OT: Two 12V motors in series?

Here's my problem: I have a 'project' that involves pumping small amounts of water in a circular fashion. The power source is 28VDC. Being economical, I am wanting to use two 12VDC 350GPH bilge pumps in series ( 2 X $15.00 ) rather then one 24VDC 700GPH pump (> $100.00). The volume of water flow is not that critical. What I'm wondering is if OHM's law applies to the Resistance through two motors like any other resistive load? I haven't purchased the pumps yet, or I could bench check them.

Won't work unless you can guarantee that the load on the motors are exactly the same all the time and that the motors are super well matched.
I'd still be leery of doing it, though, as you might get a really unstable system with slow oscillation between the motors.
You don't say anything about the size of the motors, but a 28->12 volt regulator would be a much better solution.

Benta.

You can run them in series if the loads are equal. Ohm's law is not the controlling factor. The resistance of the windings will be low, which is why motors draw a large starting current. When the motor turns, it generates a reverse voltage that reduces the current drawn to match the load. If the two pumps are not pumping equal amounts, the lighter loaded one will have a higher voltage across it and conversely the other one will be underpowered. That effect will tend to balance the two, but there could be some odd effects, such as surging with the load bouncing back and forth between the two.

Bill

Glad we agree, 9100 :-)

You may be already aware of this since your question was about connecting the pump motors in series and not placing the pumps hydraulically in series. But I'm going to talk a little bit about the hydraulics.

I assume these are centrifugal pumps and not positive displacement, since they are bilge pumps.

Two centrifugal pumps in series will not double the flow, it will double the head (pressure) and the flow will be the flow that one pump is rated for.

If you place the pumps in parallel, the flow will be additive but will not double. The two-parallel-pump flow rate will be the intersection of the system head-flow curve with the pumps' head-flow curve. For most systems and pumps you can expect somewhere around 30% to 60% more flow by running two identical pumps in parallel. The only way to get around this is to design the system head loss so that the system-pump operating point for two pumps in parallel is at the 700 gpm point.

If I'm preaching to the choir, I apologize.

Ed in Florida

The 12V pumps I'm considering supposedly have a 2.5A draw each. I think there is an "amp penalty" for dropping the voltage down. I have a 24-12V power supply (for another project) and it includes a large heatsink in the package. Not to mention the added complication & weight. I'm pretty much limited to 10A at 28.5V, or I will have to rewire......I might as well bite the bullet & find a suitable 24V pump. Thanks for y'all's advice.

and for Ed, yeah, I had planned to just 'T' the outlets of the pump into a common discharge. I'm basically circulating water through a heat exchanger (salvaged trans oil cooler), and back to the tank, in a loop. Not sure what the pressure drop across the cooler would be, but aren't centrifugal pumps (which they are) sort of 'self regulating? That is, do the electrical loads increase much if the flow IS restricted?
steve

What if you connect the rotors of the pumps ???
That might work

Peter from Holland

Steve;

Centrifugal pumps are sort of "self-regulating" but in a very defined manner, depending upon the specific pump design. In general, the more flow, the less the HP required. In general, the more head (i.e. pressure developed across the pump) the more HP required. But the pumps must operate within their flow and head design envelope or bad and unpredictable things happen (cavitation, fluid overheating, etc.)

But within the realm of centrifugal machinery, there are exceptions to the above and without seeing the pump curves, I cannot tell you how your pumps will perform. Being bilge pumps, they most likely have an open impeller and act more like vertical turbine pumps which often require MORE HP with more flow. This is what I would assume for your design purposes.

Your pump supplier may be able to supply you with a curve and get it if you can. But even without a curve, a bilge pump should have a specification that says something like: "350 gpm at 20 feet head". What that means is that when the system head (pressure drop from pump discharge to pump suction) is 20 feet (8.65 psi), then the pump will move 350 gpm. If the system head is less than 20 feet, the pump will move more fluid. If the system head is more than 20 feet, the pump will move less fluid.

In a situation like this, it is best to make your flow loop with as large a piping as practical and as few fittings as possible to minimize system head loss (pressure drop) and then install a valve at the outlet of the pump(s) so that if excessive flow occurs you can then throttle things back and get the pumps back operating on their curve.

As a general design guide, try to design your piping system to average 5 feet per second or less fluid velocity at the desire flow rate.

"Cameron's Hydraulic Handbook" is good reference for primary information on pumps and piping systems.

Ed in Florida

800 GPH doen't sound like a small amount of water to me. I am used to steam heat exchangers where 60 GPH ~ 100Kw ! Main point is that pumps like a positive head on their input because they are not very good at "sucking" water in. My other point is that I would put a resistor and a capacitor across each motor, not so much to provide a 12V supply to each one but rather to absorb commutation spikes and "holes". If any brush sparks, then the current through both motors drops to zero, so the other motor slows down as well. I would use 100 ohms and 2000mF 16 V working (watch the polarity).
FWIW I tried to use a pair of 12V computer cooling fans in series from 24V. They would not run. On examination of their "works" they used a magnet on their rotors with a chip and some printed circuit coils as their stators. Using the above trick did NOT work, I had to build a proper half voltage point using power transistors.
Frank

As usual, the simplest solution is the best....a little searching ( Google failed miserably, showing nothing but large manufacturing concerns, BING went right to a dealer) and I found a 500 GPH 24VDC bilge pump for $41.54, including shipping. (Draws 1 amp.) Within $15.00 or so of the 12V I found on ebay.

Thanks to everyone for their advice!

You might be better off to find a auto heater core. Trans coolers are high pressure normally 3/8". Heater cores are 5/8"-3/4" and would probably even cool better and have more flow.

Motors in series

Connect the Armature and Series (if it has a Series) to Armature and Series (if it has a Series) of the 2nd, Connect from 28 Volt the shunt of one to the shunt second.

The Shunt will be balance between the unit.
The Shunt may not be found in cheap unit.

Stan