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fairbank56 · 17-08-2011, 04:49

Quote:

Originally Posted by osirissail

- - But in all cases motors, etc. use "watts" not amps. As explained in several posts to consume the needed watts to continue to operate, if the voltage drops the amperage will increase to maintain the required "watts" needed.

Only if there is regulation at the load, which is most likely the case with the OP's refer and explained by senormechanico. With a dc motor, the current does not increase when the supply voltage is reduced. With a resistive load, the current decreases. It would be real nice if the load would magically maintain it's power consumption, but it doesn't happen without electronic circuitry to provide regulation. This is easily proven with some simple real world testing.

Eric

osirissail · 17-08-2011, 05:11

Marine refrigeraton systems that use the Danfoss BD50 or BD35 compressors are not "DC" systems. They are actually 120VAC compressors and have a "Power Module" that contains circuitry including an invertor to convert your DC supply to AC. They also contain control circuitry that turns off the unit if the electrical supply drops below a preset point. So as "seniormechanico" mentions the unit uses "watts" of power and if the voltage drops too much the power module will turn off the system just as your DC/AC invertor will do.
- - This is why the size of the DC wiring is very important for these type units to minimize any line loss voltage drop.
- - As a side note: Since these "power modules" contain an invertor they are very RF noisy which is why we have to turn off the refrig/freezers when using a SSB radio.

Captain Bill · 17-08-2011, 05:31

For those of you arguing that conventional DC motors increase their current draw as voltage goes down, you're wrong. They simply slow down. I've used voltage as a speed control on DC motors many times. Depending on the motor design one gets a given speed for a given voltage. The motor will draw more amps under load attempting to reach that given speed. Conventional AC motors are controlled by phase matching with the power cycle and will draw more amps as voltage decreases. That's why AC motors burn out in brown out conditions. Power companies may lower the voltage in a brown out but they don't change the Hz (50 or 60 depending on what part of the world you live in). The Danfoss compressor is an AC motor and the controller is a small 3 phase inverter. I agree that the expected behavior under reduced voltage conditions would be to draw more amps to maintain the power requirements of the AC motor. The controller is designed to provide constant voltage, Hz, and vary the amperage to match the load on the motor, allowing the compressor to run at a constant speed, all while dealing with variable input voltages on the DC side. The only way it can provide constant power to the AC motor at lower voltages is to draw more amps from the dc side.

fairbank56 · 17-08-2011, 06:24

Excellent post, and FINALLY, someone else who knows how dc motors operate.

The Technautics "Cool Blue" system uses a Danfoss BD35F compressor and according to the manual, it is a brushless dc motor. I just did some testing on one and it does draw more current as the supply voltage is reduced. This is because it has an electronic control module that has a dc-dc converter and regulator.

Eric

MarkJ · 17-08-2011, 06:47

Quote:

Originally Posted by senormechanico

I knew this thread would be full when I got home!

Gee. I thought it was a dumb question with a one post simple answer including D'oh!

Thanks for the help, everyone.

Its this afternoons job.

(unless its too hot.)

Mark

osirissail · 17-08-2011, 06:54

Quote:

Originally Posted by fairbank56

. . . The Technautics "Cool Blue" system uses a Danfoss BD35F compressor and according to the manual, it is a brushless dc motor. I just did some testing on one and it does draw more current as the supply voltage is reduced. This is because it has an electronic control module that has a dc-dc converter and regulator.
Eric

Don't believe everything you read from the Technautics company. I have two of their "Cool Blue" units purchased a decade ago and the compressors are Danfoss and they are 120VAC units. It took a lot of digging through Danfoss technical data to find this out. I did because the $250 price of the Power Module really shocked me. I couldn't understand what could cost that much in a little 2" x 3" by 1" plastic box.
- - Ask RParts for a definitive answer since they are pretty much the authorities on such things these days. But my "Cool Blue" units are AC compressors. http://www.rparts.com/product_info.p...cmpteho13vu640

fairbank56 · 17-08-2011, 06:59

A one post answer, what's that?

A brushless dc motor is essentially the same thing as a synchronous AC motor but really isn't considered AC because the dc motor is fed with switched dc, not sinusoidal alternating current.

Eric

Blue Stocking · 17-08-2011, 07:01

Quote:

Originally Posted by MarkJ

Gee. I thought it was a dumb question with a one post simple answer including D'oh!

Thanks for the help, everyone.

Its this afternoons job.

(unless its too hot.)

Mark

You are priceless. Are you a Zen master.

fairbank56 · 17-08-2011, 07:42

According to Danfoss, the BD35 is a brushless dc motor using an external solid state controller and three phase pulse technology to propel the motor. Don't let the "three phase" fool you. It's pulsed, or switched, dc, not sinusoidal ac.

Eric

osirissail · 17-08-2011, 08:06

Yep, you are quite correct! - I finally found a good Danfoss tech sheet at: http://www.ra.danfoss.com/TechnicalI..._pk100c902.pdf

Top of page 6 is the description "The BD compressors are fitted with a brushless direct current motor which is electronically commutated by an electronic unit."

Seems the things will run on just about anything from DC to AC depending upon that very expensive "electronic unit." And the compressors, etc. are now made in China . . . I did easily find tech info but it was in Chinese. Finally found an English one. Great "cut-a-way" diagram on page 1.

DotDun · 17-08-2011, 09:28

Let's stop the techno-babble gobbledygook and get back to answering the OPs real question.

For a set of circumstances, size/insulation of refer box, compressor characteristics, how much beer is inside, desired temperature, environment, yada, yada, yada, it's going to take X amount of power to keep the beer/eggs cold. It doesn't matter if the compressor slows down/runs longer or draws more current to maintain speed as the batteries are depleted, that's included in the compressor characteristics.

A portion of X is consumed to heat the wire from the power source to the unit in question. Since heating of wire isn't normally considered a desired feature, it behoves one to eliminate as much of the wire heating as possible.

Let's pick a number of 50ah/day using 18awg wire to keep the beer to the desired temperature. Let's use the voltage drop calculator and ampacity tables at:

Genuinedealz - Technical - Calculators

It really doesn't matter if the calculator/tables are totally accurate, so don't reply stating you disagree with them, they do demonstrate the main point, the effects of voltage drop in wire.

Back to the 50ah number. Per the tables, ABYC allows 18awg wire to handle 20amps when placed outside the engine space. We also see that 18awg copper wire on a 20 foot one-way run @ 12v & 5amps has 10.97% voltage drop. In our theoretical scenario, that means that 5.5ah/day is consumed heating wire, leaving 44.5ah for keeping the beer cold. If you substitute 8awg wire for the smaller 18awg we see only 1.08% voltage drop, equating to .54ah/day heating wire, added to the 44.5ah for cooling the box and you are now consuming ~45ah/day for your refrigerator. Yes, heating wire costs you $$.

So, pick your poison, leave the small wire (and drink the beer) or spend the $$ and do the work to install bigger wire (and save the beer for tomorrow).