Soft Starter, what it is and how to install!

There are many people who have never heard of soft starters and many who have heard of them still have many doubts. And I’m going to say that, amazingly, the soft starter is much simpler than it looks.

Anyone who has studied any about electric motors and electric drives has heard of star triangle starting and what it is for.

The advance in electric motor starters was only possible thanks to the advance in the studies of power electronics, which is the area of ​​electrotechnics and electronics that study the semiconductors that support large electrical currents such as thyristors, which are essential components in soft starters.

Types of electric motor starters

There are several ways to start induction motors, for example, direct starter, compensating key starter and others, including the star delta starter .

Each of these starts has an effect on the power grid and the motor.

These effects can be less or more harmful to the electrical system and the engine, as they can cause several side effects. The best known of all is the peak current during engine start.

I don’t know if you’ve noticed, but often when a big engine is started, does the brightness of the lamps decrease? Much like what happens when the electric shower is turned on in a home.

This is mainly because the peak current that a motor causes in the electrical network is very high.

The image below shows a graph with the peak current of a motor connected to direct start. You can see right at the start of the start that the current is very high at the first moment of the engine running, then the current goes down to the point of the rated motor current and remains until the engine is turned off.

Current graph as a function of direct start time.

There are several ways to avoid electrical and mechanical problems that can occur when starting electric motors, and the use of the soft starter is one of them.

What is a soft starter?

The soft starter is also known as a soft starter and is a device that has the objective of limiting the electric current and the starting torque by accelerating and decelerating the motors, controlling the electrical voltage sent to the motor and protecting it during its operation. nominal.

The soft starter basically starts the motor with a voltage below the rated voltage and this voltage will gradually increase over a certain period of time, this happens until at the end of this time the rated voltage is applied to the motor. This reflects directly on the motor current graph as shown in the image below.

Current graph as a function of soft starter start time.

Using the soft starter is the best option in relation to the other types of starters, since in the direct starter the motor receives rated voltage in a very short period almost instant, where the negative effects are greater in relation to the others.

The star-triangle starting voltage has basically two stages, one where we initially apply a lower voltage and then a higher voltage, which is the nominal motor voltage. In the current graph we can see how the current creates an instantaneous peak during the change of state from star to triangle, much smaller than the current peak of the direct start, but it is still not ideal.

Current graph as a function of star-delta departure time.

Parameterization of a soft starter

In the soft starter, the initial voltage value can vary between 40% and 100% of the nominal voltage and the start ramp time can vary between 0 and 20 seconds. These values ​​are selected in the soft starter itself, usually and trimpotes.

To select the appropriate soft starter voltage and time parameters, we use the table provided by the manufacturers.

The engine starting parameters are defined according to your application. To do this, just find the correct application or one that has the greatest possible relationship with the one in the table. The table below is an example provided by a manufacturer.

Adjustment proposal and starting parameters for soft starter.

We will use as an example the application to drive a hydraulic pump. Just find the application of the motor in the table, then enter the parameters in the trimpots of the soft starter and then we have the initial voltage of 40% of the nominal value and the starting time in seconds is 10.

Selecting the soft starter parameters.

When the soft starter is connected, the initial motor voltage is 40% of its nominal voltage, so this value increases over time and after the selected time the motor is operating at 100% of its nominal value, according to the parameters set in the soft starter.

This occurs so that the peak current at the moment of the motor start is lower, because according to the ohm law, the lower the voltage the lower the current, considered a fixed impedance.

After the soft starter applies the rated voltage to the motor coils, it jumps with an external contactor and turns off its thyristors to preserve the softstarter’s life, reducing heat losses.

In some soft starter models, the bypass is integrated and internal, that is, it eliminates the use of complex controls and auxiliary contactors, which generates enormous practicality and space savings inside the panel. After the starting ramp is finished, the bypass automatically enters.

Advantages of using a soft starter.

The soft starter allows for better system operation, reducing maintenance and extending the useful life of all equipment. Among the various benefits of the soft starter we can mention:

  • Soft start and stop of three-phase asynchronous motors , eliminating mechanical peaks in the motor and in the load;
  • Reduction of inrush current peaks;
  • Space saving when compared to other starting methods; In many softstarter models it is necessary to carry out what in electrical engineering is called a by pass.

Advantages of soft starter starting.

Diagram and wiring diagram for a soft starter.

Load diagram.

Here we have a connection diagram for a motor driven by the soft starter. Note that the connection is quite simple.

Soft starter, load diagram.

The power cables are connected to the input of the motor circuit breaker.

From the motor circuit breaker output, the load cables are connected to the load terminals of the soft starter.

From the output terminals of the soft starter, three cables supply the terminals of the three-phase induction motor.

Command diagram.

The command part is even simpler, see the diagram below:

Soft starter, command diagram.

From the outlet of a circuit breaker pole, we connect a cable to the A1 contact of the soft starter and derive a cable for the 13 NO contact of the selector switch.

From contact 14 NA of the selector switch, we connect a cable to terminal 1 of the soft starter.

From the second pole of the circuit breaker we connect a cable directly to the A2 of the soft starter and that’s it … it’s connected.

Soft starter uses.

The soft starter is widely used in the industry in connection of pumps, compressors, conveyor belts and several other types of controls for motors. So it is worth understanding how they work and how they should be connected! Without a doubt it is a good option for existing conventional matches.

Examples of using the soft starter.

Although soft starter startups are mostly in the industrial area, there are commercial and residential applications, such as use in large pool pumps, building and condominium pumping systems, conveyor belts in airports, escalators and others .

The use of the soft starter in addition to increasing the useful life of motors also makes the electrical energy in motors to be optimized and better taking advantage of, even saving electricity.