How do I use a multimeter to test a contactor?
The contactor is detected using the resistance block of the multimeter, and the detection process is as follows:
① Detect the resistance of normally open contact and normally closed contact under normal condition. The figure below shows the resistance of normally open contact of AC contactor under normal condition, because the normally open contact is in open circuit under normal condition, so the normal resistance should be infinite, the digital multimeter will show the out of range symbol "1" or "OL" when it detects, the resistance value should be close to 0 when it detects the normally closed contact under normal condition. When detecting the resistance of normally closed contacts under normal condition, the normal resistance value should be close to 0 Ω. For the AC contactor with linkage frame, press the linkage frame, the internal normally open contact will be closed and the normally closed contact will be disconnected, so you can use the multimeter to detect whether this point is normal or not.
② Detect the resistance of the control coil. Detect the resistance of the control coil as shown in the figure below, the resistance value of the control coil should be a few hundred ohms, in general, the greater the power of the AC contactor, the greater the suction force of the coil on the contacts (i.e., the coil is required to flow a large amount of current), the coil resistance is smaller. If the resistance of the coil is infinity then the coil is open, and if the resistance of the coil is 0 then the coil is shorted.
③ Energise the control coil to detect the resistance of the normally open and normally closed contacts. The figure below shows the detection of the resistance of normally open contacts by energising the control coil of an AC contactor. In the control coil is energised, if the AC contactor is normal, it will emit a "click" sound, while the normally open contacts are closed, normally closed contacts are open, so the measured resistance of the normally open contacts should be close to 0 Ω, normally closed contacts should be infinity (the digital multimeter will display the out-of-range symbols when detecting "1 " or "OL"). If there is no change in the measured contact resistance before and after the control coil is energised, the control coil may be damaged or the transmission mechanism may be jammed.
