[GreenKeys] energizing relays and questions on voltage and current

[Douglas W. Jones]

On Thursday, April 10, 2003, at 03:56 PM, eugenehertz@optonline.net 
wrote:

> The ticker has an electromagnet (it actually has three, but my question 
> is general).  Last night I wanted to see if the darn thing had any life 
> left in it, so I measured the resistance of one of these electromagnets 
> and came up with about 10 ohms.  The point was to attempt to 
> re-energize these electromagnets and see what happens!.  I have no 
> technical information on these and was afraid that I might burn out the 
> coil.

Here's how I'd go about it.  I'd dust off my trusty bench Variac,
hook its output to a 12-volt step-down transformer, attach a voltmeter
across the transformer output in parallel with the stock ticker's coil,
and then turn up the voltage until the coil pulls in.

(Pull-in is a well-defined term for any electromagnet actuated system
like a solenoid or relay.  It's the voltage or current required to make
the electromagnet pull the moving part home, against the resistance of
whatever springs, dashpots and apparatus may be connected to the moving
part.  The pull-out voltage or current is also useful to know.  That's
the minimum voltage or current needed to keep the moving parts pulled
in.  Lower the voltage below the pull-out voltage, and the spring wins.
Raise the voltage above the pull-in voltage and the magnet wins.

This experiment can be done in seconds, literally.  Just twist the
variac knob until you hear a click, record the voltage, turn it down
until you hear the counterclick, record the voltage again, and turn it
off.

The stepdown transformer is used between the variac and load because
I suspect that the required voltage is low (because it's a low resistance
coil).  Were it a high resistance coil, I'd still use the step-down
transformer just in case, and only if I couldn't make it pull in would
I take the transformer out of the circuit and try it with just a variac.

Also, since it's pre 1920's technology, I'd put a bridge rectifier in the
circuit because I suspect design for DC power and batteries.

(You know the voltage, from this you can infer the current -- most
telegraphic systems are intended to be current driven, not voltage
driven.)

Finally, there are thermal considerations.  Some electromagnets, relays
and solenoids are designed for continuous duty.  Those you can leave
energised forever.  Others are designed for intermittent duty.  With
those, you should only pulse the power or you risk burning things up.
That's why I recommend that the variac experiment be done quickly, in
seconds, and then turn off the power.

To find the maximum continuous current that you can safely put through
a solenoid, coil or whatnot, you use the variac setup in exactly the
same way, but you don't pay attention to pull-in and pull-out events.
Instead, you turn up the voltage very gradually (goose it up a a bit
every minute or two over a period of hours, while you're doing something
else), and constantly monitor the temperature of the coil.  The best
place to monitor any electromagnet is the center of the volume of packed
wire, but that's hard to get to.  The center of the pole piece on an
electromagnet or relay is a good substitute.

You don't need a thermocouple or thermometer, you can do this with your
fingertip.  You risk burnout when the temperature goes up somewhere near
the temperature required to boil water.  Above that, varnishes, shellacs,
rubber and similar materials begin to fail.

The reason I trust my fingertip is simple.  The insulators we use are
organic, like my finger.  If I can hold my finger against the pole piece
for an extended time withoug going ouch or smelling smoke, chances are
that the insulation in the coil is also comfortable and not about to
carbonize.

So, slowly goose up the voltage until you don't feel comfortable holding
your finger against the pole piece, and then declare that voltage (and
the inferred curent) to be the maximum continuous operating voltage.
If that's above the pull-in voltage, your coil is safe for continuous
operation.  If that's below the pull-in voltage, your coil is safe only
for intermittent operation, you can actually compute the safe duty cycle
by working out the safe maximum power in watts and then making sure that
that's the average power you send in in your intermittent signal.

> So not knowing what I was doing, I found a 7.2V camcorder battery and 
> tried to touch it to the ticker contacts just momentarily to see what 
> would happen (figuring I was generating less than 1A).  Sure enough! 
> The electromagnet energized and it "ticked!" (actually it just blotted 
> the type wheel on the paper and advanced, which is awesome!).

So, my guess was right, use the step-down transformer between variac
and coil.  I think I actually answered your question.  Variable voltage
and variable current are interchangable in this experiment, control 
either
one and you can infer the other using Ohm's law.

				Doug Jones
				jones@cs.uiowa.edu