BFoelsch said:
I suspect you are missing the whole point. A NEMA size 5 starter with a DC
coil will definitely have some kind of circuit to reduce the coil voltage
after the coil pulls in. Assuming that you are measuring the current
correctly, I would say that 3 amps is actually a little low for the pull-in
value, but after the contactor is closed I would expect the current to drop
to maybe 1/4 amp.
The Square D NEMA 6 & 7 contactors used to pull in at about 125 -140 VDC and
reduce the holding voltage to around 8 volts. Your 3 amps at 125 volts gives
you 375 watts, which will burn out a coil of that size very quickly.
Interesting. I have never seen a contactor with a DC coil - or how it
works. I looked at the SquareD catalog and all the NEMA 6 & 7 contactors
have a DC coil. Do you know what is used to reduce the current, like
shunted resistor?
To the OP. If this seems odd, DC contactor coils behave somewhat
differently form AC coils.
In AC coils the current is limited by resistance and inductance. When
the coil is energized, the magnetic path has an air gap because the
armature is not pulled in. When the contactor closes, the armature
closes the magnetic path, the inductance goes way up, and the current
goes way down. This results in a desirable high current to close the
contactor and low current to hold it. For a NEMA #3 SqD contactor the
inrush is 700 VA and closed is 46 VA. Closed is 14 Watts indicating a
relatively large inductance.
In DC coils the current is limited only by resistance (ignoring some
inductance with rectified AC) so the current is the same open and
closed. Thus per BFoelsch, a means is used to reducing the holding
current. The SqD catalog for a NEMA #6 contactor gives 1780VA inrush and
48VA closed. Closed is 32 Watts compared to 48VA indicates low inductance.
The SqD coil is about 120 VDC so 1780 VA inrush is about 14.8A. If the
coil were totally resistive its resistance would be about 120/14.8 = 8
ohms. If totally resistive the closed voltage would be about 19.7 V and
current 2.4 A.
The SqD catalog says the contactors I looked at are always supplied by a
transformer with 120 VAC secondary, and are supplied by transformer even
if the supply voltage is 120 VAC. The coil voltage is not actually given.
bud--