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Motor Technologies Compete for Space in Small
Variable Speed Drives
Alternately called speed controls, small variable speed
drives (VSDs) typically range from 10 to 2000 watts
in output power. The core of this lower power class
is best represented by VSDs ranging from 75 to 750 watts,
often described as the fractional horsepower segment
of this important family. Applications can be found
in many markets; some of the most popular uses include
variable speed conveyors, multi-axis conveyors, bar
feeders, fans, compressors, pumps, mixers, food processing
equipment, automated ticket handling equipment, body
scanning actuators, and centrifuges.
Regardless of the type of application, a VSD must control
the speed of the motor over a wide speed range. It holds
the motor’s velocity signal within a specified
range as a function of voltage, temperature, and load
changes. Actual performance parameters include speed
range and speed regulation. Other important performance
parameters include motor torque, power levels, input
current levels, and motor efficiency. Two control strategies
are used in industry: open-loop control and closedloop
control. Open-loop control depends on the electric motor’s
internal regulation. Closed-loop control techniques
measure the motor’s speed and compare it to the
desired speed values.
Most smaller VSDs in use today continue to employ brush
DC motors for a majority of applications due to their
lower unit cost and ease-of-use, but technology change
is on the immediate horizon. Two different VSDs —
brushless PM and AC induction driven VSDs — are
competing technology replacements. Both provide performance
enhancements and better long-term cost savings than
equivalent brush DC VSDs.
At the beginning of the design process, users may select
a brushless PM VSD (employing a brushless DC motor)
because they need high performance capability and are
willing to pay more to obtain it. AC induction VSDs,
on the other hand, are selected for longer life requirements
and acquisition pricing similar to brush DC VSDs.
Emerging Pressure Points
Factory automation machines used in lower intermittent
duty cycle applications will continue to use brush DC
VSDs as their first choice; however, an increasing number
of factory machine builders are looking at lower life
cycle costs. Key elements for better life cycle costs
include longer life expectancy, lower maintenance costs,
better power efficiency, lower electromagnetic interference
(EMI), and lower thermal stress. These elements provide
the user with cost savings over a longer time period.
The rapid increase in the use of nonlinear power electronics
found in today’s DC and AC VSDs has significantly
increased the level of EMI in industry. Theimpact of
government regulations and industry standards (FCC,
IEEE, and IEC) concerning EMI levels places a premium
on lower EMI performance. As a result emphasis is now
placed on implementing new circuitry dedicated to minimizing
EMI and using electric motors with lower EMI signatures.
AC Induction VSDs
The popularity of smaller AC induction motors in lower
cost and higher volume markets (appliances, vending
machines, etc.) has fostered its use in VSDs in industrial
and factory automation markets.
The AC induction motor is a brushless AC motor with
demonstrated longer life and lower maintenance performance.
Power electronics packages continue to drop in price
as quantities begin to climb and the semiconductor industry’s
manufacturing processes shrink electronic device size
and package more components on a smaller volume of silicon.
Of the many types of VSDs used, the inverter driven
threephase VSD and the singlephase control VSD are currently
the most popular.
Benefits and disadvantages of the brushless AC VSD versus
the brush DCVSD can be seen in the following comparison.
The 90-watt, single-phase ES01+VS1590A VSD and the 200-watt,
three-phase BHF62AT VSD from Oriental Motor represent
two different AC closed-loop control schemes. The ES01
AC VSD uses a pulse generator to control speed regulation
over a 17.8 to 1 speed range. The larger three-phase
BHF62AT AC VSD uses a sensorless vector closed-loop
control with inverter drive. The two AC VSDs show similar
regulation capabilities to the 90-watt brush DC VSD
using a closed-loop back EMF control scheme. The DC
motor has a wider speed range and better power efficiency,
but its major limitations are in life cycle costs and
EMI performance. (The brush DC motor generates large
amounts of EMI.) While the DC VSD has lower acquisition,
installation, and operating costs, the unit life expectancy
is much shorter (due primarily to motor brush life).
The lower performance AC induction motors achieve the
same acquisition costs as the brush DC VSD at a longer
life expectancy and much lower EMI levels.
Brushless PM VSDs
Brushless PM VSDs, using either brushless PM or brushless
DC motors, achieve the highest overall power efficiency
over wide speed and torque ranges. (Higher motor power
efficiencies lead to better energy savings.) The brushless
PM motor will develop the highest power per-unit-volume,
or power density, of the three motor technologies represented
in this article.
Two brushless PM VSDs, the 120-watt BX5120 and the 90-watt
AXU590A, also from Oriental Motor, illustrate this point.
The higher performance closedloop BX5120 uses an optical
encoder to provide the necessary velocity feedback and
a three-piece inverter as the power electronics drive.
This higher cost VSD creates the highest precision speed
regulation signals (0.05%) as a function of load or
voltage or temperature variations over a wide speed
range of 100 to1.
The lower cost AXU590A uses a hall device as the closed-loop
feedback to achieve a smaller speed range, but has better
speed regulation (as a function of load, voltage, and
temperature variations) than the brush DC VSD. This
unit also has the smallest volume and the highest power
density of all the devices discussed in this article,
and is often used in smaller-sized, more portable machines.
Both three-phase inverter driven brushless PM VSDs possess
superior life cycle performance and much lower EMI signatures
than the DC VSD.
Future Industry Needs
Tomorrow’s process and machine builders require
an infinitely flexible VSD that can vary speeds, control
torque ranges, operate at best efficiency levels, and
perform under a wide range of loads, voltages, and temperatures
over a longer time interval. It must simultaneously
meet more stringent industry and government regulations.
The enhanced performance levels of both AC induction
and brushless PM VSDs provide users with improved alternatives
to the venerable DC VSD.
This article was written by Dan Jones of Motion
Media Group for Oriental Motor USA Corp., Torrance,
CA. Dan Jones can be contacted at Dan.Jones1@verizon.net.
For product information, contact Nick Johantgen, engineering
manager for Oriental Motor USA Corp., at (310) 325-0040
or NickJ@orientalmotor.com.
Visit Oriental Motor online at www.orientalmotor.com.
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