Building a new ballmill - need some advice

[Anacron]

Hello.

 

I'm building a new ball mill from scratch. I have most construction materials ready, but there's one piece I haven't got yet. I'd like to know what electro-motor to use. I've got a few water-tight plastic jars with 6 liters of volume to use for milling. Their diameter is about 20 centimeters. Suppose they're filled 1/3 with lead, which makes them weigh about 22 Kg each. The ideal speed for milling is about 60 to 90 RPM. Let's say the shaft diameter is 4cm (I'll probably want a large diameter here to increase the grip) and the engine RPM is about 1700. This means the shaft should rotate at 300-450 RPM and the diameter ratio for the pulley between the shaft and the engine should be 3.8 to 5.7.

 

Now here's the thing I'd like to know. How powerful engine do I need (in kiloWatts) to power my ball mill? Our power supply is single-phase 230VAC. I've seen 1.4 to 4.0 kW engines with 2000-4000 RPM on the market, but I'm guessing those RPM are a bit too high. Those of you who have ball mills with similar amounts of milling media, what is the power rating of your motors?

 

Thanks in advance!

[swervedriver]

A 1/3 HP 1750rpm single phase motor turns a 30 pound jar very easily, so .5 kW should turn 22 Kg easily when geared right.

[Anacron]

Only half a kW? Are you sure about that? I've seen many people use much more powerful electro-motors to drive their ball mills, so I'm in a kind of doubt against those numbers. Personally I would go for at least 1 kW. I've been reading a little bit so I'll likely want a continuous-duty TEFC motor. What's the deal with capacitors here? ~Alex

[Peret]

One horsepower is equivalent to 750 watts, so 1/3HP is 250W. You can of course use a larger motor - it will only draw the power it needs to turn the load.

 

The deal with capacitors is that induction motors won't turn on their own, they need a rotating magnetic field. Three phase motors get this for free by having one winding on each phase, 120 degrees physically apart around the stator. With single phase motors, the capacitor produces a phase shifted current in a second winding (ideally) 90 degrees out of phase with the first, giving you the necessary rotating field. Sometimes they're just "capacitor start" and the second winding cuts out once the motor has enough inertia to keep up, but they have much less torque. Small induction motors don't have a capacitor, they have divided poles, one half of which is wrapped with a thick copper short circuit. The induced current in this short circuit causes a phase shift between the pole halves to give the necessary rotating field.

 

Do make sure whatever motor you pick is continuous duty. Check the rating plate. Quite often they're only rated for an hour or two before needing a rest to cool down, though this is the time at full rated load, and if you're not working it very hard it won't get so hot. A small fan on the shaft is generally a good idea.

[Arthur]

If your mill jar is going to weigh 20+ kilos then it's getting to be time to consider two smaller jars, ceramic media or some manual handling method. You will likely not be wanting to pick up that drum and hold it over a sieve to separate the balls from the powder.

 

Motor - Lots of people in reality use much less power that you think the small rock tumblers have 30w motors the bigger ones 40w. Look for a TEFC motor from 1/4 to 1/2 HP, for a big heavy drum look for a three phase motor with speed control inverter running off single phase. Strangely these turn up on ebay for small money. This way you can soft start a mill without clearing breakers with a DOL start.

 

Before settling on a motor consider the motor speed and whether you are in 50Hz land or 60Hz land as the running speeds are different.

[Mumbles]

Honestly, I would have suggested the same thing. Maybe .75kW if you REALLY want to be sure. I'll have to check, but I think my ball mill is only 0.2 kW and it turns 15 kilos with ease.

 

I am no good at electronics, so I really don't much about capacitors besides the fact that they help with start up, and everything has to be installed on delivery if I have any chance of making it work.

[Anacron]

Very well. I'll ask a colleague of mine about the capacitor, he studied electronics so he'll know, but I'd probably want to buy a motor that includes one. I've also seen options to use a tri-phase motor along with a frequency regulator that can run on a single-phase AC power supply. The advantage is that it lets you regulate the RPM easily, but it's also more costly. So I'll settle with a TEFC continuous-duty 230VAC 50Hz single-phase motor with 0.5 - 0.75 kW and cca 1700RPM. Thanks for the answers.

[Anacron]

Question. I admit I'm not really an electro-engineer, but how big of an impact does the motor's starting current have when talking about installation fuses? I found a 0.55kW 1400RPM motor, and the manual states that the starting current is about 21A, but our fuses are only 16A. This is a problem, no? ~Anac

[Arthur]

Get an electrician in your country to fit the correct type of motor starter and a motor start rated fuse or breaker.

 

This is one good reason to use a small motor! Unless you are going into serious production a 100 - 200w motor will be plenty, and this will start with a simple switch, consuming less current than your rated outlet circuit.