Motors - ABB MS116 Manual

MA N UA L MOTOR START ER G UI D E

3.2 Motors

Due to their high inrush peaks, locked rotor currents, and high potential for overheating, motor loads represent one
of the most demanding load types. The figures below show an overview of an across-the-line motor start-up. The
starting current is a characteristic of the motor. The starting time is a function of the load torque, inertia and motor
torque and is influenced by the motor technology. As the starting current ratio (6-13 x I ) is higher than the rated op-
e
erational current I , an excessively long starting or braking period can cause an overload (temperature rise) in the
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motor. This can create electromechanical stresses or damage the motor's insulation if it is not properly protected.
There are many different manufacturers represented on the market, selling at various prices. Not all motors have the
same performance and quality as motors from ABB, for example. High efficiency enables significant savings in en-
ergy costs during the motors' normal endurance. In the IEC 60034-30 standard for rotating electrical machines, four
different efficiency classes have been defined. The classes are called IE1, IE2, IE3 and IE4, where motors belonging to
IE4 are the most efficient. See the graph below for details. A low level of noise is something else that is of interest
today, as well as the ability to withstand severe environments. There are also other parameters that differ. The de-
sign of the rotor affects the starting current and torque and the variation can be quite large between different
manufacturers for the same power rating.
Figure 12: Diagrams of the different currents at the start-up of a motor.
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