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Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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 Posted: Sun Aug 30, 2015 3:31 pm Post subject: Hydraulic Pump Start-Up Amperage |
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My Lancair Legacy has a nominal 12 volt electrically powered hydraulic pump that raises and lowers the landing gear. When the airplane has sat in the hangar overnight, the hydraulic pressure bleeds down enough that the pump turns on momentarily when I turn on the master switch. Master relays aren’t designed to make or break large electrical currents and I always wondered if I was damaging it by hitting it with the large surge current from the hydraulic pump turning on as the master relay closes.
A friend has a fancy little electronic box that measures electrical current, sampling many, many times per second. Here is a trace of the current flow through a shunt between the battery and master relay. The blue trace shows an 80 amp surge as the master relay closes and the hydraulic pump almost simultaneously turns on. In this case, with the airplane on jacks, I had the gear down and the landing gear selector switch in the “up” position when I turned on the master switch. That means that as soon as the master relay closed, the hydraulic pump also turned on to raise the gear.
The 80 amp surge lasts for about one or two milliseconds. About ten milliseconds later, the hydraulic pump obviously starts and the current surges to 155 amps. Landing gear extension and retraction each take about six seconds. In order to focus on the very brief surges, this graph shows only a fraction of the first second.
The other two traces show the current draw from raising and lowering the landing gear in the customary manner, after the master relay has already closed. Their two traces were overlayed with the first one.
I’m not sure what this means. However, years of trouble free operation for my airplane and many similar ones show that the master relay isn’t damaged by the hydraulic pump turn on surge.
Dennis
Lancair Legacy, 860 hours
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nuckolls.bob(at)aeroelect
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| Posted: Sun Aug 30, 2015 5:34 pm Post subject: Hydraulic Pump Start-Up Amperage |
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The other two traces show the current draw from raising and lowering the landing gear in the customary manner, after the master relay has already closed. Their two traces were overlayed with the first one.
I’m not sure what this means. However, years of trouble free operation for my airplane and many similar ones show that the master relay isn’t damaged by the hydraulic pump turn on surge. |
Yup, that's what they do . . . and it's
generally a good thing for battery master
contactors NOT to make them close against a heavy
load.
The fact that you've not had a problem may
be a classic demonstration of "who wins the
relay race". There is a natural delay time
between first voltage on a contactor to first
-touch in the bounce then some more time until
bouncing ceases.
It is possible that your pump controls have
sufficient energize delay time that they
don't begin to electrically hammer the battery
contactor until after it's reasonably stable.
Its a function many variables that can make
one configuration eat battery contactors while
others are not so problematic.
There are simple circuits you can put in
series with the gear-down pump contactor
to delay power to the coil by a few hundred
milliseconds or so. Totally transparent
to the system and crew but in SOME cases
may have a profound benefit to battery
contactor life.
I did the sketch for a Glasair owner about
20 years ago . . . didn't hear from him again
after he installed "The Mod". I'll dig
it up for you if you're interested.
Bob . . . [quote][b]
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