Soft starter selection
The correct soft starter model needs to be selected according to the user's intended application and installation environment.
The selection of the soft starter model can also be driven by a cost compromise between an expensive motor control centre
enclosure / cabinet / panel design / long cables, or a higher rated soft starter model.
The selection of the soft starter model is dependent on the following characteristics of the user's installation:
1) Motor size and connection type – in general the soft starter current must be greater or equal to the motor full load
2) Typical operating temperature – the soft starter is rated at a nominal operational temperature of 50°C but may be
operated up to 60°C. Implement de-rating of the soft starter maximum current at temperatures above 50°C. The
operating temperature is determined by the environmental factors as well as the soft starter workload.
3) Soft starter workload – The workload is defined in two categories: Normal duty and Heavy duty. This comprises the
typical starting time, the typical number of starts per hour and the running time (if no Bypass Contactor is installed)
Typical starting time – the shorter the starting time, the lower the thermal loading on the soft starter. A starting
time of less than 30 seconds is considered normal duty. 30 second-starts and longer, are considered heavy duty.
b. Typical number of starts per hour – frequent starting increases temperature of the heat sink in the soft starter.
Number of starts less than 10 per hour (or 6 minutes or longer between starts) are deemed normal duty. 10 or
more starts per hour (or less than 6 minutes between starts) are considered heavy duty.
Bypass contactor installed – if the soft starter is used with an internal or external bypass contactor, the 'on-time'
of the soft starter is only during starting and stopping – this reduces thermal loading and a smaller soft starter
model may be appropriate. A bypass contactor is recommended for applications where the motor is started and
then runs for a long time before it is stopped. For applications where frequent start-stop cycles are required, a
bypass contactor is not recommended.
4) Altitude above sea-level – the MEsoftstart range of soft starters are designed and tested for operation up to 1400m
above sea level. De-rating of the soft starter maximum and operating currents is needed for operation at higher altitude.
Definition of motor duty applied to MEsoftstart soft starters:
1. Normal duty is defined as no more than 10 starts per hour (> 6 min between starts) and start ramp shorter than 30s.
2. Heavy duty is defined as more than 10 starts per hour (<6 min between starts) or start ramp longer than 30s.
The tables below serve as a guideline for selecting the correct model soft starter for the application according to the motor
starting current and type of application. The tables below show the allowable starting current for the listed motor sizes as a
percentage of their maximum normal running current at power factor of 0.8.
A bypass contactor is recommended for applications where the motor runs for a long or continuous periods. A bypass contactor
reduces the current through the soft starter switching elements and can allow the use of a smaller soft starter and increases the
efficiency of the installation.
Applications with short, frequent start and stop ramps should not use a bypass contactor for mechanical life reasons of the
contactor. However, the soft starter needs to be sized correctly to handle the switching current for such heavy duty use.