Items to consider when buying a motor

July 19, 2000
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Over the past 10 years, most motor manufacturers have introduced a “higher-than-standard-efficiency” line of motors in industrial (1- to 200-hp) sizes, according to Donald A. Lanser, director of marketing for the Dayton Motor Division, Emerson Electric Co. (St. Louis).

Through the education of motor buyers and users, these high-efficiency motors have risen to nearly 20% of the market, he added. “That leaves 80% of 1- to 200-hp motors sold still on the lower, standard-efficiency platform.”

Both Canada and the United States have developed regulations and laws that help reduce energy consumption by mandating new efficiency standards, he continued. Starting in late 1997, all general-purpose three-phase induction motors manufactured for use in the United States and Canada have to meet these minimum efficiency levels.

“Most standard-efficiency motors do not meet the minimums required,” Lanser stated. “Besides the reduction in energy costs, energy-efficient motors operate cooler, resulting in longer bearing and insulation life.”

Figure 1 (page 10) shows typical industrial applications in which motors are used; 75% of all motor applications are in pumps, compressors, and blowers.

Motor classifications

In selecting electric motors, “Users need to understand how motors are classified,” Lanser said.

The most widely used classification is by operating characteristics and/or by type of power required.

The most common type is the induction motor of ac operation. In this type, speed remains relatively constant as the load changes. The following are some classifications of this motor type.

Single-phase motors:

Shaded pole — Low starting torque and low cost; usually used in direct-drive fans and blowers and in small gearmotors;

Permanent split capacitor (PSC) — Performance and applications similar to shaded pole but more efficient, with lower line current and higher horsepower capabilities;

Split phase — Moderate starting torque, high breakdown torque used on easy-starting equipment, such as belt-driven fans and blowers, grinders, centrifugal pumps, gearmotors, etc.;

Capacitor start — High starting torque and breakdown torque, medium starting current; used on hard-starting applications, compressors, positive-displacement pumps, farm equipment, etc.;

Capacitor start, capacitor run — Similar to capacitor-start, except they have higher efficiency; generally used in higher-hp, single-phase ratings.

Three-phase motors operate on three-phase power only. High starting and breakdown torque; high efficiency; medium starting current; simple, rugged design; long life; generally for industrial uses.

Direct current (dc) are usable only if dc power is available. Use with a DC/SCR control enables adjustable-speed applications.

Users also need to determine in what environment the motor will be operating so that the type of enclosure (the motor’s housing) can determined:

Dripproof (DP or ODP) — Ventilation openings in the end shields and shell are placed so drops of liquid falling within an angle of 15 degrees from vertical will not affect performance. Usually used indoors, in fairly clean locations.

Totally enclosed (TE) — No ventilation openings in end shields or motor housing (but not airtight or waterproof). Used in locations which are dirty, damp, oily, etc. There are several types that fall under this category.

Totally enclosed fan cooled (TEFC) — Includes an external fan in a protective shroud, to blow cooling air over the motor.

Totally enclosed nonventilated (TENV) — Not equipped with an external cooling fan; depends on convection air for cooling.

Totally enclosed air over (TEAO) — Air flowing from driven equipment or an external device provides cooling airflow over the motor.

Hazardous location (explosion-proof) — A totally enclosed motor designed to withstand an internal explosion of specified gases or vapors and does not allow the internal flame or explosion to escape.

“As you can see,” Lanser pointed out, “there are a lot of things to consider when you are buying an electric motor.”

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