnbecanabovecircuitthelarge,isLSince
LLs
RRs
VRPT
s
RIP
LLs
RR
VI
oe
o
Approximate SPEC
+
-
1V ML
1R 1L 2L
s
R2
8/9/2019 Drive Ac Basics 1
13/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
13
Single (fixed supply) frequency
characteristics
.0speed,syncat,1standsillat;
:thatNote
below.asshownbecansticscharacteri
slip)(versustorquethe,frequencygiveaFor
1
1
1
==
= sss m
e
m
PLUGGING
TORQUE(+)
MOTORING
emT(max torque or
pull-out torque)
esT (starting torque)
SLIP,s
TORQUE(-)
e
rated slip0 unity slip
(standstill)
GENERATING
e
m
e
m
zero slip
(sync.speed)
SPEED
8/9/2019 Drive Ac Basics 1
14/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
14
Single frequency characteristics
CURRENT TORQUE
operating point
(rated torque)
rated slip
rated current
POWERFACTOR
SLIP
1.00
Standstill synchronous speed
EFFICIENCY
8/9/2019 Drive Ac Basics 1
15/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
15
Single frequency characteristic
As slip is increased from zero (synchronous), the
torque rapidly reaches the maximum. Then it
decreases to standstill when the slip is unity.
At synchronous speed, torque is almost zero.
At standstill, torque is not too high, but the current
is very high. Thus the VA requirement of the IM is
several times than the full load. Not economic to
operate at this condition.
Only at low slip, the motor current is low andefficiency and power factor are high.
8/9/2019 Drive Ac Basics 1
16/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
16
Typical IM Drive System
IDC
Modulation Index,
VDC
n
IM
Supply Rectifier and Filter 3-phase Inverter
CIRCUIT
BLOCK DIAGRAM
8/9/2019 Drive Ac Basics 1
17/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
17
Variable speed characteristics
For variable speed operation, the supply is aninverter.
The frequency of the fundamental AC voltage will
determine the speed of IM. To vary the speed of
IM, the inverter fundamental frequency need to be
changed.
The inverter output frequency must be kept close to
the required motor speed. This is necessary as the
IM operates under low slip conditions.
To maintain constant torque, the slip frequency has
to be maintained over the range of supply
frequencies.
8/9/2019 Drive Ac Basics 1
18/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
18
Variable voltage, variable
frequency (VVVF) operation
In order for maximum torque production,
motor flux should be maintained at its
rated value.
( )
operation
Volt/HertzconstanttheasknownpopularlyisThis
constant.keptbetohasratiothe
flux,motorhemaintain torder toinTherefore,
44.4or
44.42
1
RMS,In
cos
:isemfbackBut the
sin
11
1
1
1
11111
111
1
fE
Nf
E
NfNE
tNdt
dNe
t
m
mm
m
m
=
==
=
=
=
8/9/2019 Drive Ac Basics 1
19/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
19
Constant Torque region
Hence for VVVF operation, there is a need tocontrol the fundamental voltage of the inverter if itsfrequency (and therefore the frequency of the IM)need to be varied.
To vary the fundamental component of the inverter,the MODULATION INDEX can be changed.
The rated supply frequency is normally used as thebase speed
At frequencies below the base speed, the supply
magnitude need to be reduced so as to maintain aconstant Volt/Hertz.
The motor is operated at rated slip at all supplyfrequencies. Hence a constant torque region isobtained.
8/9/2019 Drive Ac Basics 1
20/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
20
Constant Torque Region
TORQUE(+)
SPEED
SLIP,s
1f
rated slip
0
2f
3f
4f
5frated torque
54321 fffff >>>>
TORQUE(+)
SPEEDSLIP,s0
rated torque
8/9/2019 Drive Ac Basics 1
21/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
21
Constant Power region
Above base speed, the stator voltage
reaches the rated value and the motor
enters a constant power region.
In this region, the air-gap flux decreases.
This is due to increase in frequency
frequency while maintaining fixed
voltage.
However, the stator current is maintained
constant by increasing the slip. This is
equivalent to field weakening mode of a
separately excited DC motor.
8/9/2019 Drive Ac Basics 1
22/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
22
Constant Power region
TORQUE(+)
SPEEDSLIP,s Base speed
0
rated torque
TORQUE(+)
SPEEDSLIP,s0
CONSTANT TORQUE
REGION
Base speed
"FIELD WEAKENING"
8/9/2019 Drive Ac Basics 1
23/24
Dr. Zainal salam; Power
Electronics and Drives
(Version 2),2002, UTMJB
23
VVVF Summary
SPEED0
CONSTANT TORQUE
Base speed
CONSTANT POWER
Electromagnetic torque, Te
Terminal (supply)
voltage, V1
slip,s
slip frequency,fs
8/9/2019 Drive Ac Basics 1
24/24
Dr. Zainal salam; Power
Electronics and Drives
(V i 2) 2002 UTMJB
24
Examples
A three-phase 4-pole, 10 horsepower, 460Vrms/60Hz (line-to line) runs at full-loadspeed of 1746 rpm. The motor is fed froman inverter. The flux is made to be constsnt.Plot the torque-speed graphs for the
following frequency: 60Hz, 45 Hz, 30Hz,15Hz.
A three-phase induction motor is using athree-phase VSI for VVVF operation. TheIM has the following rated parameters:
voltage: 415V (RMS)
frequency: 50Hz
slip (p.u) 5%
pole pair 2
If the inverter gives 415V (RMS) with
modulation index of 0.8, calculate the requiredmodulation index if the motor need to beoperated at rotor mechanical speed of 10Hz.