Electrical Machines - Motor Soft Starter

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A motor starter is an important device used for starting and stopping electric motors in a smooth and controlled manner. There are several different types of motor starts available for this purpose, but there is a special type of motor starter called a soft starter, which is used for controlling the input voltage and current, and the torque developed by the motor.

A soft starter performs several functions like protection and optimization of the motor. It smoothens the starting of the motor and reduces the maintenance due to wear and tear.

Read this article to learn more about a soft starter and its working principle.

What is a Soft Starter?

As the name implies, a soft starter is a device used for starting an electric motor. A starter is provided for controlling the input voltage and current and start up the motor smoothly. A soft starter is an advanced version of a normal motor starter which provides some advanced mechanical and electrical features.

The main purpose of using a soft starter is to reduce the strain that motor experiences during starting phase. A soft starter reduces this strain by gradually applying the supply voltage to the motor and allows it to accelerate smoothly. This helps in reducing the overall wear and tear in the motor.

Working Principle of Soft Starter

The working of a motor soft starter is based on the principle of controlling the amount of applied voltage to the motor. This controlled voltage is achieved by limiting the torque of the motor. This limiting of the torque allows the soft starter to decrease the voltage and permits it to gradually stop dropping the voltage for obtaining a smooth progression of the current.

In many other models of soft starters, a series of solid-state devices called thyristors is used. These thyristors are used for limiting the voltage to a more manageable level when the motor begins to start up. These thyristors have a ON state and allow the flow of current and an OFF state to limit the flow of current. When the thyristors are ON, the motor is powered up and once the motor attains its full power, these thyristors are relaxed. This is how a soft starter keeps the motor temperature low and reduces the strain on it.

The above theory explains the working principle of an electrical soft starter. There is another type of soft starter as well called mechanical soft starter.

Where, the working of a mechanical soft starter is based on the cultches and different types of couplings which are employed for limiting the torque in the motor. As discussed above, the controlling of motor torque limits the voltage in the motor circuit and allows it start up smoothly.

Components of a Soft Starter

A motor soft starter has two main components which are

Power Circuit

This is the main circuit which supplies operational power to the electric motor. In a soft starter, the power circuit consists of a series of thyristors, protecting devices, and current transformers.

Control Circuit

This circuit is responsible for controlling, monitoring, and protecting the power circuit and its components. The control circuit also provides a used interface circuit for operating the motor. When the gate voltage is applied to the series of thyristors, they turn ON and allow the flow of current in the power circuit and energize the motor.

Let discuss about these two main components of a soft starter in detail.

Advantages of Soft Starter

Motor soft starters reduce the strain put on the motor during powering up phase. The soft starter does this by gradually increasing the voltage across the motor circuit. Soft starters make the starting up of motor smooth and controlled that increases the life span of the motor.

Soft starter reduces the inrush current during powering phase of the motor. This reduces the electrical stress on the power supply. Soft starter smooths the starting of the motor, this reduces the wear and tear in the motor and reduces the overall maintenance cost of the motor.

Soft starters provide a better control over starting and stopping of the motor. Soft starters are equipped with built-in protection features like overload protection, undervoltage protection, and more. Hence, they are capable in providing better protection to the motor.

As we know, motor draws a high inrush current during starting phase. Soft starters reduce this inrush current and hence the power demand. In general, soft starters are compact in size and are suitable for applications where space is a limitation.

Soft starters generally use advanced control algorithms and help in achieving a reliable motor operation.

Disadvantages of Soft Starter

Soft starters are mainly used for starting and stopping of the motors. They are not able to provide a continuous speed control during the operation. They are relatively expensive than other types of motor starters like DOL starters, star-delta starters, etc.

Soft starters are not suitable to used in applications where high torque is required at starting or at low speeds. Soft starters require a thermal management system because they produce heat during operation.

Soft starters require a proper configuration setting to achieve optimal performance of the motor. They have some compatibility issues as well with certain types of electric motors.

Applications of Soft Starter

Soft starters are mainly used for starting and stopping of electric motors. They are also used for controlling the operation of motors used for driving water or liquid pumps.

Soft starters are required to control the motors used in conveyor belts. These are used for smoothing the operation of motors because sudden starting of conveyor belts result in jerks, misalignment, and strain on the motors.

Soft starters are used in motor circuits which are used in large fans to operate them smoothly at a constant speed. Soft starters are also used to start and stop the motors used for driving the belts and pulleys to smoothen their operation. Because belts and pulleys cannot withstand rapid jerks.

Conclusion

A motor soft starter is a type of motor starter that is used for the smooth starting and controlling of electric motors. It is also used for protecting the electric motors against different kinds of faults like overload, undervoltage, phase loss, etc. By providing all these protections, soft starters significantly extend the life of an electric motor.