Simplicity Yields Cost Savings

By Kim Kowalewsky, LEESON Electric Corporation

Permanent magnet DC motor teamed with SCR drive provides cost-saving advantages in low-horsepower applications.

Whenever technological change sweeps an industry, there's also a paradigm shift, as new ways for solving problems are discovered. But in the rush to take advantage of the new technology, it's easy for designers and manufacturers to overlook simpler and more cost-effective solutions.

Here's an example: With the emergence of AC adjustable speed drives, many applications have been specified as requiring the newer technology.

But it's a fact that for many applications, the most straightforward, cost-effective way to put together a small-horsepower, adjustable speed drive is by teaming the proven technology of a permanent magnet DC motor with a packaged SCR control.

With a minimum of fuss, the PMDC/SCR combination provides an industrial-duty drive with smooth, stepless speed control throughout the speed range, plus these key capabilities:

• Precise speed regulation
• Simple connections
• Reversing and braking options.

The Simpler, The Better

What makes DC SCR drives so easy to apply?

With DC motors, the speed is controlled simply by varying the amount of voltage applied to the armature. The more voltage, the higher the speed.

By adding an inexpensive packaged control, the user can vary the output voltage -- and thereby the speed of the motor-- simply by turning a knob on the control unit.Such SCR controls get their name from the silicon controlled rectifiers (SCRs) they use to convert the AC current supplied by the power company to DC

DC SCR drives offer good speed control and good efficiency. Commonly, a 60:1 speed range is attained, with speed regulation of 1% or better.

A Cost-Efficient Alternative

While lacking some of the specialized motion control features of brushless DC drives or vector-type AC inverter drives, DC SCR drives cost considerably less -- making them ideal for a variety of industrial and commercial applications. DC SCR drives are an excellent choice for small horsepower applications (sub-fractional to approximately 2 horsepower), where only basic control functions are needed, such as: stop/start, forward/reverse, and adjustable speed control.

Such applications might include:

• Conveyors for general material handling (including food processing)
• Pumps, sprayers, fans, and mixers
• Machine tools, textile machines, and other factory equipment
• Packaging machines
• Medical and laboratory equipment
• Exercise equipment
• Office machine

So where you're dealing with a low horsepower application, don't forget to consider the simplicity and cost effectiveness of a DC SCR package. In many situations, it will do the job every bit as well as the more sophisticated control packages--with considerable cost savings.

Other PMDC Motor Uses

Aside from being used in control packages, permanent magnet DC motors are commonly used in applications needing high starting torque or predictable torque throughout the speed range.

PMDC motors are also known for delivering high power in a compact package. In intermittent duty applications, it's common to see as much as three to four times the continuous duty rating, depending on the duty cycle.

In addition, permanent magnet motors use less energy than other types of DC motors, because no current is needed to "excite the field," as in field-wound types of DC motors. This added energy efficiency can be especially important in low voltage applications, where the power source is often a storage battery.

Low-Voltage Applications Expand

Demand for low voltage PMDC motors is growing rapidly and involves a number of applications, such as:

• Mobile hydraulic pumps
• Mobile hoists and lifts
• Forklift trucks and other vehicles
• Solar-powered applications
• Anywhere battery back-up power is use

The popularity of PMDC motors in general is enhanced by their wide availability as a standard, off-the-shelf item. A number of models and types are generally available, including general purpose NEMA and IEC metric frame motors, washdown designs for food processing environments, explosion-proof motors, and gearmotors.

Original equipment manufacturers often have specialized fit and performance requirements, however. Several motor manufacturers offer design services to accommodate customer needs for special shafts, mounting provisions, electrical characteristics, tachometers, custom gearboxes, or other application-specific features such special paint colors.