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Choosing a Motor for Efficient Operation

Jul 1, 1999 12:00 PM, By Robert J. Lawrie, Consulting Editor

To get the most out of your motor, here's a review of different types of NEMA design motors as well as special motors and drives for various torque loads.

For most loads in a typical plant, the constant-speed, general-purpose, NEMA Design B squirrel-cage induction motor will meet your needs while giving you the most for your money. Due to its popularity, NEMA established this particular design for a broad range of applications. (See the May, 1999 issue for selection considerations.)

But just because it's the most common, the Design B motor is not for every application. Sometimes special conditions warrant additional NEMA designs; however, these usually come with a premium price tag.

The NEMA Design C induction motor is a "double-cage" motor. Its primary advantage is higher overall accelerating torque, with good full-load operating efficiency. Because one of the "cages" contains small conductors, this motor cannot tolerate long acceleration time. So don't use it for high inertia loads or those requiring frequent starting and stopping.

The NEMA Design D induction motor has a high-resistance rotor that gives it the high starting-torque. It has a higher full-load slip. This means greater losses if operated continuously at full-load. \

Usually, this motor finds its best and most efficient application on machine tools with pulsating loads. These use the "flywheel" effect to smooth out peak demands.

The Multispeed squirrel-cage motor can sometimes be effective for certain hard-to-start loads, by making use of two different characteristics on the two speeds (Fig. 3, original article). The extra torque of the low-speed winding provides good breakaway and acceleration, while the high-speed winding can match the lower steady-state requirement at full-load. Using this technique can save you big money; however, make sure you factor in the extra cost of the required special controller.

The PAM, or "Pole Amplitude Modulated" motor is a special variation of multispeed motor. It is not limited to the fixed speed choices of standard multispeed motors. It's ideally suited for use on some cooling loads, where the high speed for initial "draw down" temperature and the lower speed for "holding" may only need to be 25% apart. An effectively designed PAM motor can fit these speeds, providing significant savings in operating costs.

The wound-rotor motor is a relatively expensive motor (including controls). It provides almost total flexibility of torque control to achieve this by control connections between the rotor and external resistors. This permits many operating characteristics.

This motor also allows close control on acceleration torque from start-up to full-speed, with the rotor losses confined mainly to the external resistors. This means there's little thermal stress on the motor itself. Cranes and hoists make frequent use of this drive; but very high-inertia systems can also occasionally justify its use just for acceleration control.

Variable-frequency drives offer another alternative. Recently, the most popular drive is the adjustable-frequency AC motor drive (occasionally used for its superb acceleration control). One of the main uses of this solid-state system is for its speed adjustment capability. However, it has found important use in providing high-efficiency acceleration of very high inertia loads.

The "family" of speed-torque curves all denote the efficient, low-slip portion of the motor's operating characteristic even from low speed to full speed.

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© 2012 Penton Business Media, Inc.

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