Machinedesign 1130 Heavy Duty Motors Treadmills 0 0

Small motors, big considerations

May 1, 2000
The littlest things can make the difference between a properly operating motor and one that can't do the job. Here’s what you need to look at before selecting a small motor

The smaller the motor, the more stringent the application requirements frequently become. That's why small motors are so often custom- designed. To further complicate matters, many small motors fall below the frame definitions established by the National Electrical Manufacturers Association. NEMA has defined dimension and mounting configuration standards for all kinds of fractional and integral-horsepower motors, but the smallest frame it covers is 42, which houses a 1/6-hp motor at the least.

So manufacturers of subfractionalhorsepower motors use their own designations. Often, these correspond to barrel diameters. For example, a 24-frame motor is 2.4 in. in diameter, and a 31-frame, 3.1 in. Of course, barrel diameter is just one dimension and often not the most critical. Issues such as mounting configurations, shaft length, and shaft diameter are frequently of greater concern, and generally these are not standard across the various brands of motors.

Know what you need

Ask most motion system designers whether they need a heavy-duty motor for the application, and they'll probably say "Yes." Indeed, unless you're going to use the motor for routine household applications or for air moving, it will likely face electrical and mechanical stresses that would quickly break down an appliance- duty motor.

But you need to be more specific. The line between commercial and industrial- duty motors is not always clear. Sometimes a commercial-duty motor will be just fine for an application that might otherwise be considered industrial-duty. Other times, it could be just the opposite: An industrial- duty might be the best choice for a commercial or home application.

A key example of the latter in recent years has been exercise equipment. Many treadmill manufacturers have opted for industrial-duty motors, even if their machines are not necessarily intended for heavy health-club use. They want a motor that can handle startstop stresses and that has a long operating life on an expensive piece of equipment.

This is where the designer's knowledge and experience of the various manufacturers is critical in examining the various features that they build into their motors. Commercial and industrial-duty motors have key differences:

Bearings. Industrial motors have ball bearings, which in smaller motors are almost always sealed or shielded. Most manufacturers include a preloaded or "wavy" spring to minimize shaft end play and reduce vibration and noise. Commercial-duty motors typically use a bronze sleeve bearing, which has the advantage of being very quiet but less durable.

End shields. An industrial motor's endshield will usually be made of very rigid cast aluminum and have machined surfaces for bearing seats. Commercial-duty motors, on the other hand, will have zinc alloy endplates, which are less rigid. Some extremely light-duty motors (usually for fans) will use stamped-steel endplates, but these seldom appear in power transmission applications.

Frames. Industrial motor frames are usually painted, often with epoxy for corrosion resistance and appearance. Commercial-duty motors typically have zinc-plated frames.

Serviceability. Because they are designed for less demanding applications and cost less, commercialduty motors often have no easily serviceable parts. Industrial-duty motors usually do. On dc motors the major wear item is the carbon brushes, and industrial-duty motors will allow brush changes without disassembly of the motor.

Electrical insulation. Insulation classes B, F, and H are considered industrial ratings. In fact, most subfractional industrial-duty motors are either Class F or H because of their relatively small heat dissipating area. This provides an extra measure of protection and gives motor designers the ability to design for a very wide speed range. Class A insulation is adequate for many commercialduty motors.

Ask the manufacturer

Remember that you have an experienced ally ready to help you come up with the right motor: your supplier. Work together with their application specialists, starting as early in the process as possible. They can develop prototypes, special electrical and mechanical designs, unique mountings, gearboxes, and other features.

There are other advantages. By planning ahead with your supplier, you can enjoy reduced costs from shorter production lead times, special delivery programs, and other factors.

Kim Kowalewsky is national sales manager at Leeson Electric Corp., Grafton, Wis. His specialty is subfractional-horsepower motors and gearmotors. He holds a B.S. in electrical engineering and has more than 15 years of experience in motors and power transmission.

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