24V DC pump characteristic at reduced voltage 1

[s0050506]

Hello,

I have 24V DC hobby pumps (10 meters, 20 liters/min manufacturer specifications).

I want to operate them at 12V to run them continuously at least 7 hours without overheating.

Application requires 10m head, 1.5 liters/min; want to put several pumps in series for this.

Question: how can I estimate pump characteristic at 12V?
Can I assume (5 meters, 10 liters/min) as a good estimate?

I want to determine how many to put in series.
I have no further specs about the dc motor.

Thank you in advance!
Sander

 

[Skogsgurra]

Half voltage usually doesn't produce half speed. The reason is that there is not only a counter-EMF, but also a resistive voltage drop.

On the other hand, if you reduce the flow from 20 l/min down to 1.5 l/min, you will have better than half the speed.

Without knowing more about the application, I think that two pumps is conservative and safe. Depending on friction being more or less dominant over static head, it could even be possible that you can do 10 m at the lower flow. Test with one first, if possible. Else, install two pumps.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[s0050506]

Many thanks!

Indeed, 10 meters is my estimate of friction in a coiled copper tube.
The coil discharges in open atmosphere, hence no static head is involved.

However I do not understand your reasoning behind "Depending if friction is dominant, you could do 10m at the low flow". I understand that at 24V and zero flow, I should get 10 meters from one pump.
So at 12V at least two pumps are required, am I wrong?

Best regards.

 

[s0050506]

I think I have been ambiguous in first sentence, sorry.
I meant: manufacturers specifications 10m at zero flow, 20 l/min at zero head.

Does this change your conclusions?
Best regards

 

[waross]

With a large shunt motor, at half voltage, field weakening may result in the same speed. The problem is that the available torque will be way down. Any load will result in slowing the motor so much that overheating is a serious threat.
You still need the same (Amps times Volts) to drive the same load and the Amps will be double.
Then, with small motors such as hobby motors the resistance that Skoggs points out becomes much more of a factor than with larger motors.
Without knowing more about the motors the effect of the resistance is unknown and may upset performance expectations badly.
The good news is that I have had good luck running larger pumps in series. They shared the load well and performed as expected, but they were running at rated voltage. I expect that your pumps will share the load well, that won't be the problem.
I would derate your pumps to a little less than half power maybe 30%.
Monitor your current and limit the current to rated full load current or less.
From the characteristics given I will assume that these are centrifugal pumps. You may have to restrict the flow to keep the current down.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

I think it is safe to assume that "24 V DC hobby pumps" are PM.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

I agree Skoggs. What was I thinking about? That leaves the motor resistance as the big unknown.

Bill
--------------------
"Why not the best?"
Jimmy Carter