When the AC electromagnet excitation coil is connected with AC constant voltage source, the coil will balance the induced potential with the supply voltage; the induced emf is proportional to the magnetic flux, slightly less than the supply voltage. ∵ if the power supply voltage remains unchanged, the magnetic flux is also approximately unchanged (if Φ decreases for some reason, e will also decrease; therefore, the current will increase to increase the magnetic potential and make Φ increase. And vice versa). It is said to be a constant flux type.

    If the armature can not pull in after power on, the current will not decrease. This will not only make the equipment not work, but also burn the coil due to overheating for a long time.