RV a/c fan starts very slow, anybody familiar with the issue?

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uncle sticky
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RV a/c fan starts very slow, anybody familiar with the issue?

Postby uncle sticky » Mon Aug 21, 2017 3:30 pm

I've got a dometic 12,500 BTU roof mount a/c. Pretty standard for a smaller trailer/caravan. I was having a hard time getting my generators to turn the a/c compressor over, so I installed a hard start capacitor in the unit. Now, when I start it up, shore power or generators, the fan starts very slow. Doesn't matter whether it's on fan only, or fan plus a/c, it seems to take a few minutes to get spun up. I don't know if the capacitor changed the way it works, I fucked up (although the fact that it works suggests it's not wired backwards), or it is just coincidence. Local RV shop said it sounded like the motor, not the compressor, was going bad. Of course, they sold me the capacitor, and a new a/c, but I haven't installed the new a/c.
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Token
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby Token » Mon Aug 21, 2017 5:15 pm

Not enough data to say.

This can get complex so you gotta give more info on where you installed the capacitor.

Some types of single pole motors are sensitive to the increased impedance a cap will create.

Can you split the connection to have the capacitor service just the compressor and put the fan in front of the capacitor, closer to the generator?

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uncle sticky
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby uncle sticky » Mon Aug 21, 2017 9:25 pm

Hmmmm, there I did it per instructions? Once I got to the wiring, there were extra spade connections that I plugged into. Makes sense that plugging the capacitor into the fan/motor line rather than just the compressor line might cause the fan to run slowly.
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby Token » Mon Aug 21, 2017 9:53 pm

There are two types of capacitors out there.

PTC Thermistor based that disconnect the capacitor after thermal criteria are reached.

Potential Relay based that use back EMF to charge a relay that disconnects the capacitor.

If the fan eventually spins up to full spec, methinks you may have the Thermal version that is perhaps oversized for the application.

You gotta check the details.

Basically, you inserted a bigger capacitor in the circuit and sounds like it takes a while for the relay to take it out of the circuit. So your impedance is higher for a while before the relay kicks in.

The hard start cap had the capacitor and PTC connected to one side of the cap, right? You just inserted two wires?

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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby BBadger » Tue Aug 22, 2017 3:56 am

This is purely from an electrical standpoint, but is there a large resistance inline on the charging circuit that causes it to take a long time to charge the capacitor (i.e. increasing time constant)?
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby uncle sticky » Tue Aug 22, 2017 11:34 am

So yes to the question! I didn't remove anything, and saw only the one resistor, which I think is a combo unit, start and run. The one I installed had a cylinder, with a smaller square attached to the top. When I installed, all I did was secure it with a strap, and plug in two spade connectors. So it sounds like it's doing it's job, and not messing up the fan, but it's taking a while for the hard start to get up to temperature, at which time it shuts off and lets' the other one go to work.
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby Just_Joe » Tue Aug 22, 2017 12:26 pm

I rarely make mistakes, but...
I installed a hard start cap in my newish trailer this spring and experienced a similar issue.
I then bought the manufacturers suggested cap, climbed back up and when I went to replace it, realized I had wired the first one to the start and run posts, not the start and common.
I installed the new one correctly and my Yammy 2400 now fires up the 13.5 AC at 7000' elevation.

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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby Traveller in Time » Tue Aug 22, 2017 12:29 pm

How about the mechanical standpoint: can it be there is some dust or lack of lubrication in the bearings of the fan?

Should easy turn a few cycles when moved by hand.
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby Token » Tue Aug 22, 2017 1:06 pm

There should be a wiring diagram in the unit itself where it shows the spot where a PTC hard start plugs in and also an alternative diagram for the potential relay as an option.

You may need to connect a couple wires together if you do not have the potential relay in the circuit. Usually a purple and black but it really depends on make and model.

Back EMF is reactive, out of phase, so can in effect reduce the net RMS voltage and cause the slow-down.

You got any electrical wizkids local to climb a ladder and take a peek?

This kind of stuff is real hard if you don't see the gizmo and wiring.

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uncle sticky
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby uncle sticky » Tue Aug 22, 2017 3:21 pm

Thanks. A buddy is an electrical engineer, I may have him take a look. Sadly, he deals mostly in micro voltages, and I worry he'll kill himself with household or higher, but we're not That close.
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby Token » Tue Aug 22, 2017 8:56 pm

uncle sticky wrote:Thanks. A buddy is an electrical engineer, I may have him take a look. Sadly, he deals mostly in micro voltages, and I worry he'll kill himself with household or higher, but we're not That close.


Oooooh, that is a dangerous thing.

I know first hand.

Was electrician before I got my EE in small circuits.

An EE don't teach you how to stay alive with 120V AC.

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uncle sticky
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby uncle sticky » Tue Aug 22, 2017 9:17 pm

Well, like I said, he's not that good of a friend...... I'll try and get a picture tomorrow. Might help. Thanks for the help.
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby FIGJAM » Wed Aug 23, 2017 6:11 am

Video will be funnier. :twisted:
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uncle sticky
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby uncle sticky » Wed Aug 23, 2017 2:02 pm

[img]IMG_20170823_080636.jpg[img]
[img]IMG_20170823_080721[/img]
[img]IMG_20170823_080813[/img]

So now that I've gotten back into the a/c, the wires go from the run capacitor directly to the hard start capacitor.
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uncle sticky
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby uncle sticky » Wed Aug 23, 2017 2:05 pm

IMG_20170823_080813.jpg
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby Token » Wed Aug 23, 2017 2:50 pm

Well dammit, looks easy as pie and I see white to white, red to red.

Plus you got the genuine Dometic part.

Doesn't make much sense.

Is the HSC doing its job for the compressor? Staring easy with genny?

Only other thing to poke at is if the run capacitor is worn down.

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uncle sticky
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Re: RV a/c fan starts very slow, anybody familiar with the issue?

Postby uncle sticky » Wed Aug 23, 2017 3:49 pm

It works, even with my crappy Smarter tools 2000 watt (ha! liars!) generators in tandem. The low fan speed may not be related, just proximite. Seems to work, just takes a little while for everything to spin up.
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