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Radioflyer
Joined: 04 Feb 2009
Posts: 43
Location: Boston
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 Posted: Tue Jan 31, 2023 1:31 pm Post subject: Master Relay Typical Resistance (long Story) |
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I've been chasing a sporadic in-flight alternator drop out issue in my LongEz. (No engine starting or dead battery issues.) The alternator was checked out to be fine and the voltage regulator is fairly new. I recently installed a small EFIS which shows the system voltage. To my surprise, it showed 14.8-15.3V when my analog Voltmeter had all along displayed just under 14V. This got me to trace Voltages from battery to contactors (both Essex 70-111225-5 units!) to bus to overvoltage relay (Lamar).
With master switch on (some small loads), engine not running:
Battery and input to master relay (not the control terminal) was showing 12.16V,
output of master relay showed .4 to .48 drop depending on gentle taps,
bus voltage at breaker panel measured an additional .2V drop,
input to overvoltage relay was 11.45V dropping to 11.41V.
Ostensibly the input to the Voltage regulator would have been about 11.4V, which would explain the high Voltage I was observing on the EFIS display while flying.
OK, my theory is that the occasional alternator dropouts (which seem more common in colder weather, BTW) are due to low voltage feed to the voltage regulator due primarily to high resistance master relay contacts. This would raise the alternator output voltage to occasionally surge to 16V, thereby tripping the overvoltage relay off. Do we concur?
So, out of curiosity what should be the proper pass-through resistance for a master relay? I used the 4-wire probe technique as suggested by Bob N. on the removed master relay and measured the resistance average to be about 133 milliOhms. (In the plane it measured more like 203 mOhms.)
Meanwhile, I'm waiting to receive a new Cole-Hersee #24115 contactor to replace the old Essex unit. If this doesn't fix the problem, I'll sell the plane.
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user9253
Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan
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| Posted: Tue Jan 31, 2023 6:23 pm Post subject: Re: Master Relay Typical Resistance (long Story) |
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With light aircraft loads, the voltage drop across the battery contactor should
be near zero. One half volt is way too much. If all of the crimps are tight and
the nuts are tight, then the contactor is at fault. Replace it.
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Joe Gores |
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bob.verwey(at)gmail.com
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| Posted: Thu Feb 02, 2023 7:49 am Post subject: Master Relay Typical Resistance (long Story) |
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Is the master contactor physically hot after extended use? Might hold a clue?
On Tue, 31 Jan 2023 at 23:35, Radioflyer <skyeyecorp(at)airpost.net (skyeyecorp(at)airpost.net)> wrote:
| Quote: | --> AeroElectric-List message posted by: "Radioflyer" <skyeyecorp(at)airpost.net (skyeyecorp(at)airpost.net)>
I've been chasing a sporadic in-flight alternator drop out issue in my LongEz. (No engine starting or dead battery issues.) The alternator was checked out to be fine and the voltage regulator is fairly new. I recently installed a small EFIS which shows the system voltage. To my surprise, it showed 14.8-15.3V when my analog Voltmeter had all along displayed just under 14V. This got me to trace Voltages from battery to contactors (both Essex 70-111225-5 units!) to bus to overvoltage relay (Lamar).
With master switch on (some small loads), engine not running:
Battery and input to master relay (not the control terminal) was showing 12.16V,
output of master relay showed .4 to .48 drop depending on gentle taps,
bus voltage at breaker panel measured an additional .2V drop,
input to overvoltage relay was 11.45V dropping to 11.41V.
Ostensibly the input to the Voltage regulator would have been about 11.4V, which would explain the high Voltage I was observing on the EFIS display while flying.
OK, my theory is that the occasional alternator dropouts (which seem more common in colder weather, BTW) are due to low voltage feed to the voltage regulator due primarily to high resistance master relay contacts. This would raise the alternator output voltage to occasionally surge to 16V, thereby tripping the overvoltage relay off. Do we concur?
So, out of curiosity what should be the proper pass-through resistance for a master relay? I used the 4-wire probe technique as suggested by Bob N. on the removed master relay and measured the resistance average to be about 133 milliOhms. (In the plane it measured more like 203 mOhms.)
Meanwhile, I'm waiting to receive a new Cole-Hersee #24115 contactor to replace the old Essex unit. If this doesn't fix the problem, I'll sell the plane.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510091#510091
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Best Regards,Bob Verwey
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Radioflyer
Joined: 04 Feb 2009
Posts: 43
Location: Boston
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| Posted: Thu Feb 02, 2023 8:51 am Post subject: Re: Master Relay Typical Resistance (long Story) |
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The master relay was not hot at all. In fact, Amp draw was in spec, <1A. I do believe the problem is corroded contacts. It was probably in there for 35 years.
I have ordered a new (but different) relay and will check its pass-through resistance before installing.
I was hoping someone could concur with my theory that large Voltage drop across this device could cause my alternator drop-out issue that started the whole investigation.
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