chapter 6 - ac drives
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SEE 4433
Power Electronics
And Drives
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Chapter 6:
AC Drives
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Engineer/Motors_and_Drives/What_exactly_is_an_ac_drive_and_just_how_does_it_work%3F/21560/
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Motor Drive
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• 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 other “external”
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
requires variable-speed operation.
• The state-of-the-art in IM drives is such that most of the DC drives will be
replaced with IM in very near future.
Induction Motor Drive
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Induction motors (IM) most widely used
IM (particularly squirrel-cage type) compared to DC motors
Rugged
Lower maintenance
More reliable
Lower cost, weight, volume
Higher efficiency
Able to operate in dirty and explosive environments
IM mainly used in applications requiring constant speed
Conventional speed control of IM expensive or highly
inefficient
IM drives replacing DC drives in a number of variable speed
applications due to improvement in power devices capabilities
Reduction in cost of power devices
Introduction
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Review on
3-phase IM
■ Balanced 3-phase winding on stator
■ Rotor of squirrel-cage (SC) type IM has conductors shorted by
end rings
■ By induction, same number of phases and poles produced by
rotor as in stator winding
■ When stator supplied by balanced 3-phase AC source (frequency
ω (rads/s) or f (Hz)) rotating field moving at synchronous speed, ωs
(rad/s) is produced
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Torque Production
• Only “squirrel-cage” IM (SCIM) is considered in this module
• Neglecting all harmonics, the stator establishes a spatially
distributed magnetic flux density in the air-gap that rotate at a
synchronous speed, ω1 or Ns
where ωe : supply frequency (in Hz);
p : pole pairs (p = 1 for 2 pole motor, p = 2 for 4 pole motor etc)
• If the rotor is initially stationary, its conductor is subjected to a
sweeping magnetic field, inducing rotor current at synchronous speed.
• If the rotor is rotating at synchronous speed (i.e. equals to f1), then
the rotor experience no induction. No current is induced in the rotor.
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• At any other rotor speed, say ωm, the speed differential ωi-ωm
creates slip. Per-unit slip is defined as:
Torque Production
• Slip frequency is defined as: ω2= ω1-ωm.
frequency rotor, f2 = sf1
• When rotor is rotating at ωm., rotor current at slip frequency
will be induced.
• The interaction between rotor current and air-gap flux
produces torque.
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Single-phase Equivalent Circuit (SPEC)
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Stator voltage equation:
Rotor voltage equation:
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SPEC, referred to stator
• From previous diagram, SPEC is a dual frequency circuit. On
the stator is ω1 and on the rotor ωm
• Difficult to do calculations.
• We can make the circuit a single frequency type, by referring
the quantities to the stator
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Rotor Current
If E is the back EMF in the stator phase, then the back EMF
in an equivalent rotor phase with the same effective turns
ratio will be E where :
Note that the quantities are now referred to the stator, but with
the rotor resistance alteration.
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Performance calculation
using SPEC
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Performance
calculation using SPEC
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Power and Torque
Power is transferred from stator to rotor via air–gap, known
as air gap power
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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.
Example :
Solution :
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Solution :
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Starting Current
For the previous example, Calculate the stating current when
motor is first switched on to rated applied voltage.
This is common for induction motors. Care should be taken
when starting induction motors.
Solution :
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Approximate SPEC
ω1=2πfe=electrical speed
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Single (Fixed Supply)
Frequency Characteristics
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Single Frequency Characteristics
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• 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 and efficiency and
power factor are high.
Single Frequency Characteristics
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Scalar
Control Of
Induction
Machine
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Variable Line Voltage,
Fixed Frequency
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Typical IM Drive System :
Variable voltage, variable frequency
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Typical IM Drive System :
Variable voltage, variable frequency
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Typical IM Drive System :
Variable voltage, variable frequency
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V V V F – Variable Speed Characteristics
• For variable speed operation, the supply is an inverter.
• 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.
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Variable Voltage, Variable Frequency
( V V V F ) Control Scheme
In order for maximum torque production, motor flux (air gap)
should be maintained at its rated value.
(assume negligible volt drop across stator impedance)
Therefore, in order to
maintain the motor flux,
the ( E1 / f1 ) ratio has to be
kept constant.
This is popularly
known as the
constant Volt / Hertz
operation
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Variable Voltage, Variable
Frequency Constant V / f
T- ω characteristic
of the motor shifted
horizontally
Small slip
frequency operation
at all operating
frequency
(high efficiency)
50 Hz
30 Hz
10 Hz
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Variable Voltage, Variable
Frequency Constant V / f
At low speed , the volt drop at Rs and Xls quite significant
compared to supply Vs, therefore to compensate the drop, Vs
normally boosted at low frequency
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Variable Voltage, Variable
Frequency Constant V / f
Three
phase
power
supply
Voltage
Source
Inverter
(VSI)
Rectifier
Induction
Motor
IM
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Variable Voltage, Variable
Frequency Constant V / f
Problems with open-loop constant V/f
At low speed, voltage drop across stator impedance is
significant compared to air gap voltage - poor torque
capability at low speed
Solution:
Boost voltage at low speed Maintain Im constant – constant
Φag
•Torque deteriorate at low frequency – hence compensation
commonly performed at low frequency
• In order to truly compensate need to measure stator current
– seldom performed
1
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Problems with open-loop constant V/f
Variable Voltage, Variable
Frequency Constant V / f
with voltage boost at low frequency
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Problems with open-loop constant V/f
Variable Voltage, Variable
Frequency Constant V / f
Poor Speed Regulation
Solution:
Compensate slip closed-loop control
TL
Slip ωr ωsyn
Motor
Characteristic after
compensation
2
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Constant V/f
– open-loop with slip compensation and voltage boost
Variable Voltage, Variable
Frequency Constant V / f
Three
phase
power
supply
Voltage
Source
Inverter
(VSI)
Rectifier
Induction
Motor
IM
Slip Speed
Calculator
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Constant Torque Region
• Hence for VVVF operation, there is a need to control the
fundamental voltage of the inverter if its frequency (and
therefore the frequency of the IM) need to be varied.
• To vary the fundamental component of the inverter, the
MODULATION INDEX, MI can be changed.
• The rated supply frequency is normally used as the base
speed
• At frequencies below the base speed, the supply magnitude
need to be reduced so as to maintain a constant Volt / Hertz.
• The motor is operated at rated slip at all supply frequencies.
Hence a “constant torque” region is obtained.
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Constant Torque Region
Rated Slip
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• 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.
Constant Power Region
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Constant Torque Region
“Field Weakening” Constant Torque
Base Speed
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VVVF Summary
Constant Power Constant Torque
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Idealized
Motor
Characteristic
DC Motor Induction Motor
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Example :
Motor is controlled by a voltage source inverter with
constant V/f
Calculate:
a) Speed for frequency of 30 Hz and 80% of full load
b) Frequency for a speed of 1000 rpm and full load
c) Torque for a frequency of 40 Hz and speed of 1100 rpm
Answer: 796 rpm, 37.67 Hz, 31.07 Nm
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A three-phase 4-pole, 10 horsepower, 460 Vrms/60 Hz (line-to
line) runs at full-load speed of 1746 rpm. The motor is fed from
an inverter. The flux is made to be constant.
Plot the torque-speed graphs for the following frequency: 60
Hz, 45 Hz, 30 Hz,15 Hz.
A three-phase induction motor is using a three-phase VSI for
VVVF operation. The IM has the following rated parameters:
• voltage : 415 V (RMS)
• frequency : 50 Hz
• slip : (p.u) 5%
• pole pair : 2
– If the inverter gives 415V (RMS) with modulation index of 0.8,
calculate the required modulation index if the motor need to be
operated at rotor mechanical speed of 10 Hz.
Example :
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a) Briefly discuss the circuit topologies for variable
voltage variable frequency induction motor speed
Drives
b) A three-phase 4-pole, 10 kW, 415 Vrms/50 Hz (line-to
line) runs at full-load speed of 1320 rpm. The motor is
fed from an inverter. The flux is made to be constant.
Plot the torque-speed (Hz) graphs for the following
supply inverter frequency: 50 Hz, 30 Hz, 20 Hz.
Also plot T-ω when f = 60 Hz and 100 Hz.
Example :
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The End
of Chapter 6
Thank You