Starter solenoid power

Discussion in 'Instruments / Avionics / Electrical System' started by DLrocket89, Nov 11, 2010.

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1. Nov 11, 2010

DLrocket89

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Quick question...in a typical GA aircraft, does the power for the starter get routed through the solenoid that controls the rest of everything, or not? I know they can't handle the amps, but for the intermittant use that is a starter, maybe it would be OK?

What's the logic behind the setup? I have ideas both ways, I don't want to bias the discussion though...

2. Nov 11, 2010

TFF

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One aircraft I work on has the battery going directly to the starter solenoid first but a jumper goes from the hot side of the start solenoid to the master. Starter gets the straightest shot that way.

3. Nov 11, 2010

djschwartz

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There are two connections to a starter. One operates the solenoid to engage the starter. This is a moderate current circuit, typically 10 - 20 amps depending on the starter, and is routed through the master switch so that the starter cannot be accidentally engaged when the master is off. The second connection is the power for the starter motor itself. This is very high current, as much as 100 amps, and comes directly from the battery to the starter. There is a sample aircraft wiring diagram is this presentation on EAA Chapter 902's web site under "Builder's Corner".

http://www.eaa902.org/WiringTips.pdf

4. Nov 11, 2010

DLrocket89

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That works, thanks!

...I was figuring on a starter being more like 500 amps.

5. Nov 11, 2010

djschwartz

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The starter will draw a brief surge as it first engages and begins to turn, and it can also draw a lot of current if it stalls. The amount of current you need to plan on, and thus the main starter current wire size does depend on the engine. It also depends on the starter. The newer lightweight starters actually draw less current for the same cranking ability than the older ones. 100 amps is a reasonable amount to plan on for a smaller engine (O-200, O-235, etc.) If you're starting an IO-540 with an older starter you might want to plan on 200 amps. You would probably want a bigger battery as well.

At 500 amps a typical 25 AH light aircraft battery would be completely dead from a fresh, full charge in 3 minutes, assuming it could deliver the current continuously without damage.

6. Nov 11, 2010

DLrocket89

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Current plan is actually to use a Mazda rotary conversion, so whatever those starters use...

7. Nov 11, 2010

djschwartz

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Hmmm. Don't know anything about those. My only advice there would be to look at the wire sized used in a car with that motor if the conversion uses a similar starter. Or see what has been done by other builders using a similar conversion.

FWIW, if you look at the standards in AC43.13b you'll see that a #4 wire would be good for 100A over a moderate run from the battery to the starter. That's what I have in my O-200 powered Luscombe with a SkyTec starter and the battery behind the seat. A #0 wire would be good for 200A over a moderate run or 100A over a long run. That's what I have in my IO-360 powered Stephen's Acro that has an older Lycoming starter and the battery back at the tail for CG.

8. Feb 12, 2011

cosy

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This contact is usualy not for a load about as you wrote, but good for 3 to 5 A.
The solenoid is a special one in case of a car-type starter with "flying wheel", they can consum about 10 A. Because there are two magnetic elements to drive: first the solenoid contact to feed big power to the motor of the starter, second the magnetic pull mechanism to engage the "flying wheel".

9. Feb 12, 2011

Toobuilder

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Many airplanes have the battery solenoid very close to the battery (often at the rear of the aircraft), in an effort to reduce the amount of "live" cable in the aircraft. The battery solenoid feeds the main battery cable some distance to the firewall, where the start solenoid is located. In this case, the starter current does run through the battery solenoid, but since the battery contacts are closed, there is little chance of damage, even at very high amperage.

BTW, I have no problem turning over my IO-360 with a Odessy 680 battery in the aft baggage compartment and #6 cable. I'm using a geared, Hitachi style mini starter from a big block Chevy.

10. Feb 13, 2011

Dan Thomas

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The Skytech permanent-magnet starters draw less than the old Prestolites, and depending on the model, the Skytec can draw as much as 285 amps. The old prestolite could eat 400 amps, IIRC. See Lycoming Starters

Wiring diagrams are here: Experimental Wiring Diagram

There's a good reason certified aircraft run all current through the master solenoid. If there's ever a short anywhere in the airplane, you can shut the master off and stop the electrical fire. If you had the starter connected directly to the battery and you developed a short between ground and the cable that runs between the battery and the starter, you wouldn't be able to shut it off in flight. Can be fatal. If the starter solenoid stuck closed (hung starter, not an unknown phenomenon) and the starter wouldn't disengage, you wouldn't be able to stop the prop turning without either waiting for the battery to die or getting out and disconnecting the battery, whose cables and terminals would be REALLY hot.

The other reason all current is run through the master solenoid is so a pilot has the ability to disconnect the battery in the event of a forced landing. Torn metal cuts wires and makes sparks that set fire to spilled fuel.

FAR 23.1361 regulates certified aircraft, and the only circuits permitted to be connected directly to the battery must be protected by a fuse or breaker of five amps or less.

Dan

11. Feb 13, 2011

Jman

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Excellent Points Dan. Thanks for that.

12. Feb 13, 2011

cosy

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Please pay attention on the currents with soleonid! The statement from Dan I'll apreciate but please note that a solenoid as (most used in aviation)
White Rogers type 70 and 71 has following specs:
Voltage: nominal 12V (up to 14 V)
inrush current: 150A max.
continuos current: 80A max

A solenoid can easy switch in at nearly no current but switch off is a big problem if current is high: the connector "glues" onto the contacts, the relay mechanism will creait a huge voltage peak and an electric arc (solenoid internal).
A current of some 400 A needs a big solenoid. I would never run the 12V strip thru the Master solenoid. The short part under Voltage from Batt to the starter contact can be done really save and large enaugh to not to fail or heat while in use.
As fare as I know a (mazda) rotary engine has a biger torque for a starter then any four stroke engine with same volume. The reason for that is while 4-strokers does 2 turns in order to get fired each cylinder one time, the rotary fires 3 times each rotation, so for the same two turns it engage not only four contractions but six. And this is why torque is higher

13. Feb 14, 2011

Toobuilder

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The probability of having to open the battery solenoid while cranking the engine in flight is so small as to be insignificant. If this happens, you have bigger problems than a welded set of contacts.

I'm not familiar with the rotary starters, but I would bet that they are the typical Hitachi or ND gear reduction units that are used on everything these days. These things have far less current draw than the old direct drive Delco units. Uness you have something really unusual, common aircraft practice of using the relays in series should prevail (IMHO).

14. Feb 15, 2011

Dan Thomas

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Failure on the ground can present significant risks, too, as I pointed out earlier.

The typical solenoids as Cosy posted in his link have manufacturer's current limits that are regularly violated by the aircraft manufacturers. Sticking to the published inrush current will get you the rated 100,000 operations, but for the light airplane this is overkill. Not many light airplanes will start 100,000 times in their lifetimes. Thoes solenoids usually have copper discs pulled down onto copper studs, and the copper oxidizes with age and loses its conductivity. A few suffer burn-welding, which can cause the hung-starter problem. As the Director of Maintenance of a flight school, looking after eight airplanes, I have learned that it's prudent to replace aging (but still working) solenoids even on airplanes with low total time because of the oxidation problem. Oxidation causes heat, which leads to welding. A $30 solenoid is a lot cheaper than a$1200 starter that burns out when the solenoid sticks shut. BTDT. The stuck starter was of a type that has a silent overrunning clutch (O-200) and as the engine was running, and the pilot wasn't paying attention to the big discharge showing on the ammeter as the alternator tried to keep up with the battery's discharge, the starter got hot and cooked itself. Brand-new starter, too. Old solenoid. The electrical noise in his headset finally caused him to shut down but the damage was done. If he'd had a starter solenoid directly connected to the battery he could not have stopped the prop on shutdown until the battery was dead and some wiring maybe cooked. Maybe a fire. Starter wiring is rated for intermittent loading, not continuous, and will get plenty hot.

Some of us learn things the hard way and then try to help other people avoid that hard way.

Dan

15. Feb 15, 2011

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