from the "overkill" department ...
Greetings to all ...
one BIG difference between a 24VAC coil and a 24VDC coil is the amount of windings that each contains ... the DC has MUCH MORE wire in it than the AC coil (for similar sized contactors) ... the reason:
in the AC coil, TWO major things limit the amount of current which flows in the circuit ... (1) the length of the wire - which gives us the “resistance” of the coil ... and (2) the amount of “counter-electromotive-force” (Counter-EMF) which gives us the “impedance” of the coil ...
you can easily measure the “resistance” of the coil with a simple ohmmeter ... on the other hand, you can’t directly measure the “impedance” ... like the “resistance”, the units for “impedance” are also generally expressed as “ohms” - but an ohmmeter won’t measure the impedance ... because:
the impedance results when the magnetic lines of flux CUT THROUGH the conductors of the coil ... this “CUTTING” of the conductors by magnetic flux lines is the basic definition of a “generator” ... in simplest terms, every time the AC circuit rises and falls (alternates), the magnetic lines of flux build up and then collapse - and in so doing, they “CUT” through the wires of the coil ... due to this action, the coil becomes a “generator” which produces a certain amount of electricity ... how much electricity comes out of the coil? ... not as much as we put INTO the coil in the first place ... and in what direction does this new “produced” electricity flow? ... (here’s the trick) ... it’s ALWAYS counter/against/opposing/reversed/fighting/limiting the original current that we put INTO the coil ... it’s this “wrong-way” current which helps to limit the amount of current which we can push through the coil with our INPUT of 24VAC ... in simplest terms, the “impedance” is like an “extra added resistance” that only shows up while the coil is energized ...
so that’s why you can’t measure the “impedance” of a coil with a simple ohmmeter ... specifically, you can’t hook up the ohmmeter while the coil is powered up ... and the ONLY time that the impedance is developed is WHILE the coil IS powered up ... (warning! if you’re determined to try measuring the impedance with an ohmmeter while the coil is energized, then kiss the meter goodbye first) ...
the most important thing here is the fact that the impedance plays a BIG role in limiting the amount of current which can flow in an AC coil ...
now for the DC coil, only ONE major thing limits the amount of current which flows in the circuit ... (1) the length of the wire - which we refer to as the “resistance” of the coil ... specifically, the “impedance” issue doesn’t come into play in the DC circuit ...
side note for picky people: this description is a slight simplification ... there IS a certain amount of impedance when the DC coil is FIRST powered up ... but then the flux lines stabilize in position, and so then there is no continuous “counter-EMF” developed while the DC coil remains energized ...
once again, you can measure the “resistance” of the coil with a simple ohmmeter ... and if you compare the measured resistance of a 24VDC coil with the resistance of the 24VAC coil, then you’ll find that the resistance of the DC coil is MUCH higher (for similar sized contactors, of course) ... that’s because of the extra length of wire which must be wound into the DC coil - in order to limit the amount of current ... incidentally, the wire in the DC coil is usually much finer too ...
one more thing ... most AC coils have a “Hertz” rating ... 50 to 60 Hz is fairly common ... if the frequency/Hertz of the AC power is too low, then the lines of magnetic flux won’t “CUT” the coils as often as with the higher (recommended) frequencies ... therefore, less “Counter-EMF” will be produced (thus less “impedance”) and so too much current will flow through the coil ... the coil will overheat and usually burn out ...
on the other hand, if the AC frequency is too high, then too much “Counter-EMF” will result ... not enough current will flow - and the coil might not develop enough “pull” to properly energize the relay ...
an interesting experiment:
remove a “good” 120VAC coil from a “bad” contactor ... connect a 120VAC lamp bulb (60 to 100 watts is best) in series with the coil - and CAREFULLY! plug the circuit into a 120VAC receptacle ... but be quick! ... don’t leave this rig connected more than a few seconds at a time or the coil will overheat ... but while it’s connected, note how brightly the lamp bulb glows ... now insert some heavy steel object through the opening of the coil ... something like the handle of a pair of pliers makes an EXCELLENT test ... note that the lamp glows less brightly ... now take something else (like a heavy screwdriver) and “bridge” the open end of the pliers’ handles ... the lamp gets even dimmer ... what’s happening is that the handle of the pliers concentrates the magnetic lines of flux - and more “Counter-EMF” is produced ... so more “impedance” limits the amount of current in the circuit - and the bulb gets dimmer ... when you “bridge” the handles of the pliers, you further concentrate the magnetic lines of flux - and even more “Counter-EMF” is produced ... the increased “impedance” limits the amount of current in the circuit - and the bulb gets dimmer still ...
suppose that you tried the same thing with a DC coil (using the proper DC voltage and lamp bulb of course) ... neither the pliers nor the screwdriver would make any difference to the brightness of the bulb ... because there is no “Counter-EMF” being generated by the “unmoving” lines of flux around the DC-powered coil ...
and one more experiment with “real-world” applications:
connect a “good” contactor with a 120VAC coil to a 120VAC circuit ... make sure that that contactor can properly “clunk on” and “clunk off” when you energize and then de-energize it ... now insert an ammeter in the circuit ... measure and record the amperage while the coil is energized (600 milliamps is a ballpark figure for many of the contactors that I use in my classes) ... now de-energize the contactor and “stall” the armature action with something like a wad of paper - or maybe a toothpick ... now energize the coil and check the amperage again ... be quick! ... if you leave the coil energized too long (I’m talking about seconds - not minutes) then it will overheat and burn out ... you’ll find that the current will be much higher with the “stalled” contactor ... because the “magnetic flux” won’t be as concentrated - and so less “Counter-EMF” (and thus less “impedance”) will be available to limit the current flow ...
this last effect is one reason why AC contactor coils (and AC solenoids too) occasionally burn out due to LOW voltage - as well as due to HIGH voltage ... many technicians can easily image why a HIGH voltage would tend to burn out the coil - but they have a problem understanding why too LOW a voltage can cause the same outward symptoms ... the reason is that if the voltage gets too low, then the contactor armature (the moving part) might not be pulled in completely ... in that case, the magnetic flux won’t be concentrated enough - and the insufficient impedance can allow the current in the circuit to climb too high ... a burnt out coil is the usual result ... DC contactor coils don’t suffer from this same “low-voltage-burn-out” problem - because the current in their coils is adequately limited by the inherent (built in) resistance of their longer windings ...
final “rule-of-thumb” thoughts:
if you connect a 24VAC coil (with less windings) to a 24VDC circuit, then the AC coil will usually overheat and burn out ...
if you connect a 24VDC coil (with more windings) to a 24VAC circuit, then the DC coil will usually not have enough “pull” and it might not properly actuate the contactor ...
apologies in advance: I’m tired (actually exhausted) ... if I’ve made any mistakes here, I’m sure someone will point them out ... but I think that I’ve got it right ...
unless you have questions, my work here is done ...