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Electric motors are the workhorse of the industry. Many applications exist where more than one motor can be used or the exact replacement is not available. LEESON makes every effort to maximize interchangeability, mechanically and electrically, where compromise does not interfere with reliability and safety standards. If you are not certain of a replacement condition, contact any LEESON Sales Office or LEESON Authorized Distributor. Identifying a motor for replacement purposes or specifying a motor for new applications can be done easily if the following information is known :
Much of this information consists of standards defined by the National Electrical Manufacturers Association (NEMA). These standards are widely used throughout North America. In other parts of the world, the standards of the International Electrotechnical Commission (IEC) are most often used. Description Nameplate data is the most important first step in determining motor replacement. Much of the information needed can be generally obtained from the nameplate of the motor to be replaced. Take time to record nameplate information because it can save time, avoid confusion and misapplication. Important Nameplate Data
Alternating current (AC) motors are divided into two electrical categories based on their power source. 1. Single Phase Motors A single phase general purpose design, with an electrolytic capacitor in series with the start winding, offering maximum starting torque per ampere. A centrifugal switch removes the auxiliary winding and capacitor when the motor approaches full load speed. The design is a heavy duty unit which has approximately 300% (of full load) starting torque. Common applications include compressors, pumps conveyors and other "hard-to-start" applications. A single phase general purpose design, with an electrolytic capacitor in series with the start winding, offering maximum starting torque per ampere. A second run capacitor remains in series with the auxiliary winding during full load operation. This type of design has lower full-load amps as a result of the run capacitor and is consequently used on most higher horsepower single phase motors.
This design has an auxiliary winding with a "run" capacitor, but unlike the capacitor start / induction run motor, the capacitor and auxiliary winding remain in the circuit under running conditions. (There is no centrifugal switch on this type of motor). A permanent Split Capacitor motor has low starting torque and low starting current. PSC motors are generally used on direct-drive fans and blowers. They can also be designed for higher starting torque and intermittent applications, where rapid reversing is desired.
NEMA Electrical Design Standards The following table can be used to help guide which polyphase design type should be selected.
This design has linear speed/torque characteristics over the entire speed range. SCR rated motor features include high starting torque for heavy load applications and dynamic braking variable speed and reversing capabilities. Designs are also available for use on generated low voltage DC power or remote applications requiring battery power. |
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Three phase or polyphase induction
motors have high starting torque, power factor, high efficiency and
low current. They do not use a switch, capacitor, relays etc. , and
are suitable for use on larger commercial and industrial applications.
General purpose, three phase motors have different electrical designs
classifications as defined by NEMA. NEMA Design C motors have higher
starting torque with normal starting current. Both types have slip of
less than 5%. ("Slip" being a term which expresses, as a percentage,
the difference between synchronous motor speed and full load motor speed,
for example, 1800 rpm synchronous versus a full load speed of 1740 rpm. 
Direct current (DC) motors are used in applications where precise
speed control is required or when battery or generated direct current is the available
power source.