DETERMINING WHAT SIZE ELECTRIC MOTOR TO USE IN A MIXING APPLICATION

[thinksabunch]

The problem is determining what size electric motor to use in a mixing application. The mixer is a battery-start, hydraulic drive, paddle-type stucco/cement mixer with a 17HP gasoline engine installed on it now. The 17HP gas engine is overkill because rather than mixing gravel, cement, sand, water - we are using the mixer to mix a powder with paper pulp. We also wanted to go with something environmentally conscious. Instead of running gasoline, we want to use electricity. We're leaning toward a DC electric motor with a smart motor control box so we can run the mixer in both directions - which it is already set up for now through the hydraulics. We know our application will require significantly less than 17HP, but were wondering how we go about figuring more precisely how many HP it will take to turn the paddle blade 50-60 turns per minute. We can control RPM with the motor control box, but we need the Goldilocks horsepower to have enough torque but not be over-powered. We have access to the end of the shaft that turns the blade on one end - so we could fab a lever or moment arm, but how to proceed? What other information is needed? Any ideas?

 

[itsmoked]

Here's the best you're going to do.

Figure out how to drive the actual drum with a wrench. Perhaps the final drive pulley is held on with a bolt or braze on a nut?

Fill the drum with "the mix".

Using a bending-bar (cheap) torque wrench turn the drum with the torque wrench. As you turn it find the hardest, highest torque point.

Record that.

Now from running it with the engine and timing how fast it turns gather the RPM number needed.

The relationship is torque x rotational speed = horsepower.

From the speed you have have the final gear ratio.

So looking at available motors 1725 or 3450rpm you figure out the pulley size needed on the motor that when matched to the one on the mixer drum gets you the drum speed you need and that matches the rpm used to calculate the horsepower above.

Once that's done get a motor that's 50% more than calculated and you should be good to go.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MintJulep]

You might possibly get there by playing with **something** in the hydraulic system until you get the mixer to stall.

 

[LittleInch]

Can you measure gasoline flow? Then assume 30% efficiency from the fuel energy.

It will get you close.

But remember the electric fixed speed motor will only take the power it needs to maintain speed. That's ac motors but I think dc is the same.

So getting a bigger motor than you need isn't a bad thing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TheTick]

If you need bi-directional, why not 3-phase AC? Seems AC has power and torque advantages over DC, plus no convertor.

 

[BrianE22]

battery-start, hydraulic drive, paddle-type stucco/cement mixer with a 17HP gasoline engine installed on it

How do these work? I'm guessing the gas engine powers a hydraulic pump that drives a hydraulic motor that spins the paddle? If so there's a few areas that will have losses. It's a lot of work but maybe measure the flow into the hydraulic motor and the pressure drop across the motor. Using those numbers and the motor's efficiency (from the vendor), you could get a pretty close measure of the power that drives the paddle.

 

[TheTick]

If you're keeping the hydraulics, why do you need a bi-directional motor?

 

[LittleInch]

I think they are replacing the whole set up. So it would look like they want a motor able to do 50-60 RPM with a hefty torque.

Hence why the hydraulic motor is there in the first place I imagine.

Buying a new electric powered mixer would seem a much better solution to me, but...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.