TORQUE PRODUCTION WITH AC DRIVES

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TORQUE PRODUCTION WITHAC DRIVES
MOTORSUnderstanding the technology

• Developed by,
• Rockwell Automation Drives Business
• Reliance Electric

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Presentation Abstract
After 25 years of AC Drive acceptance, drive
manufacturers offer the industry many types of
control methods. Well review some motor drive
basics and then discuss the technologies offered
in AC Drives along with the selection process.
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AC DC Motor Basics
REVIEWING MOTOR FUNDAMENTALS
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Motor Basics
Motor nameplate HP is achieved at Base RPM HP
Torque Speed / 5252
Constant Torque Range
Constant Horsepower Range
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Motor Basics - AC Motor Construction
3 phase stator winding circuit w/ connections T1,
T2 T3
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Motor Basics - AC Motor Operation
2 Pole Motor
Motor RPM is equal to
Rotating Magnetic Field of a 2 Pole AC Induction
Motor
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Motor Basics - DC Motor Construction
Field Poles Assemblies
NOTE The Armature Field Circuits are
mechanically fixed at 90 at all times
Distinct Armature Field Circuits are
mechanically separated
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Motor Basic - DC Motor Operation
Simple Model
Motor RPM is equal to
Both Armature Terminal Voltage Field Strength
affect DC Motor speed
To create motor torque at the shaft, we increase
Armature Current
Rotating Magnetic Field of a 2 Pole AC Induction
Motor
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Motor Basics - AC DC Summary
Key Points of Understanding

• AC Induction Motors have one circuit to connect
• Connection to T1, T2 T3 for the stator
• DC Motors have 2 separate circuits to connect
• Connection to F1 F2 for the Field
• Connection to A1 A2 for the Armature
• To make AC Motors perform like DC Motors
• Treat the AC motor like a 2 circuit machine

Mechanical differences must be overcome
mathematically
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AC Drive Basics
PWM AC DRIVE FUNDAMENTALS
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Drive Basics - PWM AC Drive Construction

• Diode rectifier converts AC line voltage to
  fixed voltage DC.
• DC voltage is filtered to reduce current ripple
  from rectification.
• Inverter changes fixed voltage DC to adjustable
  PWM AC voltage.

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AC Drive Basics - PWM AC Waveforms
PWM waveform is a series of repetitive Voltage
pulses
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AC Drive Basics - V/Hz Operation
At 100 of the motors base speed, the V/Hz ratio
is determined HP 100 of motor nameplate
Motor speed is controlled by ramping Voltage
Frequency
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AC Drive Basics - V/Hz Operation
At 50 of the motors base speed, the V/Hz ratio
is maintained HP 50 of motor nameplate
Operation at 50 Base Speed
At 50 of base speed, Voltage Frequency
decrease by 1/2
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AC Drive Basics - V/Hz Operation
Operation at 25 Base Speed
At 25 base speed, Voltage Frequency decreases
by 3/4s
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AC Drive Basics - V/Hz Operation
To increase starting torque, V/Hz Drives use
Voltage Boost to over-flux the motor to increase
starting torque
Offsetting the voltage ratio increases motor
starting torque
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AC Drive Basics - V/Hz Operation
Voltage Boost over prolonged operating periods
may result in overheating of the motors
insulation system and result in damage or
premature failure.
CAUTION Motor Insulation Life is decreased by
50 for every 10?C above the insulations
temperature capacity
Unable to perform like DC, the industry looks to
Vector Control
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AC Drive Basics - Vector Operation
If we can de-couple and Regulate Current, the
component that creates torque at the motor, we
can regulate motor torque, not just motor speed!
This is the premise for Vector Control
Current Regulation allows Torque Control
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AC Drive Basics
AC VECTOR DRIVE FUNDAMENTALS
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AC Drive Basics - Motor Modeling
AC Drive Parameters create a Motor Model based
on data entered in the drive parameters

• Motor Magnetizing Current
• Motor Full Load Amps
• Motor Voltage
• Motor Base Frequency
• Motor Base (Slip) RPM
• Motor Horsepower

Correct Motor Data is the most important factor
for success
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AC Drive Basics - Motor Modeling
AC Drive Parameters Magnetizing Current

• Magnetizing Current is the current required to
  excite the motor laminations and copper winding
  w/o doing work.
• Magnetizing Current is NO LOAD AMP draw less
  friction and windage
• Establishes the motors Flux
• (FLA - Mag. Amps) 100 Torque Current
• Wrong data will reduce motor torque production

Magnetizing Current will range from 35 to 50 of
FLA value
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AC Drive Basics - Vector Operation
Torque is produced, as well as regulated even at
0 RPM
Magnetizing Current Motor No Load Amps a
fixed value from 0 RPM to Motor Base RPM
Magnetizing Current is the equivalent of Field
Current
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AC Drive Basics - Motor Modeling
AC Drive Parameters Full Load Amps

• The motor FLA value may set the scaling for
• Motor Overload
• Drive Overload
• Torque Current Available
• (FLA OL) - Mag. Amps Max. Available Torque
  Current
• Wrong data affects available torque current and
  may allow damage to the motor.

Since every Vector algorithm is unique, check w/
manufacturer
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AC Drive Basics - Motor Modeling
AC Drive Parameters Voltage Base Hz

• Voltage Base Hz values will
• Establish the motor V/Hz ratio for the drive
  output
• Wrong data will cause motor heating and possibly
  reduce motor torque as well as shorten insulation
  life.

Needed to assure proper motor operation w/o
over-heating
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AC Drive Basics - Motor Modeling
AC Drive Parameters Base HZ RPM

• Base Hz RPM values will set the scaling for
• Calculation of motor slip
• Identifies expected motor RPM at Frequency
• Allows for speed error detection correction
• Establishing the point of field weakening
• Wrong data here can cause excessive current draw

AC Drives regulate speed based upon motor slip
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AC Drive Basics - Motor Modeling
AC Drive Parameters Horsepower

• The Horsepower value may be used to
• Estimate the expected motor impedance
• Estimate the expected motor inductance
• Calculate the torque loop gains
• Wrong data here can cause poor speed and torque
  regulation

Horsepower information gets us in the Ballpark
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AC Drive Basics - Vector Operation
Flux Vector Drives act very much like DC Drives
Magnetizing Current is decreased above Motor
Base RPM
Field Weakening occurs whenever we exceed Motor
Base RPM
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AC Drive Basics - Vector Operation
Torque at the motor shaft based upon load
Torque Current Motor Load at the Shaft a
variable value during speed regulated operations
Torque Current increases or decreases dependent
upon load
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AC Drive Basics - Vector Operation
Torque at the motor shaft based upon Torque
Reference
Torque Current Reference setting a fixed
value during torque regulated operations
Torque Current can be commanded as a reference
value
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AC Drive Basics - Vector Operation
Torque production suffers if 90 is not maintained
Improper tuning, incorrect motor parameters,
problems with motor speed feedback or undersized
drive applications will result in poor load
(torque) regulation.
Motor torque is optimized ONLY when 90? is
maintained
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AC Drive Basics - Vector Operation
Load Type Forward Speed Reverse Torque
How a load becomes applied to the drive system
can be critical to system success. A load where
there is Forward Velocity Reverse Torque is the
most difficult load to handle.
Time to find motor rpm position is limited by
inertia speed
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AC Drive Basics - Vector Operation
Motor Current is Vector Sum of Torque
Magnetizing
This is where the term VECTOR DRIVE is derived
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A² B² C²
Motor Current is whats measured with a clamp-on
meter
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AC Drive Basics - Flux Vector Operation
Flux Vector Drives regulate current torque
using rotor speed position to optimize torque
at the motor shaft along w/ current feedback from
the motor.
Encoders provide rotor speed position
information
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AC Drive Basics - Rotor Temperature Torque
As motor temperature reaches nominal operating
values, torque linearity and accuracy improves in
FVC operation
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AC Drive Basics - Field Oriented Control
Field Oriented Control uses the same basic
technology as Flux Vector Control, but adds
Voltage Feedback to optimize / adapt to changes
in motor temperature.
The drive continuously adapts to motor
temperature change
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AC Drive Basics - Summary
Key Points of Understanding

• Errors in Encoder Feedback affect the
  Micro-Processor
• Speed instability will occur
• Encoder Feedback Signals must be NOISE FREE
• Select an appropriate encoder for Vector Motor
  use
• Proper grounding is very important
• Motor Data programmed in the drive must be
  accurate

Motor information, measured or programmed is key
to success
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AC Drive Basics - Sensorless Vector Operation
There are actually 2 types of drives advertised
as Sensorless Vector

• Those with a V/Hz Core

• Those with a Vector Core

All Sensorless Vector Drives are NOT the same!
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AC Drive Basics - Sensorless Vector Operation
SVC with V/Hz Core Technology

• Use sophisticated Current Limiting algorithms
  to improve constant torque starting torque
  operation
• Typically needs less motor information for setup
  adding some simplicity
• Can operate multiple motors from one drive
• ONLY regulates V/Hz output, clamps CURRENT
• Can only operate as a Speed Regulator, NOT TORQUE

V/Hz Core SVC Drives can operate multiple motors
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AC Drive Basics - Sensorless Vector Operation
SVC with Vector Core Technology

• De-couples Torque Magnetizing Currents to
  maintain 90? alignment
• Typically needs more motor information for setup
  adding some complexity
• Can operate only one motor per drive due to the
  information required to regulate current
• Regulates SPEED and Regulates TORQUE

Vector Core SVC Drives can operate only one motor
at a time
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AC Drive Basics - Sensorless Vector Operation
SVC Drives w/ a Vector Core estimates rotor speed
position
A Speed Estimator calculates rotor speed
position
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AC Drive Basics - Control Loops
There are 3 Basic Control Loops in High
Performance Drives
1,000 rad/sec
100 rad/sec
10 rad/sec
Bandwidth ratio between loops ranges from 31 to
101
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AC Drive Basics - Regulator Diagram
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AC Motor Basics - Inverter Duty
INVERTER DUTY MOTORS
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AC Motor Basics - Inverter Duty
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AC Motor Basics - Inverter Duty
Types of AC Motors
T-Frame Construction Motors allow commonality in
footprint shaft height.
Definite purpose laminated frame designs
provide higher power densities improved torque
to inertia performance.
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AC Motor Basics - Inverter Duty
Rotor Designs Vary by motor type
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AC Motor Basics - Equivalent Circuit Diagram
Equivalent Circuit Diagram of an AC Induction
Motor
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AC Motor Basics - Drive Operating Region
Peak Torque capacity is dependent upon the motor
BDT
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AC Motor Basics - Drive Operating Region
NEMA Design B Motors vary in Breakdown Torque
capacity
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AC Motor Basics - Operating Range
Speed / Torque Curve of an AC Drive Inverter
Duty Motor
Inverter Duty Motors operate at 1/10th Base RPM
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AC Motor Basics - Operating Range
Speed / Torque Curve of an AC Drive Inverter
Duty Motor
CHp Operation above Base RPM is typically limited
to 150
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AC Motor Basics - Operating Range
Speed / Torque Curve of a Vector Drive Vector
Duty Motor
Vector Duty Motors operate at 0 RPM w/ 100
Torque Cont.
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AC Motor Basics - Operating Range
Speed / Torque Curve of a Vector Drive Vector
Duty Motor
Special motor drive designs can allow operation
up to 8 Base RPM
Some Vector Duty Motors can provide CHp ( 2
Base RPM )
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AC Drive Performance
COMPARING AC DRIVE PERFORMANCE
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Control Selection

• FVC operation is best since the position and
  velocity of the rotor is known and restarting is
  immediate.
• V/Hz being a soft speed regulator is very
  forgiving for restarting into loads with high
  inertia.
• SVC may be more difficult to implement due to
  limitations by manufacturer. Processor
  algorithm dependent.

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Control Selection

• V/Hz operation inheriently controls multiple
  motors.
• SVC or FVC operation with multiple motors is only
  possible when motor shafts are mechanically
  locked together and assumptions are made about
  total motor current values.

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Control Selection

• V/Hz is typically good for up to 101 Constant
  Torque.
• SVC is typically good for up to 401 Constant
  Torque.
• FVC is typically good for up to 1,0001 which
  includes continuous operation at Zero Speed.

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Control Selection

• V/Hz has no quantifiable response time or
  bandwidth.
• Typical SVC specifications may state 100
  Radians/second.
• Typical FVC specifications may state 1,000
  Radian/second.

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Drive Selection
Both AC DC Drives have specific areas of merit
to consider
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Drive Selection
Both AC DC Drives have specific areas of merit
to consider
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Drive Selection - Speed Range
Digital DC Drives AC Vector Drives performance
similarly
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Thank You!
Any Questions?