control of drives
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CONTROL OF ELECTRICAL DRIVES
CONTROL OF ELECTRICAL DRIVES
Controlling of electrical drives is a vast area. Now a days electrical drives are mostlyused in the industries.
Normally we are going to electrical drives is to vary the speed. Electrical drives are mostefficient to vary the speed when comparing to the other drives.
Consumption of Electricity in INDIA in 2006
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CLASSIFICATION OF ELECTRICAL DRIVES
Drives are normally classified in to two types namely
AC Drives
DC DrivesNow a day we are mostly using the AC drives.
Varying the speed is very easy in DC MOTORS. But we have only AC source. If we want
to use DC motors we must place a rectifier circuits. It will become uncomfortable.
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Electrical Drives
Drives are systems employed formotion control
Require prime movers
Drives that employ electric motors as
Prime movers are known as Electrical Drives
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Advantages Inter-disciplinary
Several research area
Expanding
Small
Efficient
Flexible
A modern variable-speed drive has four components:
Electric machines - ac or dc
Power converter - rectifiers, choppers,
inverters and cycloconverters
Controllers -matching the motor and
power converter to meet the load
requirements
Load
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Modern electric drives
Non-linear controlReal-time controlDSP applicationPFCSpeed sensorless
Power electronicconverters
Utility interfaceRenewableenergy
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Components in electric drives
Motors
DC motors - permanent magnet wound field
AC motors induction, synchronous (IPMSM, SMPSM), brushless DC
Applications, cost, environment
Power sources
DC batteries, fuel cell, photovoltaic - unregulated
AC Single- three- phase utility, wind generator unregulated
Power processor
To provide a regulated power supply
Combination of power electronic converters
More efficient
Flexible
Compact
AC-DC DC-DC DC-AC AC-AC
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Before semiconductor devices were introduced (
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Electrical motors for control Electrical motors for control- velocity
drives (1990)
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Electrical motors for control Electrical motors for control- velocity
drives (2001)
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Flow chart of reluctance motor control system
Advantages of Switched Reluctance Motor
Simplicity and reducibility of construction
Low manufacturing and operating costs
Fault tolerance
High torque/inertia ratio and efficiency
Disadvantages of Switched Reluctance Motor Reluctance Motor
High level of torque ripple
High level of acoustical noise
Normally we are using mechanically operated switches to turn on or to off the motors. Nowa day we are using electronic devices to switch off or to on.
In this paper we are going to see about switching control of electrical machines.
Categories of Switches
There are three categories of switches:
Diodes (rectifiers) - on/off determined by the power circuit.
Thyristors (SCRs, Triacs) - latched on by a control signal but turned off by the
power circuit. Controllable Switches (BJTs, MOSFETs, GTOs, IGBTs, MCTs) - turned on and off
by control signals.
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Power Diodes
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Circuit Symbol:
Current-Voltage Characteristics:
iiDD
++vv
DD
--
AA
KK
iiDD
vvDD
II
vvFF
vvratedrated
ReverseReverse
BlockingBlocking
iiDD
vvDD
RealReal IdealIdeal
ReverseReverse
BlockingBlocking
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II00
++--vvdd
iiTT
++
vvTT
--
ControlControl
SwitchSwitch
The current sourceThe current source
Approximates theApproximates the
Current that wouldCurrent that would
Actually flow due toActually flow due to
Inductive current storage.Inductive current storage.
ControllableControllable
switchswitch
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Gate Turn-Off Thyristors (GTOs)
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Switching Waveformsoff on offoff on off tt
SwitchSwitch
controlcontrol
signalsignal
vvTT,, ii
TT
tt
IISS
ttDD(on)(on) tt
DD(off)(off)
VVSS
VVSS
VVonon
ttriri
ttfvfv
ttCC(on)(on)
ttrvrv
ttfifi
ttCC(off)(off)
ttcc = cross over ON and OFF times= cross over ON and OFF times
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Switching Waveforms for GTOs
GTOs are sensitive to dv/dt. Therefore, snubber circuits are used to minimize dv/dt
and di/dt.
GTOs are available to handle 1000s of V, A up to 10 kHz.
An example for controlling the electrical drives
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Consider an AC motor. Figure shows the flow chart.
EMI filter
L1, L2, C1, C2, C3 and C4 constitute a LP filter
Keeps asymmetrical conduction interferences within the converter
Uncontrolled rectifier
Uncontrolled rectifier with a C filter
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DC link
EMI
filterInv
erter
L 2
L 1
C 1
I N 2
C 2
C 3
E A R T H G N D
I N 1
C 4
E
d
c
D 2
Idc
+ C d c
D 3D 1
D 4
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EDC is a DC voltage with a ripple. CDC is chosen to minimize the ripple (under1% EDC)
DC link
Minimizes the ripple of the DC voltage
Contains a shunt resistor RS used to gain
information on DC link current (this informationis used to compute rotor speed)
Inverter
Six IGBT transistors with anti-parallel diodes
Microprocessor controls the driver circuit, which turns transistors no or off
These are the types of switching control of electrical drives.
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C
2
+
C
1
R S
Q 6
Q 1 Q 5
Q 2 Q 4
Q 3
W
Q 6
Q 1 Q 5
V
Q 2 Q 4
U
Q 3