Selection of Electrical Motor 

     Identifying a motor for replacement purposes or specifying a motor for new applications can be done easily if the correct information is known.

This includes:

• Mechanical requirements of the driven load

• Physical and environmental considerations

Failure to ground a motor properly may cause serious injury.

Efficiency and economic considerations

• Electrical Characteristics and Connections

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.

Driven Load - Mechanical requirements

• For a motor to drive a load properly, It must produce enough torque to accelerate from standstill to operating speed, and to supply enough power for all possible demands without exceeding its design limits.

To specify the motor properly, the following characteristics of the load should be considered:

1) Running characteristics:

  • Continuous running, constant load.

- horsepower requirement

  • Continuous running, varying load

- peak horsepower requirement

  • Cyclical load

- peak torque and horsepower requirements

2) Speed

  • Constant speed

  • Multi-speed

- what speeds required?

  • Adjustable speed

- determine needed speed range

3) Starting and Stopping

  • Frequency of starting and stopping

  • Starting torque requirement

  • Acceleration restrictions

  • Requirements for braking

- mechanical

- plugging

From this information the size and design characteristics of the motor, as

well as control and braking requirements can be determined.

Physical and Environmental Consideration

Usual Service Conditions

Motor ratings apply to motors operating under usual service conditions.

NEMA and EEMAC (Electrical Equipment Manufacturers Association of

Canada) standards specify usual environmental conditions as:

1. Exposure to an ambient temperature in the range of 0º to 40ºC or

when water cooling is used, in the range of 10º to 40ºC.

2. Exposure to an altitude which does not exceed 3300 feet (1000

meters) (see MG1-14.04)

3. Installation on a rigid mounting surface

4. Installation in areas or supplementary enclosures which do not seriously interfere with the ventilation of the machine

Unusual Service Conditions

The manufacturer should be consulted if the motor is to be operated in

unusual service conditions.

NEMA and EEMAC standards also specify typical unusual service conditions.

1) Exposure to:

  • Combustible, explosive, abrasive or conducting dusts

  • Lint or very dirty operating conditions where the accumulation of

dirt may interfere with normal ventilation

  • Chemical fumes, flammable or explosive gases

  • Nuclear radiation

  • Steam, salt-laden air, or oil vapor

  • Damp or very dry locations, radiant heat, vermin infestation, or

2) Atmospheres conductive to the growth of fungus

  • Abnormal shock, vibration, or mechanical loading from external sources.

  • Abnormal axial or side thrust imposed on the motor shaft Operation where:

 •Excessive departure from rated voltage or frequency exceeding 10%

• Unbalanced Voltage between legs by more than 1%

3) Operation of speeds above the highest rated speed

4) Operation in a poorly ventilated room or an inclined position

5) Operation subjected to:

  • Torsional impact loads

  • Repetitive abnormal overloads

  • Reversing or electric braking

Enclosure

• The enclosure for the motor should be chosen to protect it from the

expected operating environment

• See Chapter IV for Enclosure listings

Mounting

Motors are generally mounted horizontally with feet attached to the

floor, but other arrangements are common:

- wall mounted

- ceiling mounted

- pedestal mounted

- face mounted

- flange mounted

The size and length of the shaft can be specified if the standard shaft

types or materials are not suitable for the required mounting arrangement or machine configuration

Insulation

- The type of insulation used in a motor depends on the operating

temperature that the motor will experience. Motors are specified

by ambient temperature and insulation class.

- Class A is an older classification. Class B is the standard for current

motor designs and class F and H are used in higher temperature applications.