1 It is the resistance of the coil that limits the current in the DC relay coil. Using AC requires a significant increase in voltage due to the inductive reactance of the coil.

2 The magnetic attraction force generated by the DC current is constant. Alternating current crosses zero one hundred times per second, which will cause vibrations and the contacts will not be firmly engaged. The AC relay has a short-circuit copper ring on the iron core, which is used to generate a magnetic lag current to avoid the magnetic field from crossing zero.

So AC and DC relays are not interchangeable. AC used for DC coils will burn quickly. DC is used for AC, the suction is insufficient or cannot be pulled in.

The iron core of the AC relay is made of superimposed silicon steel sheets and has a short-circuit ring. It is to reduce the eddy current and hysteresis loss generated by the alternating magnetic flux in the iron core. Shape like "E" shape. In order to reduce the disappearance of magnetic force and vibration noise when the AC frequency crosses zero, short-circuit rings are added on both sides of the core.

The core of the DC relay is made of a single piece of cast iron or stainless steel, and there is no short-circuit ring. Shaped like a "cylinder".