I have a 12V DC pond circulation pump that I'd like to slow down. It runs off about 350W of solar, a solar controller, and a 12V LiFePO4 battery.

When running off 12V (actually between 13.2 and 13.8V) it draws approx 14 Amps.

In order to slow it down, I was thinking of dropping the voltage down to 8-10V, but have some questions:

1-The pump is completely submerged - will lowering the voltage like this damage the pump?

2-If I drop the voltage, will that also drop the current (reducing the overall power requirement)?

3-Now the big question: Assuming the answers to 1 and 2 are favorable, how can I drop the voltage without wasting much power (I know I could add a high wattage resistor, but that wastes a lot power). I'm looking for a simple circuit - doesn't need to be very efficient, but needs to be better than just dissipating power through a resistor.

I know I could also gate down the flow, but that also wastes a lot of power - perhaps even more than a resistor.

Thanks in advance.

 

Will the pump even start at 8V?

Using PWM and varying the duty cycle would probably be a better idea.

 

Using PWM, as dl324 suggested would likely be best.
It's reasonably simple and is quite efficient.

You could use a 555 timer configured as an astable oscillator with adjustable duty-cycle, driving a MOSFET to control the motor current.
If motor start-up is a problem, the full voltage could be applied for a few seconds before the PWM starts.

If that sounds like something you can/want to do, I can post a circuit.

 

Have you tried blocking the output to obtain the flow you want?
The current will normally reduce proportionately also.

 

You could use a 555 timer configured as an astable oscillator with adjustable duty-cycle, driving a MOSFET to control the motor current.
If motor start-up is a problem, the full voltage could be applied for a few seconds before the PWM starts.

If that sounds like something you can/want to do, I can post a circuit.

If it's easy for you to post, this sounds like what I'm looking for. Thanks

 

The only thing is the load remains the same, unlike constricting the outlet flow which will reduce the required load and current normally.

 

The only thing is the load remains the same, unlike constricting the outlet flow which will reduce the required load and current normally.

I'll give this a try - certainly easy enough to test. I would have thought that with increased resistance to flow, the power requirement would increase. But apparently not?

 

I'll give this a try - certainly easy enough to test. I would have thought that with increased resistance to flow, the power requirement would increase. But apparently not?

Only on certain types of pumps, gear etc, on typical vane impellor type etc, the current goes down.

 

The only thing is the load remains the same

Don't see that.
It you reduce the power to the motor and it slows down, then obviously there's less water flowing and a lower load.

 

If it's easy for you to post, this sounds like what I'm looking for.

Below is the LTspice simulation of the circuit:
C3 delays the start of the PWM so full power is applied during motor startup (yellow trace) and the motor starts at 14A, then dropping to the lower average value after about 2.4s (green trace).
The PWM frequency is about 1.3kHz as determined by the total resistance of pot U2 and C2's capacitance.
The duty-cycle of the PWM signal (and thus the average motor voltage) is determined by the wiper setting of U2.

N-MOSFET M1 can be just about any with a 30V or greater Vds rating, and a max on-resistance of ≤5mΩ (to avoid the need for a heat-sink).

 

PWM motor speed controllers are (relatively) cheap things you can buy on ebay etc, based on a similar 555 circuit.
40A versions are about $10. You could modify the board to add soft-start if necessary.