Selecting soft starters for use with IE3 motors

Posted by AuCom on 14/07/16 12:48

IE3 motors are now mandatory in Europe for new installations between 7.5 k W and 375 kW. This is in line with elsewhere in the world. (And for a time in Europe, IE2 motors with a variable speed drive may be used for drive-suitable applications)

IE3 motors are more efficient than IE2 motors. The improved efficiency results in lower lifetime energy usage and energy costs, which has obvious value. However, other IE3 motor characteristics are affected by the changes made to achieve this efficiency. For example, the starting torque is lower, and the starting current is higher. 

These motor changes affect the entire motor driven system, including the control method. The IE3 motor characteristics must be fully considered when selecting any motor control method, to ensure sufficient acceleration torque within the current limit. This means care is needed when selecting the motor and soft starter.

In general terms, when comparing IE3 motors to IE2 motors:

  • Operating current is lower
  • Starting current is higher
  • Inrush current is significantly higher

And the degree of change in start and inrush current varies:

  • Larger power motors see a smaller increase in start current and inrush current. 
  • Smaller power motors see a greater increase in start current and inrush current.
  • Depending on the cable characteristics, long cable runs may dampen the increased inrush current.

The traditional ‘rule of thumb’ selections (eg pump 350%) are no longer always accurate. The soft starter must be sized to deliver the higher starting current within its standard ratings, eg 100 A at 450% FLC rather than 100 A at 350% FLC.  A soft starter should handle the inrush current as part of its starting function.  

Motor information can be found on the motor data sheet and nameplate. You may also need information from the motor supplier, for example, inrush current depends on the motor, application and operating conditions.

Topics: Energy Efficiency, IE3 Motors

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