# Amps for a coil!

Discussion in 'Electrical Engineering' started by CLT, Mar 20, 2006.

1. ### CLTGuest

How much amps can take a coil for a nema size 5 contactor. The coil is
125 vdc! I am reading 3 amps and the coil gets hot, but i have no
baterries, just a variac with a diode bridge to feed the coil!

CLT.

2. ### daestromGuest

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

3. ### Guest

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

4. ### CLTGuest

I am reading the 3 amps in the primary side from de variac, but it
cant be so diferent in the coil. The question is, how many amps
should i expect to read, lets say 0.25 amps? or less?

5. ### Bud--Guest

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).

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 <gfretwell>'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--

Thanks Bud!

7. ### Bud--Guest

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--

8. ### daestromGuest

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

9. ### Bud--Guest

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--

10. ### daestromGuest

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

11. ### Bud--Guest

'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--

12. ### daestromGuest

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