What is a Soft Starter? Principle, Advantages, Disadvantages, and Applications.
Soft starters are the upgraded and modern method for starting induction motors. They are basically used for squirrel-cage AC (alternating current) induction motor starters, replacing the star-delta, compensating switch, or direct starter methods.
The above-said methods achieve a reduction in the starting current, but the switching is by voltage steps. However, none compares with the soft start method (which uses the soft-starter).
A soft starter limits the starting current of the motor to determine the torque and also sets the starting time so it is possible to start the motor very softly.
The starting principle of a soft-starter is based on a fast-acting semiconductor, called a thyristor. It is also known as silicon controlled rectifier (SCR) that can have four terminals.
The starting voltage of an electric motor is reduced depending on the angle of conduction of the thyristors, thus decreasing the peaks in the current, one of the duties of a soft-starter is to control the power of a motor without changing its frequency.
To accomplish this, the thyristors act at two points. One is zero voltage control and zero current control.
The control circuit has to time the firing pulses from the last zero value of the waveform, both voltage, and current. The sensor can transform the current either in a single-phase or for each phase that the system has.
Soft starters mainly consist of two parts.
The power unit consists of a heat dissipater and thyristors, which are controlled by a logic circuit card that makes the control unit.
Generally, a microprocessor-based control is used. Sometimes, a circuit breaker is used in place of fuses to guarantee perfect protection of the thyristor during the fault period. further, a thermal protection relay is provided on the supply side.
However, this protection is provided within the soft starter, so it is not required to install externally.
This starter is also used in the star-delta form. During the starting period, contactor A is closed and B is open, hence the power is given to the motor via a soft starter which starts the motor softly.
After some time B is closed and the soft starter is cut off from the supply and power is directly given to the motor.
By adjusting the firing angle of thyristors, the voltage applied to the motor is controlled. With the correct adjustment of the variables, the torque and current are adjusted to the load’s needs, that is, the current required will be the minimum necessary to accelerate the load, without frequency changes.
Soft-starters, which through a micro-processed command, control thyristors that adjust the voltage sent to the motor stator. In this way, protect the electrical network from high starting currents.
Soft-starters can be used in the most diverse applications.
Its main ones are in:
Reference: Electrical Installations by John Blaus.
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View Comments
Thanks for useful information
I want to make Soft starter
want soft starter making guide
I would like to know more about soft starters
Very nicely explained, Thank you.