Noisy DC motor

System · July 2008

Hhi

I have a Solar Chill evaporative cooler in my house and have been running it with at 120 volt AC benchtop 12 volt regulated DC power supply (the cabling to run the cooler on DC from my battery bank would cost $250 due to the the amp draw/distance involved and the power supply was $29!!).

This has worked very well for the last two years. Last night while the unit was powered down, I splashed water onto the power supply and rather than risking a certain "smoke check". I connected my small 12v Battery charger and used it to power the cooler for a while.

The problem is that the cooler being run on the battery charger, is very much noisier, a sort of "growly howl" comes from the motor. Very unnerving! I ran it only a very short time until I got the the regulated DC power supply dried out.

I also noticed in the battery charger powered mode that the current draw is almost one third less than using the regulated power supply.

Obviously I would like the best of both worlds the energy savings of the battery charger and the quietness of the regulated power supply!

Thoughts?

Thanks

Mike

BB. · July 2008

Re: Noisy DC motor

A regulated Power Supply has filters/circuits (capacitors and inductors, etc.) to give you a "smooth" flat DC voltage...

The typical AC battery charger just has a full wave rectified output--which in the simplest case is just the AC wave form with the "negative" AC wave form flipped over... So now you have a 120 Hz modified sine pulse train--typically, with the Vmax=sqrt (2) * Vdc...

If the motor is a "universal" type AC/DC motor, the rectified DC should not be a problem (same as a brushed drill motor). If the motor is an electronically commutated type motor (similar to DC computer fans)--then the pulsed DC may be confusing it.

You could put a capacitor on the battery charger output--but then you may end up over voltaging the motor (by sqrt of 2) and the battery charger may become a very inefficient method of making the power (things get hot).

-Bill

Should add that many regulated power supplies are not digital switching supplies but use an analog output instead--which can also be less efficient.

System2 · July 2008

Re: Noisy DC motor

Thanks Bill

Makes sense on the power supplies. The regulated PS has a cleaner output at the cost of efficiency compared to the battery charger.

You are right, the PS is a cheapy analog type off ebay. A DC switching PS may be more efficient then, (a thought to pursue!)

The motor is a 12 volt, brushless, PM type. I erred on the side of caution because I believe the motor to be very pricey (more so than I care to replace right now!).

Maybe, I'll invest in some copper wire! I hate to turn perfectly good electricity to wasted heat any more than necessary.

Mike

BB. · July 2008

Re: Noisy DC motor

Mike,

Take a very close look at your DC motor's specifications... 12 volt (or any voltage) battery systems have a very wide operating range (for example ~10.5 to 15.5+v--dead battery to equalizing a cold battery bank)...

It may be possible that the output of a simple battery charger with a filter cap may not be much worse--you can always connect it up and test (perhaps use another 12 volt load so you don't accidentally kill your $$$ motor).

If you have, for example, a 24 volt bank and a 12 volt load, there are devices that can efficiently down convert the DC voltages (switch mode power supply--the DC equivalent of a transformer--or the same unit can take a 12 volt battery and run a 24 volt load). I think this one is unregulated (simply 1/2 the 24 volt battery voltage)--but they also have other versions too.

-Bill

mike95490 · July 2008

Re: Noisy DC motor

For best efficiency, use a 120V AC - 12V DC switcher power supply. They are fairly common, and are usually pretty efficient >80% for new designs.

Even computer supplies, are good, regulated, and have a 12V circuit, if you have a computer nearby, you may be able to sneak power off it.

What is the power draw on the fan ??

example 12V 2.5A $35 ZWS30-12 75% eff.
http://dkc3.digikey.com/PDF/T082/P2288.pdf

System2 · July 2008

Re: Noisy DC motor

Bill, Mike

The whole shebang draws about 12 amps at 12 to 15 volts.

Took the battery charger apart and found that it does NOT have any big ole caps, hence the dirty power and noise.

I guess I'll look at buying a switching power supply, maybe off ebay. Building a PS with a fullwave rectifier and a big cap is not so difficult, so I guess I'll look into an appropriate transformer. Looks like 9 volts and 180 watts is a good number for a 14 volt output.

Only drawback to flea bay is most everything is from China and I really would like to buy American if possible.

Mike

icarus · July 2008

Re: Noisy DC motor

Wouldn't it be simpler to just run the wire?

Tony

mike95490 · July 2008

Re: Noisy DC motor

That's a fair amt of power to transport at 13V.

here's a AC-DC supply 90% eff
http://www.newark.com/jsp/search/productdetail.jsp?SKU=85K4826
bit pricey at $150

System2 · July 2008

Re: Noisy DC motor

Bill/Tony

Indeed it is! Much larger than I originally appreciated!!!

Using our old friend the voltage drop calculator, it works out to AWG 2 wire over a run of 150 ft to get 3.11% loss or better. WHEEEE!!!! At local prices, a couple hundred bucks or more to purchase the wire, then the conduit, and breaker, and disconnect box, and surely the yada yada!

90% efficiency strikes me as pretty good for $150.

As it is, the little bench top PS is working well in the current four day, 107 degree, hot spell.

I'm just offended at having to draw 20 amps off the batteries when I can get the job done for 11 or 12 amps.

Making my own is probably not going to be. A decent toroidal transformer is $80 to $100. A full wave rectifier and cap is not so much, but I can easily see spending a couple of bills for a nicely turned out unit. Boxes, switches indicator lights warning beacons, audible horns, etc. Plus I'm so damn slow and finicky... everything has to end up a Taj Mahal, dammit. A little OCD I suppose!!

Thanks guys

Mike

BB. · July 2008

Re: Noisy DC motor

Since you have 120 VAC available--can you retrofit an AC motor into your cooler?

-Bill

icarus · July 2008

Re: Noisy DC motor

BB. wrote: »

Since you have 120 VAC available--can you retrofit an AC motor into your cooler?

-Bill

Bingo!

Once again, Bill wins!

Tony

System2 · July 2008

Re: Noisy DC motor

In a word, efficiency!

Must'a never seen a Solar Chill Evaporative cooler! It's all Stainless steel, and uses 8" Celdek media. With the 12 volt permanent \magnet motor it produces 4000 CFM airflow while drawing only 120 to 150 watts of power to cool my 850 sq. ft off the grid house.

It uses a 24" four bladed fan instead of a squirrel cage blower for efficiency. It also has an automatic timer controlling the water pump to pump water for one minute and then off for three for maximum cooling without waterlogging the cooling media.

Bottom line is, it uses about 1/4 the electricity than my old conventional swamp cooler used. Works really well especially when I have to run it 5-6 hours a day during this hot spell. It'll be 107 degrees again today.

thanks

Mike

mike95490 · July 2008

Re: Noisy DC motor

There is no reason for a DC motor to be any more efficient than an AC motor. maybe a few percent, but not 30%

System2 · July 2008

Re: Noisy DC motor

Right, Mike. There is no free ride! The DC motor is way smaller!

The cooler is more efficient because it is better optimized for the job at hand. The 12 volt motor in the Solar Chill is FAR smaller than 1 hp motor in the old swamp cooler because everything else is designed for efficiency, as I mentioned, the fan, the motor, the evaporative media, the water system are all more tuned to their purpose.

My Trimetric shows 10 amps running the Solar Chill direct off the battery bank, and 15 amps running off the 12 volt power supply.

My old Adobe Air window mount cooler showed 40 amps draw off the battery on the Trimetric.

The difference is dramatic! I was saw a sustained 30 degree temp drop through the cooler last night.

Sadly, I haven't been able to contact the manufacturers in the last few weeks. A niche may have closed.

Mike

Noisy DC motor

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