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  #31  
Old 06-13-2018, 08:25 PM
Mich48041 Mich48041 is offline
 
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Bob N commented on the starter voltage spike myth HERE.
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  #32  
Old 06-14-2018, 05:31 AM
Mich48041 Mich48041 is offline
 
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The starter contactor in most airplanes is energized by a switch that gets positive voltage from a fused source. If the wire shorts to ground, the fuse blows and no harm is done. Wiring the starter contactor as described below is dangerous. If the wire shorts to ground, the starter engages.
Quote:
if the solenoid is powered directly from the bus, the grounding starter switch, on my aircraft at leased, is not fused.
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  #33  
Old 06-14-2018, 02:01 PM
Cumulo Cumulo is offline
 
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Quote:
Originally Posted by Mich48041 View Post
If the battery is so weak that it can not hold the master contactor closed (less than 1/2 amp), then how can the battery send hundreds of amps to the starter motor?
Well, a starter motor can be thought of as a constant current device. With a given load, more voltage = faster, less voltage = slower, aprox same current. Stalled starter amps is about the same as turning amps. This relationship will not exist with a fixed resistance load, but does with a DC motor.

Again, the internal resistance of a starter is only milliohms, so it can flow beaucoup amps with only a couple of volts. What limits the current of a starter motor is the generated back emf (voltage), not the motor resistance.

Edit: Yet again, if the starter pulls the battery voltage down to below the dropout voltage of the master solenoid , the master disconnects. It does not depend on the division of current, just too low of a bus voltage produced by starter current and the internal resistance of the weak battery.

Re: fusing a starter solenoid for safety, if I have the picture right, the fuse could protect the starter switch wire or in case of an internal case to coil short of the starter solenoid, but I have not been aware of any such failure. I would think the potential for such a failure is very low. Further, such a failure would likely occur just outside of the hanger door, not a 12k ft. So, maybe keep it simple is better in this case, but unlike Nuckoll I could be wrong.

Last edited by Cumulo : 06-14-2018 at 02:20 PM. Reason: clarity
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  #34  
Old 06-14-2018, 02:20 PM
rv7charlie rv7charlie is online now
 
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Yeah, you could. :-)

Let's start with why we install protection devices at the supply end of the wire (instead of on the device).

Explain that, and then we'll move forward.
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  #35  
Old 06-14-2018, 04:29 PM
Mich48041 Mich48041 is offline
 
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Stalled starter current is greater than turning current because there is no back EMF. That is why lights dim when motors start.
A stalled motor is a fixed resistance (but will increase slightly as the wire heats up).
Stalled starter motor current is directly proportional to the voltage.
-
The danger of switching the negative wire of the start contactor is that there is greater chance of a shorted wire energizing the contactor. Then the propeller could move unexpectedly and possibly strike someone. A switched negative wire will be two or three feet long and pass through the firewall. That long negative wire is exposed to danger.
Now compare that to a positive switched wire. It is still exposed to danger for two or three feet. But if it shorts to ground, the propeller will not move because the fuse will blow.
A person might argue that the chances of that wire shorting to ground are slim. True, but if using that reasoning, then no fuses or circuit breakers are necessary for any wires.
Many accidents happen due to a chain of events. That accident chain could be:
An electrical design error.
Vibration that wears wire insulation.
The master switch being left on.
Touching and moving the damaged wire.
A person standing in the propeller arc.
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  #36  
Old 06-14-2018, 04:36 PM
Cumulo Cumulo is offline
 
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Quote:
Originally Posted by rv7charlie View Post
Yeah, you could. :-)

Let's start with why we install protection devices at the supply end of the wire (instead of on the device).

Explain that, and then we'll move forward.
LOL , Yes the standard aircraft starter ckt is wired through a fuse. Autos and tractors aren't. And, yes, always protect the wire. (Genesis 41:30)

The issue(side) is: can a starter event damage small electronic devices?

The answer is yes. In the 1960's when marvelous solid state avionics began appearing, starter zaps caused havoc. Avionics/radio bus switches were then installed to isolated these delicate critters. When alternators became ubiquitous the problem faded. Serendipity.

The "kick" of a released starter is a NEGATIVE spike of whatever amperage the starter was drawing when released. If not snubbed it will generate hundreds of volts and possibly resonate into a positive excursion as well. The alternator diode bridge, if present on the bus, will effectively snub the initial negative excursion since the spike is shorted out by the alternator's 3 sets of 2 diode to ground. this will limit the spike to about 2 or 3 volts negative for about 100 millisec or so.

However, since diodes take a finite amount of time to conduct, some spike could sneak through but will be only microseconds or so. It would take some lab work to know how fast alternator diodes really are.
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  #37  
Old 06-14-2018, 05:32 PM
rv7charlie rv7charlie is online now
 
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Correlation cannot be assumed to be causation.

Can you cite lab tests demonstrating starter current spikes failing current technology a/c electronics? Be aware that there *were* lab tests demonstrating early solid state a/c electronics failing during the start sequence. But the cause wasn't flyback spikes from the starter. Hint: Silicon isn't the only substance from which you can make a semiconductor, and wasn't what was used in early solid state a/c electronics.
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  #38  
Old 06-14-2018, 05:41 PM
Cumulo Cumulo is offline
 
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Quote:
Originally Posted by Mich48041 View Post
Stalled starter current is greater than turning current because there is no back EMF. That is why lights dim when motors start.
A stalled motor is a fixed resistance (but will increase slightly as the wire heats up).
Stalled starter motor current is directly proportional to the voltage.
.
Yep, All true. The sequence usually is that the starter is drawing it's near normal current and due to the weak battery's internal resistance, the bus voltage falls, the starter slows, less back emf - high current, contactor drops out Still lots of amps flowing in the starter. BY the way, if you have ever been zapped by a starter as I have, I doubt you will be anxious for an encore.

And, yes , agreed , a fuse is the safest way to wire a starter switch. However, injecting the fuse issue into this tread is tangential.
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  #39  
Old 06-14-2018, 06:03 PM
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vlittle vlittle is offline
 
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Quote:
Originally Posted by rv7charlie View Post
Correlation cannot be assumed to be causation.

Can you cite lab tests demonstrating starter current spikes failing current technology a/c electronics? Be aware that there *were* lab tests demonstrating early solid state a/c electronics failing during the start sequence. But the cause wasn't flyback spikes from the starter. Hint: Silicon isn't the only substance from which you can make a semiconductor, and wasn't what was used in early solid state a/c electronics.
I still have CK722 transistors and 1N314 diodes. Hoping they make a comeback, like vinyl records. Anyone who understands the last sentence is ooold!
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  #40  
Old 06-14-2018, 06:48 PM
moosepileit moosepileit is offline
 
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https://en.m.wikipedia.org/wiki/CK722

Is that up the alley?

Break...

12V, one, two, 12V starter shock? How? Did you become the transorb/snapjack/diode?
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Last edited by moosepileit : 06-14-2018 at 06:50 PM.
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