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rick_od · 12-10-2007, 01:25 PM

This is a crazy one. I have 3 different (small) inverters from the same manufacturer. I get 113VAC when the engine is off and when I start the engine, it drops to 103.5VAC. I installed one to operate my fridge when in transit and it doesn't like to operate under 108VAC.
What would cause the voltage to drop when the engine is running ??

rick_od · 12-10-2007, 01:25 PM

This is a crazy one. I have 3 different (small) inverters from the same manufacturer. I get 113VAC when the engine is off and when I start the engine, it drops to 103.5VAC. I installed one to operate my fridge when in transit and it doesn't like to operate under 108VAC.
What would cause the voltage to drop when the engine is running ??

M&EM · 12-10-2007, 01:48 PM

Answer may depend on where the invertors are supplied from. The coach or chassis.
If chassis, then maybe starting motor loads chassis battery voltage too low for the inverters to maintain output. Then they can't get back up to normal. If tied to an ignition switched accessory point, that point may momentarily go off during starting.

rick_od · 12-10-2007, 03:20 PM

M&EM
1 inverter is the cigerette lighter type. The other 2 are hard wired to the coach batteries. But, when you turn the ignition key on my RV, a relay connects the chassis and coach batteries together so the alternator can charge all the batteries. So in responce to your reply, the inverters are connected to the coach batteries with the engine off and both the chassis and coach batteries when the engine is on.

exrench · 12-10-2007, 04:08 PM

Check the voltage reading to the inverter-since the fridge is probably a fair distance from the batteries the gauge of the wire may be a factor in the voltage to the inverter.You did not say in your post wether the fridge was calling for cold or not.It is normal for the fridge to load the inverter voltage down only when it it trying to cool.To get a true picture you must test AC voltage as well as DC supply voltage under both circumstances(1)fridge trying to cool.(2)fridge not trying to cool.The engine running may have nothing to do with the voltage- Larry

rick_od · 12-10-2007, 04:38 PM

Larry,
Thanks for the help. Sorry I forgot to mention...the AC voltage readings I mentioned were with nothing plugged into the inverter. Just the Fluke meter....no load at all. AC voltage drops as soon as the engine is started.

M&EM · 12-10-2007, 05:02 PM

Waveform coming out of the inverter is not sinusoidal and will give fluky readings unless using a true rms responding voltmeter. No pun intended.

rick_od · 12-10-2007, 05:18 PM

It is a RMS 'Fluke' meter. Before I used this meter, I used an analog 'Simpson' meter and got the usual lower 'modified sine wave' readings.

TandW · 12-10-2007, 05:38 PM

The AC an absorption reefer uses supplies current to a 300 or so watt resistance element. A 500 watt inverter is needed in this application. Good luck!!

rick_od · 12-10-2007, 05:56 PM

You guys are not understanding me.

I have an inverter plugged into the coach batteries. With nothing plugged into the inverter (ie. NO LOAD), and the engine off, the inverter produces 113 VAC. When I turn the engine on, which in my RV, automatically connects the coach and chassis batteries thus charging all batteries from the alternator, and STILL nothing plugged into the inverter (ie. NO LOAD), the inverter output drops to 103.5 VAC.

I AM using a good quality comercial grade RMS 'Fluke' brand meter. I AM aware that using a junk meter on a modified sine wave inverter will give lower than normal readings, that is why I am using a quality meter.

The question really is, if the DC voltage increases slightly when the engine and alternator are running, why would the AC voltage decrease from the inverter....STILL NO LOAD ON THE INVERTER (except for the 'fluke' meter of course) ??

Thanks
Rick

M&EM · 12-10-2007, 06:18 PM

Try putting a load on the inverter such as a 100 watt light bulb.
On my large inverter, if there is insufficient load to keep the inverter in output mode it goes to idle and the rms goes down.

BTW, There is a difference between RMS and true RMS responding meter. Nothing to do with junk!

NeilV · 12-11-2007, 12:08 AM

Rick,

What are the capacities and brands of the inverters. The inverters voltage regulation may be in steps and you could be hitting the bottom of the next step due to the voltage increase when running the engine.

rick_od · 12-11-2007, 01:34 AM

M&EM,
Didn't know there was a difference...looked at my Fluke and it was a true RMS....Thanks

NeilV,
All 3 are Vector. 2 are 400/800 waats with 800 being peak. The other 1 is a cigarette lighter type that is 95 waats. The label on the fridge says 2.5amps 300 waats.
All 3 react the same as far as AC voltage output with the engine on or off and all 3 were tested with nothing plugged into them (ie. no load).
Maybe I'll try plugging something into them with the meter and see what happens.

Thanks
Rick

Bob (WA0MQE) · 12-11-2007, 01:49 AM

Rick, 2.5amps 300 watts doesn't sound right for an AC load on a refrigerator. I'd say it should be closer to 7 to 9 amps on AC. That's closer to 800 to 1000 watts.

2 amps is closer to the DC requirement of the refrig. The DC is required whether you're running AC or on Propane.