question for the electrically inclined

[dmurdock]

I have a small generator which will start and run the a/c in my travel traielr with no problem. I'm guessing it starts fine due to capacitors, however, I would like to verify that my a/c is getting enough voltage and is not being "browned" out while it is running. Is there some kind of guage I can plug into an electrical socket in the trailer which will give me this info? I have a real fancy multimeter with about a million different settings, would this work? (hehehe, i've just used it for continuity testing so far... )

 

[JWhitcomb]

I see meters that plug in as you described. I see them in RV supply catalogs.

 

[Don M]

What is the total output of the generator in watts?

What is the current draw in amps of the AC unit?

General rule for AC appliances that need to start motors is to multiply the current draw by about 3 for starting current.

Some guys say you can get away with less. I like a fudge factor.



If your genny is big enough you probably have little to worry about with a single AC unit.



your multimeter is not the greatest thing to measure amp draw. You need an amp probe to measure it safetly. If you diont have one... the data plate on the AC unit will almost always give you the current draw in amps.



Don~

 

[M Barnett]

Also be sure to not use a small gauge extension wire between the A/C and the gen. Wire can only carry so many amps.

Michael

 

[DMalone]

Use your meter and make sure you are getting a constant voltage. If the voltage drops too low it can cause problems. Anything lower than 10% below the voltage i would be concerned about. All so if you can borrow amp probe meter it would be good to check and make sure the A/C is running within nameplate amps. As the voltage drops the amps go up, thus using more power and at some point the heat is transfer to the wiring and all componets. There are amp gauges and digital meters that you can install so that any time you can monitor at a glance.

 

[B.G. Smith]

oltage

Put the multiimeter on AC volts and plug the leads in accordingly, check the voltage in one of the outlets in your home to get an idea where the meter is reading, should be 115 to 125 volts. Be careful while doing this (don't touch the bare leads) as you have the common side of the circuit in one hand and the hot line in the other. Now check the voltage on your generator while it is running unloaded, start the Air Cond. and set the thermostat so you know the compressor is running and check the voltage again. I think most RV AC units can stand voltage down to about 105 volts without damage but if the voltage is that low you need to know what the ac cycles are. What sometimes happens with a generator is that a large load will slow the motor so that the cycles per sec. will drop also and if they drop much you will burn up your unit. You can check the rpm of the generator with a hand held tachometer or a reed type vibration meter. An Onan 4kw unit should maintain 1800 rpm, some of the industrial type units run 3600 rpm. Ain't no electrician but I used to change clothes next to one when I had to work for a living. bg

 

[Tim1]

dmurdock, in the latest issue of jc whitney that i recieved they have a nice voltmeter that plubs into a wall socket an has a range from 96 to 135 volts. It costs 16. 95 and the order number is 48uy2463B Its a good looking unit and will tell you if you are having a voltage drop real quickly. I agree to check the nameplate of the cooling unit for amp draw. If your gen set is 2000 watts, just divide by 100 giving it a 20 amp rating. Thats pretty close. Tim

 

[redramnc]

B. G. has a good point about checking the frequency of the AC under load. You check it the same way you do for AC voltage, just select the frequency setting on the meter. The nameplate of the AC unit will probably say "60 Hz" somewhere on it - the generator nameplate shoud match. Motors can tolerate a little variance but more than 1 HZ +/- (<59 or >61) needs to be checked in to. I would be surprised if you find more than a . 5 Hz variance. It is a quick fix until you can get someone to look at it but you can set your rpm on the generaotr by looking at the voltage output under load and adjusting rpm to bring it in spec.



With the solid-state regulators these days most gen sets are pretty stable and work properly as long as they has fuel and oil available to run on.



However, it doesn't hurt to check.

 

[Skydiver]

Water and electricity

Water and electricity really do not have much in common,but they behave much the same.

Volts = pressure

Amps = Quantity or gallons

Watts =total of above or total product produced

(I think the above is correct. It has been about 12 years since trade school so my memory may be slipping. The following I know is correct)

You can't get 80 gallons of water per minute through a 1/2 inch garden hose at normal line pressure. Just as you can't get 80 amps of electricity through a 14 guage wire at 115 volts.

I see alot of people that will "turn up" the speed of there generator not knowing the outcome.

A cheap and simple way to check frequency on a gen is to .

Get an analog 115 volt clock with a second hand. First plug it into the home currant and "test the clock" with a stop watch. Then plug it into the gen and do the same test over 5 or 10 minutes to check the speed or cycles of the gen. If the analog clock and the stop watch are recording the same amount of time your gen is running at the correct rpm.

Tim

 

[fest3er]

Originally posted by dmurdock

I have a small generator which will start and run the a/c in my travel traielr with no problem. I'm guessing it starts fine due to capacitors, however, I would like to verify that my a/c is getting enough voltage and is not being "browned" out while it is running. Is there some kind of guage I can plug into an electrical socket in the trailer which will give me this info? I have a real fancy multimeter with about a million different settings, would this work? (hehehe, i've just used it for continuity testing so far... )

In a sentence, yes, you can use your multimeter to check the voltage. Set the meter to A/C Volts. If it is not auto-ranging, set the range to at least 150 Volts. Turn on the generator and check the voltage at an outlet nearest (or on) the genset; it should be around 110-120 VAC. Then check the voltage at pseudo-random outlets in the trailer; the readings should all be within a couple volts of each other.



Now start turning on appliances and measure the voltage. At some point as you are turning on appliances, you may see the voltage starting to drop. As long as the voltage does not drop below the minimum required voltage of any appliance, you should be OK.



There is one caveat. Remember that air conditioners, refrigerators and other motors can draw a significant amount of current when starting up. If you are already drawing bear the max current capacity of the genset, this start-up draw could well drop the voltage below the minimum; this is a condition you should try to avoid as much as possible. (To be more specific, an occasional short drop in voltage shouldn't hurt anything; repeated often, you could be headed toward trouble.



A short definition of the limitation of a genset would be that it is designe dto produce only so many amperes of current. Once you try to draw *more* than that, the voltage will drop more than is desired.



Go through your TT and find the allowable voltage range of all the appliances. THen you will know how low you can let the voltage sag befire shutting stuff off.



I helped my brother wired in a 10KW Generac for his house in MI. When we tested it, we found we could easily run the range and oven, but trying to run the whole house as though it were connected to the AC mains would result in the voltage dropping to 104 or less. We didn't have a scope handy, so we didn't measure the frequency output.

Finally, in response to Skydiver, Watts (power) is Volts*Amperes (P = I * V).



Fest3er