nuckolls.bob(at)aeroelect
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 Posted: Mon Jun 28, 2021 9:41 am Post subject: Interav OV relay: how to do an integrity test. |
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At 02:50 PM 6/27/2021, you wrote:
| Quote: | --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
Are the wires shorted together that you suspect are the contacts?
If not, maybe the 3 ohms are the contact wires. 3 ohms could be from corroded contacts.
3 ohms is too low for a coil. It would draw almost 5 amps and get very hot, 70 watts of heat.
Assuming the contacts are normally closed, then:
If one pair of wires is open, it means that either the contacts are not making contact or the coil is open.
In either case, the relay is defective.
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Joe Gores |
Joe's observations are spot on.
The OV management product you're evaluating has a history that
dates back 50 or so years. I'm aware of at least two
OV management devices based on relays. One such product
launched my aviation engineering career in 1972. My first
task at Electro-Mech was to design a replacement for this
device:
http://aeroelectric.com/Pictures/Relays/RBM138-1_A.jpg
http://aeroelectric.com/Pictures/Relays/RBM138-1_B.jpg
In the second image, we see two relays. A normally
closed device with rather hefty contacts incorporated
to disconnect field excitation on the runaway alternator.
The second relay is interesting. It's a reed relay
wrapped in mu-metal to ward off effects of external
magnetic fields. There is a capacitor across the
reed relay coil for what I assume is intended to
offer some time delay for energizing the relay.
There's a resistor/thermistor network in series with
the relay's coil which is also in series with a
voltage calibration potentiometer.
With set-point calibration voltage applied, the
potentiometer is adjusted until the sensing relay
just trips thereby energizing the field disconnect
relay.
The resistor/thermistor network provides temperature
compensation for the rather strong, positive resistance vs.
temperature coefficient of the sense relay's copper
coil.
Using relays offers a latching feature in that
the drop-out voltage of the sense relay is a small
fraction of the pull-in voltage. Hence, if the
sense relay closes at 16.0 volts, it probably won't
release until the voltage drops below 4-6 volts.
This means that the ship's battery voltage will keep
the relay closed until the alternator field supply
switch is opened.
I've not had occasion to inspect an InterAv-Alcor
OV relay. Here's what I have in the library with
respect to this OV management device and it's
ancillary system components:
http://www.aeroelectric.com/Installation_Data/Interav/
Two versions of the OV relay are illustrated: one
with all white wire and a second with colored wires.
As Joe has noted, you should measure a very low resistance
between two of the four wires connected to the
normally closed contacts of an excitation disconnect
relay. The other wires should have a rather high
resistance . . . probably on the order of several
hundred ohms. This would represent connections to
the sense coil.
We don't know if this product has two relays. In
theory, the whole thing can be implemented with one
relay.
The design we ultimately offered Cessna had a field
disconnect relay driven by an SCR which latches
in an on-state after trip. The SCR was triggered
through a resistor, capacitor, and zener diode network
tailored to optimize temperature stability of the
setpoint.
Check the resistances between wire again. If you
don't see obvious 'coil' and 'contact' values then
Joe's hypothesis of relay failure is correct.
If you DO see appropriate resistances between the
wire pairs, then attach a variable power supply
to the HIGHER resistance pair and bring voltage
up slowly until you hear a 'click'. This should
occur between 15.5 and 16.5 volts according to
the OV Relay's placarded performance numbers.
You could also wire a tail-light bulb in series
with the other pair to the test power supply.
The bulb should be illuminated until the 'click'
is heard.
Is this a 'potted' assembly? If you look at the
under side, do you see a metal close-out or is it
filled with pour-in plastic?
If it is failed and not potted, I'd like to have
the carcass. It would be interesting to see how the
'other' competition in 1972 was doing things. I will
publish a Shop Notes on the teardown inspection.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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