Friday, April 2, 2021




A soft starter is a device that reduces the motor's inrush current and torque during startup. This is accomplished using a soft starter, which regulates the amount of voltage flowing through the motor windings during start-up.


As an electric motor accelerates to normal speed, it consumes a lot of electricity during start-up. To limit the surge of current known as inrush current and the torque of the motor, a soft starter can be employed. As a result, the start-up is safer, smoother, and more progressive.

Soft starters safeguard your electric motor from harm while also extending the life of the motor and the system as a whole by minimizing the warmth created by frequent start and stop actions. As a result, the mechanical stress on the motor and shaft would be reduced, as would the electro-dynamics on the power lines.

In addition, a large inrush current exerts a considerable demand on the electrical supply system, resulting in additional costs. Soft starters are often employed in industrial applications that demand a substantial inrush current during start-up and have a high initial load.


Soft starter control can be either open-loop or closed-loop.

OPEN CONTROL - Two thyristors are connected back to back in an open control. This results in a half-wave delay in conduction (180 degree). The delay is gradually increased until the input voltage reaches full load voltage.

CLOSED-LOOP CONTROL - A sensor monitors the output current and speed of the motor shaft in a closed-loop control. A feedback signal is supplied to a controller, which compares the value of the feedback signal to a set point value and adjusts the starting voltage to get the desired response.

The soft starter can also be operated over ethernet or through direct start/stop wiring. Both techniques of control have advantages and disadvantages. A PLC is not required for direct starter/stop signals. Using ethernet connection and devices to control where a PLC is necessary might be less expensive. Feedback would be possible, as well as flexible, customizable, and monitoring control capabilities.


A TRIAC, which is meant to limit the applied voltage to the motor, is the main component of a soft starter. Two thyristors or SCRs are placed back-to-back in a TRIAC (Semiconductor controlled rectifier). The gate voltage to the thyristor is controlled by controller logic in the form of PID controllers or microcontrollers. When an internal pulse is delivered to its gate, current is allowed to flow, and current is subsequently sent to a motor. The pulses are supplied on a ramp time basis so that the current can be applied to the motor gradually. The motor would be able to start slowly, lowering torque and inrush current.

Soft starter and DOL both work when the motor reaches full speed. The distinction is in how they behave before they reach top speed. On the DOL starting graph, we can detect a straight inrush voltage when comparing the voltage and time characteristics of DOL and soft starting. The soft starter, on the other hand, takes some time to reach maximum voltage.

With a soft starting, the current is more regulated, but with the DOL, there is a huge surge in current as the engine starts.

The time it takes for the motor to reach full speed is slower and more regulated with the soft starter. A DOL starter, on the other hand, gets the motor up to full speed almost instantly. This may have unfavorable consequences.

When employing DOL, the torque applied to the motor increases as the speed increases. When employing a soft starter, this resists a steady torque increase.


We now understand how a soft starter works. Let's take a look at some industrial applications and see how we can use what we've learned about why a soft starter can help prevent unwanted mechanical and electrical effects.

1. A dust collector in a factory is an example of a soft starting application. It is equipped with a big fan. It will take some time to get the fan running in this case, but once it does, the current and torque burden on the motor will be lowered. The fan in the system will pull air into the filters, which will capture dust particles before returning clean air to the factory.

If a DOL starter is used in this application (dust collector), the load will be affected, as well as the rapid acceleration, which will cause excessive wear and tear. A motor with belts and pulleys would drive the air cleaner fan. When using a DOL, the belt may slip and wear.

2. Another fantastic application for soft starter is in the water supply. Pumps must be brought up to full speed gently and gradually when used in industrial processes. If not, pressure surges in the water system may occur, potentially resulting in dangerous conditions.

When the DOL starter is utilized, pressure surges in the water pipes occur. This might lead to line breaks due to severe wear.

3. Soft starters can be utilized to improve the performance of a conveyor system that is used to carry large loads. A soft starter might be installed inside the control panel and fed three-phase electricity through the input terminals. A soft starter on the output connector would deliver three phase power to the motor.
The employment of a DOL starter in a conveyor system would put strain on components such as couplings and bearings that drive the conveyors. This unfavorable mechanical impact can result in a constant threat, unscheduled shot-down, and a reduction in component life. Using soft starts also has an electrical consequence that we wish to avoid. The DOL starter's large current surges can be dangerous motor's contacts and parts.


1. By progressively changing the starting voltage, the starting current may be smoothly controlled.

2. It makes bad power factor correction easier.

3. The soft starter's low start current contributes to the motor's mechanical parts having a longer life span.

4. The motor shaft is gradually accelerated, which helps to prevent jerking.


A soft starter is an ideal technique for regulating the system when industrial action necessitates a substantial starting load. We also talked about the soft starter's wiring and control, as well as the interior components. We'll go through how TRIAC uses ramp time to control voltage to the motor. We also discussed several mechanical and electrical side effects that can be avoided by employing a soft starter.

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