Asynchronous motors (full page)

The asynchronous (i.e. induction) motor is robust and reliable, and very widely used. 95% of motors installed around the world are asynchronous. The protection of these motors is consequently a matter of great importance in numerous applications.

Asynchronous motors are used in a wide variety of applications. Here are some examples of driven machines:
- centrifugal pumps,
- fans and blowers,
- compressors,
- crushers,
- conveyors,
- lifts and cranes,
-  …
The consequence of a motor failure due to an incorrect protection or inability of control circuit to operate can include the following:

• For persons:

  - Asphyxiation due to the blockage of motor ventilation
  - Electrocution due to insulation failure in the motor
  - Accident due to non stopping of the motor following a control circuit failure

• For the driven machine and the process:,

  - Shaft couplings, axles, driving belts, … damaged due to a stalled rotor
  - Lost production
  - Delayed manufacturing

• For the motor itself:

  - Motor windings burnt out due to stalled rotor
  - Cost of repair
  - Cost of replacement

Therefore, safety of persons and goods, as well as reliability and availability levels, are highly dependant on the selection of protective equipment.
In economic terms, the overall cost of failure must be considered. This cost is increasing with the size of the motor and with the difficulties of access and replacement. Loss of production is a further and evidently important factor.
Specific features of motor performance influence the power supply circuits required for satisfactory operation
A motor power-supply circuit presents certain constraints not normally encountered in other (common) distribution circuits. These are owing to the particular characteristics of motors directly connected to the line, such as:

• High start-up current (see Fig. N62) which is mostly reactive, and can therefore be the cause of important voltage drop
• Number and frequency of start-up operations are generally high
• The high start-up current means that motor overload protective devices must have operating characteristics which avoid tripping during the starting period.

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Fig. N62:Direct on-line starting current characteristics of an induction motor

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Motor control systems

Different kinds of motor control solution are compared in the following tables.

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	Is / In	Ts / Tn	Speed control	Torque control
Direct on line	5-10	5-10	No	No
Star – Delta	2-3	1-2	No	No
Auto-tranformer	2-3	1-2	No	No
Soft starter	3-5	1.5-2.5	No	Yes
Variable speed drive	1.5	1.5-2	Yes	Yes

	Intérêt principal	Inconvénient
Direct on line	Reduced cost Hight starting torque	Hight in-rush current
Star – Delta	Reduced in-rush current	Reduced starting torque
Auto-tranformer	Reduced in-rush current	Hight weight
Soft starter	Reduced in-rush current controlled start and stop	Reduced starting torque
Variable speed drive	Controlled speed Energy saving at reduced speed	Higher cost

Fig. N63a: Comparison of different motor control solution

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