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Old 10-10-2019, 06:37 PM   #1
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Default ProAir Undermount compressor failure #3

I just had my third undermount compressor replaced. Indirectly, the cause is running the AC while on shore power. Under battery or engine it runs great. On shore power the compressor will cycle on/off in about 30 second periods. It only does this under shore power. Evidently, this overheats the compressor motor (I have a 24 volt system) and after a short time it is dead. The motor shows signs of overheating.

Pro Air cannot explain why this happens. I'm an electrical engineer and do not have have a clue... They are pointing to the inverter/charger. Maybe, but I can't think of a failure mode that would cause harm to the motor. The charger has no problem charging my lithium batteries (CS2015).

Anybody out there have a similar story??
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Old 10-10-2019, 08:16 PM   #2
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Weird. It almost has to be voltage-related, somehow.

I would recommend setting up some kind of data logger and observing the voltages present at the compressor under the various power-source scenarios.
Arduino, maybe?
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Old 10-10-2019, 08:17 PM   #3
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I just had my third undermount compressor replaced. Indirectly, the cause is running the AC while on shore power. Under battery or engine it runs great. On shore power the compressor will cycle on/off in about 30 second periods. It only does this under shore power. Evidently, this overheats the compressor motor (I have a 24 volt system) and after a short time it is dead. The motor shows signs of overheating.

Pro Air cannot explain why this happens. I'm an electrical engineer and do not have have a clue... They are pointing to the inverter/charger. Maybe, but I can't think of a failure mode that would cause harm to the motor. The charger has no problem charging my lithium batteries (CS2015).

Anybody out there have a similar story??

You need to get a view of the power cleanliness on it for all three sources and what is different between them. Dirty power is often a cause for motor overheating. It probably has been cycling on the thermal cutout on the motor and they are not designed for that.


Is it a DC motor or a variable frequency drive?
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Old 10-11-2019, 02:27 PM   #4
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Does the presence of DC voltage on the lithium module charge line from the inverter/charger trigger a relay in the lithium module that promotes the use of one DC source over another?

I recall some discussion about the charge port and discharge port on the lithium modules and which port the the inverter/charger output was connected to. IIRC RT might have changed the recommendation of which port to connect to at some point.

Maybe someone here with more knowledge of the Ecotrek modules can enlighten us.
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Old 10-11-2019, 04:13 PM   #5
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Does the presence of DC voltage on the lithium module charge line from the inverter/charger trigger a relay in the lithium module that promotes the use of one DC source over another?

I recall some discussion about the charge port and discharge port on the lithium modules and which port the the inverter/charger output was connected to. IIRC RT might have changed the recommendation of which port to connect to at some point.

Maybe someone here with more knowledge of the Ecotrek modules can enlighten us.

It does make one wonder about something like that, perhaps the charger isn't even in the circuit at all. Is the shore charger actually big enough to run the AC continuously. Some chargers also put out really dirty power at full load, too.


The fact that it is OK on battery alone would remove bad battery from consideration, so it would have be not supplying the power, I would think.


Does the Proair, use the DC directly or through variable AC drive, which should clean up any power issues and also shut off if low voltage or bad power. If straight DC, it makes more sense, and would very possibly be Marko's suggestion.
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Old 10-11-2019, 08:10 PM   #6
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I don't think the charger side of those inverters has high enough output to start the A/C compressor. I seem to recall them being fairly low output compared to better known brands like Magnum or Samlex.

Looks like it could be a Benling compressor: DM18A7_Shanghai Benling Scroll Compressor Co., Ltd.

based on these two links for the 12V versions:

https://www.ebay.com/itm/ProAIR-12V-...YAAOSwUshdPaSE

https://www.ebay.com/itm/ProAIR-12V-...gAAOSwp3FdMy1g
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Old 10-11-2019, 09:27 PM   #7
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Quote:
Originally Posted by markopolo View Post
I don't think the charger side of those inverters has high enough output to start the A/C compressor. I seem to recall them being fairly low output compared to better known brands like Magnum or Samlex.

Looks like it could be a Benling compressor: DM18A7_Shanghai Benling Scroll Compressor Co., Ltd.

based on these two links for the 12V versions:

https://www.ebay.com/itm/ProAIR-12V-...YAAOSwUshdPaSE

https://www.ebay.com/itm/ProAIR-12V-...gAAOSwp3FdMy1g



If it is a pure 12v DC motor and the charger is one of the small, lower quality ones that they tended to use a lot of, I would have to guess that the batteries are at a lower voltage than the charger is set for and the charger is trying to be at (13.6 vs 14.7v maybe) so the charger is totally maxed out and probably putting a lot of ripple on the DC output to the batteries. Ripple will overheat DC motors in most cases quite quickly.
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Old 10-12-2019, 12:34 PM   #8
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In response to BOOSTER...

The motor is 24vdc, however it would not surprise me if that is internally converted to 3-phase AC. An earlier version of the compressor had a separate power supply to the motor with what looked like three power inputs to the motor.

The inverter/charger cannot supply the full 70amps that the compressor needs. It does lower the overall battery current draw by about 30amps (I have an ammeter in the RV). If sunny, another 10 to 15 amps is further reduced (24v).

I, too, am leaning towards the theory that the charger's output is dirty. Switching power supply noise may cause big issues the the compressor's electronics. If I had an oscilloscope I could check this out. But I still wouldn't have way to fix the issue...

This is what the compressor looks like:
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Old 10-12-2019, 02:08 PM   #9
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I'm sorry to hear of your problems. Your a/c system, if working properly, should be a worthy alternative to rooftop a/c's. Perhaps this is why it hasn't caught on, but I though this was a standard on many buses and shuttles. Reliability should have been proven by now.
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Old 10-12-2019, 02:11 PM   #10
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Quote:
Originally Posted by robkrohn View Post
In response to BOOSTER...

The motor is 24vdc, however it would not surprise me if that is internally converted to 3-phase AC. An earlier version of the compressor had a separate power supply to the motor with what looked like three power inputs to the motor.

The inverter/charger cannot supply the full 70amps that the compressor needs. It does lower the overall battery current draw by about 30amps (I have an ammeter in the RV). If sunny, another 10 to 15 amps is further reduced (24v).

I, too, am leaning towards the theory that the charger's output is dirty. Switching power supply noise may cause big issues the the compressor's electronics. If I had an oscilloscope I could check this out. But I still wouldn't have way to fix the issue...

This is what the compressor looks like:

Very interesting, thanks for the information. As you know well, DC powered motors are very often not DC motors any more, so that is always a possible thing to consider. Does the unit have multiple compressor speeds?


You might be able to get a rough idea of the cleanliness of the power going to the compressor by using a multimeter on AC to look for superimposed AC voltage on the DC lines. I have been able to see dirty power with my Fluke meter most of the time. Might at least be good for comparison of the shore to other power sources.


You may wind up finding out your only solution is to get a higher quality, and hopefully larger output, charger.


What charger is in there now?
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Old 10-12-2019, 02:58 PM   #11
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You will never see a true DC motor in a modern compressor. The reason is that DC motors generally require commutators and brushes. These are unacceptable for use in a sealed compressor, since (a) they cause contamination and (b) cannot be replaced in this application, as a practical matter.
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Old 10-17-2019, 04:14 PM   #12
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What brand and model RV is this on?
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Old 10-17-2019, 06:04 PM   #13
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Page 5 of the pdf for the 18A7 has a section called "driver parameter" which calls out "frequency" of 15Hz to 120Hz. Looks like this is an inverter style AC motor as several have suggested.

I agree with all who said that dirty power from the charger is causing the issue. A potential fix would be to add some kind of LC filter to the input, it would definitely help to get a scope on the 12V at the compressor to get an idea of what you will need.
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