As more and more machine and panel builders become aware of variable speed
control for small ac motors, the demand for products that hit the market "sweet
spot" increases dramatically. This sweet spot is a combination of product
attributes that customers really care about, such as ease of use, economy,
reliability, flexibility, physical size, and performance. The marketplace has
many innovative problem solvers, but few that meet the demand for all these
attributes. Following is a discussion of these attributes' importance and how
they can be incorporated.
Ease of use. As the drives market continues to offer a broad
product selection, a constant challenge that drive manufacturers face is making
ac drives easier to install, set up, and operate. The Reliance Electric drives
team took this challenge seriously with its new SP200 micro-drive by keeping the
parameter adjustment set down to a level appropriate for many standard
applications--purposely omitting the features required only for complex
applications. In addition to this carefully chosen set of adjustments, Reliance
Electric engineers made use of Rockwell Automation's usability lab by observing
customers while they used a prototype product. This supplied useful information
about user needs that was later incorporated into the design.
But no matter how intuitive and easy to use a manufacturer makes its product,
customers request even simpler and more efficient means of setup. This is
especially true for OEM applications where multiple units require the same
settings. OEMs wanted a handheld programming device that could store in memory
the settings of one programmed drive and then duplicate these settings to
multiple units. The Reliance Electric CopyCat, a future option for the SP200,
was designed for this purpose. It is capable of storing, sending, and receiving
parameter settings. The CopyCat will allow efficient setup of multiple units
that need to be programmed with identical (or similar) parameter settings. As
time progresses, the demand for more products that offer ease of use will
increase, and manufacturers will differentiate their products based on this
demand.
Economy. In many cases, such as large pump and fan
applications where the benefits of variable speed far outweigh the cost of an ac
drive, the justification is easily made based on energy savings alone. For
example, operating a 100-hp fan with variable speed may reduce energy costs by
thousands of dollars per year. Small horsepower applications also reap the
benefits of variable speed, but often for different reasons.
The small horsepower variable speed justification is more heavily dependent
on other advantages such as increased throughput, less downtime, and component
substitution (e.g. mechanical speed reducers). Sometimes the justification is
dramatic and obvious. Other times, more detailed machine and system analysis is
required to find the answer. This is where ac micro-drive manufacturers strive
to offer low-cost, yet capable products that can compete with other components
such as motor starters, mechanical adjustable-speed drives, and dc drives. It
truly becomes an issue of cost versus benefit.
Reliability. Over the last 20 years, ac drives have emerged
into a respectable and mature technology. Today, well-documented, proven
designs, along with robust power device technology, have yielded a high degree
of reliability. However, the design envelope is now being stretched to the
extreme limits of compactness, temperature range, shock and vibration, and
source voltage fluctuations. This requires a high degree of design engineering
and prototype and production testing to verify that every product meets and
exceeds customer expectations.
Reliance Electric engineers went beyond these steps when designing the SP200.
They used highly accelerated life testing (HALT). With this test method,
products are stressed well beyond their design specifications, up to the point
of failure. Following failure, they are repaired and taken to the next extreme.
The process is repeated until the product can no longer be repaired. This extra
effort permits design changes prior to final production and provides an
additional level of reliability comfort.
Flexibility. Today, application requirements are becoming
widely understood. Be-cause of this, and the maturity of ac drives, a wide
selection of good products is now available. At the same time, machine builders
are demanding more from a drive as they strive to be more competitive on not
only price, but also performance. Examples of some customer requirements that
are becoming more popular, but not currently designed into most products, are as
follows:
Expanding voltage requirements. Ac variable-speed drives are now becoming
more attractive to use in both industrial and commercial settings. Products
that operate on traditional industrial 3-phase 230 and 460 supply voltages
have been available for quite some time. Until recently, if the supply voltage
was single-phase and 230V, a 230V 3-phase drive would have to be de-rated by
50%. Now, specific models are designed to accommodate single-phase in-put
without de-rating, making the drive easier to specify and install. In fact,
due to demand from the commercial segment, a few manufacturer's brands also
offer connection to 115V, while still delivering 230V 3-phase output for
operation of a standard 3-phase motor. This negates the problems of
single-phase motors not being well-suited for ac drive variable-speed control,
and the general unavailability of 3-phase 115V motors.
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Focused speed control. A large percentage of drive applications have fairly
specific speed control requirements and often don't need many control inputs
and functions found on standard general-purpose drives. Some drive
manufacturers have taken extra steps to insure that customers don't buy more
control than what is needed for the application. The SP200, for example, is
offered in three different control types to most appropriately address the
needs of customers. One model exists for simple open-loop speed control from
one analog signal. Example applications include fans, pumps, conveyors, and
treadmills. A second model is commanded to specific speeds by opening and
closing switches. Example applications include commercial laundry machines,
packaging machines, and car wash machines. A third model performs an operation
between two analog signals. PI setpoint control can be defined with this model
and can be used to control things like temperature or pressure. And this can
be accomplished without the need for a standalone loop controller, resulting
in reduced system cost.
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Adaptable and robust control inputs. In many cases customers want to use
their own low-cost and robust 24V dc discrete signals to control a drive. Many
drives cannot directly accept these types of signals and must be used with an
adapter card. However, some designs can directly accept 24V dc signals without
the additional card and its associated cost. In addition, often the control
inputs need to be isolated to accommodate both grounded and floating control
signals without encountering "ground loop" problems that can cause complete
system malfunction.
Physical size. Consider ac drives of the past. Only 10 years
ago, the smallest drives consumed more than six times the space required by
recent ones such as the SP200. Drives are now approaching the size of a motor
starter. This is important for machine and panel builders who want to conserve
panel space and receive all of the benefits of variable speed control.
Manufacturers have been able to dramatically reduce drive size for a number
of reasons. For example, the power silicon components (diode bridge rectifier
and IGBT inverter sections) that convert power from the utility and deliver it
to the motor have improved designs with respect to both package size and power
loss. The reduced power loss significantly reduces the size of a drive by
lowering the demand for cooling devices such as heat sinks and fans. However,
the size reduction in the power section is only one part of the overall
product size reduction. The integration of more features into microprocessors
and other logic components reduces total discrete part count. This along with
the general miniaturization of other surface-mount and power supply components
results in a large savings of circuit board space. Finally, when designed
concurrently with the electronics assembly, the drive's enclosure itself
becomes a key factor in keeping the overall dimensions to a minimum.
Performance. Customers must also consider the level of
torque performance expected from today's micro-drives. Many tout current
overload capacity numbers such as 150% for one minute or 200%, instantaneous.
These estimates predict a drive's ability to produce high torque levels needed
for startup or dynamic load response. Unfortunately, these specifications are no
guarantee of actual performance. Unless proper hardware and software designs are
optimized for torque performance, some designs may not be adequate for many
constant-torque applications. When a design is implemented to produce high
torque over a wide speed range, the performance difference can be very large.
Users should consider drive and motor choice, software, and mechanical elements
when designing a control system.
The high demand for ac micro-drives has forced manufacturers to "hit the
sweet spot"--in other words, to provide more optimized designs that offer the
best in ease of use, economy, and small physical size, while continuing to
provide reliability and performance at an economical price.
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