Doctor Herpes

Question:

– Hide quoted text — Show quoted text – What are the typical startup surge currents for a Dometic Duo-Therm BriskAir AC (model 57912.621)? The voltage in the campground here measures typically 116V, and the AC current nominally changes from about 2 A (fan only) to 8 – 9 A when the compressor starts, and the voltage dips to 114V.  No problem. However, I’m seeing some occasions where the current jumps to 45A, the AC fan slows, the compressor hums and fails to start.  The line voltage during these incidents drops to 102 (at power pole) – 104V (in RV) during this starting surge.  I’m on a 30A circuit. The peak starting current will be a little less than the locked rotor current or LRA.  the LRA of the compressor is expressed on the nameplate.  You can pull the cover off and look if you really want to know.  there will be an "LRA" with a number beside it on the nameplate.  I’m going to guess about 50 amps.

I’ll have to check for this when I climb on the roof.  This seems to agree with the currents I was measuring. You can calculate the impedance of the campground supply from the information you have provided.  The formula is: impedance = delta voltage/delta current.  Your voltage drops from 116 volts to 114 volts when you draw, let’s average your numbers and say, 8.5 amps.   the delta voltage is 2 volts and the delta amps is 8.5 so 2/8.5 = 0.235 ohms. This quite "soft".  Typically I’d expect the impedance at the end of you RV power cable to be 0.03 or less.

I guess being on vacation, I must have turned my brain off.  I should have though of this, but I had no reference for "good" values.  Is there something in the electrical code which covers this?  Campground power supplies are so variable, unfortunately. I think the actual "delta I" is more like 6.5A.  This would give 0.31 ohms, which is even worse. To check our math lets use the same formula to compute the starting voltage drop problem.  If your voltage drops from 114 to 104 during starting and the LRA is about 50 amps, then 10/50 = 0.2 ohms.  Given the estimated numbers, this is quite good agreement.  Since the voltage at the pole and the RV is practically the same, one may assume that the CG wiring is where the excess impedance (softness) is.

Yeah, but the campgrounds always say that it is the power company’s fault. Your AC is obviously on the edge.  I know that mine will start at 104 volts but probably won’t at 100.  I imagine your voltage varies that much as the load on the whole park changes so sometimes your AC will start and sometimes it won’t.

Fortunately it only drops this low when attempting to pull 45A.  This does not seem to be the normal situation, unless the compressor is short cycling, as mine was. I initially though that the campground’s power line was a bit ’soft’, but now I’m not so sure.  I suspect that the compressor is attempting to start under a load, and drawing the extreme current.  The travel trailer is only 19′ and the small size may be contributing to short cycling of the AC and causing this problem.  The AC is a non-ducted model with the standard roof vent mounting. Any ideas?  Fixes?  Will it smoke during one of these surges? Last question first, yes this situation poses a risk for the AC.  It isn’t that the motor will necessarily be  harmed by the low voltage – it probably won’t.  The problem is that after so many operations the Klixon thermal overload breaker in the compressor will weld its contacts on one of the no-start events.  When that happens, the LRA current will flow until something gives.  Hopefully the RV breaker but maybe the compressor winding.

Would you know if the Klixon breaker is accessible and replaceable?  I don’t know how long this problem has been going on – but I suspect some time.  If replaceable, I just may replace it for peace of mind. Fixes?  Best bet is to move.  If you can’t do that then you’ll need to do something to boost your voltage.  You can buy one of those very expensive autotransformers from $CW$ or equiv.  

I’ve already "moved", as I’m now back home.  :-(  I’ve seen the autotransformer in the CW catalog.  It looks to be $$, large, and heavy.  No thanks. – Hide quoted text — Show quoted text -If you’re handy you can build something very cheaply.  All you really need to do is boost your voltage about 10 volts.  To do that you simply need to obtain a 10 volt, 30 amp transformer, hook the 120 volt primary to the RV power and hook the secondary in SERIES with the RV supply, observing the polarity so that the voltages add. You can get such a transformer surplus for $20 or so from places like http://www.meci.com.  Mount it in a metal box along with a switch to cut it in and out of the circuit (30 amp DPDT switch from Lowe’s, etc – about $6) and install it near the breaker panel.  When the supply voltage is low, flip the switch to boost the voltage 10 or 12 volts.  When it isn’t, flip the switch the other way and switch the transformer out of the circuit.  If you’re interested in doing this I can supply you with a circuit sketch.  Of course, if you’re not comfortable twisting wires together then the autotransformer from $CW$ would be appropriate.  (I suppose that disclaimer is necessary for those who don’t know how to do things but criticize those who do….)

Actually, I’ve considered making one from my old Magnetek converter and a SPDT relay.  Should be quite easy, but I’m not so sure that I want to add another large gadget to the RV (due to limited storage space and weight), unless low voltage and AC operation becomes a more common problem for me.  I only used the AC about a week out of the last 6 weeks – and that was primarily for humidity reduction.  I generally just use the TurboMaxx Fan. There is another option you might want to check into.  There is a gadget available called a hard start kit or "kick starter" that attaches to the compressor terminals and generates additional torque in the motor.  This device helps tight compressors or compressors operating on low voltage to start.  The device is available from HVAC supply stores and costs <$30.  A kick starter is a good thing in general to have on the AC because it also lowers the peak starting current.  It will let you run on lower supply voltage or at the end of longer extension cords.  It is a simple 2 wire device that simply clips to the compressor terminals.

Is this the same item which Chris was referring to?  A capacitor and a PTC? For voltage as low as you’re reporting this would merely be a patch to cover over the real problem.  But it will be useful for other CGs with higher voltage.  Also reduces light dimming when the compressor starts.

Light dimming?  My lighting is all 12V!  The battery makes for an excellent brownout protector.  :-)

Response:

Last question first, yes this situation poses a risk for the AC.  It isn’t that the motor will necessarily be  harmed by the low voltage – it probably won’t.  The problem is that after so many operations the Klixon thermal overload breaker in the compressor will weld its contacts on one of the no-start events.  When that happens, the LRA current will flow until something gives.  Hopefully the RV breaker but maybe the compressor winding. Would you know if the Klixon breaker is accessible and replaceable?  I don’t know how long this problem has been going on – but I suspect some time.  If replaceable, I just may replace it for peace of mind.

Almost always this round black device is located in the compressor terminal box.  Just pop the box cover and there it is.  If you’re lucky there will be a Klixon number on the device.  If you’re unlucky it will be a house number.  If you can’t find the Klixon part number, I recommend getting one directly from the Mfr, as many times the house numbered parts are custom-tailored for the application.  I would NOT trust a cross-referenced general purpose part for this application. There is another option you might want to check into.  There is a gadget available called a hard start kit or "kick starter" that attaches to the compressor terminals and generates additional torque in the motor.  This device helps tight compressors or compressors operating on low voltage to start.  The device is available from HVAC supply stores and costs <$30.  A kick starter is a good thing in general to have on the AC because it also lowers the peak starting current.  It will let you run on lower supply voltage or at the end of longer extension cords.  It is a simple 2 wire device that simply clips to the compressor terminals. Is this the same item which Chris was referring to?  A capacitor and a PTC?

yep, same thing.  The original Kick Starter brand used a capacitor and a mechanical potential relay but all the ones I’ve purchased in the last couple of years have been solid state.  Frankly, I like the mechanical ones better. The PTC solid state ones require a bit of time to cool off and reset.  If you try to restart too soon, the kickstarter won’t have any effect.  The downside of the mechanical one is that orientation matters. For voltage as low as you’re reporting this would merely be a patch to cover over the real problem.  But it will be useful for other CGs with higher voltage.  Also reduces light dimming when the compressor starts. Light dimming?  My lighting is all 12V!  The battery makes for an excellent brownout protector.  :-)

Well yeah, if your converter design leaves the battery in the circuit.  Mine does.  My mother’s MH’s magnetek converter doesn’t. When AC is applied the relay transfers the 12 volt lighting from the battery to the rectifier. Lights very much DO dim with (not so) heavy loads like the AC starting or using the microwave.  Annoying.  Didn’t know which kind you had, which is why I mentioned it. — John De Armond http://bellsouthpwp.net/j/o/johngd/ Cleveland, Occupied TN

Response:

– Hide quoted text — Show quoted text – Would you know if the Klixon breaker is accessible and replaceable?  I don’t know how long this problem has been going on – but I suspect some time.  If replaceable, I just may replace it for peace of mind. Almost always this round black device is located in the compressor terminal box.  Just pop the box cover and there it is.  If you’re lucky there will be a Klixon number on the device.  If you’re unlucky it will be a house number.  If you can’t find the Klixon part number, I recommend getting one directly from the Mfr, as many times the house numbered parts are custom-tailored for the application.  I would NOT trust a cross-referenced general purpose part for this application. There is another option you might want to check into.  There is a gadget available called a hard start kit or "kick starter" that attaches to the compressor terminals and generates additional torque in the motor.  This device helps tight compressors or compressors operating on low voltage to start.  The device is available from HVAC supply stores and costs <$30.  A kick starter is a good thing in general to have on the AC because it also lowers the peak starting current.  It will let you run on lower supply voltage or at the end of longer extension cords.  It is a simple 2 wire device that simply clips to the compressor terminals. Is this the same item which Chris was referring to?  A capacitor and a PTC? yep, same thing.  The original Kick Starter brand used a capacitor and a mechanical potential relay but all the ones I’ve purchased in the last couple of years have been solid state.  Frankly, I like the mechanical ones better. The PTC solid state ones require a bit of time to cool off and reset.  If you try to restart too soon, the kickstarter won’t have any effect.  The downside of the mechanical one is that orientation matters. For voltage as low as you’re reporting this would merely be a patch to cover over the real problem.  But it will be useful for other CGs with higher voltage.  Also reduces light dimming when the compressor starts. Light dimming?  My lighting is all 12V!  The battery makes for an excellent brownout protector.  :-) Well yeah, if your converter design leaves the battery in the circuit.  Mine does.  My mother’s MH’s magnetek converter doesn’t. When AC is applied the relay transfers the 12 volt lighting from the battery to the rectifier. Lights very much DO dim with (not so) heavy loads like the AC starting or using the microwave.  Annoying.  Didn’t know which kind you had, which is why I mentioned it.

Actually, my original converter was the Magnetek 6332 which used a 12V switchover relay.  I’ve replaced it with the 7345RU which no longer uses this relay – the full converter output is applied directly to the battery & house circuits. Thanks for the advice.  I’ll be looking for the Klixon breaker and replace it for safety sake.  I’ll also verify that the capacitor and ptc device which Chris referred to is actually installed, and install one if it’s not.

Response:

<<..        It basically just needs to be in the return air flow- the should be a spot to hold the tube in the sheet metal of the inside assembly right at the front- it is just a small piece bent at a 90