---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Sun 12/18/11: 5 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 05:20 AM - Re: Starter contactors (Jan de Jong) 2. 07:48 AM - Re: Starter contactors (Robert L. Nuckolls, III) 3. 07:53 AM - Re: Starter contactors (Robert L. Nuckolls, III) 4. 10:08 AM - Re: Purpose of Diodes on Relays (Noel Loveys) 5. 01:28 PM - Re: Starter contactors (user9253) ________________________________ Message 1 _____________________________________ Time: 05:20:31 AM PST US From: Jan de Jong Subject: Re: AeroElectric-List: Starter contactors A solid state loadswitch as starter contactor could also be an option (attached). I would be inclined to make my own though by parallelling some BTS555. Jan de Jong ________________________________ Message 2 _____________________________________ Time: 07:48:51 AM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: Starter contactors At 09:05 PM 12/17/2011, you wrote: >I've considered the "Little Switch" on this page: >http://www.flamingriver.com/batterydisconnect This appears to be the same switch offered by Harbor Feight at a fraction of the cost . . . http://www.harborfreight.com/heavy-duty-battery-cutoff-switch-66789.html Bob . . . ________________________________ Message 3 _____________________________________ Time: 07:53:14 AM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: Starter contactors At 02:47 PM 12/17/2011, you wrote: > >My aircraft has a Rotax 912ULS engine with an 18 amp alternator. I >am considering adding lighting and additional avionics that would >load the alternator to near maximum. Can you share your load analysis with us? >Option 3 is the simplest and lightest weight and least >expensive. But I am concerned about the starter contacts welding >themselves shut with no way to shut off the power. > Is the likelihood of the starter contactor failing closed, great > enough to warrant the use of two contactors in series with the starter motor? Not if you can climb out quickly and disconnect the battery without undue hazard. But the incidences of sticking starter contactors of the S704 variety are exceedingly rare. How about a battery switch and S704-1 style starter contactor. When you say your alternator is loaded to "near max" . . . under what conditions and for how long? Keep in mind that legacy design goals for engine driven power sources is to keep 20-25% of alternator capacity in reserve for battery replenishment. A load analysis of ENERGY requirements for various phases of flight might yield some less stressing facts about your night flight capabilities . . . ESPECIALLY with LED nav lights. Landing lights are a toss up . . . their current demand is high but the duty cycle can be exceedingly low. You don't really NEED light until you're in the flare. Bob . . . ________________________________ Message 4 _____________________________________ Time: 10:08:26 AM PST US From: "Noel Loveys" Subject: RE: AeroElectric-List: Purpose of Diodes on Relays One thing that really surprises me on your lower trace is the fact there is no variance in the lower trace as the engine cranks. I know in my plane the lights in the panel dim as the engine is cranked just because of the current drawn by the starter motor really loads down the battery. This could be because of the speed of the trace which I make out to be around 1.500 sec left to right. The only place I've actually seen a diode on a switch is on turbine engines where the starter motor is also the generator (old 206). The MM specifies the diode be in good working order to protect from reverse EMF. I was told that from time to time I would see the diodes again but haven't Noel From: owner-aeroelectric-list-server@matronics.com [mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Robert L. Nuckolls, III Sent: December 16, 2011 6:27 PM Subject: RE: AeroElectric-List: Purpose of Diodes on Relays At 01:28 PM 12/16/2011, you wrote: A starting motor is essentially a coil. When you activate the starter (read coil) draws down on the current of the battery. The more load on the starter the greater the current drawn by the starter. When you release the starter button, key or whatever, all the energy stored up in the coil (or starter motor) is immediately dumped into the circuit, with extremely high voltage in reverse polarity. The diode allows this reverse potential to be drained off safely without frying all your expen$ive avionics and other digital equipment on the buss. A few years back (about 11) when we were exploring the behavior of energy stored on inductive devices, we discovered (and demonstrated on the bench) that un-suppressed spikes from inductive devices are almost totally dissipated across the spreading contacts of the switching device. For example, here's one of many plots we gathered while stirring the Contactor-Spike- Stew pot: http://www.aeroelectric.com/Pictures/Curves/CH_Bus_Noise_w_0p1_Cap.gif There are two traces. The top one shows voltage developed across the contacts of the controlling switch . . . the lower trace shows voltage that made it out to the bus . . . barely perceptible. Know that electronic devices designed to run directly from the bus of any vehicular DC system is (1) easily designed to stand off the (2) minuscule packet of energy that makes it across the opening switch. In short, there is no risk of "frying" anything. The major risk is shortened service life of the switch that controls the contactor. The same conditions apply to energy dumped from a starter motor . . . it's the starter contactor that takes the hit, not electro whizzies running off the bus. One might think that an inductor charged to hundreds of amps represents the grand dragon of spike generators. Motors have a unique feature called counter-emf. A 12v motor really runs on the DIFFERENCE voltage between applied voltage (battery) and counter-emf due to rotation. The real free inductance model of a motor is rather small in comparison with contactor coils having hundreds of turns of wire. This is why you never see spike suppressors on a motor . . . they're just not a potential threat. Bob . . . ________________________________ Message 5 _____________________________________ Time: 01:28:12 PM PST US Subject: AeroElectric-List: Re: Starter contactors From: "user9253" > Can you share your load analysis with us? Aircraft load at present time 0.3A SL 40 com 1.6A GTX 327 X-PNDR 0.2A 496 GPS 1.5A Dynon D-180 EFIS 2.6A Autopilot w/ 2 servos 0.1A Intercom 0.5A Misc 0.7A Battery Contactor 1.5A Fuel Pump (continuous) ---------------------------------------- 9A Total 3.5A Proposed Dynon SkyView (additional 1.5A during backup battery charging) 3.2A Proposed 35W HID Landing light (continuous for collision avoidance) ----------------------------------------------------- 15.7A New Total The voltage regulator requires dedicated cooling at higher loads. The Rotax 912ULS starts so quickly that main battery charging load is negligible. The autopilot will not be turned on until after climbing to cruise altitude. The battery will be charged by then. I never fly at night, thus no position lights. The electric fuel pump runs continuously per Van's RV-12 design. > How about a battery switch and S704-1 style starter contactor. I assume that you meant to type S702-1 starter contactor. I am reluctant to use a manual battery disconnect switch. It would add another failure point to the electrical system. In the event of a failed closed contactor, it would take me a minute or more to exit the plane and operate the battery switch. The cost to buy a second starter contactor is not much more than a quality manual battery switch. > But the incidences of sticking starter contactors . . . are exceedingly rare. That answers my question. If it is not a safety of flight issue, I am inclined to keep the electrical system simple with one starter contactor connected directly to the battery and add an indicator light showing starter energized. Thanks Bob, for the reply. Joe -------- Joe Gores Read this topic online here: http://forums.matronics.com/viewtopic.php?p=361150#361150 ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message aeroelectric-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/AeroElectric-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/aeroelectric-list Browse Digests http://www.matronics.com/digest/aeroelectric-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.