# Running Two Brushed 12V DC Motors

1. Aug 26, 2016

### Bryan Deily

I am trying to figure out the best way to hook up two 12v DC peristaltic pumps to run as close to the same speed as possible. The manufacturing tests had the pumps running within 10ml per minute which is acceptable; when I hook the pumps up in series they are not even close. When hooked up in parallel they are in sync however they run intermittently 1 second on 1 second off. I am assuming I'd need more volts or amps due to internal resistance of the motors? The individual motors are 12v 3a max 1.5a continuous. The power supply I was testing was the same 36w. Do I need to get a 72w(12v 6a) to run both in parallel?

Also, what would be the best way to get the motors running dead in synchronized? I'm looking to use these pumps for a water exchange; pump old water out and new water in. Ideally the volumes are identical to keep parameters from creeping over time.

2. Aug 26, 2016

### billy_joule

Yes.

Without feedback (measuring tank level or flow rates) you won't get them balanced.
A cheap float switch (>$10) would do. Or a different design, why do you need to pump out? Why not an overflow? Alternatively, you could arrange the pumps so the outflow pump flowrate will always be greater than the inflow while maintaining a set tank level. 3. Aug 26, 2016 ### Bryan Deily I was thinking of wiring a float switch in to power down both pumps in case there is not enough new water in the resivour. This would prevent pumping water out without replacing it. If the numbers work out the pumps will be off by about 1/4 cup per day. My tank would never overflow due to evaporation however salinity would climb slowly. I may try an inline flow restrictor after each pump at 750 ml/min to see if that gets it any closer. I was thinking of using a system of float switches and relays I just felt there were too many parts involved and was worried about failure. 4. Aug 26, 2016 ### Bryan Deily Thank you billy. 5. Aug 26, 2016 ### Bryan Deily It seems most float switches couldn't handle 72w... so the plot thickens. 6. Aug 27, 2016 ### Baluncore Peristaltic pumps are not calibrated if the grade or age of the flexible tube used is different. To match two pumps you should first fit them with tube from the same batch. 7. Aug 27, 2016 ### Nidum (1) Fit feed back devices and use some very simple electronics to sync the motors . Disc with a few slots or holes and a photo switch would be ok for the feedback devices . (2) Use electronic variable speed drives and match the motor speeds manually . 8. Aug 27, 2016 ### jim hardy Can you mechanically couple the shafts together ? 9. Aug 27, 2016 ### Nidum Or even use one motor ? 10. Aug 27, 2016 ### Baluncore If it is the mass transferred that must be the same then the pump revolution count is not important as errors will accumulate due to differences in tube characteristics and the temperature / density in the fluids. A counter-flow heat exchanger might improve things but the only reliable way is to regulate the level in the common tank with a float switch. 11. Aug 27, 2016 ### jim hardy i think we all agree,, and our thought processes would benefit from an error budget. How much flow are we considering and how much volume does it take to reach the 'worry point' ? I like clever mechanical gizmos. One can then build a circuit that does the same thing. https://en.wikipedia.org/wiki/Differential_(mechanical_device) A differential gear connecting the peristaltic pumps would lock them in step when the averaging shaft is locked. Rotating the averaging shaft would raise speed of one and lower the other. Averaging shaft's speed would be set to a function of level error, the tank will provide integration. RC cars have small differential gears, Ebay shows them in the$10 to \$25 range..

Same thing could be done electronically by precisely measuring rotation of each shaft, perhaps with old mouse wheel optocouplers, controlling motor voltages individually. I like the gears, though, it'd be more fun to watch !

.................................................... That said, moving on to your other question................

Yes you do need a larger supply.
Try your 6 amp car battery charger .

12. Aug 28, 2016

### Bryan Deily

If I were to use variable speed drives and manually match speeds do you think it would need to be recalibrated down the road? Thanks for your input.

13. Aug 28, 2016

### Bryan Deily

I am trying to use the pumps for automated water changes in a saltwater fish tank. My tank is small relative to most which means the pumps would run a short time to exchange the desired amount of water. I'd like to see this scale to larger tanks as well which would require more precision. I will be running the pumps for about 6 minutes a day to exchange 7 gallons a week. The pumps are currently off by about 3.7% in volume. I'd prefer about a 1% difference or less.

I like the differential idea; I guess ideally I would run both pump heads off of one motor. Not sure what type of gearing this would require. The differential has three shafts? Two in with one out at the average speed between the two?

Last edited: Aug 28, 2016
14. Aug 28, 2016

### Bryan Deily

I spoke with the manufacturer of the pumps and they have stepper motors available. If I were to switch to stepper motors and have them run the same amount of steps would that be easier? I'd just have to figure out how to control them as I intended on running the brushed motors on a timer 6 minutes per day.

15. Aug 28, 2016

### Baluncore

Stepper motors will not solve the problem. It would be easier to use one motor with the pumps on a common shaft. Water temperature and density will continue to vary as will the characteristics of the peristaltic tube and roller pressure.

If the waste pump drew water from the top of the aquarium and was run slightly faster, or with a bigger diameter peristaltic tube, it would always outperform the injection pump and could not lower the water in the tank below the waste inlet level. The waste pump would suck some air, but no float switch would be needed.

16. Aug 28, 2016

### Nidum

Yes - stepper motors would be a very good solution . No problem with finding controllers for them .

With software control you can set up any timing schedule and still have the facility to make small adjustments as nescessary to balance the system .

It would be quite easy to add some feedback as well and get a self balancing system if you wanted to .

17. Aug 28, 2016

### Baluncore

Can you put three smaller tubes in one peristaltic pump? Two would pump waste water while one injected clean.
With two tubes having slightly different diameters but the same wall thickness, the same could be done.

18. Aug 28, 2016

### Nidum

This type of set up generally works quite well for long periods but certainly occassional adjustments would be needed .

Stepper motors would be my preference though .

19. Aug 28, 2016

### Bryan Deily

The tubing is identical; water temperature and density should be close as the salinity and temperature are matched. I feel like if these were my concerns I'd be working in the 1% error range.

It is a great thought to have the waste pump at the surface of the return portion of the tank so it can not pump out more than is pumped in. I have an optical sensor that maintains evaporation by topping off with fresh water so if it were to pump more out than is coming in it would trigger fresh water to run and keep the level the same. I could run the pumps on opposite schedules to avoid that problem though.

I'd like to entertain running both pump heads off of one motor shaft. First, because I have the motors on hand. Second, because I think it is the more simple approach. I really don't know where I would start here however. Should I look at modifying the pump heads and existing shaft or would I use some type of gear box?

Thanks for all the great help.

20. Aug 28, 2016

### Baluncore

We cannot answer that question until we do not know what make and model peristaltic pump you are using.
Can you post a link to the technical data sheet.