Starting Methods of Squirrel-cage Three-phase Asynchronous Motors

Starting for any motor is particularly important, we have talked about in the previous article, starting torque, starting current is small, starting time is short and less heat, is what we want more. How to achieve this goal, especially for the frequency of the squirrel cage rotor motor, starting performance control and improvement is more important. For squirrel-cage rotor motor starting, we have two common methods, namely, direct starting and reduced voltage starting.

Direct start is the motor and the rated voltage of the grid through the switch is directly connected to the starting equipment involved in this start is relatively simple, but the disadvantage is that the starting current is particularly large, the grid capacity is not enough, the motor starting difficulties, while the impact of the motor windings is particularly large, and therefore in the design of the motor will be fully taken into account in the starting process of heat, electromagnetism and other factors. In order to ensure that the motor can start normally, the voltage drop of the grid is clearly defined, when the voltage is low, the motor may have problems starting.

For squirrel-cage rotor motors, the starting current is generally 5-7 times the rated current, so the starting time should not be too long, and is not suitable for frequent starting.

Motor can be started directly under the condition that the grid capacity is sufficient, when the grid capacity is insufficient, we must consider other ways to start the motor, which is often used in a reduced-voltage starting method, the current application of more widely used and frequency starting.

Stator reactance method. The reactance is strung in the stator circuit and is removed when the speed reaches the rated speed. The purpose of this method is to reduce the terminal voltage of the motor winding by distributing part of the voltage through the series reactance, thus reducing the starting current. However, this method is only suitable for the starting torque requirements are not high occasions, such as no load or light load occasions.

Transformer step-down method. Through the transformer to adjust the end voltage added to the motor winding, this method in the motor of the test process is used more, also applies to the starting torque is not high for no-load or light-load occasions.

Star-delta conversion starting. For the same motor when using different connection, the corresponding stator current is different in the same power state, the stator voltage under star connection is higher than the voltage of delta connection, and the stator line current is smaller than the line current of delta connection. This starting method is suitable for normal operation of the delta connection motor, the principle is the same as the transformer step-down method.

Yanbian triangle starting. The starting mode by the star-delta connection conversion starting mode evolved, start a part of the stator winding connected to the triangle, another part of the stator winding connected to the star. This starting method, the starting current and starting torque than direct starting are small, but higher than the star-delta starting, and can be adjusted according to different starting requirements stator winding turns. However, after solving the starting problem, the stator winding will be more complicated.

Frequency conversion starting. This is a feature of modern power supply, for different power motors, many manufacturers have adopted the frequency conversion power supply, a better solution to the motor starting problem. However, the operating process will be frequency motor according to frequency motor use also led to the occurrence of a number of electrical failures and bearing system failures. In this regard, we will choose another section to communicate with you.