Different Types of Soft Starters
Friday, December 28, 2018
The device that reduces the power supplied to the motor during the start-up is called a soft starter. Basically, soft starter reduces inrush current to the motor to reduce overall load on the electrical circuits of the motor. Inrush current or initial current is the surge of high current into motor’s windings that’s required to power the motor by overcoming inertia at the startup. Soft starter protects the motor from high inertia. It increases the speed slowly which reduces stress on motor parts and prevents overheating. Soft starters comprise of overload protection and controller.
Overload protection ensures that a high amount of current isn’t drawn by the motor during the start-up so that it doesn’t damage itself. Overload relay in the soft starter is used for overload protection.
Controllers are used to control the current being supplied to the motor.
Soft starters are placed between the utility line and the motor so that they can regulate the current being fed to motor. With the help of soft starters, the speed of an AC motor can be increased gradually while drawing less current. Due to the low voltage, torque also reduces which allows for an easy or soft start. Soft starters can be used on many different types of motors. They are usually used with AC inductor motor due to its reliability, ruggedness and simplicity.
Types of Soft Starter
1. Primary Resistor
Primary resistor was developed during 1900. Resistor for each phase of the current is there in the circuit. When motor starts, the resistors resist the current resulting in the drop of voltage. About 70 percent of the total line voltage is sent to motor at startup. After the AC motor has accelerated, sets of contacts are closed by the timer. Thus, the resistors are removed from circuit and the motor is allowed to reach full power.
2. Auto Transformer
Auto transformer is an effective method of soft startup. It is better than primary resistor soft starting. Resistors are not used in auto transformer. Instead, taps on windings of transformers are used for controlling the power supplied to the AC motor. Usually, taps are set to provide 80 percent, 65 percent and 50 percent of the total line voltage.
3. Part Winding
In this method, the windings of the motor are divided into two separate sets or more. These identical sets of windings are intended to operate in parallel. At startup of motor, power is only applied to one winding set. As the speed of the motor increases, other winding sets are also powered for normal motor running. Energizing windings this way reduces starting torque and starting current.
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