I can't find what i'm looking for in the engineering stack exchange because it's all way over my head. need basic help answer the basics to get started. so maybe a link or video that helps me understand how to even ask the right question on motor needed. I'm not going to become an engineer or learn everything i simply want to understand how to solve one type of problem.

so here's my basic need:

I have a water pump bought at a store that needs to be driven by a drill. But i want to use a motor from another appliance to run the water pump so i can use my drill for other things and have the motor that runs the water pump just working on that. I took apart a spice/ coffee grinder and rewired it and hooked it up the water pump and its simply doesn't move from lack of power ( it works fine and spins fine not attached to the water pump). So how do i calculate the motor i need to run the water pump?

also i will need this calculation to serve other projects... I'm also going to want to make a yeast stirer and use the motor from an orange juicer to spin a magnet that will affect a stirer stick through a glass container to stir yeast for 24- 48 hours. Not a lot of power needed but will needed to run all day and night.

so I want to understand just the very basics i need to determine how to choose a motor or how to very simply adjust or reconfigure any motor set up (i.e. use cogs or some kind of added system of wheels to increase the motors power) to: 1) be able to run a water pump or spin anything that requires a lot of strength and can run continuously for an hour to two hours. 2) Secondly, I need to calculate and determine needs to run a motor that spins a light load but continuously for 48-72 hours without burning out.

I cant find answers on google or stack exchange because when i ask the question i get a ton of irrelevant or very complex answers. i'd like help in getting enough info to keep my projects going without having to study engineering for weeks. i dont need to understand every single thing but enough to get these jobs done and repeat the process for similar future projects. Thank you!

  • $\begingroup$ Let me ask the obvious question. Why not simply get a cheap secondhand drill and use that to drive your pump? $\endgroup$ – DLS3141 Jul 26 '17 at 6:17
  • $\begingroup$ Also, for a stir plate, you'll want speed control, that makes it a bit more complicated. My stir plate uses a magnet glued to a PC fan with the blades removed and has a speed control circuit attached to that $\endgroup$ – DLS3141 Jul 26 '17 at 6:21
  • $\begingroup$ @DLS3141 you should post your pc fan & magnet with sped control as an answer -I'd vote... $\endgroup$ – Solar Mike Jul 27 '17 at 16:00
  • $\begingroup$ @SolarMike I would, but it's not my design and I didn't even build it, I just took it apart enough to see how it worked. The guy who designs and builds them started a company and sells them for less than I could make one. stirstarters.com $\endgroup$ – DLS3141 Jul 27 '17 at 16:19

So how do i calculate the motor i need to run the water pump?

The ideal hydraulic power required to drive a pump is simply a function of:

  • Mass flow rate
  • Fluid density
  • Total head loss of the system.

So, the ideal power required is:

$$ P_{hydraulic-kW} = (q*\rho*g*h)/(3.6*10^6)$$


$q = flow rate (\frac {m^3}{hr})$

$ \rho = density(\frac {kg}{m^3})$

$ g = gravity(9.81 \frac {m}{s^2})$

$ h = Head Loss (m)$

It's pretty much plug 'n chug to get this far, but this is just the amount of power that the pump has to impart to the fluid in order to achieve the flow rate. Since you're interested in the motor and the pump is not perfect, the efficiency of the pump in converting mechanical power fom the motor into fluid power has to be taken into account.

$$ P_{shaft-kW} = P_{hydraulic-kW}(\eta_{pump}) $$


$\eta_{pump}$ is the pump efficiency

Now, the value for $\eta_{pump}$ of your drill pump is either something you'll have to look up, estimate or measure. (I'd guess it's 0.4-ish, but that's a guess, nothing more)

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  • $\begingroup$ I dont understand the math formulas above. i was hoping my question could have a simpler answer. for example: a 600 watt motor from any standard appliance can run a water pump spinning 300rpm and pump 3 litres of water per minute. if its 1200watts it will spin 600rpm and pump 6lpm. ..an answer that sounds like something along those lines. but thank you for taking the time and helping. I'm very grateful but feel i would spend hours and hours looking up each aspect of the formula above and not actually build anything and i wont nec get it. looking for a simple version of the above 4a novice. $\endgroup$ – djalexis Jul 29 '17 at 9:01
  • $\begingroup$ @djalexis that is the simple formula, and yes it includes unknown things such as efficiency. That is usually the case as you do not know the inner losses of your system. $\endgroup$ – joojaa Jul 29 '17 at 16:42
  • $\begingroup$ That's as simple as it gets. $\endgroup$ – DLS3141 Jul 29 '17 at 23:42

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