Tiny motors on new grinders? - Page 5

Grinders are one of the keys to exceptional espresso. Discuss them here.
buckersss

#41: Post by buckersss »

DaveC wrote:This is a tricky discussion due to the confusion between power, torque and the way different motors handle this. Add a gearbox (Torque Multiplier) and the confusion gets worse. A commercial grinder motor has to do 2 things....start turning and be able to keep turning, plus grind quickly. Motor shafts are "usually" directly connected to the burrs. For an AC motor to start turning and keep turning through pull up torque to max torque, requires a motor of size n....but to grind fast enough might need a motor of size 2n. A motor consuming more watts, so it can do more work.

the same grinder in home use, may require a motor of size n and because it doesn't need to grind in 4 seconds, but 8 seconds a motor of size n is sufficient.

The same grinder with a gearbox able to generate the required torque with a motor of size n/2, might grind in 16s

In all cases the same total work is done over a different unit time The motors can be of size 0.5, 1 or 2. In a grinder of good design, the size of the motor should only dictate it's capacity to do work. If a grinder is prone to stalling, it's because the design of the grinder (or gearing) is such that there is insufficient torque to turn, the power of the motor is not relevant if it does not have the capacity to do any work.

A "give me a lever long enough and I will lift the world" kind of thing...For any specific grinder, the motor power (plus gearbox if any) design, ultimately comes down to, does it have sufficient capacity (torque), to do the work (weight of coffee ground per unit time) required of it.

P.S. It is also important to consider optimum burr "packing" as I call it in the design....which may well affect grind.
how difficult is it to find out if grinders have a gearbox? i expect its really just a conical thing, no?

ira
Supporter ♡

#42: Post by ira »

Monoliths, Robors and Settes have a gearbox as I'm sure do many more I don't know.

Ira

Decent Espresso: espresso equipment for serious baristas
Sponsored by Decent Espresso
User avatar
Jeff
Team HB

#43: Post by Jeff »

Power (Watts) is an instantaneous thing. Work (Watt-seconds) is what it takes to grind an amount coffee.

A hand-crank generator is reported to be somewhere around 10 W, so maybe 20 W input. So, a "20 W motor" is sufficient to grind coffee as evidenced by all the successful hand grinders. It will take a lot longer to do the work than a shop grinder that can supply more power.

canatto

#44: Post by canatto »

buckersss wrote:
im curious what the effect of rpm has on all of this - to your point on grind time. if you need 200W at 120rpm for those 68mm conicals, but instead the rpm of the motor jumped up 4 times, to close to 500rpm (many conicals are around there). At that point, shouldnt you need 200W x 4 = 800W just to maintain the power per revolution you were achieving before? so while you are quicker, you arent any stronger persay.
I'm not quite sure what you meant by "stronger", in our grinder case however, strength = power rating, meaning the work a person (or a machine) capable of putting out in unit time, measured with units like watts, Kgfm/s, horsepower, torque*rpm, and so on.

The total energy (or work) needed for a grinder to put out to grind a certain dose of beans of certain roast is
E = torque * rpm * time

Noted in the above equation, torque * rpm = power rating of the grinder, it does seems for a given dose of same beans, a more powerful grinder will finish the job sooner, assuming the total work E remains constant.

rpm is part of the equation, but it does not have a direct bearing in the mix by itself.

ira
Supporter ♡

#45: Post by ira »

There is also inertia, especially with something like a bigger Mazzer grinder, once that motor is spinning at full speed it's going to take a lot to bring it to a halt, which is why some lower powered grinders suggest turning them on before adding the beans. How good the burrs are at feeding also makes a huge difference in the power required.

Ira

DaveC

#46: Post by DaveC » replying to ira »

Sure, those larger motors have more inertia, and the fact that you're out of the "pull up torque" range. Iv'e been doing a lot of testing/work these past few years and a lot of "forum knowledge" about motors, grinders, burr sizes/types and speeds, doesn't quite work out in practice....what does happen is fully explainable by Physics though.

coffeeOnTheBrain

#47: Post by coffeeOnTheBrain »

Thanks to all if you for the great read. And please excuse my lengthy post ;)

There are many different kinds of motors out there with very different characteristics. A 200w AC motor is very different to a 200w DC motor. The 200w are only the peak wattage. The characteristic is very different. A DC motor has the highest torque at the lowest speed and looses torque gradually with increasing speed. An AC motor has quite low torque at startup and a much higher torque at a specific speed, but the startup is the issue. There are some electronics tricks that help with the startup but the characteristic doesn't change significantly.
To be able to grind beans during startup AC motors need to have much more Watt than DC motors.

I actually didn't research any of this but I am pretty sure most boutique grinders have DC motors and most commercial grinders have AC motors. I believe the reason for this is that AC motors have a pretty significant benefit, they stay at a defined speed automatically by design, while DC motors need to have a speed controller to ref up without load or small load. Electronics got so good that this is really insignificant, but an industry doesn't change that quickly.

Personally I like the oomph of an EK or similar and dislike the sound of the changing speed of an eg-1. Would I get an eg-1 if I would win the lottery, sure!

There is a new kid in town though. The Ultra has a different kind of motor which basically has full torque from almost startup and you can set the speed and it won't need to hunt to stay there exactly. Obviously this comes with a drawback, the electronics are much more complicated than even the speed controller of a DC motor. I personally prefer my grinders to be as simple as possible and hence find the Ultra to be overengineered, but it is a great concept and I appreciate it's existence and I can't say I am not tempted at times.

On the other hand there is a completely different discussion happening at the same time, the speed of grinding. There are people arguing that lower grinding speed is better for the coffee. Actually different people have very different arguments for it, like temperature or uniformity of grinds. Nonetheless as pointed out previously lower grind speed needs less torque needs less Watts.
In this context I am still waiting for a vibratory conveyor to feed the beans into the grinder :D

ECM Manufacture: @ecmespresso #weliveespresso
Sponsored by ECM Manufacture
User avatar
Jeff
Team HB

#48: Post by Jeff »

The torque required seems like it will be primarily related to the type of bean, roast level, burr cut (force) and burr diameter (lever arm). There's another impact of larger diameter as, in many designs, you've got more grinds being sheared or shattered at the same time (increasing total force).

User avatar
yakster
Supporter ♡

#49: Post by yakster »

coffeeOnTheBrain wrote: A DC motor has the highest torque at the lowest speed and looses torque gradually with increasing speed.

A DC motor has quite low torque at startup and a much higher torque at a specific speed, but the startup is the issue.

There are some electronics tricks that help with the startup but the characteristic doesn't change significantly.

To be able to grind beans during startup AC motors need to have much more Watt than DC motors.
Did you mean to say AC instead of DC in the underlined part above?
-Chris

LMWDP # 272

coffeeOnTheBrain

#50: Post by coffeeOnTheBrain » replying to yakster »

Yes, thank you!

Will edit it above.