Specifying a Motor

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In writing a motor specification there are various items that must be considered to ensure that the motors are specified correctly.

These are as follows:

  1. AREA CLASSIFICATION THAT MOTOR IS TO BE USED IN.
  2. kW RATING AND SPEED REQUIRED.
  3. MOUNTING REQUIRED
  4. ELECTRICAL SUPPLY DETAILS
  5. CONSTRUCTION OF MOTOR.
  6. BEARINGS
  7. PAINTING
  8. TESTING
  9. INSULATION
  10. DOCUMENTATION

Each of these will now be considered.

AREA CLASSIFICATION

This must be taken into consideration as it determines the type of motor that can be used.

For normal non-hazardous area a Totally Enclosed fan Cooled (TEFC) motor is suitable. These are available with various enclosure ratings such as IP54, IP55, IP56, IP65, IP66 and IP67. The difference in the rating is explained in AS1939.

As a rough rule of thumb the following is applicable.

  • IP54 Weather proof
  • IP55 Light hose proof
  • IP56 High pressure Hose proof
  • IP65 Complete protection against entry of dust and light hose proof
  • IP66 Complete protection against the entry of dust and high pressure hose proof
  • IP67 Protection against immersion

For hazardous areas it is extremely important that the correct motor be chosen. Please refer to our section on Hazardous Locations which details in depth how to correctly select the correct motor.

Also the ambient temperature range of operation must be stated as this can determine the class of insulation to be used and, in some instances, may require special grease in the bearings.

kW RATING AND SPEED.

These are determined by the driven equipment and are generally a mechanical consideration rather than electrical.

Their are standard ratings available as set down in AS1359. 30.

The speed of the motors is determined by the number of poles and the frequency of the supply.

The following approximations are applicable.

  50 Hertz 60 Hertz
2 Pole 2880 rpm 3460 rpm
4 Pole 1440 rpm 1750 rpm
6 Pole 960 rpm 1150 rpm
8 Pole 720 rpm 860 rpm
10 Pole 580 rpm 690 rpm
12 pole 480 rpm 570 rpm

The actual full load speed will depend upon the motor design.

MOUNTING REQUIRED.

These are explained in detail in AS1359.22.

The usual methods are as follows:

  • Horizontal foot (B3) IM1001
  • Horizontal flange (B5) IM3001
  • Horizontal foot and flange (B3/B5) IM2001
  • Vertical flange (V1) IM3011

ELECTRIC SUPPLY DETAILS.

The voltage, phase and frequency of supply must be stated complete with tolerances required.

The correct voltage, on which the motor is required to operate, is important as this can change the characteristics of the motor for no apparent reason. If the voltage decreases by 5% the starting torque of the motor will decrease by 10%. This could mean that the motor will now not develop enough torque to start the load

The voltage tolerance must be also stated, such as +/- 10%. This will ensure that the motor will operate correctly over the range.

It must be shown in the specification if the motor is to operate on three phase or single phase as different motors are required.

The frequency of the supply must be stated as this can change the speed. Again the tolerance must be stated

Please refer previous comments under kW rating and speed for examples of how frequency can affect the speed..

CONSTRUCTION OF MOTOR.

The physical requirements of the motor must be shown. This will include such items as cast iron frame, plastic fans etc.

Please refer to the examples of specifications which provide the details required.

BEARINGS.

The requirement for the bearings must be stated. This should include Brand name, ball or roller and method of greasing.

Again refer to the examples of specifications which provide the details required

PAINTING.

Clear and precise details of the paint finish must be given. For severe chemical duty it may require the motor to have a special epoxy paint finish. The type of paint required, Brand name and final colour should be stated.

Again refer to the examples of specifications which provide the details required

TESTING.

It is important to state what testing is required. This can vary from a routine test to a full performance test. Please refer to AS1359.60 which details the different type of tests.

INSULATION

This is very important as it will determine the final surface temperature that is acceptable and also affect the expected life of the motor. The types available are:

  • A Class 105
  • E Class 120
  • B Class 130
  • F Class 155
  • H Class 180

The selection will depend upon the ambient temperature in which the motor is to be used. Please refer to AS1359.32 which provides details of the different types of insulation.

The normal standard insulation is Class F with Class B rise.

DOCUMENTATION.

It is important to state in the specification what documentation you require to be submitted. This can vary from simple outline dimension drawings and type test certificates to exploded view drawings, SAA certification and as made drawings.

 

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Copyright 2006 Electric Motors International Ltd
Last modified: 03/19/06