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Building my own motor setup. (not the motor :P )

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twist2b

Mon May 18 2009, 08:02PM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

Ok, so my brother has an old HAND CRANKED coffee grounder. It produces REALLY good coffee.

During college his roommate and him used a drill to get the coffee without physical labor. Good idea, but kind of a hassle.

SO, we are designing a method by taking apart a drill and having a whole geto setup :P

But, we have ALL summer. Might as well be a little more engineer-like (since I am an EE and he is a ME... use what we learned right?)

Ok, so I want to just buy a motor, and create an electrical schematic. I know how to get the right voltage and Amperage from an AC supply... (plug it in and what not so it lasts forever)

Do you guys have recommendations on what motor would be best and how to setup the motor for efficiency?

Just an idea right now... we are prob just going to use the drill, but it would be fun project if we went through it.

BTW - there is a nut at the top of the grinder.. thats how we use the drill.

Thanks,
Twist.

Mattski

Mon May 18 2009, 09:23PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

There are several important factors in choosing your motor: the best RPM for the coffee grinder, the torque needed at that RPM while grinding coffee beans, and the motor's input (DC or AC and what voltage).

Now since you already have a working setup with the drill, I would suggest staying with that but making it more permanent by taking the motor and gearbox out of drill, putting them in some kind of enclosure, and using a button or switch instead of the drill's controls. I suspect the main drawback of this is drills are loud usually.

If you want to do it from scratch you'll need to choose a motor with the right torque and RPM. The RPM can probably be estimated by you with the speed that you hand crank it or use on the drill. The torque is harder to measure, but I'd bet it's reasonably high. There are many ways you could hack together a rough measurement, such as hanging a weight off the hand crank handle, and measuring the weight at which it turns, or if you had an adjustable torque wrench that would work too.

For a hand-cranked grinder the RPM is going to be smaller than what the motor directly produces I bet, so you probably want to buy one with an integrated gear box. If I were you I'd just order a geared motor that produces the right RPM and is about as big as my fist. Then if it has trouble buy a bigger one.

The input to the motor can be AC or DC. I personally would go with DC, as it's easy to control them with a simple PWM controller. Choose a voltage that you have or can make a power supply for. With an AC motor than runs off your house voltage you don't need a separate power supply, but then controlling it becomes harder. But if it's the right size, you might not need to control it except for the on/off switch.

Efficiency is an unusual thing to be concerned with in a coffee grinder. Motors are pretty efficient at turning electricity into work., so you'd want to mainly make sure that your controller is efficient, and that you are grinding at the optimum speed. Gearbox will also have loss, but all you can do is spend more money for a better one. But it's not something I'd worry about.

Steve Conner

Tue May 19 2009, 09:34AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

How do you measure the efficiency of a coffee grinder anyway?

Once you get this working, you need to make an induction heated espresso pot next.

twist2b

Tue May 19 2009, 05:15PM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

Mattski - THanks, yeah I was thinking DC as well. I am NOT an ME... so I am not amazing at this stuff... but the I was thinking efficiency as in constant speed. When grinding it by hand.... to get good results (espresso needs a fine grind) you want to go at a constant rate.

Basically I was wondering the mathematics in choosing the right motor.

Its pretty easy to turn the grinder, but choosing a too small motor and the speed could fluctuate.. because the level of stress on the turning force fluctuates depending on how far down the bean is before getting crushed.

Yeah, I think we will just go with a drill, and setup a side box and all that. It runs on DC so to last forever I will just add a "Transformer/Rectifier/Capacitor Smoother/Voltage Regulator" and a switch.

I am just thinking.. since its summer why not go all out. It would be a huge learning experience... and be fun IMO.

BTW - The SLIGHTLY newer model of the one I have is almost the same and I saw it here:

It looks 90% the same except the gold piece on top is a nut.... which is better for us.

Sulaiman

Tue May 19 2009, 06:35PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I'd have a look at sewing machine motors.
They are built for long life, are reasonably quiet,
(compared to grinding coffee anyway)
and a light dimmer will do for speed control.

Electric drills are built down to a cost!
Sacrificing longevity, quietness and efficiency for cost and power.

For best efficiency use a three-phase induction or servo motor with a suitable electronic drive.

The easiest would be to scavenge a motor and associated controller from some other (unused) domestic appliance, a coffee grinder for example ;)

twist2b

Tue May 19 2009, 08:42PM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

Thanks Sulaiman! That is a smart idea!

Yeah, its just that electric ESPRESSO grinders are $100+
Normal electric coffee grinders are just 20 bucks.
The one we have was $2 (ebay)
Sewing machine is genius though! I will look for a scavenged one :)

Mattski

Wed May 20 2009, 01:54AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

twist2b wrote ...

Mattski - THanks, yeah I was thinking DC as well. I am NOT an ME... so I am not amazing at this stuff... but the I was thinking efficiency as in constant speed. When grinding it by hand.... to get good results (espresso needs a fine grind) you want to go at a constant rate.

Basically I was wondering the mathematics in choosing the right motor.

Its pretty easy to turn the grinder, but choosing a too small motor and the speed could fluctuate.. because the level of stress on the turning force fluctuates depending on how far down the bean is before getting crushed.

...

To avoid speed fluctuations there are some things you can do. You can buy a bigger motor with more torque, or change the gear ratio for more torque. There will still be speed variations but they will not be as big as the motor gets bigger. If you design an active motor controller you can reduce the speed variation to almost nothing, but that is more complex requiring either tuning of a "general purpose" controller (such as PID) or design of a custom controller. It also requires either a microcontroller or some decent analog circuit design - all in all I only suggest this approach mainly if you would want to do it for fun :)

A flywheel coupled to the motor shaft would also help smooth out these bumps, and reduces the size of the motor you need.

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