# Is AC relay "inrush" current caused by more than the mutual inductance effect?

My interest is in the "inrush" current of an AC relay. I took apart a 24VAC one. It's energized current was about 40mA but when I removed the plate that is pulled by the electromagnet, the current doubled (roughly). This leads me to believe that part of the inrush is at least due to fact that the plate has not been pulled into the closed position.

I'm curious as to why less current flows through the coil after the plate is pulled against the electromagnet.

• Good question Doug. Commented Sep 17, 2020 at 21:48

The "plate" is the relay armature.

Figure 1. Image source: D&T Online.

When the armature is pulled in by the coil it closes the magnetic circuit. This results in an increase in coil inducatance and since the impedance (resistance to current flow) is given by $$\ Z = 2\pi fL \$$ it should be clear that for a given frequency the impedance will increase with the inductance. Your hunch is correct: the current will be high initially and reduce when the relay picks up.

This has the advantage that AC relays can be made to pull in strongly and automatically reduce their current when pulled-in. Be aware though that if the relay binds and remains energised while not pulling in that the coil may overheat and burn out. This can be a particular problem on larger contactors.

Is AC relay “inrush” current caused by more than the mutual inductance effect?

There's no "mutual" in a relay. Mutual inductance would require one coil affecting another - and there is no other in this case.

When the armature pulls in, it connects the two sides of the core - does this make the core in effect longer? That would explain the higher L!

No, it eliminates the air gap. Think of the air gap as a large magnetic circuit resistance due to air's poor permeability relative to the iron core. It's making a poor inductor. Close the air gap and now you have a good low reluctance magnetic circuit so the inductance increases.

• Armature, right, I should have looked it up! Yes, a higher XL makes some sense to me. The core has a trench through the center with a copper ring around one side to create the pole effect. Commented Sep 17, 2020 at 16:17
• Oops hit send by accident. At any rate, when the armature pulls in, it connects the two sides of the core - does this make the core in effect longer? That would explain the higher L! Commented Sep 17, 2020 at 16:19
• See the update. Commented Sep 17, 2020 at 16:50

My interest is in the "inrush" current of an AC relay.

@Transistor has clearly dealt with the situation of inductance increasing when the magnetics don't have a gap anymore - just regard the closing of the gap as producing a lot more magnetic permeability and, given that inductance of a coil is proportional to permeability, the AC current reduces when the gap is minimized.

Another problem is that inrush current also occurs when you activate the relay coil when the applied AC voltage is passing through 0 volts. This will cause the current in the first few dozen cycles to approximately double. If you activated the relay coil at the peak of the AC voltage then you would get no peaking of the current: -

Picture taken from here.

• I think I'm getting the picture. L = N^2 X u0n X a / l Commented Sep 17, 2020 at 16:45
• Great answers Andy and Transistor. That has always bugged me - how current is higher when mag circuit air-core then much lower when changed to low reluctance core. Is there an intuitive “why” we can state? Not just equations say so, but physical why. Commented Sep 17, 2020 at 17:36
• @relayman357 not sure what it is you want stating in more detail. Can you be clearer cause I'd hate to drone on about this or that when you meant something else. Commented Sep 17, 2020 at 17:45
• Thanks Andy, you are very clear. It just bothers me that the current is higher for air core. Then, when we put a nice magnetic material for the flux to flow through the current goes down. Something about that is counterintuitive to me. Commented Sep 17, 2020 at 17:54
• @relayman357 OK I see what you say. With a closed magnetic path the magnetic reluctance is much lower therefore, for a given current, more flux is produced (LINK1). If there is more flux per amp then there is more inductance (LINK2). Commented Sep 17, 2020 at 18:03