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Soft Starter 50A STO050 with Motor Protection

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The STO soft starter is designed to smoothly start (and stop) three phase AC induction motors by utilising a constant current system (closed loop). As well as efficient soft start and soft stop, the STO range adds essential motor protection with excess start time, phase loss and motor overload functions.

Availability: In stock

Excl. Tax: £630.00 Incl. Tax: £756.00

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Excl. Tax: £630.00 Incl. Tax: £756.00
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Product Description


In addition to the benefits of soft start and soft stop, which improve the operating life and efficiency of your equipment, the STO also protects your three phase motor and load from damage during operation.

With built-in motor overload protection, the STO050 continuously monitors the motor current and then calculates the expected temperature. The user sets the motor trip class via the front panel and the STO will trip when the calculated temperature reaches a set value, preventing operational overload and ensuring user peace of mind.

Further benefits of using the STO soft starter include the bypass overload protection that safeguards the starter from severe spikes in current. The starter also monitors the time taken to start and will trip if the motor cannot start within the allowed time, protecting against stalled loads.

Suitable for Squirrel Cage Motors the STO050 soft-starter works by gradually applying reduced voltage, essential in protecting the motor from overheating causing winding damage (AC Motors that start at full voltage can experience 5 to 9 times their nominal current).

The STO050 has several adjustments for more control:

  • Current ramp (soft start).
  • Current limit.
  • Motor full load current (FLC).
  • Soft stop.
  • Excess start time.
  • Motor trip class.
  • Start current.

Why use a soft starter?

A soft starter is a device which controls the acceleration of an electric ac motor by means of controlling the applied voltage.

Without this control the interaction between the rotating magnetic field flux and the rotor winding flux will result in a high rotor current as torque is increased. By the time the motor reaches full speed, a current greater than the rated current will be drawn resulting in overheating and likely motor damage. To prevent this, a soft starter is needed.

Specification, Dimensions & Downloads

Additional Information

Starter Guide


Product Manual


Product Videos No

Soft Starter 50A STO050 with Motor Protection

Mains Supply  
Mains voltage (L1, L2, L3) STB-006-2 3 x 208Vac to 240Vac max. (+10% / -15%)
Mains voltage (L1, L2, L3) STB-006-4 3 x 400Vac to 480Vac max. (+10% / -15%)
Mains frequency (at start) 47 Hz to 70 Hz
Rated insulation voltage 600 VAC
Operating maximum current 50A AC3
Maximum start current 96A
Leakage current 10mA
Minimum working current 600mA
Starts per hour 6
Motor rating 240V 21.3 HP / 15.9kW
Motor rating 480V 37.0 HP / 27.6kW
Ramp time 2 to 20 seconds
Kick start at 70% (set by dip switch) 0, 100, 200, 300mS
Initial torque 30 to 70%
Control Input  
Manual Start/Stop (terminals 4 & 5) 4 = ground 4 to 24Vac/dc
Automatic Start/Stop Auto start on power up
Main contactor relay (terminals 1, 2, 3) NC, NO, C Max. 500mA, 125Vac
Degree of protection IP20
Operating temperature 0°C to +40°C
Storage temperature -25°C to +70°C
Humidity 5% to 95% Relative Humidity
Standards met  
Low-voltage switchgear and controlgear. Contactors and motor-starters. AC semiconductor controllers and contactors for non-motor loads
Electrical Safety Standard EN6097-1:2008
Generic Emission Standard EN6097-4-3:2000
Generic Immunity Standard EN6097-4-3:2000


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