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Electric motors

03.09.2021

Electric motors

Electric motors convert elec­trical energy into mechanical (rotating) energy and with that have the reverse function of gen­erators.

The major categories are related to an AC or DC power supply but then the choice is endless from the very small step-motors used in robotic applications to very large motors in the MW range.

Electric motors come in all shapes and sizes and suitable for a wide range of power supplies. As with generators the applied frequency and the number of poles in the sta­tor determine the speed of the mo­tor. Nowadays the most widely used electric motors are the 3-phase alternating current asynchronous motors with a squirrel cage rotor. An overview of this type of motor, in the range from approximately 0.3kW to 160kW, for various volt­ages, frequencies and speeds is shown on the next page.

This chapter will concentrate on this type of АС-motors. When us­ing variable speed drives AC-mo- tors can be precisely controlled for starting, speed and torque.

Electric motors are available in different housings for foundation or flange fitting. They are also available with differ­ent protection classes against the ingress of solid particles and water (IP-class) and for use in an explo­sive environment (Ex-class).

Ex-motors are available with the following classes :

  - increased safety Ex-e
  - flameproof Ex-d
  - pressurized Ex-p.
Electric motors are available in IEC standard machines, suitable for 45 °C cooling air or 32 °C cool­ing water temperature. When the temperatures for cooling air or water are different from the standard values correction factors must be used for which the appli­cable Rules & Regulations must be consulted.
When additional cooling capacity is required an extra cooling fan can be installed on the main electric motor. When such a motor is also totally enclosed these motors are also referred to as TEFC for Totally Enclosed, FanCooled [motors].

Testing AC-motors

All AC-motors have to be tested and when the power rating is above lOOkW they have to be certified by the classification society.

All AC-motors have to be tested and when the power rating is above lOOkW they have to be certified by the classification society.

The basic AC-motor test consists of:

 -  Meggertest,
 -  High voltage test
 -  Meggertest again
The second meggertest is to verify if the isolation values are still intact after the high voltage test.

The following tests and measure­ments are to be documented at nominal voltage and frequency:
 - start current
 - no-load current
 - full-load current
 - consumed power
 - supplied power
 - efficiency
 - power factor
 - start torque
 - nominal torque
 - speed range
 - housing temperature
 - winding resistance cold
 - winding resistance hot after the full-load test

 - heat run to determine the maxi­mum winding temperature un­der continuous load.

The maximum permissible winding temperature depends on the type of winding insulation used, the temperature of the cooling air or the temperature of the cooling water. As an example the table on page 105 gives an overview of limits to temperature rise for air­cooled rotating machines. The maximum temperature rise is determined in a heat run. The heat run is a test where the mo­tor is loaded with nominal load until the temperature of the housing sta­bilizes. Before the start of the test, the temperature of the motor and resistance of the windings at this temperature is measured.
When the housing temperature stabilizes the resistance of the winding is measured again. From the two values obtained, the tem­perature rise can be calculated

Electric motors
A motor test stand at a motor man­ufacturer showing the motor
          under test and the water brake (dynamo metre)

TIME 
 AIR IN 0 C  AIR OUT ° C  DIFF " С
8:00 18 18 0
8:30 18 20 2
9:00 19 22 3
9:30 20 25 5
10:00 21 30 9
10:30 21 36 15
11:00 22 43 21
11:30 23 44 21
The equipment necessary for a heat run is called a dynamometer, a brake which converts power pro­duced by the electric motor into heat. This brake is also free moving so that torque can also be meas­ured.

For large motors the heat run, with a mechanical load, can be replaced by using two frequency converters to supply the motor. One frequency converter supplies the motor with the rated voltage and frequency and the other with a lower than nominal voltage and frequency. With the motor running at no-load speed on the first frequency con­verter the variable voltage is in­creased so that the total current of the two power sources is equal to the rated current of the motor. The advantage is that the power consumed comes from the losses that produce the heat. The rest of this test is the same as for the heat run as described above.   

             Code of standardized frames for the various types of standardized electric motors
Electric motors

Electric motors
       
     Squirrel cage motor
 1. Shaft with bearing
 2. Squirrel cage rotor
 3. Stator Windings
 4. Cooling fan
 5. Connection box
 6. Protection cover
 
Windings of electrical machines can be insulated with different materials. The properties of the insulat­ing material determines the maximum allowed tem­perature. Insulating materials are divided in classes. When a higher insulation class is selected this will al­low a higher temperature when in operation. The higher temperature allows a higher current, which is the source of the heat, and with that a higher power rating of the motor. This also applies to other electrical equipment such as generators and trans­formers.

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