Soft starter

Starting the work of powerful electric drives with asynchronous motors has an impact on the supply network, especially at the end of electricity transmission line, voltage drop during large starting currents leads to a voltage slump in the network, which ad-versely affects the operation of other connected equipment (computers, communica-tions, relay protection terminals, etc.). To resolve this issue, our plant has developed a soft starter.

SS (soft starter, thyristor)

SS is intended for a control of start-up process, work and a stop of electric motors

  • Smooth/soft acceleration;
  • Smooth/soft stop;
  • reduction of starting current;
  • matching of motor torque with load torque.

During start-up, the torque often reaches 150-200% of the rated torque in a split second, which can lead to a failure of drive’s mechanical part. In this case the starting current can be 6 to 8 times higher than rated one, because of this a voltage drop occurs in a local power network. Voltage drop can cause problems for other network loads, and if voltage drop is too much, then the motor itself may not to be started. The use of soft starter devices ensures a limitation of rate of rise and amaximum value of starting current from zero up to rated value within a given time.
Depending on load nature soft starters provide different motor control modes realizing a particular rela-tionship between motor rotation speed and output voltage.

Scheme and a principle of operation

Voltage regulation is controlled by thyristors.
Such control provides voltage increase at the terminal blocks of the motor from a certain predetermined primary level up to the supply network voltage. Thus, an optimum starting current and starting acceleration torque are regulated for a specific application.

SS additionally provides a “smooth stop” function. The rated voltage of motor supply is gradually reduced, mirrored opposite to the start process. This prevents a sudden stop of the drive, what is especially important when working with pumps (prevent hydraulic shock) or a conveyor.
In distinction from other manufacturers of this type of products, our units are equipped with FCD (filter-compensating devices) upon customer’s request.

FCD performs two main functions when starting and running the motor:

  • maintaining the required cos φ
  • reduction of harmonic current and voltage components
Soft Starter
Electric circuit diagram of SS connection

SS main electromechanical characteristics

Soft Starter

Fig. 3 – voltage applied to the motor at start-up from SS

Fig. 4 – a dependence of torque on the rpm at different values of applied voltage

Fig. 5  – current dependence on motor rotation rate

SS main characteristics

There is a possibility of current control. Essentially this is an ability of soft starter to regulate voltage so that the current varies according to a given characteristic.
Digital SSs are usually implemented on a microprocessor and allow very flexible control over the operation of the device and realize many additional functions and protections, as well as provide convenient indica-tion and communication with the upper-level control systems.
Protection. In addition to its main function – organization of a soft start – SS contains a set of motor and mechanism protection. As a rule, this complex includes an electronic protection against overload and faults of the power circuit. The additional set can include a protection against start time exceeding, from phase misalignment, phase sequence change, too small current (protection against cavitation in pumps), from overheating of SS radiators, from the decrease of network frequency, etc. It is possible to connect to many models a thermistor or a thermo relay built into the motor.
Engineering support during commissioning works includes preliminary calculations of the start-up process in this or that mode with all parameters determination: a speed and torque of motor, voltage curves and motor current accounting limited power and voltage drop on the internal resistance of voltage source.
For this purpose, a process simulation program has been developed when motor starting from the soft starter. It allows to select an optimal start-up algorithm and current limitation level, to calculate an acceler-ation time, taking into account voltage drop applied to the motor, and also to check the motor condition from the terms of allowable thermal overload. Soft starter programming is for motor running in a starting mode of tachogram that satisfies a starting criterion for a specific application of actuating mechanism (a requirement that a motor speed increases linearly, exponentially, etc.).
For personnel safety – cabinet doors are equipped with mechanical or electromagnetic interlocks with the following operating principle:

  • if a power voltage is applied to the device, the cabinet doors can not be opened;
  • If the doors of any cabinet are open, you can not apply power voltage to the device.

In addition, the doors of high voltage sections are equipped with handles with a possibility of locking them with a padlock, and low voltage sections are equipped with an integrated lock to prevent unauthor-ized access. Input of power voltage and cable outlet to the engine is from above, from below the cabinet or at the top of both side walls of the cabinet. There is a space for cables connection inside the device. De-gree of protection – IP54 (IP21 for SS with voltage 11-15 kV). Operating temperature range is from 0 to +40 ° C (optional – -20 to + 40 ° C with a heater).

Technical characteristics

   are meant for electric drives with the following characteristics:
Power, kW Voltage, V Power, kW Voltage, V Current, A
2,2 – 2500 230 – 690 30 – 50 000 1500- 15000 30 – 3000

The device functionally includes:


Soft Starter A view with open doors of medium voltage ele-ments and user terminal blocks:
User terminal blocks are located in the upper sec-tion of the cabinet.
The linear contactor is located in the left section of medium voltage.
EPT-Tx is located on medium voltage busbars under a line contactor at the entrance to HRVS-DN power section for input voltage measuring.
Two optical EPT-Tx outputs are connected through optical wires to EPT-Rx (Electronic Po-tential Transformer Receiver), which is located in the low-voltage section.
The main part of soft starter is a power section.
A bypass contactor is located on the right side of power section.
  Additional equipment is filter compensating de-vice.
Soft Starter


Soft Starter Control module is a heart of soft starter, it controls the angle of ignition of thyristors (lo-cated in the power section).
Caution: fiber optic wires at the top part of control module are sensitive to bending and heating.
Intermediate relays are controlled from the control module.
Switches (Direct / Soft start and local / remote control) are mounted in the low-voltage sec-tion to prevent an accidental change of their position while the linear contactor is closed.
Note: it is prohibited to change a position of these switches during
closed linear contactor.
Additional equipment is filter compensating device.
Soft Starter