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P-MAG Question

blueflyer

Well Known Member
I was reading the FAQs about the PMAG, and came across this one:
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Q: Will the P-MAG work when I slow the engine on final approach?

A: The P-MAG is designed to operate at less than typical in-flight idle speeds (1,000 rpm) . It will operate at 700 to 800 rpm, and sometimes less. If a particular craft is known to have an in-flight idle below the speed the P-MAG alternator will sustain the ignition, it should be noted so the pilot can stay above that speed if the emergency condition is ever encountered.

This minimum P-MAG speed is easy to ascertain on the ground. Your ground idle will be significantly less than your in-flight idle. When doing a run up, simply isolate the P-MAG side of the ignition (if not running dual P-MAGs), and gradually slow the engine toward idle. Note the speed where the engine quits (if at all). If this speed is comfortably less than your in-flight idle speed, you should be covered. If not, note the operating limitation so you'll be prepared in the even of a power emergency.
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I know some folks have their ground idle set at 500RPM or so, but can you get your in flight RPM's below 700-800? I would hate for my mags to quit on me if I chop the power for some reason.....what are your thoughts?
 
The FAQ answer is referring to the self generating power capabilities of the P-mag. It is my understanding that the P-mag will idle the engine slower than 7-800 rpm provided that "ship's" electric power is available as would be normally anyway. The internally generated power becomes primary power above 7-800 RPM.

Bevan
 
As long as the Pmag is supplied with ships power, it will not die below those quoted RPMs. Your particular engine/prop combination may or may not be able to sustain such a low RPM, but the Pmag will still fire appropriately.

Only when the Pmag is depending on its internal generation and has a loss of ship's power (emergency scenario?) will the RPM get low enough that the internal power generated will be insufficient to sustain firing.
 
My ground idle is set to ~500, and even with a C/S prop I can't see less than 1000 RPM in flight. Slowed to taxi speed, yes, but you would have to really work hard to get the prop to 7-800 RPM in flight. As discussed, the low RPM cutout is only an issue if you have lost all electrical power anyway.
 
got it now. Thanks for the clarification. The mouse wheel running my brain turns really slow sometimes!
 
Here's the deal...

The 114 series P-mags have an internal generator. Once they are spinning fast enough to produce enough power to fire the plugs, they automatically switch over to internal power. This is typically around 800 RPM +/- and the transition is seamless. You will not notice when they switch over and there is no way to know when they are running off of internal power.

This the key thing, when running on internal power and you throttle back while on the ground and the RPM drops below the magic number (around 800 RPM, as I said above) the ignition will stop firing the plugs. The part you must understand is that while throttled back in the air, such as when on short final, your prop is still spinning faster than 800 RPM, regardless of what your ground idle speed is. Thus, you have a functional ignition that will work fine in the event you need to go around. However, once on the ground, unless you realize there is a problem, the engine will probably die on roll out. Not necessarily a bad place to have your engine die.
 
Here's the deal...

The 114 series P-mags have an internal generator. Once they are spinning fast enough to produce enough power to fire the plugs, they automatically switch over to internal power. This is typically around 800 RPM +/- and the transition is seamless. You will not notice when they switch over and there is no way to know when they are running off of internal power.

This the key thing, when running on internal power and you throttle back while on the ground and the RPM drops below the magic number (around 800 RPM, as I said above) the ignition will stop firing the plugs. The part you must understand is that while throttled back in the air, such as when on short final, your prop is still spinning faster than 800 RPM, regardless of what your ground idle speed is. Thus, you have a functional ignition that will work fine in the event you need to go around. However, once on the ground, unless you realize there is a problem, the engine will probably die on roll out. Not necessarily a bad place to have your engine die.

OK Bill - I know that you know a lot more about the P-Mags than I do... but my idle on the ground is less than 800 RPM's, and the engine keeps running, even when idling before shutdown after the flight. Are you really saying that the P-Mags don't switch back to ship's power when the RPM gets too low for internal power? Or is the internal power drop-off at a lower RPM?

I'm now confused!:confused:

Paul
 
Post #3 seems to indicate that the P-mag will only quit at low RPM if ship's power has failed.
 
I have called P-mag on this same question, With dual P Mags, with the power at idle on the ground (Rpm's around 700) when you cycle the mags, one would continue to run and the other the motor would quite. Thought it might be something wrong with the mag so I made the phone call. It was explained that the internal power generation RPM is not an exact science, they have seen the internal power kick on as low as 600 on some and as high as 800 on other, it will vary with each individual mag. This is not a concern because as said previously in the post that you cannot get the idle RPM that low in flight. With a IO-360 and a 3 blade Catto Idle RPM over the fence was around 1050 rpm.

Therefore for my normal runup, i would always do a mag check at 1600 rpm as well as right around 1000 to check both.
 
OK Bill - I know that you know a lot more about the P-Mags than I do... but my idle on the ground is less than 800 RPM's, and the engine keeps running, even when idling before shutdown after the flight. Are you really saying that the P-Mags don't switch back to ship's power when the RPM gets too low for internal power? Or is the internal power drop-off at a lower RPM?

I'm now confused!:confused:

Paul
Paul,

Yes, you are correct. The P-mags convert back to ship's power when the RPM's drop to a point where the internal generator can not support the ignition. This happens automatically and unfortunately the P-mag does not report its power source, internal or external. Thus, the EICommander cannot tell you which power source it is running off of. It does report the voltage but this is the voltage on "the board" and not its source.

As stated by some of the follow-on posts, it really depends on your P-mag. For some reason the exact cut off seems to vary between P-mags. That is why it is important for each individual to test their P-mags at some point after installation so they know what to expect.

Regarding runups, I perform mine at 1400 RPM and due to my installation, I test them prior to each flight using this sequence.

That will test the internal generator but it really doesn't test to see if ship's power is reaching each ignition. This is because the P-mags automatically cut over to the best power source. Meaning, I can drop ship's power and that test will tell me if the internal generator is working or not but since I start on both P-mags, it will not tell me if both are receiving ship's power. The only way to test that is to leave one switch up, turn the other off, and reduce my engine RPM below the magic number and see if the engine keeps running. Then turn on the previously off ignition, turn off the previously on ignition with the engine still at low idle RPM and see if it continues to run.

I hope that clears up your confusion.
 
Some people also don't realize that one difference between the 113 and 114 is the source of "backup" power. In the 113, backup is the internal alternator while the ships power is primary. The 114 is the opposite. And Brad tells me both have the same alternator fitted.
 
Some people also don't realize that one difference between the 113 and 114 is the source of "backup" power. In the 113, backup is the internal alternator while the ships power is primary. The 114 is the opposite. And Brad tells me both have the same alternator fitted.

Correct and this is a great way to think about it.
 
simple test if you RTFM

For my setup (in accordance with the diagram provided in installation manual) it's both simple and easy to confirm:
a) the source of p-mag power (internally generated, or 12V bus),
b) if p-leads ground (shut off) correctly,
c) at what RPM internally generated power is sufficient.

I have a breaker attached to ship's power for each p-mag; with the engine operating at 1000 RPM, if I pull each breaker in turn, removing ship's power and the engine continues to run, I know each p-mag:
I. was running normally on ship's power (breaker ON),
II. can generate voltage (breaker OFF), and
III. can generate voltage sufficient to keep my engine turning (breaker OFF) at that RPM.

(After restoring both breakers), If I pull the left breaker and reduce the idle until that p-mag drops out, I have established my lower RPM limit for internally generated power/operations for that p-mag. After restoring the left breaker, I can repeat the procedure, and results, for the right p-mag.

For my configuration, cutoff for both is below 800 RPM.
 
For my setup (in accordance with the diagram provided in installation manual) it's both simple and easy to confirm:
...
That is a good pre-flight test but the issue is that there is no way to verify which power source they are operating off of in flight. This is not necessarily a bad thing but it would be nice to know if you have an electrical issue or not.

Also, as I mentioned in a different thread, I had a P-mag generator fail right at 500 hours. I caught it on the next run up but it would have been nice to know in flight that one of my P-mag's had a generator fault. The good thing is that the P-mags never skipped a beat when that single generator failed and it switched over to Ship's power.
 
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