Selecting soft starters for use with IE3 motors

The start current of the motor depends on the motor design, rotor speed and stator voltage from zero speed until full speed is reached. The load only influences the time taken for the motor to reach full speed. The current/speed curve of the motor is independent of all external influences other than stator voltage.

The motor will always initially draw locked rotor current (LRC) when started direct-online (DOL). The current will stay at LRC as the motor begins to accelerate, and fall as the motor approaches full speed. For many induction motors the current does not fall significantly until the motor speed is over 90%.

Starting an unloaded motor DOL is just as severe on the supply as starting a fully loaded motor, except that the overload is for a shorter time. However, some clamp type ammeters with slow response times do not show the true start current.

Incorrectly installed and poorly calibrated electromechanical starters eventually reach full voltage and DOL start the motor, acting as expensive DOL starters.

IE3 motors are now mandatory in Europe for new installations between 7.5 and 375kW. This is in line with elsewhere in the world. 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 100A at 450% FLC, rather than 100A 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.

As an example, many applications within the water & wastewater industry can benefit from the use of a soft starter, from pumps and separators to aerators and milliscreens.

Correct application of a soft start can protect equipment, maximise productivity, and provide intelligent control over an entire system. Soft stop functionality prevents water hammer, prolonging the life of pumps, pipes, valves and headworks, while gentle acceleration reduces mechanical stress on a system during start.

The starting function of motors is often misunderstood, impacting motor performance and compromising energy efficiency.