Start with electronic soft starter (ESS)

The soft starter reduces the starting current for the motor by increasing the motor voltage up to the nominal value according to predefined functions. Compared to direct-on-line start, starting current peaks and torque surges are reduced. This is usually implemented with phase-angle control of the sine half-waves: Each half-wave of the AC voltage is not switched through to the motor with the zero crossing, but later. The resulting reduced root mean square (RMS) of the voltage is increased in a ramp up to the full supply voltage.

Soft starter: phase angle control and bypass contact (TOR: top of ramp, full sine half-waves).
Soft starter: phase angle control and bypass contact (TOR: top of ramp, full sine half-waves).

2- or 3-phase controlled

A distinction is made between 2-phase and 3-phase controlled soft starters depending on the structure of the power sections.

Comparison of 2- and 3-phase controlled soft starters

2-phase controlled soft starter

3-phase controlled soft starter

  • mostly for lower motor power
  • only in line connection possible
  • higher unbalance of the starting currents
  • cheaper to buy
  • mostly for higher motor power
  • more extensive parameterisation
  • lower starting current than with 2-phase controlled soft starter
  • depending on type, in line and inside delta connection possible
Schematic structure of a 2-phase controlled soft starter with integrated bypass contacts
Schematic structure of a 2-phase controlled soft starter with integrated bypass contacts

With 3-phase controlled soft starters, 2 different wirings are possible:

Soft starter: In line connection and inside delta connection

In line connection (standard connection)

Inside delta connection (root 3 connection, √3 connection)

  • Corresponds to direct-on-line start, 3 cables are led to the motor.
  • Load contactors can be dispensed with for devices with safe torque off (STO).
  • Only possible with star-delta motors in delta wiring. The motor windings are connected in series with the thyristors (term from "thyratron" and "transistor") in delta, 6 cables are led to the motor.
  • The soft starter is only loaded with 58% (1/√3) of the current and can be designed accordingly for a lower current.
  • Requires a 3-phase controlled soft starter and is only supported by some manufacturers and types.
Zoom
Comparison of in line vs. inside delta connection of a 3-phase controlled soft starter

Soft starter vs. frequency inverter

Purchase and maintenance costs are lower for soft starters than for frequency inverters, but a soft starter is less flexible: It can only be used for starting and not for part load operation, for example. In addition, the soft starter (unlike the frequency inverter) also reduces the torque of the motor, and the lower starting torque may not be sufficient to start the motor against high condensing pressure if the configuration is unfavourable.