Low Voltage Technology

Current limit

The voltage increases linearly from the “Initial Voltage“ pre selected point. If the adjusted “Current Limit” is reached, the voltage will stop ramping and remain at its level until the current is reduced below the adjusted “Current Limit“ level. In that case ramp up time is prolonged until the current has reduced below “Current Limit” level. 

Soft starter output voltage

Although the current is limited the motor provides suitable torque to accelerate the driven load and reach “End of acceleration”. If the motor torque does not exceed the load torque the motor will not start. In the so rare cases severe damages might occur to the motor and starter.

Motor start measured with an oscilloscope
(Ch1, voltage; Ch2-4 current L1 – L3)

The oscillogram shows, the soft starter providing a smooth and stepless current and voltage ramp to the motor. The voltage ramps up until “Current Limit” is reached. After the motor is at nominal speed, the current will decrease, to the nominal motor current.

Pump control

Our digital Soft Starters line of products ISA-D, ISA-DS and ISA-HD provide an advanced microprocessor based control algorithm, enabling the sophisticated pump control feature. The motor torque is continuously monitored to eliminate peak
torque from stressing the motor, pump and pipe system. These soft starters provide 3 field selectable pump control algorithms.

Pump and special load control

Two major problems are associated with starting and stopping of pumps.

Over-pressure during starting

The sharp increase of torque towards the end of acceleration can cause high pressure and damage the pipe system. The pump control enables selection between three voltage ramp-up curves as well as torque curve to reduce peak torque. Current ramp is available for special loads.

Water hammer during stopping

During soft stop, when voltage is decreasing, motor torque may fall below load torque causing abrupt stalling instead of smoothly decreasing speed to zero. This creates the “water hammer” phenomenon (resulting in a loud noise and damage to the pipe system). The pump control algorithm enables selection between three voltage ramp-down curves or torque curve pre venting stall condition and eliminating water hammer.


  • Illuminated LCD with 2 lines of 16 characters each
  • Selectable languages: English, German, French or Spanish (aditional on request)
  • The LCD indicates the motor current, fault descriptions and statistical data.
  • The LEDs provide a quick status indication. (control voltage on, motor soft start, motor on, motor soft stop, motor stop, fault)
  • Easy settings by six clearly marked keys, user-friendly software with factory default parameters.


The analogue product line provides simple potentiometer settings for all the parameters:

  • Familiar setting, similar to standard overload relays
  • Easy commissioning
  • Prior knowledge is not necessary
  • Status display via LEDs
  • ISA-A include motor protection functions

Communication RS 485

Modbus Profibus

RS 485 with MODBUS or PROFIBUS protocol enable:

  • Remote (start, stop, etc.)
  • Monitoring (motor current, faults, statistical data)
  • Remote parameter setting
  • Remote reset


The product lines ISA-B, -B2P, -A, -SL2, -SL3 and -DS include an internal bypass.

  • This reliable components bypass the power electronics after the motor is at speed.
  • In addition the starters provide an auxiliary contact, allowing signaling of a successful motor start. Attention! Many of our competitors call these auxiliary relays “a bypass contact“!
  • The bypass function reduces the heat dissipation and eliminates the EMC load of the system.
  • As an option, ISA-DS series can be designed for DOL-start (available from ISA-DS 390).

ISA-D options

Analogue option card

  • Function 1: Thermistor-input, PTC or NTC
  • Function 2: Analogue-output, based on the motor current, programmable from 0 – 10 VDC, 4 – 20 mA, 0 – 20 mA or inverted

Motor insulation option card

This unique feature guarantees the availability of important plant components, such as fire-extinguishing systems, redundant systems as well as submersible pumps. The system measures the motor insulation level while the motor is not in operation. Two programmable steps are available:

  • Alarm-Level, adjustable 0,2 – 5 MOhm
  • Start inhibit, adjustable 0,2 – 5 MOhm, avoids motor starting if the insulation is below the critical level.