Amps for a coil!

If you're using a diode bridge to rectify AC to DC, how are you measuring the DC?

Rectified DC fed into a highly inductive load (such as your coil), means you can't just use a simple voltmeter to get the voltage reading.

Different types of meters work differently, but for example a meter using a D'Arsonval movement (old style analog meter) responds to the *average* voltage, not the RMS voltage. So adjusting your variac to get 125vdc (rms) is a bit trickier.

daestrom

What voltage are you developing across the coil and how are you measuring it?

Measure the coil resistance. The normal coil current would be 125 divided by the resistance (DC circuit and Ohm's law).

Reading the current at the input of the vairac includes the current to energize the variac which may be a significnt part of your reading (and can't just be subtracted off because it is at a different phase angle). Note cautions about using an RMS reading meter.

The coil will act as an inductor to smooth out the rectified AC supplied to it. In addition the diode bridge will act as a 'freewheeling diode' (conducting current from the coil when the supply pulses fall below the coil voltage) to further smooth the current. As a result you may have a fairly constant voltage across (and current through) the coil - I don't know how much smoothing results. Measuring the voltage across the coil, as in 's post, is probably the best idea. You want this to be

125V using a RMS reading meter (an average reading meter is probably quite close because of the smoothing).

Significant ripple in the coil current can produce heating from eddy currents in the core unless the core is laminated.

The 2 times I supplied a DC coil through a full wave bridge, the coil ran at its rated current when the AC supply voltage was very near the DC coil rating. I don't think the physics requires this.

bud--

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--

Yep. Worked with a lot of such contactors. The older styles I've worked with have one large contact on each contactor, with arcing contacts, main contacts and often an arc-chute with blow-out coils. Then off the 'tail' side, there is a couple of pairs of 'auxilary' contacts. And one of those NC contacts is used to short out a resistor in series with the coil. The resistor is sized to provide just holding current when the NC contacts open up.

Because they only have one large current carrying main/arcing contact, you often will see two of these beasts in the controller as a minimum. If its for a DC motor that has starting resistors, you may see more large contactors that are used to short the motor starting resistor as the motor accelerates.

While an integral horsepower AC contactor may have all three phases routed through one contactor that has three/four pairs of contacts each the size of your fingernail, the equivalent horsepower DC starter would have two contactors, each having a pair of contacts the size of your entire thumb for arc interruption (picture holding up both thumbs facing each other and bring them together), and possibly smaller contacts underneath these for carrying the load current (these contacts break before the arcing contacts so there is no arc on these, then the 'arcing contacts' open up to interrupt the DC current).

Such are the 'joys' and pitfalls of interrupting large DC currents.

daestrom

The OP has a NEMA 5 contactor with DC coil. SqD NEMA 6 & 7 contactors are only supplied with DC coils. Why switch to DC coils with large contactors?

bud--

Well, I admit I may have rambled off topic a bit. But is this NEMA 5 contactor with a DC coil meant for a DC motor or an AC motor? It occurs to me that the OP didn't specify the type of load. Interrupting DC load current takes quite a different contactor construction than AC.

My 'rambling' was about the unusual size/construction of contactors used for DC motors versus those for AC motors. Obviously, the most often used coil for a DC motor controller is also DC and has auxilary contacts to put a resistor in series with the coil to limit 'holding current' while still providing the higher 'pickup current' needed.

daestrom

'Rambling' is often, including here, an interesting feature of threads.

The OP didn't specify what is being interrupted. However the SqD NEMA 5 & 6 contactors, the largest in that line, are for AC circuits. The coil has a required transformer feeding a "solid state rectifier circuit" to a DC coil. It is not obvious to me why SqD converts to DC for the largest AC circuit contactors. Could just be hum/armature vibration.

bud--

Well, one thought is that 'amp for amp' you can get more mechanical force with DC. Any AC coil needs a 'shading coil' to even out the magnetic pull, otherwise they chatter at 120hz. This reduces the instantaneous magnetic flux available across the coil armature face. DC coils don't have that particular problem (although they do need external current limiting).

Perhaps the larger contactors, with stronger springs for quick-opening, need more 'pull' to pick up?

daestrom