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Old 16-08-2011, 17:21   #16
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Re: Voltage Drop . . . How Does it Affect Amps ?

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The reason a motor runs hotter when running at slower than it's design speed is because there is less cooling effect of the rotating armature.
No, no, no...

fairbanks, that back emf stuff is all well and good for an unloaded motor. What happens when you put a load on it, such as a compressor? Does the current go up? You betcha it does...
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Old 16-08-2011, 17:21   #17
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Re: Voltage Drop . . . How Does it Affect Amps ?

The design of the internal electronics will increase the current to provide the same wattage. Also the capacity of the battery will also be reducing as the voltage drops. There are user selector resistor values to change the cut in & cut out voltage selected points to make the Danfoss units more useful on house batteries as opposed to the higher voltage of cars. Mark you are heading for the need of better performance obtained from LIFEPO4 batteries for house batteries!!!

In a pure resistive load & by Ohms Law the current is dropping exactly the same ratio as the voltage.

Regards Bill
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Old 16-08-2011, 17:46   #18
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Re: Voltage Drop . . . How Does it Affect Amps ?

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fairbanks, that back emf stuff is all well and good for an unloaded motor. What happens when you put a load on it, such as a compressor? Does the current go up? You betcha it does...
Sure it does, as long as the supply voltage remains the same. Didn't say it wouldn't. Wasn't talking about a motor with no load. The question wasn't about varying the load, it was about varying the supply voltage.

Eric
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Old 16-08-2011, 18:09   #19
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Re: Volt drop. How does it affect amps?

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So the battery drops faster?

Mark

You're not really expecting an answer are you?

I say no; an amp is an amp and power used is power used. But I'm not joining the technical battle beyoud this.

But since I would assume at the core of the question there is an unspoken problem, why do you feel there is a voltage drop that wasn't there before?
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Old 16-08-2011, 18:09   #20
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Re: Voltage Drop . . . How Does it Affect Amps ?

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But can I get a simple answer to this question?
At least the answer to your question is simple: No.

Michael
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Old 16-08-2011, 18:17   #21
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Re: Voltage Drop . . . How Does it Affect Amps ?

Mark,

Where are You?--I will send you some ice.
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Old 16-08-2011, 18:48   #22
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Re: Voltage Drop . . . How Does it Affect Amps ?

senormechanico has the most correct answer. The rest of us are just talking about dc motor theory which is probably not even what's being used. Need more info from the OP about his unit.

Eric
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Old 16-08-2011, 18:50   #23
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Voltage drop is a resistive load. Hence power is lost to heat in the cable. So to answer the OP yes the battery discharges a little bit faster.

Dave
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Old 16-08-2011, 20:01   #24
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Re: Voltage Drop . . . How Does it Affect Amps ?

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Mark,

Where are You?--I will send you some ice.
Thats the only answer I can understand

Can you email the ice?

Bash said "so if you're trying to talk yourself out of replacing wiring where you've HYPOTHETICALLY got too much of a voltage drop, don't."

I feel the consensus is that it would probably use more battery or run the compressor more slowly and less efficiently. So I do need to do something about it.

It isn't a new problem as I have felt there's a voltage drop on that side of the boat. Its just not quite so easy to work out why and how to fix it. One can't just run a new cable.

Thanks for everyones thoughts. Its always a learning curve. And Cost Vs Value. Bloody boats!!



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Old 16-08-2011, 20:25   #25
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Re: Voltage Drop . . . How Does it Affect Amps ?

Two different things - if you are measuring the voltage at the refrigerator compressor and it is significantly less than the voltage at the batteries, then you have wiring that is too small - Line loss.
- - If the voltage drop is observed at the batteries then your batteries are either too small or too old.
- - 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.
- - Since batteries also supply "watts" - even though we measure the battery capacity in "amp-hours" you should not drain the batteries at a faster "rate". That said if the supply of power in the batteries is decreased then the frig will drain the power in the batteries quicker - but because there was less energy available in the batteries. With the same size siphon a half full bucket will empty before a full bucket.
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Old 16-08-2011, 20:37   #26
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I'm an idiot so proclaimed. If the voltage drops the amp draw would increase battery drains more amps and discharges faster.the real challenge is where that battery should stop being discharged and how the cycles get figured.. That gets beyond me. I think you want fast full recovery and a limit on how low you take the bank. Is there a condition that is occurring that non idiots could help with. Like my batts are
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Old 16-08-2011, 22:10   #27
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Re: Voltage Drop . . . How Does it Affect Amps ?

I've been gone all day setting up for a fair.
I knew this thread would be full when I got home!

Think of it this way:
The electronic module sees incoming battery voltage and it knows from its software commands that it has a (usually 3 phase AC) motor to run and it's supposed to stay within a certain RPM range.
As the battery voltage drops, it therefore ups the incoming current by reducing its input impedance by increasing the duty cycle of the Pulse Width Modulation of its inverter section.

A lot of electronic items such as a Bullet M2 long distance Wifi unit do that.
Mine is rated from 8 to 24 volts, but the power consumption is fairly flat over that range. The same thing goes for Sailor's Solutions LED lighting. and newer Bernina sewing machines with PWM power supplies.
There is heavier current draw at lower voltages and less at higher ones, but the power consumption stays virtually the same.
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Old 16-08-2011, 22:19   #28
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Re: Voltage Drop . . . How Does it Affect Amps ?

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Originally Posted by MarkJ View Post
One can't just run a new cable.
Can't just run a new cable?

Well... err..... ummm.....

Read this: kollmann-marine.com :: View topic - Number One Danfoss BD compressor Problem
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Old 16-08-2011, 22:33   #29
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Re: Voltage Drop . . . How Does it Affect Amps ?

The battery will drain faster.

The wire (or connections) are loosing 0.7x 4=2.8watts.
To those who do not feel the battery will drain faster where does the 2.8w that’s heating up the wire, instead of cooling down the fridge, come from?
The other concern is the lifespan of DC motors becomes shorter (when run at load) if the voltage is too low. 11.7v is getting a bit low and may be is at a level where the fridge life will be noticably shorter.

Some voltage drop in the wiring is inevitable, but the more its minimized the longer your batteries will last.
0.5v (some people use 3% boat builders often use 5%)can used as the maximum acceptable voltage drop, but if it is practical lowering this is beneficial especially on items that contribute heavily to the energy budget such as the fridge.
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Old 16-08-2011, 23:17   #30
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Re: Voltage Drop . . . How Does it Affect Amps ?

0.7v is not unusual, but is too high. Check the drop in the + and wires separately to try and isolate the problem. Use the multimeter on volts one end on the + battery terminal the other on the + connection of the compressor. Repeat for the negative. Repeat for smaller sections of wire (say between the battery and fuse panel) to further isolate the loss.
Then first step is cleaning all the connections, but the most likely cause is wiring that is just a bit too small for the load. Replace the wiring with thicker gauge or leave the existing wiring in place and add an additional wire, particularly over the longer runs.
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