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What is a Star-sealed Contactor KFX?


What is a Star-sealed Contactor KFX?

In most of Bluelight control cabinets for synchronous machine project, you will find this white color contactor labeled KFX, you may wonder what it is and what does it do? We call it a Star-sealed Contactor. Star-sealed is a direct translation from Chinese words actually mean ‘bypass the wye(Y) connection’. In other words, to short connect all three motor cables UVW when motor is not running.


What does KFX do?

Most of the Permanent Magnetic Synchronous machines are “Y” connection for their armature coils, and the other side UVW are the motor power cables that connect to the inverter. What does Star-sealed contactor KFX do is just bypassing cables UVW together thus the motor stator coil can form close circuits. If during the time motor is rotating unexpected, the rotating magnetic field cased by permanent magnet rotor will generate current in the stator because there are close circuits. That current will then produce inductive magnetic field that will counter rotors rotation. Thus, motor cannot run fast. 

 star contactor

Why do we need KFX?

KFX is a safe protection method for PMS machine. It has the following two purpose:

1. If machine brake malfunction, elevator cannot stop and will run towards the heavier side. With KFX on, motor cannot rotate very fast, so elevator can stop eventually at a relative slower and safer speed.

2. During power failure, rescue teams can release the brake and let elevator slip to door zone at a slow and safe speed, especially for MRL project.

Please be aware, geared inductive machines do not have this feature. KFX does not work for geared inductive machines because there is no permanent magnet in the motor, rotating the rotor will not generate magnetic field.  



1. KFX must be very reliable, because if elevator is running and KFX is malfunctioned, inverter outputs are short circuited. The high current can easily break the inverter or even the motor. So we use original Japan made high quality contactor with NC contacts for KFX.

2. We design the interlock for KFX contactor and run contactor KDY in our Bluelight control cabinet to prevent the accident that both contacts of KDY and KFX are conducting, in case it short circuit the inverter outputs.

star seal


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