ch09 - ac drives
TRANSCRIPT
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Chapter 9
AC Drives
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AC Drive Fundamentals
• The speed of an AC motor is largely
dependent upon two factors: – Frequency of the incoming AC power
– Number of poles
N =60f
P
N = speed (RPM)
f = frequency
P = number of pole pairs
60 = constant
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AC Drive System
• Operator control -• Drive controller - converts
fixed voltage/frequency
into adjustable
voltage/frequency
• AC motor - typicallythree-phase squirrel cage
motors
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Variable Voltage Inverter
• Also known as six-step voltage source inverter
• The VVI has four main sections: – Converter - switches AC to pulsating DC
• For higher power applications, a three-phase rectifier is used.There are two types:
» Controlled rectifier
» Uncontrolled rectifier
– Intermediate Circuit
– Inverter
– Control Circuit
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Rectifiers
• Uncontrolled Rectifier
• Controlled Rectifier
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The Intermediate Circuit
• Transforms the pulsating DC to a smooth
DC waveform• Output is called the DC Bus Line
• Three types of circuits typically used:
– Inductive Filter – LC Filter
– Chopper
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Intermediate Circuit Examples
Inductive and LC filters Chopper Circuit
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The Inverter • The function of the inverter is to invert the DC bus line voltage back
to a simulated AC voltage
• An inverter consists of six electronic switching devices used toapproximate a three-phase AC source for the motor
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Inverter Operation
• The VVI Drive inverter controls nothing but
frequency.
• The diagrams on the following slides
demonstrate the operation of a typicalinverter
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Inverter
Operation
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Inverter
Operation
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Control Circuit• The control circuit performs the following functions:
• Receives the operating commands from the control paneland sets the system variables
• Generates the command pulses which turn the
semiconductors off and on
• Produces outputs for the control panel to display variousoperating conditions
• Performs an orderly shutdown procedure in the event of an
abnormal condition
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Control Circuit Operation
• The speed control (rheostat) feeds an inputsignal into the control circuit input terminal
• This control signal is delivered, after conditioning, to a shift register
• Control circuit operation is illustrated in thenext slide
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DriveController Section
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Voltage/Hz Ratio
• If the frequency of the drive voltage is changed without
changing the motor voltage, the current, and therefore thetorque, of the motor will change
• If the frequency is lowered and the voltage remainsconstant, torque will rise
• However, the motor may overheat from excessive current
• To maintain constant current and torque under varyingfrequency conditions, the bus voltage must be varied to theinverter output frequency
• This variation is called voltage-to-frequency ratio
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Overcurrent Protection
• To protect motors from overcurrent in malfunction
situations, circuitry must be included to preventovercurrent
• Overcurrent conditions may arise because:
– Physical load is excessive
– Starting inertia of the motor is excessive
– A short circuit exists in the motor windings
– A device in the drive inverter has short-circuited
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Auto Boost Circuit
• Used when starting a
motor or when a motor isrunning at a slow RPM
• The auto boost circuit provides additionalcurrent to overcome
inertia• Typically located in the
control section of thedrive
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V/Hz Ratio Control
• V/Hz ratio iscontrolled by one of
two methods in a VVI
drive
– Phase control
– Diode rectifier and
chopper configuration
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Pulse Width Modulation Drives
• VVI Drives are being replaced with PWM Drives
• Single-phase bridges used in VVI drives may notgenerate enough current for high-horsepower motors
• A three-phase rectifier provides more power thana single-phase rectifier because its pulsations arecloser together and require less filtering
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PWM Drive Circuit
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Flux Vector Drives
• VVI and PWM drives are adequate for steady-
state conditions or where slow response times inspeed changes when load variations areencountered
• Conditions that have abrupt load, speed, and
position changes require a different drive system• The Flux Vector Drive is better suited to handle
these condition requirements
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Flux Vector Characteristics
• Able to drive squirrel cage motors so that its
performance is equal to DC motors and drivesystems
• Maximum torque is available at all times
• Dynamic response is quick
• Constant velocity is maintained under varyingload conditions
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Vector ControlBlock
Diagram
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Motors Driven by AC Drives
• Standard AC motors are designed to operate with fixed
frequency/fixed voltage supplies• AC drives generate high frequency pulses that result in
voltage spikes being produced by the stator coil; these may‘punch through’ or stress the insulation
• Specially designed motors are required for operation withAC drives that have heavier insulation on the windings
• These motors are labeled as inverter duty, or inverter rated motors
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Control Panel Inputs and DriveFunctions
• Provide the interface between the operator and the drive systemfor establishing the
operational parametersand providinginformation about thesystem
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Drive Functions• Acceleration - provides soft start or ramping
• Deceleration - rapid stopping of the motor
• S-Curve - alters the start/stop curve of the motor
• Slip Compensation - reduces slip in critical applications
• Critical Frequency Rejection - prevents resonantfrequencies from developing
• Power Loss Ride-Through - provides continuous operationof the motor during power loss for brief periods of time
• Automatic Restart - in cases of power loss, auto restart may be a requirement of the process
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Inverter Self-Protection Function
• Must protect itself against various faults
– Overcurrent protection
– Overload protection
– Overvoltage protection
– Undervoltage protection – Overheating protection
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Motor Braking
• Too large a load can cause overhauling in amotor
• Typically, dynamic braking is used in ACdrive systems
• Other methods include antiparallel andregenerative braking