dcdrive
AC and DC Variable Speed
Drives Application
Considerations
Application Solution
Introduction
Application Discussion
Knowing what controller/motor package to use in a given situation
Which drive is right for you?
is just one of the subjects covered in this overview of AC and DC
Your choice depends on many application-specific factors such as
drive application considerations. Published originally as a direct
ambient conditions, type of loads, duty cycle, maintenance
mail piece for distribution use, it has been updated with additional
accessibility, horsepower range, sequencing, and more. The
information on drive selection to make it a more useful reference
following brief guidelines have been developed to provide you
and training tool. This has been previously released as publication
with a basic understanding of the differences between AC and DC
D-7151, but is updated here.
drive technologies. If you have specific questions, or require
This information also appears in the introduction section of the
Standard Drives Catalog (D-406).
Drive products support diverse applications with a wide variety of
products. AC drives serve processing needs and industrial appli-
application/selection assistance, please contact your nearest
Reliance Electric Sales Office or authorized Reliance Distributor.
AC DRIVE CHARACTERISTICS
•
which adjusts frequency and voltage for varying the speed of an
cations such as fans, pumps, mixers, conveyors, and extruders,
otherwise, conventional fixed speed AC motor. This is achieved
plus many more. DC drives control processing equipment such as
through Pulse-Width Modulation (PWM) of the drive output to
for sheet metal, material handling, and center winders/unwinders.
the motors.
Any drive featured in this publication will provide some form of
motor speed regulation and variable speed operation. Within
•
known as the volts per hertz ratio.
ranges, drive interconnectivity capabilities, and flexibilities.
•
is whether to use an AC or DC drive. Either drive is practical from
the following standpoints:
Voltage and frequency are maintained at a constant relationship
at any motor speed to maintain a constant torque. This is
these categories, we offer a broad spectrum of horsepower
When selecting a drive for your application, a major consideration
AC drives utilize a solid-state adjustable frequency inverter
Integrated drive/motor packages available.
Standard AC Drives are often the best choice when:
•
The environment surrounding the AC motor is corrosive,
potentially explosive, or very wet, and demands special
•
Wide constant horsepower speed range
enclosures such as explosion-proof, washdown, X-Extra Tough,
•
Proven performance with matched drive/motor packages
etc.
•
High speed regulation capabilities
•
Worldwide sales and technical support
•
inaccessibility of the motor or poor maintenance practices.
•
The considerations involved in choosing either AC or DC drives are
discussed in the following section.
Motors are likely to receive little regular maintenance due to
The motor must be small in size and weigh as little as possible.
•
Motor speeds can reach 10,000 RPM.
Drive Comparison Chart
•
Multiple motors are operated at the same speed by a single
Use this chart as a quick, basic reference guide to help you
drive.
determine the drive best suited for your application needs.
•
Speed regulation of 1 % is acceptable.
•
Existing fixed speed (Design B) AC motors can possibly be used.
•
A UL listed AC drive and motor package for hazardous
classified locations is required.
•
C-E compliance is required at 2 HP or less.
Standard
DC
V/Hz AC
Sensorless
Vector AC
Flex
Vector AC
Speed Regulation
0.01% (2)
1%
.5%
.01%
Speed Range
100:1
10:1
120:1
>1000:1
Yes/No
No
No
Yes/No
4:1
2:1
4:1
4:1
Encoder/Tachometer Required?
Constant HP Range
Additionally, Vector AC Drives are often the best choice
Starting Torque
150%
110%
150%
150%
for:
High-Speed Capability (1)
<3000
<6000
<6000
<6000
•
Applications requiring full load torque at zero speed.
Regeneration
Line
•
Fast changing loads.
Dynamic Braking w/o Regulation
Yes
•
Tight speed regulation.
•
Coordinated speed control for multiple drive axes.
•
Applications requiring increased starting torque.
•
Precise closed loop speed regulation (to 0.01% and less) is
AC MOTOR SELECTION
required.
Various types of AC induction motors are suitable for operation
•
High dynamic response.
with AC drives. It is essential that the user understand the nature
•
Web processes, material handling sorter conveyors, metering
of the application in terms of load characteristics, speed range,
pumps, extruders, and test stands.
and drive requirements, as they relate to the AC drive system, so
DC DRIVE CHARACTERISTICS
•
•
DC drives utilize a converter to transform AC current into DC
No
No
Snubber/Line
No
(1) Speed rating in RPM with standard motors.
(2) Dependent on encoder or tachometer used.
that the proper combination of motor and drive can be selected
for optimum performance.
current which is then fed to the DC motor which is designed for
The following motor performance graphs represent the maximum
adjustable speed operation. Speed changes are made by
continuous capability of the respective motors when operated
increasing or decreasing the amount of DC voltage fed to the
with AC drives. These guidelines are conservative and are based
motor from the drive.
upon full rated conditions (i.e., full horsepower requirements
Usually offer the lowest cost for medium and high HP
operating continually in a 40°C ambient. Full rated input voltage
applications.
is assumed). Since motors are frequently sized larger by the user
DC Drives are often the best choice when:
•
Snubber/Line Snubber/Line
Environmental conditions surrounding the DC motor are
reasonably clean, dry, and allow the use of DPG, DPG-FV, TENV,
or TEFC motor enclosures.
than the actual horsepower required to provide a performance
safety margin, the actual motor performance may be less than
100% full load capability. This should be considered in using the
data on the graphs.
The application requires a wide range of changing loads such
Most standard AC motors are designed to operate at a fixed,
as center driven winders.
rated frequency and speed. At this fixed speed, the built-in cool-
•
Motor speeds can reach 2500 RPM.
ing system will keep the motor from overheating. When operated
•
Starting torques are greater than 150% or unpredictable.
as an adjustable speed device at slower speeds, the motor cool-
•
Application HP requirements are medium to large.
ing action will be reduced. On such applications, the motor may
•
need to be a motor specifically designed for AC drive operation
•
VXS inverter duty motors to 150 HP.
such as the RPM AC motor. RPM AC motors offer premium perfor-
•
Explosion-proof energy efficient XE motors 1/3-150 HP (check
mance on Reliance AC drives
SIZING THE AC MOTOR
motor nameplate to verify CT rating).
•
TENV-EM and TENV EMT motors (for use with SP100 drives).
The following procedure gives a conservative, engineering-based
approach for sizing and selecting various AC motors for use with
the AC drive.
WARNING
MACHINERY BUILDERS AND/OR USERS ARE RESPONSIBLE FOR
INSURING THAT ALL DRIVE TRAIN MECHANISMS, THE DRIVEN
MACHINE, AND PROCESS MATERIAL ARE CAPABLE OF SAFE
OPERATION AT THE MAXIMUM SPEED AT WHICH THE MACHINE
WILL OPERATE. FAILURE TO OBSERVE THESE PRECAUTIONS
COULD RESULT IN BODILY INJURY.
1. Determine the drive motor output horsepower and continuous
torque over the total speed range and the starting torque
requirements.
2. Select the type of motor and drive.
3. Using the following graphs for the type of motor selected,
confirm that the required load torque from the motor selected
falls within the "acceptable region" of the graph.
GRAPHS 1 THROUGH 4 SHOW TYPICAL CONSTANT TORQUE
SPEED RANGE CURVES WITH GENERAL PURPOSE REGULATION GV3000/SE (With Parameter P.048 Set For"U-H") AND SP500.
GRAPHS 1 AND 2 ARE ALSO TYPICAL FOR BOTH THE SP100 AND
SP200 DRIVES.
Continuous motor performance for constant torque to base speed
and constant horsepower above base speed. Wider constant
torque ranges and/or horsepowers are available but application
assistance will be required.
Graph 1. 10:1 CONSTANT TORQUE
This graph applies for the following motors used with an AC
PWM drive:
•
VXS vector-ready motors to 10 HP.
Graph 2. 4.1 CONSTANT TORQUE
This graph applies for the following motors used with an AC
PWM drive:
•
TENV and TEFC energy and premium efficient motors 1-350 HP.
•
TENV-EZ easy-clean washdown duty motors.
•
Explosion proof energy-efficient XE motors 11/2 - 150 HP
(check motor nameplate to verify CT ratio).
Graph 3. 2:1 CONSTANT TORQUE
For use with explosion-proof energy efficient XE motors
Graph 5. Reliance Vector Duty Induction Motors (TENV,
11/2
- 150
TEA0-BC and DPG-FV Enclosures) with Encoder Feedback
HP (check motor nameplate to verify CT ratio).
SIZING THE AC DRIVE
The capabilities of the AC drive are determined by its output current rating. The drive chosen must have a continuous current rating equal to or more than the maximum motor load current. Be
sure to consider all loads including startup acceleration.
Graph 4. Reliance RPM AC Motors (TENV, TEA0-BC and
DPFV Enclosures) and VSM500 Integrated Drive/Motor
Packages
Single Motor and Drive Applications
NEMA Design B motors, will generally perform as shown in the
engineering data section. Note that all references to HP are for
single motor, standard NEMA B, 1.0 service factor, non-explosion
proof induction motors only.
General Sizing Method for Use with Multiple Induction
Motors
To size the six to 60 Hz drive for multiple motor applications or for
any applications for six to 120 Hz, the following procedure is used.
1. Examine each motor to be driven and determine motor full-load
amperes at line voltage. Determine the total full-load current
requirements for all motor(s) to be controlled by the drive.
2.To the current determined in step one, add the high currents of
any overloads which may exist such as acceleration, peak load,
etc., - and determine maximum short-term load at line voltage.
(Note: motor acceleration is by linear timed-rate acceleration
control. Therefore, locked-rotor amperes normally associated
with across-the-line starting of AC motors are not
encountered.)
3. Select the AC drive rating from the table with a current
capacity that will support the required currents as calculated in
Open Loop AC PWM Control Block Diagram
the previous steps.
AC Line Input*
4. If other than NEMA Design B - 1.0 Service Factor Induction
motors are to be used, or if explosion proof listed motors are
Rectifier
required contact your Reliance Electric sales office or Standard
Accel/Decel
Drives application engineer for application assistance.
Scale
Limits
Speed
Reference
AC DRIVE LEAD LENGTH
Standard AC drives utilize IGBT technology for rapid switching of
PWM devices to produce accurate sinusoidal drive outputs.
DC Bus
Gate
Signal
Speed
Regulator
PWM
Inverter
Current
Feedback
Local
Signal
Remote
Signal
AC
Motor
Typically operating at carrier frequencies of 8 kHz, low motor
Invertor Duty
acoustic noise is achieved. However, PWM devices can also
cause undesirable side effects such as motor stress, high peak
voltage and possible reflected waves that exacerbate the peak
voltage problems.
* Can be single phase input depending upon the type of drive
selected.
C. The speed regulator section provides gate pulses to the
Reliance Electric's matching drive/motor packages offer superior
continually alternating Insulated Gate Bipolar Transistors
design and proven performance. All drive/motor combinations
(IGBTs). The relative on-to-off times for pulses to be successively
have been tested for dynamic stability. When applied properly,
fired is continually alternating so as to create a sinusoidal
motor stress effects and high peak voltage should be minimal.
voltage pattern at the IGBT output. The wave frequency is
The table below highlights maximum motor lead lengths allowed
altered to produce variable speed outputs.
D. Rectified DC voltage (or DC bus supply voltage) is the source for
without external filtering.
the IGBTs. They are arranged in three pairs whose wave patterns
Maximum Lead Length* (Feet)
are each 120 degrees apart. The power output signals to the
230VAC
460VAC
575VAC
GV3000/SE
500
500
N/A
SP500
500
250
150
SP200
100
100
N/A
motor create sinusoidal wave patterns to each phase. Wave
frequency changes to change the motor's speed.
AC PWM Vector Control Block Diagram
AC Line
Input
*Maximum lead lengths vary also with carrier frequencies and
horsepower ratings. Additional external filters can be specified
Local
Signal
Remote
Signal
Limits
Scale
to extend lead lengths. Please see Application Note D-154 for
more detailed information.
+
Speed
Reference
Open loop AC PWM Control Block Diagram
A. The user supplies a speed reference to the drive. There is
always a choice between supplying a remote analog
reference or a local reference via keypad selection.
B. The speed reference may be conditioned for acceleration and
deceleration rates as well as maximum and minimum speed
settings.
-
Speed
Loop
Field
Controller
Torque
Command
Flux
Command
Rectifier
Gate
Signal
Vector
Torque
Controller
PWM Inverter
Current
Feedback
Compare
GT
Speed
Feedback
d
dt
Rotor
Postion
AC
Motor
Differential
Encoder
AC PWM VECTOR Control Block Diagram
A. The user supplies a speed reference to the drive. There is
always a choice among supplying a remote analog reference or
local reference via keypad selection. Also, it could be supplied
across a network interface if an optional network interface card
is included with the drive.
B. The speed reference may be conditioned for acceleration and
deceleration rates as well as maximum and minimum speed
settings.
C. The conditioned speed reference is compared to a speed feed
back signal as supplied by the motor encoder. Any error signal
between the reference and the feedback signal is amplified in
this outer (or major) control loop via proportional and integral
gains. Stability adjustments change the gain of this control loop
to match the desired dynamic response. The resulting signal is
applied as a reference to the current regulator. Motor current
will provide torque, which will in turn change the motor speed
to satisfy this outer loop, driving its error to zero.
D. This inner current control loop receives a reference from the
speed loop. This reference is compared to current feedback
from the motor. Any error is amplified and used to change the
relative on and off times for gate pulses to the continually
alternating Insulated Gate Bipolar Transistors (IGBTs). The
relative on-to-off times for pulses to be successively fired is
continually alternating so as to create a sinusoidal voltage
works as follows.
A. The current feedback from a motor can be broken into its
components: magnetizing and torque producing currents.
Picture these as the legs of a right triangle, with the actual or
measured current as the hypotenuse.
B. The magnetizing current can be shown as equivalent to the
motor's no-load current. (Less windage and friction losses).
This is a function of the motor's rotor design and is constant.
C. The torque producing current can be measured based on
knowing item B. Furthermore, it can be demonstrated that
motor slip and torque are related. This means that a particular
load current measurement at a given commanded speed will be
used to calculate the motor's slip at that moment. This will
provide an accurate determination of actual motor speed.
D. Based on the actual vs. desired speed, the reference is
continually adjusted to get the proper speed given the
calculated slip.
E. This method of control will provide speed regulation to 0.5%
on a steady state basis. Operation at or near zero speed is
possible, but accuracy falls off because slip measurement at
low frequencies becomes more difficult to measure.
pattern at the IGBT output. The wave frequency and its
For further reading on the basics of operation of vector control,
amplitude are altered to respectively produce variable speed
please refer to D-7161, Vector Operation Basics.
and torque outputs.
E. As an alternative, vector operation without encoder feedback,
How A Phase Controlled DC Drive Works
called sensorless vector mode, is also available and discussed
below.
F. Rectified DC voltage (or DC bus supply voltage) is the source for
the IGBTs. They are arranged in three pairs whose wave patterns
are each 120 degrees apart. The power output signals to the
motor create sinusoidal wave patterns to each phase. Wave
frequency changes to change the motor's speed.
G. Vector drives provide a way to regulate field supply voltage or
current. These drives have the capability to limit the flux or
field command and effect a field weakening condition to allow
* If drive is equipped with motor field control hardware or
software.
extended speed operation. This can be seen as the optional AC
drive operating range in the motor speed-torque curves.
** If motor has a wound field (not permanent magnet).
Sensorless vector operation, which is used to achieve dynamic
Items A-I in this block diagram illustrate the basic functional
speed and torque and control without a speed feedback device,
blocks of a DC drive.
A. The user supplies a speed command to the drive.
Field Supply Bridge Types
B. The speed reference may be conditioned for acceleration and
deceleration rate, as well as maximum and minimum speed
settings.
Vin
Single-Phase
HalfWave
Vout DC = .45VinAC*
Vout
C. The conditioned speed reference is compared to the voltage
(voltage regulator) or speed (speed regulator using a
tachometer) feedback signal from the motor. Any error
V in
between the reference and feedback is amplified in this outer
V out
Three-Phase
2/3 Wave
Vout =.67 Vin
DC
AC*
(or "major") control loop and applied as a reference to the
current regulator. Motor current will provide torque which will
in turn change the motor voltage and/or speed to satisfy this
outer loop, driving its error to zero. Stability adjustments
change the gain of this control loop to match the desired
V in
V out
Single-Phase
Full Wave
Vout DC =.90 Vin AC*
dynamic response of the motor and load.
D. This inner (or "minor") current control loop receives a reference
from the speed or voltage outer loop. This reference is
compared to current feedback from the motor. Any error is
V out
amplified and used to change the thyristors' firing angle. This
causes a change in current flow in the motor to satisfy the
Three-Phase
Special
Vout =1.12 V in
DC
AC*
V in
current loop, driving its error to zero. Some drives permit
stability adjustments which change the gain of this control loop
to match the electrical characteristics of the motor armature.
DC MOTOR TYPES
The inner current loop provides smooth drive performance and
The following section provides descriptions of the three most
current limit capability.
common types of DC motors used with Reliance DC Drives.
E. The driver section of the regulator provides gate pulses to the
thyristors that are synchronized to the power line phasing. The
firing angle of the thyristors is determined from the current
loop output and operating conditions such as the counter EMF
of the armature.
F. The AC line power is converted to DC by the rectifiers in the
power bridge. Phase controlled drives have some complement
of thyristors, and sometimes diodes, depending on the type of
bridge.
G. Some drives provide a way to regulate field supply voltage or
current. These drives have regulator functionality to
accomplish this.
H. DC motor armature.
I. Tachometer or encoder if supplied.
Permanent Magnet DC Motor
Permanent magnet motors have a conventional wound armature
with commutator, brushes, and a permanent magnet field. This
motor has excellent starting torques but speed regulation is less
accurate than for compound motors. However, the speed regulation can be improved with various designs, such as corresponding
lower rated torques for a given frame. Because of the permanent
field, motor losses are less with better operating efficiencies.
Stabilized shunt motors utilize a field winding in series with the
These motors can be dynamically braked and reversed at reduced
armature in addition to the shunt field to obtain a compromise in
armature voltage (10%) but should not be plug reversed with full
performance between a series and shunt type motor. This type
armature voltage. Reversing current cannot exceed the full load
offers a combination of good starting torque and feed stability.
armature current.
Approximate constant torque speed ranges of Reliance DC motors
used with Reliance DC drive controllers.
Enclosure Constant Torque Speed Range
Straight Shunt Wound DC Motor
Shunt wound motors with the armature shunted across the field
offer relatively flat speed-torque characteristics. Combined with
inherently controlled no-load speed, this provides good speed regulation over wide load ranges. While the starting torque is com-
DPG
60-100%
DPFV
1-100%
DPSV
1-100%
TENV
5-100%
TEFC
60-100%
TEAO
5-100%
TEAAC
5-100%
TEWAC 5-100%
DC Motor Full Load Operating Characteristic
paratively lower than the other DC winding types, shunt wound
motors offer simplified control for reversing and regenerative
service.
Stabilized Shunt Wound DC Motor
NOTE: This material is not intended to provide operational instructions. Appropriate Reliance
Electric Drives instruction manuals precautions should be studied prior to installation,
operation, or maintenance of equipment.
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Publication D-7725 – March 2000
© 2000 Rockwell International Corporation All Rights Reserved Printed in USA
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