| General Information |
| |
| Introduction |
| 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. |
| |
| Selection |
| Identifying a motor for replacement purposes or specifying a motor for new
applications can be done easily if the following information is known : |
- Nameplate Data
- Motor Type
- Mechanical Construction
- Electrical and Performance Characteristics
|
| 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. |
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| Nameplate Data |
|
| 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 |
| Catalog Number |
|
| Motor Model Number |
|
| Frame |
|
| Type |
(classification varies from manufacturer to manufacturer.) |
| Phase |
single, three or direct current. |
| HP |
horsepower rated at full load speed. |
| HZ |
frequency in cycles per second. Usually 60hz in USA, 50hz overseas. |
| RPM |
revolutions per minute of the shaft at full load. |
| Voltage |
|
| Amperage (F.L.A) |
full load motor current. |
| Maximum ambient temperature in centigrade |
usually +40° C (104° F). |
| Duty |
most motors are rated continuous. Some applications, however may use motors designed for intermittent, special, 15, 30 or 60 minute duty. |
| NEMA electrical design |
B, C and D are most common. Design letter represents the torque characteristics of the motor. |
| Insulation class |
standard insulation classes are B, F and H. NEMA has established safe maximum operating temperatures for motors. This maximum temperature is the sum of the maximum ambient and maximum rise at ambient temperature. |
| Code |
indicated locked rotor kVA per horsepower. |
| Service factor |
a measure of continuous overload capacity. |
|
| |
| Major Motor Types |
|
| Alternating Current (AC) Motors |
| Alternating current (AC) motors are divided into two electrical categories based on their power source. |
| 1. Single Phase Motors |
|
| 2. Polyphase (three phase) |
| |
|
Capacitor Start/Induction Run |
| 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. |
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|
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| Capacitor Start/Capacitor Run
|
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 |
| |
| Permanent Split Capacitor (PSC) |
| 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. |
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|
|
| Polyphase (Three Phase) Motors |
| 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. |
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|
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| NEMA Electrical Design Standards |
| The following table can be used to help guide which polyphase
design type should be selected. |
| Classification |
Starting Torque% |
Breakdown Torque % |
Starting Current |
Slip |
Typical Application |
Design B
normal starting
torque & normal starting current |
100 - 200 |
200 - 250 |
Normal |
< 5% |
Fans, blowers centrifugal pumps & compressors,
etc., where starting torque requirements are relatively low. |
Design C
high starting torque & normal starting current |
200 -250 |
200 - 250 |
Normal |
< 5% |
Conveyors,
stirring machines, crushers, agitators, reciprocating pumps etc., where
starting under load is required. |
Design D
high
starting torque & high starting current |
275 |
275 |
Low |
> 5% |
High peak loads,
loads with flywheels such as punch press, shears, elevators, extractors, winches, hoists,
oil well pumping & wire drawing machines. |
|
|
| Direct Current (DC) Motors |
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.
Permanent Magnet DC MotorsThis 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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