JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

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JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby mantry » Mon May 16, 2011 7:41 pm

Howdy, we have noticed that our JASCO 50 AMP Alternator that was installed many years ago has started developing a weird problem....

I had noticed before that on takeoff/climbout that the AMP meter was showing a discharge but once leveled off the next time I looked it was fine...

So the other day out flying I decided to "twiddle" with it some. If I run the RPM's at 2450, the AMP meter will show a discharge of 4-5 amps for about 15 seconds and then the Alternator will come on line and show just a few amps of charge and that will last about 5 seconds and then it will drop out and show discharge and this will repeat over and over.

But, however if I pull the power back to 2350 RPM then the Alternator behaves normal. I have checked for any LOOSE connections on the Alternator and the firewall mounted regulator but all looks OK.

Anyone have any ideas? I attempted to call SkyTronics today but the support person for the JASCO alternators was not in today.

Is this like an OVERVOLTAGE protection kicking in until it resets?

It doesn't seem to be influlenced by hot or cold.

Thanks in advance..
I'll try the SkyTronics support group tomorrow..

Mark
N3861V '49 Cessna 170-A
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby minton » Mon May 16, 2011 8:53 pm

I would say you suspicions are correct, Overvoltage trip and automatic reset. Any chance you could hook up a digital multi meter and go run it up??
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby mantry » Mon May 16, 2011 9:38 pm

Yeah, I kind of figured that was next.. May take me a few days to do that though...
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby gahorn » Tue May 17, 2011 12:01 am

This is very typical of WORN BRUSHES ! Replace them.
High rpm moves the brushes off the commutator.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight.
An originality nut (mostly) for the right reasons. ;)
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby minton » Tue May 17, 2011 12:18 am

Geo. could be right :?: :?:
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby blueldr » Tue May 17, 2011 2:19 am

George,
Alternator ---- Commutator?
BL
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby LBPilot82 » Tue May 17, 2011 3:48 am

Most common failure on any alternator should be diodes, (since there aren't any brushes.... right George? :wink: ). I would be suspicious of the regulator. We know that, for a short while anyway, the alternator is capable of putting out sufficient voltage which tells us that it's output should be good. Check wiring for clean connections first. I also like the idea of hooking up a meter too. I am running the same Jasco 50a alternator and skytronics regulator.
Richard Dach
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SN 18762
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby gahorn » Tue May 17, 2011 12:58 pm

LBPilot82 wrote:Most common failure on any alternator should be diodes, (since there aren't any brushes.... right George? :wink: ). I would be suspicious of the regulator. We know that, for a short while anyway, the alternator is capable of putting out sufficient voltage which tells us that it's output should be good. Check wiring for clean connections first. I also like the idea of hooking up a meter too. I am running the same Jasco 50a alternator and skytronics regulator.


Alternators have brushes. (Although I've never seen inside a Jasco unit, I've never seen an alternator that did not have brushes...I suppose it's technically feasible.) Dick is technically correct as regards "commutator" (normally used in conjunction with generators)... in alternators they are usually referred to as "brush rings" or "slip rings" but they serve the same purpose of transferring (or "commuting") current to/from a rotating armature or rotor. The term may be used with either type of generator, including "AC generators". :twisted:
Worn rings (or commutator) will have slight irregularities (not perfectly round) and a worn brush will have less mass and less brush-spring tension to keep them in contact with the rings/commutator.... so higher rpms may cause the brushes to "bounce" and reduce the transfer of current....resulting in a loss of efficiency or "discharge" indication.

Here's an illustration of two popular Cessna Alternators. You can see the two brushes within their holders on the right end of the rotors.

Cessna Alternators .jpg
click to ENLARGE click AGAIN to further ENLARGE


Skytronics claims to be the only authorized Jasco overhaul/repair facility in the U.S.
You may contact them at:
Skytronics Inc.
227 Oregon Street
P.O. Box 807
El Segundo, California 90245

Contact Numbers:
Phone:(800) 421-6846 US/Canada (310) 322-6284
Fax:(310) 322-6160
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight.
An originality nut (mostly) for the right reasons. ;)
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby c170b53 » Tue May 17, 2011 5:22 pm

What is happening under large loads at high RPM ? If a diode pair has failed then could the peaks of the working pair exceed the OV protection causing a trip? Just asking the question.
Jim McIntosh
Director, TIC170A
1953 C170B S/N 25656
1979 172XP
02 K1200RS
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby gahorn » Wed May 18, 2011 2:20 am

c170b53 wrote:What is happening under large loads at high RPM ? If a diode pair has failed then could the peaks of the working pair exceed the OV protection causing a trip? Just asking the question.


Usually if a diode-pair fails it manifests itself as a "hum" or "buzz" due to the vacant rectification....not a voltage spike.

The major design difference between a generator and an alternator is this:

Generator: Coils of windings on an armature are spun past the electromagnetic force-field of the FIELD windings which induces current in those armature windings. Since the windings approach and then pass-by the force-field... the current rises then falls in the winding...and as that winding then approaches the opposite magnetic pole in the field the current is induced to reverse itself (into AC current)...except for the fact that the communator/brush relationship is simultaneusly switched to the next commutator-bar, thereby keeping the current Direct and flowing in one direction as it leaves the brushes and is brought into service.

Alternator: The windings which are ordinarily spinning round and round on an generator's armature...are instead stationary (and why it's called a stator) and are on the outer-circumference of the alternator shell instead of on the inner-armature as in a generator. In an alternator the "field" windings are on a "rotor" which spins at the center of the unit. Brushes running on slip-rings bring (battery) power to coils on the rotor-blades making them into electro-magnets spinning past the stator windings. This is both a mechanical and electrical advantage of the alternator allowing it to produce more current at slower rotational speeds. (In other words, instead of a generator moving smaller windings inside a field of magnetism...the alternator moves a field of magnetism past larger/outer windings.) As the rotor brings opposite-polarity portions of the rotor past the stator windings... instead of a commutator with parallel bars mechanically switching the rapidly-reversing (AC) current into Direct Current.... the AC current induced in the stator is sent thru a rectifier-diode set which acts like electrical one-way check-valves and emits/allows only Direct Current into service on the system.

If a diode-set should fail...there would be momentary "outages" of DC current...not a significant voltage spike.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight.
An originality nut (mostly) for the right reasons. ;)
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby zaki213 » Wed May 22, 2013 7:36 am

The generator, a 35 amp Delco Remy, was replaced on my 172B with a 60 amp alternator back in 1976. The work done under a form 337 and Cessna service service kit SK172-22F. If you would like a Pdf copy of the paper work given your e-mail address and I will send it to you. The alternator provides amply power for IFR radio equpment( one comm, two navs, markerbeacon, transponder) nav lights, tail and belly strobes and two landing lights.
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Re: JASCO 50amp ALTERNATOR GOES OFFLINE AT HIGH RPM

Postby blueldr » Wed May 22, 2013 4:55 pm

I'd bet on worn brushes on the slip ring.
BL
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