[Richard]

Chester,

If you want to chat. Eight one seven 223 two 056

[Chester Stone]

Here is the latest in the saga of my A/C headache. Attached is a photo of a schedule I made of amp readings for both the front and rear units. The rear seems to be normal, 8 amps for compressor #3 and 11 amps for #4 when operated on high fan. The front unit is another story. Running at low fan, compressor #1 started at 13 amps, increased one amp per second, and shut down at 31 amps. I have to wonder why the 20 amp breaker did not trip. Running at high fan, #1 went to 23 amps and #2 went to 19 amps when #2 breaker tripped. In both low and high mode, after the compressors or breaker shut down, the amps were 4.6 until the thermostat was turned off. Note that the condenser fan makes a noise, but seems to put our sufficient air volume. Could a bad bearing in this fan cause the compressors to climb the amperage curve? Any help would be much appreciated.
   

[RussWhite]

Chester,

As I think though this latest information I need you to answer a question. The current measurements you report were "made at the breaker box", is that correct? I am asking because if you make them there it is impossible to separate the currents that are being consumed by the three motors fed from that breaker - 1. Compressor 2. Condensing fan 3. Evaporator fan

The current measurements I gave earlier in the thread were make at the air conditioner control board measuring each motor separately.

Thanks, Russ

[Chester Stone]

Russ, you are correct, I made the amp measurements at the breaker box at the breaker exit for the four compressors. I will have to look at the control board to figure out which cables to measure. I will do this in the morning. Again, thanks for all of your help.

[encantotom]

a little lesson in circuit breakers. russ, correct me if i get it a little wrong.

i used a square D trip curve and looked at a 20a breaker. a trip curve tells you for a specific size breaker how long it takes to trip at different currents.

a motor has a startup current that can be as high as 3x the running current for that motor. so if it is a motor that runs at 8 amps (just a number i picked), then at startup it can be 20-25 amps. but that is only for 1 second or less.

a 20a square D breaker is rated to run for 13 seconds at 25 amps.

if you started at 13amps like you said, and climbed 1 amp per second, i did some averaging. it would take 7 seconds to get to 20a. the rating of the breaker (no calculus needed to do that, ha). then at 25 amps (5 more seconds) the trip curve says 150 seconds to trip. at 31amps which is another 6 seconds (for a total time of 18 seconds) the trip curve shows it can run at that amperage for 65 seconds.

so some kinda awful averaging says that it could run for over a minute at that amperage without tripping the breaker.

btw, it can handle 100amps for 2.5 seconds (a 20a breaker)

i dont know what brand breakers you have but the trip curves for residential breakers should be similar.

russ, am i close?

tom

[bikestuff]

Chester,

Just wanted to let you know that I am following this thread closely. I used to have problems with both of my AC units, but they are *I believe* working correctly now. When I had a weird noise coming from the front AC unit, it turned out to be the blower fan. That was replaced. I also ended up with new control boards.

If you cannot figure out where to measure the amps on the control board, I am at my coach and can take a picture if you need it.

Cheers,
bill

[RussWhite]

Hi Tom,

Nice post. It is an easy misconception to think a 20A breaker trips right at 20 amps. Seems logical doesn't it. I wish I thought Chester's problems had anything to do with the breaker, but I don't. How he ever had 10 amps breakers in there I will never understand. It says all over the documentation and on the front of the A/C itself that each circuit requires a 20 amp breaker. But, we are beyond the easy fix now and into a more difficult problem.

Just for a recap - the control board with all its problems has very different results on the system than Chester is experiencing. When lands and relays and connectors fail on the board, the result is the voltage is not delivered to the motor and the motor just doesn't run when it is supposed to. The sad thing is this testing and high amps he is reporting will do no good at all to the control board, and could easily result in damage to the board. None of the switching components on that board are rated for the currents reported. I am worried about that.

Back to your primer on CBs. Locked rotor current and starting current are pretty much the same thing when voltage is first applied to a motor. My experience says at least 3 times normal running current, and often more. It seems to me that the only way these high currents might exist is through locked rotor on a motor. I do not believe that either of the fan motors not running could cause the compressor motor to produce such currents in the short time things stay on.

I will post a document with the wiring diagram and schematic. The wiring diagram will show what each connector goes to and where one would clamp wires to read the current one motor at a time.

But as I said, I am reluctant to recommend more testing as I fear for the circuit board. I have really only one last suggestion before I think it would be prudent to pull the unit from the coach to see what is going on. That is, replace the starting capacitors for all the motors. This can be done without removing the unit and actually might be successful if one of the motors is in locked rotor due to not starting up.

[Richard]

For those of you with scs frigette units, do they have taps for measuring pressure OR are they sealed?

A stalled condenser motor could cause what you are seeing. First, the stalled motor will cause a higher amp draw by it self, and the as we have talked about earlier, the compressor amp draw will climb sharply if the condensor fan is not blowing because the head pressure will spike. Gauges will tell you what is going on. It's hard to diagnose an ac problem without both electrical info AND refrigerant pressure info.

[Richard]

I agree with Russ. Put start caps on as the next step.

Those of us with dometic basement units would tell you the condenser fan motor bushings are kaput. It is a common failure. The motor does not have sealed bearing but an oilite (bronze oil impregnated) bushing instead. Only problem is that you cannot get to it easily to lube, so it eventually fails and the motor won't start. My symptoms were loud buzzing from basement unit,, followed eventually by tripped breaker. Replaced the start cap, added hard star cap no go. Replace fan motor and all was well. I too was freaked that I had the dreaded locked rotor on the compressor, but it was the fan motor.

[folivier]

Sorry to hijack but if anyone needs one I have a brand new fan motor for the Dometic basement units.
PM me if you are in need, if not I'll put it up for auction in Spearfish