2. AC coil pull-in (VA)/holding(VA) ratio is about = to DC coil pull-in (W)/holding(W).
Is this a typo? An AC coil sees an increase in impedance due to increased inductive reactance as it moves to shorten the magnetic circuit.
The DC coil does not change impedance.
A failure to pull in does not affect the impedance of a DC coil.
Therefore, AC coil is very much less affected by the winding temperature.
That is a design consideration. The strength of a DC electro-magnet depends on the number of turns, and the gauge of the wire within reason.
Select any wire gauge and any number of turns. Calculate the resistance and the Amp Turns.
Now double the number of turns, with the same gauge wire, and recalculate.
Resistance is doubled, current is halved.
Amp turns remains the same.
Magnetic force remains the same.
Heat generated is 50%
Now halve the number of turns, with the same gauge wire, and recalculate.
Resistance is halved, current is doubled.
Amp turns remains the same.
Magnetic force remains the same.
Heat generated is 200%
And this compares with an AC coil how?
The same relationship does not apply to AC coils.
The effect of increasing the number of turns on the inductive reaction of an AC coil is a square function rather than a linear function.
In the above example, increasing the number of turns on the DC coil and the increased induction does affect the buildup of current and thus the pull-in time.
I am not privy to the design of modern DC solenoid coils, but I suggest that if the solenoid may be subject to Pulse Width Modulation control, a designer may opt for as little resistance as possible with the resulting high temperature to get the fastest action possible under PWM control.
Dropout time may be even more affected than pull in time by increased induction. It depends somewhat on the construction of the magnetic circuit.
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Ohm's law
Not just a good idea;
It's the LAW!