Easy Way to Determine the Current Load of a Relay Coil

please tell me this was an attempt at humor and you are not seriously trying to tell someone this is factual

Mine was both, it is a fact you can determine power dissipation in a device that way. Heat is heat. It was humorous, in that I did not really expect you would go out and do it that way when there are easier, better ways to get the data.

I hope you got a good chuckle.

What is wrong is using rms values? You are talking a single phase, correct?

Dear Wannabecontroller,

It is I expect, practice of contactor/relay makers to simply measure the coil voltage applied and the current drawn on the actual coil/armature when it is wedged open (maximum air gap and minimum inductance and maximum current) and when contacts are closed (minimum air gap and current). Hence the specification volts x amps = VA, with two numbers e.g. "inrush"/"pull-in" and sealed/closed. That means rms volts x rms amps for AC service. The mechanism only cares about the magnetic pull due to the ampere-turns.

Consider a contactor with two 100 turn windings - both energised at 120V rms operate the contacts and draw 1 amp rms each - that is 120VA x 2 . If you put the two windings in series you need 240 volts and still get 1 amp in each winding (same ampere-turns) and VA is 240 x 1 = 240VA.

So it is practical for the maker to make 110V, 120V, 230V and 240V coils and just give one VA figure.

The user just divides the nominal voltage into the given VA to find the current drawn -the manufacturer will have applied worst case tolerances of "gap" and resistance to give a VA figure which can be used safely.

If you have only been taught basic theory as a student (and of course worry about power factor) you may not be aware of such "common practice" in specifications or have not yet seen that VA rating is the biggest factor for size and cost of transformers and other supply equipment.

In the case of AC contactor coils, the dominant factors are the "pull-in" current drawn (the VA, when adding the nominal supply voltage) and necessary supply power transformer rating VA to feed them and that the power factor is low - these decide the necessary rating of the contact which switches the coil, also the (LV contactor/relay) supply transformer rating and regulation. An important factor in transfo rating is the number and VA of contactors which will be operated simultaneously - it is well worth sequencing operation to avoid heavy surges and volt drops and oversized transfo, which also affect primary supplies and motor starting.

If you are concerned about what it takes to switch a contactor coil, you should take more account of the saturation when a coil is energised at zero volt point in supply cycle. The safe viewpoint is that the peak current is limited only by the coil resistance - this is important for solid state switches and less important for relay/switch mechanical contacts.

I am sure you will find, if you research the point, that solid state switch/PLC makers are aware that a 1 amp rated AC switch/output will be expected to reliably control a "1 amp inrush specification" contactor coil and build-in suitable ruggedness, including voltage surge protection and have 40 years or more of experience that their design margins give reliability. Long before "solid state" the necessary size of a relay contact to reliably switch certain loads was also established by experience.

67model