im speed control sep2023
TRANSCRIPT
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Induction Motor Drives
Speed Control
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Induction Motor Drives
Advantages of Induction Motor
RuggednessLower Maintenance
Better Reliability
Lower Cost
Higher Efficiency Ability to operate in dirty and
explosive environment
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The motor drives a load which has a certain
characteristics torque-speed requirement.
In general,
where, k may be an integer of a fraction.
Load
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In a feed drive,
In fans and pumps,
The motor can be connected to the load through a setof gears
The gears have teeth ratio and can be treated as torque
transformers
Load
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Load horsepower and torque
characteristics
Constant torque, horsepower varies as the speed
A pplications: General machinery hoists, conveyors,
printing press
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Load horsepower and torque characteristics
Horsepower varies as square of the speed, torque
varies with speed
A pplications: Pumps, some mixers, some extruders
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Load horsepower and torque
characteristics
High inertial loads
A pplications: Are typically associated with machines
using flywheel to supply most of the operating energy,
punch press
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Load horsepower and torque characteristics
Shock loads
A pplications: Drives of crushers, separators, grinders,conveyors, and vehicular systems
Power converters and motors can be damaged if they
are not protected from the overload conditions
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Methods of Speed Control of
Induction Motor
Pole Changing
Stator Voltage control Supply Frequency Control
Rotor Resistance Control
Slip Power Recovery Eddy-Current Coupling
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Pole Changing
Sometimes induction machines have a special
stator winding capable of being externally
connected to form two different number of pole
numbers.
Since the synchronous speed of the induction
machine is given by
this would correspond to changing the synchronous
speed.
P
f ms
T
[
4!
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The two coils on A & C may be connected in series in twodifferent ways ² A2 may be connected to C1 or C2.
A1 with the other terminal at C then form the terminals of
the overall combination.
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For a given direction of current flow at terminal A1, say into terminal A1, the
flux directions within the poles are shown in the figures.
In case (a), the flux lines are out of the pole A (seen from the rotor) for andinto pole C, thus establishing a two-pole structure.
In case (b) however, the flux lines are out of the poles in A & C. The flux lines
will be then have to complete the circuit by flowing into the pole structures on
the sides. Pole configurations produced by these connections is
a two-pole arrangement in fig. (a) and a four-pole arrangement in fig. (b).
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Variable Terminal Voltage
The Torque Developed in IM is
Proportional to the Square of the Terminal
Voltage
The Rotor current is directly Proportional
to the Terminal Voltage
Thermal Over Loading decreases
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A.C. Voltage Controller
AC Voltage
Controller
Fixed AC
Voltage
Source Variable AC Voltage
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THREE PHASETHREE PHASE
AC VOLT AGE CONTEROLLER CIRCUITS AC VOLT AGE CONTEROLLER CIRCUITS
Speed-Torque
Characteristics for
Stator Voltage
Control Method
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THREE PHASE AC VOLT AGE
CONT
ROLLER CIRCUIT
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AC VOLTAGE CONTROLLERFour ± Quadrant Control
Forward Motoring and reverse plugging
operations
Low speed motoring and high speed braking Thyristor pairs A,B and C provide operation
in quadrants I and IV
Thyristor pairs A¶ , B and C¶ provide
operation in quadrants II and III
Zero current sensor [ dead time 5 to 10 ms]
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W AVE FORM OF PHASE CURRENT FOR =60
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SINGLE QUADRANT
CLOSED LOOP CONTROL
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VARIABLE FREQUENCY
CONTROL The synchronous speed is directly
proportional to supply frequency.
The back emf induced
E= 4.44 N f NT kw
E E N f
E
N E ---------
f
V
N E ----------
f
[ if stator drop is neglected]
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VARIABLE FREQUENCY
CONTROL
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Constant Volts / Frequency Operation
Speed
Td= 3(V m/se)(p/2) {R r r /( R r+rLlr)2 }
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Power Converters (contd¶)
2. Inverters ± Voltage and current source converters are
fed from a DC link. The DC link is generated with
either a controlled or uncontrolled rectifier.
Vc: controlled magnitude command
f c: frequency command
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3. Cycloconverter
Power Converters (contd¶)
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The controllers implement the control strategy
governing the load and motor characteristics
To match the load and motor, the input to the power
converter is controlled (manipulated) by the
controller
Controllers
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Voltage Source Inverter
Low Internal Impedance
Any Short Circuit across its terminals
causes current to rise very fast The fault current can be cleared by Fast
acting Fuse links
T
heT
erminal Voltage remains constantwith variations of load
Inverter Grade SCR
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SIX ± STEP INVERTER
MODE OF SEQUENCE OF FIRING
OPERATION
I 1 5 6
II 1 2 6
III 1 2 3
IV 2 3 4
V 5 4 3
VI 5 4 6
Tripplen harmonics are absent
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Classification of Converter Systems
PWM-VSI is now most commonly use
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Draw Back of Six Step inverter
Derating of the motor
Torque pulsating
Over heat the machine at low speeds Commutation capability of thyristor inverter
reduced
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PWM-VSI System
Diode rectifier for unidirectional power flow
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PWM-VSI System
Options for recovered energy during
regenerative braking
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PWM Inverters
Low harmonic content in the output
voltage
Lower derating of the motor and higher efficiency
No commutation problem
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GENERATION OF CONTROL SIGNALS FOR
SPWM
MODULATED W AVEFORM
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SINUSOIDAL PULSE WIDTH MODULATION
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OPEN-LOOP VARIABLE FREQUENCY
PWM INVERTER DRIVE
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CLOSED-LOOP SPEED CONTROL WITH
PWM INVERTOR DRIVE
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Advantages
1. The circuit for CSI, using only converter grade thyristor,
which should have reverse blocking capability, and also
able to withstand high voltage spikes during commutation,
is simple.
2. An output short circuit or simultaneous conduction in aninverter arm is controlled by the µcontrolled current source¶
used here, i.e., a current limited voltage source in series
with a large inductance.
3.T
he converter-inverter combined configuration hasinherent four-quadrant operation capability without any
extra power component.
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Disadvantages
1. A minimum load at the output is required, and the
commutation capability is dependant upon load current.This limits the operating frequency, and also puts a
limitation on its use for UPS systems.
2. At light loads, and high frequency, these inverters have
sluggish performance and stability problems.
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