Soft Starter vs. Auto Transformer

Operational differences

Soft Starter	Auto Transformer Starter
Easy starting-by a push button.	Similar method of starting.
Motor starts without any jerks i.e. smoothly and gradually picks up to the synchronous speed. The life of the motor is thus increased significantly and the maintenance problems are drastically reduced.	Motor is subjected to heavy jerks during starting and a bit later because of the sudden inrush of current. This causes severe damages to the motor winding which ultimately burn off.
The valve on the delivery side need not be operated while switching the motor ON or OFF.	This is mandatory on both occasions i.e. ON or OFF.
Soft starting and soft stopping avoids back thrust on the NRV or the pipe line which is therefore protected against the back thrust during switching OFF.	Such a protection is not provided and therefore, proper operation of the valves is absolutely essential.
No electrical moving contacts and no oil hence no sparking and hazards.	Circuitry consists of several moving parts and hence sparking is unavoidable.
No limitations on the No. of starts and No. of stops per hour.	Limited No of starts and stops per hour because of: motor windings getting heated.
Acceleration time for motor to speed up from rest to rated full speed can be adjustable from 2 secs to 90 secs. This will facilitate selection of appropriate ramp times to suit the desired load/torque requirements.	Such a facility is not available with conventional timers used in this method of starting.
In rush (starting) current restricted to 200% of rated full load current (FLC).	Inrush current restricted to maximum of 400% of the FLC.

Protections

Soft Starter	Auto Transformer Starter
Phase reversal, single phasing, overload, over voltage, under voltage and short circuit are available.	Phase reversal and short circuit protections not available.
Current limit facility is available which limits the current drawn by the motor plus the load to a desired safe value.	This facility is not available.

Economics

Soft Starter	Auto Transformer Starter
Approximately Rs. 15000/- to Rs. 50000/- per annum, per equipment is saved by virtue of energy saving. The exact amount of energy saved depends upon the loading conditions.	Energy saving not achieved.
By using this the power factor can be improved by .05 which will cost further saving and will also avoid KEB penalties.	Provision for power factor improvement not available.
Reduction of maintenance costs on account of: - No necessity of transformer oil. - No moving parts present (like contactors, relays etc.). - Motor fully protected and hence no rewinding costs.	This method of starting is prone to all these problems occurring very frequently and hence involves tremendous expenses on maintenance.
Minimum down time.	Down time too large.

This comparison is biased towards soft-starter. Disregarded is total cost of investments and lifetime of the system.

Especially if the motor is infrequently started and usually runs on full power, the "power saving" is insignificant. Very large motors/large inertia doesn't "jerk" with autotransformers and for very small systems it doesn't matter. An autotransformer can last for 30+ years in a good case. Dry transformers or environmentally friendly liquids are possible if presence of transformer oil is considered a problem

For large systems, an autotransformer system can be 10 times cheaper than a frequency drive system.

I am for soft starters/VFD rather than autotransformer or other solutions, but here where there are 14 points on behalf of soft starters and not a single word on the behalf of autotransformers, I feel compelled to advocate.

Best regards,
Daniel, engineer on a large a well regarded power & automation company.

- - - -> by: daniel