1. Developed by, Rockwell Automation Drives Business Reliance Electric
TORQUE PRODUCTION WITH AC DRIVES & MOTORS: Understanding the technology
Developed by,
Rockwell Automation Drives Business
Reliance Electric
Spring Update CD, May 2001

2. Presentation Abstract
After 25 years of AC Drive acceptance, drive manufacturers offer the industry many types of control methods.
We’ll review some motor & drive basics and then discuss the technologies offered in AC Drives along with the selection process.
Spring Update CD, May 2001

3. REVIEWING MOTOR FUNDAMENTALS
AC & DC Motor Basics
REVIEWING MOTOR FUNDAMENTALS
Spring Update CD, May 2001

4. Constant Horsepower Range
Motor Basics
Motor nameplate HP is achieved at Base RPM:
HP = Torque * Speed / 5252
Constant Torque Range
Constant Horsepower Range
Torque
RPM
Base Speed
100%
Horsepower
Nameplate HP is only achieved at base speed, NOT BEFORE!
Spring Update CD, May 2001

5. Motor Basics - AC Motor Construction
Motor Frame Assembly
Rotor & Shaft
Assembly
Stator Winding Assembly
3 phase stator winding circuit w/ connections T1, T2 & T3
Spring Update CD, May 2001

6. Motor Basics - AC Motor Operation
2 Pole Motor
Motor RPM is equal to:
Note that Frequency is the only variable to affect motor speed
120 * Frequency
# Motor Poles
Rotating Magnetic Field of a 2 Pole AC Induction Motor
Spring Update CD, May 2001

7. Motor Basics - DC Motor Construction
Commutator & Brush Assembly
Armature Assembly
Field Poles Assemblies
NOTE: The Armature & Field Circuits are mechanically fixed at 90° at all times
Distinct Armature & Field Circuits are mechanically separated
Spring Update CD, May 2001

8. Motor Basic - DC Motor Operation
Simple Model
Motor RPM is equal to: S N V
Voltage ( Voltage Drop )
Field Flux
Arm
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
Spring Update CD, May 2001

9. 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
Spring Update CD, May 2001

10. PWM AC DRIVE FUNDAMENTALS
AC Drive Basics
PWM AC DRIVE FUNDAMENTALS
Spring Update CD, May 2001

11. Drive Basics - PWM AC Drive Construction
Motor
AC Line
IGBT
Inverter
Diode
Rectifier
DC Bus
Filter
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.
Spring Update CD, May 2001

12. AC Drive Basics - PWM AC Waveforms1 3
+ DC Bus
- DC Bus
Drive
500 Volts / Div.
Phase Current
10 Amps / Div.
M2.00s Ch V
PWM waveform is a series of repetitive Voltage pulses
Spring Update CD, May 2001

13. AC Drive Basics - V/Hz Operation
At 100% of the motor’s base speed, the V/Hz ratio is determined: HP = 100% of motor nameplate
Output
Frequency
Base Frequency 60
Output Voltage Hz 30
460
230
115 15 90
460VAC
= 7.67 V/Hz
Operation at Base Speed
Motor speed is controlled by ramping Voltage & Frequency
Spring Update CD, May 2001

14. AC Drive Basics - V/Hz Operation
At 50% of the motor’s base speed, the V/Hz ratio is maintained: HP = 50% of motor nameplate
Operation at 50% Base Speed
Output
Frequency
Base Frequency 60
Output Voltage Hz 30
460
230
115 15 90
460VAC
= 7.67 V/Hz
At 50% of base speed, Voltage & Frequency decrease by 1/2
Spring Update CD, May 2001

15. AC Drive Basics - V/Hz Operation
At 25% of the motor’s base speed, the V/Hz ratio is maintained: HP = 25% of motor nameplate
Output
Frequency
Base Frequency 60
Output Voltage Hz 30
460
230
115 15 90
460VAC
= 7.67 V/Hz
Operation at 25% Base Speed
At 25% base speed, Voltage & Frequency decreases by 3/4’s
Spring Update CD, May 2001

16. 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
Output
Frequency
Base Frequency 60
Output Voltage Hz 30
460
248
138 15 90
460VAC
= 7.67 V/Hz +
% BOOST
Voltage Boost
Offsetting the voltage ratio increases motor starting torque
Spring Update CD, May 2001

17. AC Drive Basics - V/Hz Operation
Voltage Boost over prolonged operating periods may result in overheating of the motor’s insulation system and result in damage or premature failure.
CAUTION: Motor Insulation Life is decreased by 50% for every 10C above the insulation’s temperature capacity
Unable to perform like DC, the industry looks to Vector Control
Spring Update CD, May 2001

18. 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
Spring Update CD, May 2001

19. AC VECTOR DRIVE FUNDAMENTALS
AC Drive Basics
AC VECTOR DRIVE FUNDAMENTALS
Spring Update CD, May 2001

20. 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
Spring Update CD, May 2001

21. 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 motor’s 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
Spring Update CD, May 2001

22. 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”
Torque Current
Magnetizing Current
100%
90
Magnetizing Current is the equivalent of Field Current
Spring Update CD, May 2001

23. 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
Spring Update CD, May 2001

24. 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
Spring Update CD, May 2001

25. 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
Spring Update CD, May 2001

26. 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”
Spring Update CD, May 2001

27. AC Drive Basics - Vector Operation
Flux Vector Drives act very much like DC Drives
Magnetizing Current is decreased above Motor Base RPM
Torque Current
Magnetizing Current
100%
90
Torque Current
Magnetizing Current
100%
90
Field Weakening occurs whenever we exceed Motor Base RPM
Spring Update CD, May 2001

28. 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
Magnetizing Current
100%
90
Torque Current
Magnetizing Current
10%
90
Torque Current increases or decreases dependent upon load
Spring Update CD, May 2001

29. 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
Magnetizing Current
100%
90
Torque Current
Magnetizing Current
10%
90
Torque Current can be commanded as a reference value
Spring Update CD, May 2001

30. 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.
Torque Current
Magnetizing Current
100%
90
Optimized Torque Production
Torque Current
Magnetizing Current ?
Poor Torque Production & Regulation
ie: Impact Load
Motor torque is optimized ONLY when 90 is maintained
Spring Update CD, May 2001

31. AC Drive Basics - Vector Operation
Load Type: Forward Speed & Reverse Torque ?
If the Nip Rolls are engaged during web travel, a condition with forward velocity and reverse torque can occur.
Use either V/Hz or a closed loop system if inertia or speed is high.
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
Spring Update CD, May 2001

32. AC Drive Basics - Vector Operation
Motor Current is = Vector Sum of Torque & Magnetizing
This is where the term VECTOR DRIVE is derived
Magnetizing Current
Torque Current
100%
Motor Current
90
100%
A² + B² = C²
Motor Current is what’s measured with a clamp-on meter
Spring Update CD, May 2001

33. 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. L1 L2 L3
Current Feedback
Motor E
Micro P
Encoders provide rotor speed & position information
Spring Update CD, May 2001

34. AC Drive Basics - Rotor Temperature & Torque
As motor temperature reaches nominal operating values, torque linearity and accuracy improves in FVC operation
Torque accuracy of 5% or better !
% Torque
Inch - Lbs
HOT Motor
COLD Motor
Spring Update CD, May 2001

35. 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. L1 L2 L3
Voltage Feedback
Motor E
Micro P
The drive continuously adapts to motor temperature change
Spring Update CD, May 2001

36. 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
Spring Update CD, May 2001

37. 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!
Spring Update CD, May 2001

38. 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
Spring Update CD, May 2001

39. 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
Spring Update CD, May 2001

40. AC Drive Basics - Sensorless Vector Operation
SVC Drives w/ a Vector Core estimates rotor speed & position L1 L2 L3
Motor
Current Sensors
Micro P
( FVC + Speed Estimator )
A “Speed Estimator” calculates rotor speed & position
Spring Update CD, May 2001

41. AC Drive Basics - Control Loops
There are 3 Basic Control Loops in High Performance Drives:
POSITION
SPEED
TORQUE
MOTOR
Position Reference is optional in most Vector Controls, internal in some
Speed Reference is typical of how we control motor operation
Torque Reference can made directly, bypassing the speed loop as a reference for applications such as Winders & Test Stands
10 rad/sec
100 rad/sec
1,000 rad/sec
Bandwidth ratio between loops ranges from 3:1 to 10:1
Spring Update CD, May 2001

42. AC Drive Basics - Regulator Diagram
Speed
Reference
Flux
Command
Torque
Gate
Signals
AC Line
Current
Feedback
Rotor Speed
& Position + -
Torque Loop
Speed Loop
Field Controller
PWM Inverter
AC Motor E
Typical Regulator Control Diagram for FVC
Spring Update CD, May 2001

43. AC Motor Basics - Inverter Duty
MOTORS
Spring Update CD, May 2001

44. AC Motor Basics - Inverter Duty
Some motor frames are sized so that just the surface area is suitable to dissipate motor heat w/o the need of a fan or blower
Blowers may be added to motors to allow operation at low speed including “0” RPM with 100% Torque continuous
Spring Update CD, May 2001

45. AC Motor Basics - Inverter Duty
Match Motor type to meet your needs!
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.
Spring Update CD, May 2001

46. AC Motor Basics - Inverter Duty
Rotor design affects torque production!
Rotor Designs Vary by motor type:
Definite purpose “single squirrel cage” rotor design for Variable Frequency Drive use
Standard Industrial AC Motor “double squirrel cage” Rotor Design for improved across the line starting torque.
Spring Update CD, May 2001

47. AC Motor Basics - Equivalent Circuit Diagram
Equivalent Circuit Diagram of an AC Induction Motor
Resistance
Stator
Inductance
Rotor
Magnetizing
AC Input
Voltage - +
Current Working
Rotor heating affects torque production!
Spring Update CD, May 2001

48. AC Motor Basics - Drive Operating Region
NEMA Design ‘B” Motor
Full Load Torque
Breakdown Torque
Rule of Thumb:
Approximately 80% of BDT (ft-lbs) is usable for PEAK Torque needs when current is available.
Therefore, current headroom from the drive can improve recovery from sudden load changes.
Peak Torque capacity is dependent upon the motor BDT %
Spring Update CD, May 2001

49. AC Motor Basics - Drive Operating Region
NEMA Design “B” Motors vary in Breakdown Torque capacity
Breakdown Torque identifies Peak Torque capabilities
Spring Update CD, May 2001

50. AC Motor Basics - Operating Range
Speed / Torque Curve of an AC Drive & Inverter Duty Motor
% TORQUE 10 20 30 40 50 60 70 80 90
100 6 12 18 24 36 42 48 54 66 72 78 84
Torque HZ
Acceptable Region
for Continuous Operation
Inverter Duty Motors operate at 1/10th Base RPM
Spring Update CD, May 2001

51. AC Motor Basics - Operating Range
Speed / Torque Curve of an AC Drive & Inverter Duty Motor
% TORQUE 10 20 30 40 50 60 70 80 90
100 6 12 18 24 36 42 48 54 66 72 78 84
Torque HZ
Torque above
base RPM =
100%
% Above Base RPM
CHp Operation above Base RPM is typically limited to 150%
Spring Update CD, May 2001

52. AC Motor Basics - Operating Range
Speed / Torque Curve of a Vector Drive & Vector Duty Motor
% TORQUE 10 20 30 40 50 60 70 80 90
100 6 12 18 24 36 42 48 54 66 72 78 84
Torque HZ
Acceptable Region
for Continuous Operation
Vector Duty Motors operate at “0” RPM w/ 100% Torque Cont.
Spring Update CD, May 2001

53. AC Motor Basics - Operating Range
Speed / Torque Curve of a Vector Drive & Vector Duty Motor HZ
% TORQUE 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90
Torque 10 20 40 50 70 80
100 96
102
108
114
120
Vector Duty Motors may have
CHP Ranges of
2 * Base Speed or more
depending on their design
Special motor & drive designs can allow operation up to 8 * Base RPM
Some Vector Duty Motors can provide CHp ( 2 * Base RPM )
Spring Update CD, May 2001

54. COMPARING AC DRIVE PERFORMANCE AC Drive Performance
Spring Update CD, May 2001

55. 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.
Spring Update CD, May 2001

56. 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.
Spring Update CD, May 2001

57. Control Selection
V/Hz is typically good for up to 10:1 Constant Torque.
SVC is typically good for up to 40:1 Constant Torque.
FVC is typically good for up to 1,000:1 which includes continuous operation at Zero Speed.
Spring Update CD, May 2001

58. 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.
Spring Update CD, May 2001

59. Both AC & DC Drives have specific areas of merit to consider
Drive Selection
Both AC & DC Drives have specific areas of merit to consider
Spring Update CD, May 2001

60. Both AC & DC Drives have specific areas of merit to consider
Drive Selection
Both AC & DC Drives have specific areas of merit to consider
Spring Update CD, May 2001

61. Drive Selection - Speed Range
Digital DC Drives & AC Vector Drives performance similarly
Spring Update CD, May 2001

62. Thank You!
Any Questions?
Spring Update CD, May 2001