Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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CHAPTER VMotor Drives
• Motor drive systems definitions
• Review of motor principles
• Mechanical Requirements of Motor Drives
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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• DEFINITION: Electric drives for motor is used to draw electrical energy from the mains and supply the electrical energy to the motor at whatever voltage, current and frequency necessary to achieve the desired mechanical output.
• General arrangement for variable speed drive.
Controller PowerElectronicsConverter
ProcessControl
Computer
Motor Process/Load
Power Source
Electric Motor Drives
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Example of Motor Drive System: Air Conditioning System
SystemController
PowerElectronicsConverter
Motor Airconditioner
Power Source
BuildingCooling
Desiredtemperature
Indoorsensors
Indoor temperatureand humidity
Temperature andhumidity
Desiredhumidity
Variable speed drive
• Three Main Elements of Motor Drives:– Electric Motor (DC, AC, SRM, Stepper)
– Load Type
– Control
• Requirements of Drives:– Variable speed
– Controllable Torque
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Motor: Review of general principles
wirecarryingcurrent oflength : Area;:
:Force
:DensityFlux
(I)Current Finger Middle )( (Flux) Field MagneticFinger Pointing
(F) Force Thumb Rule Hand"Left "
lA
IlBFA
B
FORCE
CURRENT
MAGNETIC FIELD
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Flux in C-core
++++++++++
----------
Leakage flux
iron
air-gap
+
-
Current into the paper
Current out of the the paper
NOTE: Use right hand "Screw Rule" to determine the direction of flux
NI
MMFFlux
R
VI
EMFCurrent
Reluctance
Resistance
analogy magnetic-Electric
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Air-gap flux densities
MMF=NI
Area A
g
1.8T)-(1.6iron theof saturation by the limited isdensity flux The
gap. theoflength theand (NI) coil theof MMF theknowingonly by calculated becan density flux gap-air that Note
,
gap),-air (in theair of Reluctance
o
o
o
g
NI
AB
g
NIAMMF
ThenA
g
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Evolution of motor geometry
++++++
++++
--
--
--
--
--
++
--
++++++++++
----------
C--Core Split air-gap
Simple motorgeometry
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Torque Production
++
++ + + + ++++
...........
Force
Force
+ current into the paper
. current out of the paper
TORQUE PRODUCTION
"SLOTTING"
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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M
F
dt
dua
Madt
duMMu
dt
dFFF
M
leM
where
:motionLinear
Mechanical System requirements for drives: Linear motion
Mf e f L
x
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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dt
dJT
dt
d
JT
J
rFT
m
m
Then,
:i.eon,accelaratiangular theis
,inertia ofmoment For
.
Rotating motion
r
f
torque
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Motor and load interaction
Motor Load
TL
Tem
Jeq
m
Jeq = combined load–motor moment of inertia
m = motor speed (rad/s)
TL = load torque
Tem = motor torque
eq
Jm
eq
Lem
LemJ
L
em
J
T
dt
d
J
TT
TTT
T
T
,accelerate toload theandmotor theof )( inertia
combined thecauses andbetween difference The
NetTorque:
(drag)resistance
windandfriction bearing theplus,LoadTorque:
motor by the produced eneticTorquElectromag:
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Example(1)
Using the motor-load structure as above, assume the motor has a combined inertia of 0.058kgm2. The load torque is negligible. Calculate the required electromagnetic torque if the speed is to be increased from standstill to 1,800rpm in 5 sec.
mNdt
dJT
radrotation
rpmtdt
d
meqem
mm
.19.17.37058.0
sec/7.372
sec60
min1
sec5
)01800( 2
Motor Load
TL
Tem
Jeq
m
Jeq = combined load–motor moment of inertia
m = motor speed (rad/s)
TL = load torque
Tem = motor torque
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Example(2)
The speed profile of the rotating system shown in previous example. (Jeq=0.058kgm2) is shown below. Assume the load torque is 5 N.m. Calculate and plot, as a function of time the electromagnetic torque required from the motor
0.2
0.1 0.3 0.4 0.5 t(s)
speed(rad/s)
300
–300
SectionB
Section A
Section C
Section D
Section E
Section F
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Torque and Power
rotation. of speedangular theis where
:as expressed becan power then , time
aldifferenti din place akesrotation t ldifferntia thisIf
:is donework
aldifferenti the, angle aldifferenti aby rotate ocylinder t
thecauses net torque a if system, rotationalIn
dt
d
Tdt
dT
dt
dWp
dt
TddW
T
m
m
Td
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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Friction
• Friction within motor and load appose rotation
• Examples:– Bearings (to support rotating structure)
– Air (drag/windage)
– Ditction (friction at zero speed)
• Moving objects:– Coulomb friction (independent of speed)
– Viscous friction (increases linearly with speed)
Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB
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• Centrifugal (squared) Torque– Load torque is a function of speed– Example: Fans
Load Types
• Constant Torque
– Load torque is independent of speed
– Example: Low speed hoist, elevator
Torque
Speed
Torque
Speed