AC or DC coils?

[boater sam]

Another musing from yesterday when the weather was horrible.

What is the difference between AC and DC relay coils?  Is there some way that AC coils are wound that is different?
I see the same relay construction listed in catalogues but with two types, one for DC and another for AC coils.
I know about contact current ratings being different due to arc suppression.

[DrStrangelove]

Good question.

This seems to go into some detail on the differences:

https://www.ato.com/difference-between-a...ylindrical.

[Kalee20]

AC through a simple solenoid coil is bad news - the current drops to zero twice each cycle, and so does the holding force. So the armature buzzes and chatters. Try it!

So, an AC relay coil has a 'D' shaped piece of copper surrounding part of the pole-piece. Eddy currents in this delay the passing through zero of the field in this part of the pole-piece. It means that somewhere on the pole-piece, there's always some field, so always some holding force.

[ppppenguin]

I once incautiously applied DC to the coil of an AC contactor. Let's just say that the current was excessive I'm not sure that I found a satisfactory voltage to run the coil on DC. There seemed to be either excessive current or not enough magnetic field.

[Kalee20]

It would make sense Jeffrey - especially when pulled-in, there would be a fair bit of inductance which would limit current. This of course wouldn't happen if you hit it with DC.

It also means that AC relays pull-in with a hefty thump, the de-energised inductance is rather smaller than the pulled-in inductance, so the initial current is higher than the steady current - exactly what's wanted for a snappy 'make' action.

[ppppenguin]

I've seen various circuits for relay drivers (DC) that hit the coil with a higher voltage to pull it in, then reduce it to the much lower voltage needed to hold the relay in.

As for the contacts, as mentioned by Sam, breaking DC (by relay or switch) at anything above very low voltages can give some impressive arcs if it isn't done correctly. The DC isolator for my PV panels is actually 3 or 4 standard AC isolator sections wired in series. It has to break up to 500V or so.

[Ed_Dinning]

In emergency stop circuits for AC motors DC injection into the windings is often used.
Stops the machine PDQ

Ed

[DrStrangelove]

I've seen various circuits for relay drivers (DC) that hit the coil with a higher voltage to pull it in, then reduce it to the much lower voltage needed to hold the relay in.

As for the contacts, as mentioned by Sam, breaking DC (by relay or switch) at anything above very low voltages can give some impressive arcs if it isn't done correctly.  The DC isolator for my PV panels is actually 3 or 4 standard AC isolator sections wired in series. It has to break up to 500V or so.

I seem to remember somewhat vaguely that some tape recorder pinch roller solenoids used the full voltage to pull the thing in, then a reduced voltage to keep it there.

I may, of course, have dreamed that.

[Mark Hennessy]

It's very commonly done. The difference between "pull-in" and "hold-in" is huge, and there are decent power savings to be had.

A capacitor in the drive circuit is the most common, but as it needs to carry a large current into a low impedance, it tends to be quite a large value. More sophisticated schemes exist, such as two voltages, or two transistor, where one maintains the current via a resistor. Also seen a PWM setup, which can be quite neat if you already have a microcontroller in the design.

One of my preferred options is to switch on the relay with a saturated transistor that applies practically the full voltage to the coil. Then after a short delay, the transistor becomes a current source. This has the advantage of being resistant to voltage drops - these circuits are often powered by unregulated supplies.

Assuming you already have a comparator to provide the usual power-on delay, it only takes another comparator, a diode or two, and a couple of passives to do this. As a dual comparator takes up no more space than a single, it's a neat solution.

Also, it's inherently self-resetting, as the same RC is providing both delays. This can be a worry with simple passive systems.

[PerdioPal]

I have seen DC relays powered from AC using both series and parallel diodes, quite neat for a small relay.