Please help - how to vary the speed on a 12v pump

Thread Starter


Joined Aug 17, 2013
Hi all, im sure this question can be answered here on this site. Thankyou to those who can provide an answer. I'm a farmer and electronics is not my game!

I have a 12v DC pump that pumps fluid at 5.3L/min @ 50psi. I need a way of varying the speed of this pump so i can pump at a lower flow rate.

Can it be done without compromising the torque of the motor?

I want to be able to drop the flow rate to 1-2L/min but i need the pump to do this at 30-40psi.

Can a 12v DC pump be converted into a variable speed pump without compromising too much torque?

I hope this makes sense?


Joined Oct 25, 2010
We could turn this into a farmers forum:)! I've not done it so take my comments with a pinch of salt...... If you are able to use a Microcontroller (PIC or perhaps a arduino board would be easier?) you could use pwm to control the speed of the motor, from what I understand you still get good torque using that method. The pressure maybe governed by factors other than the speed of the motor though? What are you pumping?
Cheers Geoff

Thread Starter


Joined Aug 17, 2013
It's a diaghram pump. I'm using it to pump liquiid fertilizers and need to lower the flow rate to pump a required volume over a given time.



Joined Dec 8, 2012
I have installed large, pneumatic, diaphragm pumps, below a certain PSI; they just stopped pumping, period. Don't know about the torque required for electrical pumps. Sorry.

Thread Starter


Joined Aug 17, 2013
I would like to be able to adjust the speed randomly. A knob light a light dimmer switch would be fine.

When I refer to the torque I simply mean the ability of the pump to maintain adequate pumping pressure at a lower voltage. If it is simply lower voltage that will slow down the pump.

Think of a fishtank pump. Or a swimming pool pump - how would you go about installing a variable control switch to slow the pump (motor) down to give you a lower flow rate. Is this as simple as reducing the voltage? Would a DC motor still turn under lower voltage?


Joined Oct 3, 2010
I'm more electrically minded than mechanically minded so bear with me.

When explaining elementary electronic fundamentals, we electron-minded types often use the fluid flow analogy to explain the happenings of current flow. For this example, I'm going to be using it in reverse.

I think (and I could be wrong) that pressure is related to flow in the same way voltage is related to current; that is, in a way that cannot be overcome without changing the load. Your load is likely spray nozzles or something, that represent a fixed load. If you change the flow through this load, the pressure is going to drop, and that is related to physics, not electrical properties of the motor. If you want to decrease the flow and maintain the same pressure, you need to install higher resistance nozzles.

If you were to do this, then yes (barring intricacies of diaphragm pumps unknown to me) you should be able to slow the speed of the pump without losing torque, by using PWM.


Joined Oct 15, 2009
A "mechanical" solution used in the aquarium pump world is to install a "T" right after the pump outlet with a valve and then you can simply return some of the fluid right back into the tank thus reducing the output from the main pipe.

Or some pumps you can just restrict the outlet (never restrict inlet or it will cavitate) with a valve.


Joined Nov 23, 2012
Different nozzles are needed if you expect the same spray pattern at lower pressures. Since it is fertilizer, spray coverage is not as critical as herbicide.

In the chemical industry, pumping is common and as mentioned above, at some point, you simply lose torque and motor stops. To prevent this, you can keep the same motor speed and just use a recycle loop and a pressure control valve. this is a simple spring activated valve that compresses when you get too much pressure from your pump when flow is restricted at the spray head. Excess pressure causes material to be diverted back to the tank. That way, the spray head can be changed to be more restrictive without damaging the pump.


Joined Jul 18, 2013
If it presently pumps at 50psi I would guess it is a Gear type pump, not a vane or impeller pump?
If impeller you can limit restrict the outlet, but the RPM will Increase with a corresponding drop in current.
Gear pump is the opposite, drop the rpm and the pressure goes down, you should not restrict the outlet but provide a relief valve as mentioned.


Joined Sep 9, 2010
Or some pumps you can just restrict the outlet (never restrict inlet or it will cavitate) with a valve.
A ball valve in line is a very common "solution". You can tell if it's going to work if you have the curve (volume versus ∆P) for the pump.