I am trying to select a relay to interface between a temperature controller output and a NEMA size 4 120VAC contactor coil. According to the NEMA spec, a 120VAC, size 4 has a 8.3A inrush current and a 0.54A "sealed" current. As I understand it, as the armature moves into the position, the inductance increases which increases the impedance and lowers the current to the "sealed" value. I am having a really hard time accepting that the inductance alone decreases the current by 15 times. Is there a component to this that I am missing?
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\$\begingroup\$ Is this a relay with an economizer built into it? \$\endgroup\$– HearthCommented Nov 4, 2023 at 0:44
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1\$\begingroup\$ Why not? The geometry is a coil on a partial core, which when closed, forms a closed magnetic path (plus a little spare winding for phase shift (shorting bar, shaded pole) to just keep it from rattling). The ratio of open to closed magnetic path reluctance, and thus inductance, can be well more than 15 times. \$\endgroup\$– Tim WilliamsCommented Nov 4, 2023 at 3:22
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\$\begingroup\$ No economizer. This is a 1980's vintage Allen Bradley contactor for a 208VAC heat treat oven. \$\endgroup\$– john8791Commented Nov 5, 2023 at 2:10
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That's the advantage of an AC contactor coil (the 'hold-in VA' being automatically lowered to less than 10 % of 'pull-in VA').
There is, of course, the danger of coil burn-out, should the magnetic circuit not close properly. Also the need for 'shading rings' to prevent contactor chatter.
In the case of a DC contactor coil the 'missing component', that you refer to, is required to lower the 'hold-in power'.
That component could be a high-resistance 'hold-in' coil that remains in-circuit when the low-resistance 'pull-in' coil is switched off.
Alternatively, it could be PWM control of a single coil.
