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Post subject: Voltage & Amperage Readings - T182T and GPU, with A/C Posted: 23 Jun 2018, 12:38 |
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Joined: 09/05/16 Posts: 39 Post Likes: +8 Company: dow.cc Location: Memphis TN & Heber Springs AR
Aircraft: 2009 Cessna T182T
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I'd appreciate feedback on voltage and amperage readings, on a 2009 Cessna T182T hooked up to a GPU. This is a new airplane (to me).
I have a 50 AMP GPU. Yesterday, I was updating the G1000 database. All electrical was off, except the G1000. Had the GPU hooked up. Voltage was steady, per the G1000, at 28 volts. Amperage was a negative 5 amps. Is this a normal reading? Seems like the GPU would take all the load off the aircraft's electrical system.
I have an electric air conditioner on the T182T. When I turned the A/C on, voltage dropped to about 25 volts and a negative 55 amps, per the G1000. Is this a normal reading w/ air?
I have also tried a significantly larger GPU, used when starting King Airs, Citations, etc. Larger GPU held the voltage at 28 volts, but amperage draw was still in the 50-55 amp range.
Air conditioning has been installed on this airplane in just the last couple of weeks.
Folks that installed the air conditioning system are coming to check the A/C next week.
The manager of the shop that maintains my plane believes something isn't right. He's expressed concerns about putting an excessive load and shortening the battery life.
In the air, w/ the A/C on, the voltage and amperage readings on the G1000 all appear OK.
Thoughts? Suggestions?
Thanks!
David Dow, N5242U, 901/351-1718
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Post subject: Re: Voltage & Amperage Readings - T182T and GPU, with A/C Posted: 23 Jun 2018, 13:27 |
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Joined: 01/11/10 Posts: 3962 Post Likes: +4137 Location: (KADS) Dallas, TX
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Your master was on correct?
Master switch on and the backup battery has to be armed to link them both to the GPU port.
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Post subject: Re: Voltage & Amperage Readings - T182T and GPU, with A/C Posted: 23 Jun 2018, 19:45 |
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Joined: 01/11/10 Posts: 3962 Post Likes: +4137 Location: (KADS) Dallas, TX
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OK, humm, well maybe this...
When they put the A/C system in did they add a second alternator?
If so do you now have an alternator selector switch?
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Post subject: Re: Voltage & Amperage Readings - T182T and GPU, with A/C Posted: 24 Jun 2018, 12:14 |
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Joined: 12/26/17 Posts: 141 Post Likes: +74
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Sounds to me like your small (50 amp) GPU was inadequate or defective. It isn't clear in my mind what your readings were with the large GPU hooked up and the AC on. If it remained at 28 volts (which I think you said it did) and the amps were 50-55 that is showing you what the total aircraft system was drawing at that moment. I think your AC is fine and the large GPU is fine. If you can hook it up to the large GPU and power it up with AC on - post a pic of the panel readings it would be useful.
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Post subject: Re: Voltage & Amperage Readings - T182T and GPU, with A/C Posted: 24 Jun 2018, 14:38 |
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Joined: 12/10/07 Posts: 30761 Post Likes: +10757 Location: Minneapolis, MN (KFCM)
Aircraft: 1970 Baron B55
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If your concern is about the negative aspect of the current readout my guess (don't have any Cessna wiring diagrams) is that the external power connection ties into the battery side of the ammeter shunt rather than the bus side (more typical, at least for Beech). With that arrangement, any current delivered by the GPU to the aircraft loads (avionics, lights, A/C, etc) would appear as a negative current (i.e. supplied by the battery if no GPU was involved). If the wiring diagram for your airplane shows the external power connected on the bus side of the ammeter shunt then I would agree with your mechanic that something (specifically the external power connection) is incorrect. IIRC, Cessna used to include a basic wiring diagram in the POH that might be sufficient for determining which side of the ammeter the external power source is connected to.
That the A/C ups the amperage above 50 doesn't surprise me, in fact I would have thought the A/C needed more than that by itself. 50A @28vdc is only 1400W which translates to a bit less than 2HP at 100% efficiency. Typical electric motor efficiencies are far less than that and there are other loads in the A/C besides the compressor (e.g. evap and condensor fans, controller, etc) so it sounds like the compressor motor is probably less than 1 HP.
And it makes perfect sense that the voltage dropped when running the A/C with a GPU limited to 50A.
_________________ -lance
It's easier to fool people than to convince them that they have been fooled.
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Post subject: Re: Voltage & Amperage Readings - T182T and GPU, with A/C Posted: 24 Jun 2018, 14:45 |
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Joined: 12/26/17 Posts: 141 Post Likes: +74
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Have you tried leaving the Battery (Master Switch) Off, hooking up the big GPU, powering up the GPU and turning on the AC externally with the polished aluminum touch switch on the left side of the cowling right by the GPU connection?
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Post subject: Re: Voltage & Amperage Readings - T182T and GPU, with A/C Posted: 28 Jun 2018, 01:06 |
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Joined: 12/26/17 Posts: 141 Post Likes: +74
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David - When you are connected to the big GPU what you are seeing is normal and correct.
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Post subject: Re: Voltage & Amperage Readings - T182T and GPU, with A/C Posted: 03 Jul 2018, 00:13 |
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Joined: 12/26/17 Posts: 141 Post Likes: +74
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David - Are you familiar with how the plane can be connected to GPU (let's say by an FBO) prior to your arrival even with the cabin doors closed and locked and power up / turn on the AC and precool the plane? It is a really neat feature!
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