DC Pumps? Why isn't anyone using them?

[Doctor mcboy]

it may make some difference but probably not much. keeping the head pressure down will help more. those are small pumps. if you go to larger pumps the power requirements will be higher. thus draining your system faster. when i was doing the solar thing i found out buying a dc to ac inverter then buying a efficient ac pump worked better than dc pumps. go figure

 

[jfoster38122]

I have a magdrive 18 that I used to use for a return pump when I had a 125 reef tank well over 10 years ago and I just found it in the garage and took it apart and rinsed dust out of the impeller and it started pumping water as soon as I plugged it in
I wasn't going to use it bc I thought it would be more watts than I wanted to run but I looked it up and it's 140 watts
I just bought a new pump for the pond that I'm stating in a few weeks when it gets delivered
I'll keep the magdrive as a backup pump
only downside is the cord isn't that long bc it's made for aquarium use mainly but still will keep it as a backup and may get a cover for the plug that would be on an extension cord if I had to use it temporarily

 

[tarry]

I was mostly interested in [DC pumps] for the energy savings and the improved safety of low voltage dc. But now that I know [DC pumps] are actually AC and that low voltage can't (as far as I know) be protected by GFCI, that only leaves us with efficiency.

I am also not an expert on safety matters with electricity in ponds, nor am I an electrical engineer. I'm that guy with a little knowledge and too much confidence. So read on at your own risk!

TL/dr: 1) The DC pump is likely to be safer than an AC pump of the same capacity
2) You can buy a DC GFCI, though I don't know how easy they are to install/use


GENERAL POINTS:

There are two things to consider: the amount of current you are exposed to, and the frequency that the current is changing at. More current == bigger effect on muscles and cardiac pacemakers (the pacemakers your are born with, not the manmade ones!). A little current is no problem (you can feel it, maybe it hurts/burns, but otherwise does not harm you), but as you increase the current, at some point you interfere with the heart.

For 'house current', at around 10-20 mA your muscles contract so that you cannot let go. Around 40-100 mA is enough to cause cardiac issues.


DC VERSUS HOUSE CURRENT:

How does DC compare with house current?

First, house current is 120 volts RMS, which is 5-10 times the voltage of a 12v or 24v DC system. With higher voltage you can drive larger currents, so this makes the DC system safer than the AC system.

Second, muscles and pacemakers respond more strongly (are more sensitive) to 60 Hz pulsating current than continuous current. The effect is big -- a factor of 3 to 5. So this also makes DC systems safer than house current, which is 60 Hz AC.
(These values are repeated in many locations across the web, often without any citation provided. Here is a publication that gives references; I have not looked them all up, but they seem reliable:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/


DC MOTOR VS AC MOTOR:

A DC motor reverses the current direction in order to drive the motor. Does this negate the DC advantage? No.

If the current leak occurs 'upstream' of this reversal, than you are exposed to DC, not AC -- the DC system retains its advantage.

If the current leak occurs 'downstream' of the reversal, then you still have the advantage that the voltage is lower.

I am not sure at what rate the DC current in the particular DC pumps we are discussing is switched at. Many DC motors are switched at very high rates, substantially over 1 KHz. At these frequencies of modulation (e.g., 5 KHz), the thresholds for muscle contraction and cardiac arryhmias are roughly comparable to DC. So the DC motor retains its safety advantage over the AC motor.

DC GFCIs:

Are there DC GFCI's? Yes, absolutely. The general principle -- is the amount of current flowing in the two conductors equal, or is more current going out than is coming back -- can be applied to AC or DC. Of course, you cannot use a GFI designed for AC on a DC circuit.
And I cannot speak to the cost or ease of installation of DC GFIs. But they definitely exist!

For more information about GFCI's, see:
https://www.westernautomation.com/wp-content/uploads/2016/03/GFCIs_For_AC_DC_Systems.pdf

 

[fishlips]

All other concerns aside

I think there is something fishy with the power stated on the pumps. If you take this standard power converter 24v 4amps mix in a little ohms law 120vac*1.8amps= 216 watts of power used on input side. the out put side is 25vdc*4amp= 96 watts. That is a drop in power that you can feel with your hands, as it feel warm and heat is power lost. I think It was said here that is converted again back to AC with a different HZ. With a power lost again?

I did find a eco-twist that reported only 200w used at 110v for 5300 max flow that is lower then my random power converter and I don't know how good the pump is.

If the industry stranded is to report the pump power usage that might make it look like power saving.

Now I have not studied ohms law or electricity for over two decades so I am open to learn.

 

[tarry]

120vac*1.8amps= 216 watts of power used on input side. the out put side is 25vdc*4amp= 96 watts.

I agree, there is something is clearly wrong there. Notice that they give the input voltage as "100-240 v". So right off the bat we can see that they are not being precise; the current draw will depend on the actual voltage input. Presumably that spec is for the lowest voltage, so that's 180 watts. 53% efficiency is too low; I'd expect 75% or higher. And as you say, the excess 84 watts would make the casing very hot.

I see at least three possibilities. 1) It is an error. 2) They provide an overly-large spec because they are too lazy to measure it exactly. 3) The 1.8 amps is a transient when you first plug it in, and then it drops down (unlikely? but possible).

I don't think this has anything to do with the reported power usage of the pumps.

 

[fishlips]

It dose not effect the power rating of the pump, but to get the pumps reported power it has to go into a transformer and a controller. So we should forget the pumps power usage and see what the transformer is using. I did a quick goggle to see the transformers used. Not surprising there is no specifications on the sites I looked at and no pictures of the it. and the couple I did see that info was on the underside.