What are the methods to reduce the starting current of the motor?

1. Direct starting

Direct starting is the process of directly connecting the stator winding of an electric motor to the power supply and starting at rated voltage. It has the characteristics of high starting torque and short starting time, and is also the simplest, most economical, and most reliable starting method. When starting at full voltage, the current is high and the starting torque is not large, making it easy to operate and quick to start. However, this starting method has high requirements for grid capacity and load, and is mainly suitable for starting motors below 1W.

2.Motor series resistance starting

Motor series resistance starting is a method of reducing voltage starting. During the startup process, a resistor is connected in series in the stator winding circuit. When the startup current passes through, a voltage drop is generated on the resistor, reducing the voltage applied to the stator winding. This can achieve the goal of reducing the startup current.

3.Starting of self coupling transformer

Utilizing the multi tap voltage reduction of an autotransformer can not only meet the needs of different load starting, but also obtain a larger starting torque. It is a commonly used voltage reduction starting method for starting larger capacity motors. Its biggest advantage is that the starting torque is large. When the winding tap is at 80%, the starting torque can reach 64% of the direct starting torque, and the starting torque can be adjusted through the tap. Official account “Mechanical Engineering Literature”, engineer’s gas station!

4.Star Delta Decompression Start

For a squirrel cage asynchronous motor with a normal operating stator winding connected in a triangular manner, if the stator winding is connected in a star shape during starting and then connected in a triangle shape after starting, it can reduce the starting current and reduce its impact on the power grid. This starting method is called star delta decompression starting, or simply star delta starting (y&starting).


When using the star delta starting method, the starting current is only one-third of the original direct starting method using the triangle connection method. At star delta starting, the starting current is only 2-2.3 times. This means that when using star delta starting, the starting torque is also reduced to one-third of what it was when starting directly using the triangle connection method.


Suitable for situations where there is no load or light load starting. And compared to any other vacuum starter, its structure is the simplest and the price is also the cheapest.


In addition, the star delta starting method also has an advantage, which is that when the load is light, it can allow the motor to operate under the star connection method. At this point, the rated torque and load can be matched, which can improve the efficiency of the motor and save electricity consumption.

5.Frequency converter start (soft start)

 The frequency converter is the most technologically advanced, fully functional, and effective motor control device in the field of modern motor control. It adjusts the speed and torque of the motor by changing the frequency of the power grid. Due to the involvement of power electronics technology and microcomputer technology, the cost is high and the requirements for maintenance technicians are also high. Therefore, it is mainly used in fields that require speed regulation and high speed control requirements.

Post time: Sep-15-2023