motor selection for ball mill

[mr_byte31]

Hi All,

I am planning to build a ball mill due to its high price. A ball mill is a device to grinder any media.
There a lot of designs on internet , I like this one : https://youtu.be/kMgItVSmGDo

I have an issue to select a proper motor. I see some challenges here :
1-The jar will contain metal balls which will be a heavy load to rotate. (I think 1/3 HP at least would be needed)
2-The jar rotation should be constant (80 RPM).
3-It will run for long time <--- at least for 11 continuous days.

I have an old washing machine (Universal motor) but I think it will not be suitable since it will overheat due to the long running time and :
[Wiki] : "universal motors are best-suited for devices such as food mixers and power tools which are used only intermittently, and often have high starting-torque demands"

I thought about fan or water pump motor (induction motor), unfortunately the torque will not be enough to drive the full load. I will also need pulleys to reduce the speed.

I thought about DC motor since they have high torque and easy to control the speed. Unfortunately the price of the motor was high !

any suggestions ?

 

[FvM]

Only induction motor achieves long life. A motor with 2 or 4 pole pairs (1500 or 750 rpm @ 50 Hz) with pulley drive seems to be a straightforward solution. Or any other kind of geared induction motor.

 

[mr_byte31]

I have induction motor from water pump. it has 2 poles and run at 2900 rpm.
to reach 80 rpm , i need speed ratio of 36.25. I can try to get this ratio as close as I can.

but , this speed will be at "no load". this means when i have "full load" the speed will change and this will be less than 80 rpm.

every load will have different RPM and it will be hard to control the speed based on pulleys

 

[BradtheRad]

How easily can you turn the container by hand? That's an indication what torque it requires, which is related to power.

I'm thinking of a ceiling fan motor. It's designed to operate for hours, spinning at just a few rps. Your spec is 1.3 rps, so it won't need an extreme gear/pulley ratio. If done right then you're multiplying torque at the same time.

 

[mr_byte31]

@ BradtheRad, The jar should be rotating 80 rpm but the shaft should rotate faster than this.

The Jar will be 10 times the diameter of the steel rod connected to the shaft of the motor.

this means the motor should rotate 10 times the jar = 10 * 80 rpm = 800 rpm (motor speed)

I am afraid that the fan motor doesn't have enough power to drive the metal rod and the jar.

The jar weight would be around 5-10kg

I can easily stop fan motor by my hand but i need effort to rotate the metal rod.

 

[c_mitra]

2-The jar rotation should be constant (80 RPM).

The rotation speed is too high: the balls (or the stones or whatever they are) will get stuck to the walls of the bottle. You should not have rotations more than 30-50 RPM.

You can certainly use a belt and pullies to rotate one of these black resting rods (see your post #1 figure).

 

[mr_byte31]

Don't worry , 85 should be the critical speed.

please check : https://www.youtube.com/watch?v=L6sgGXXYdEU

 

[FvM]

every load will have different RPM and it will be hard to control the speed based on pulleys

I see that you don't know much about induction motors. Speed changes only by a small amount between zero and full load, e.g. 10%.

 

[mr_byte31]

I am not expert in motors stuff but I learn.
I was thinking to connect the motor directly to the metal rod and control the motor speed through electronic circuit like this :

 

[c_mitra]

The circuit simple reduces the effective voltage across the motor which is not the best way to control the speed (but keep the torque same). Most likely the motor will stall.

- - - Updated - - -

Don't worry , 85 should be the critical speed.

please check : https://www.youtube.com/watch?v=L6sgGXXYdEU

I hate to comment on other's creative work but the video is not useful for any serious work. It does not tell what is the critical speed, what is the speed in the figure and how the speed to be chosen for a given grinding. You can grind many things (other than grains) in a ball mill. You can use broken ceramic pieces instead of steel balls in a ball mill...

 

[c_mitra]

The Jar will be 10 times the diameter of the steel rod connected to the shaft of the motor.
.

You need to slip on short lengths of rubber tubes on the steel rod so that the grip is good (prevents slippage).

You need to compute the diameter with the rubber tubes (tyres) and not of the steel rods alone.

The centre of gravity of the jar should be below the plane containing the two supporting rods.

It will be wise to keep some provision for speed adjustment of the setup.

 

[mr_byte31]

@C_mitra, you can't make the center of gravity of the jar below the plane containing the two supporting rods !
There is a document that shows the critical speed for each jar diameter, I just added the video for simplicity to show that 80 rpm is still a valid speed.

I think to control the motor speed I need a VFD, its price is really high !
I need to find simple solutions... any suggestions ?

 

[c_mitra]

@C_mitra, you can't make the center of gravity of the jar below the plane containing the two supporting rods !

When the jar is empty, the centre of gravity is on the centre line of the jar. This is rather obvious.

When the jar is 100% full, the centre of gravity is again on the centre line of the jar.

When the jar is 50% full, the centre of gravity will be BELOW the centre line (the axis of the cylinder).

If you put the two rods rather close and the centre of gravity is above the plane containing the two rod, the jar will have a tendency to climb up *OR* get jammed.

You need to do some "thought" experiment.

You may add a third rod that may prevent toppling but will not touch the jar under normal operation.

 

[mr_byte31]

I got your point.
it is valuable observation.
thanks.

Lets get back for the motor selection

So induction motors should be the target here. I can get fan motor or water pump motor (2 poles) with 2900 RPM.

should I use a pulley to reduce the speed by factor of 3 = 966 RPM ( triple the torque and quarter the speed) and then use the circuit in my previous post to fine tune the speed ?

 

[FvM]

A triac controller doesn't work for induction motors in most cases. Only exception are fans or some rotating pumps that have strongly speed dependent load torque.

 

[BradtheRad]

I was doing tests with a ceiling fan (mounted on my ceiling). As we expect, it's easy for my hand to stop the blades from spinning. Trying to get close to the center, I can still stop it with my hand, 1 inch from center.

As for the motor shaft, I am not able to access it, but I picture if I were to grip it with my hand, I will just barely be able to stop it. If geared down 2:1, I believe it would have sufficient torque to turn your jar.

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There is also the idea of a windshield wiper motor. 12V 1A. I have tested one and I seem to recall that it continues turning no matter how tightly I try to grip the attached gear. Speed is related to voltage.

 

[c_mitra]

Fans are also induction motors that can tolerate high degree of slippage and the load is forgiving. The same motor connected with a belt and pulley to a fixed load will refuse to budge.

 

[chuckey]

A windscreen motor has an integral worm drive reduction (100:1?). I think this would be an excellent choice. Just need a 12V 10A PSU
Frank

 

[mr_byte31]

As I understand , Fan motors (high speed motors) are not strong enough for this purpose !

what should be my selection then ?

 

[mr_byte31]

A windscreen motor has an integral worm drive reduction (100:1?). I think this would be an excellent choice. Just need a 12V 10A PSU
Frank

windscreen has very small speed around [100-30] RPM.

I know that DC motors have very strong torque thats why i prefer them. I don't know about the induction motors torques. I am afraid a fan motor is not able to run because of the load