Stopping a Centrifuge

Yep. I've used braking resistors on human centrifuges (when they vomit, you've got to get that baby slowed down ASAP). I've also (once!) used a big disk brake - baaad idea.

This is a left field response, with a Green Spin so feel free to have a laugh.

Why not put a electromagnetic break or small generator on the main shaft and when you start the "Spin-down" cycle, this can then create electomagnetic Drag without physical conatct, with a side effect of putting some power back into the system through regenerative breaking.

This would reduce spin-down cycle times and put something back.

Similar idea to hybrid cars.

Just a though.

Regards,
Sapper

I have done this many times for what are called Gold Concentrators, which are essentially a large tapered centrifuge bowl doing batch processing. They want to cycle through as many batches as possible, which means fast braking. From my experience, here are your braking options:

Mechanical brakes; bad idea on a centrifuge as mentioned above. Too much friction heat, wear parts etc.

DC Injection Braking; putting DC onto one set of windings of the motor in order to create a non-rotating magnetic field and pull the rotor to a stop. Better than mechanical, but not realistically viable for batch process centrifuge machines because the kinetic energy is converted to heat in the motor, especially the rotor where it has trouble dissipating. Very short motor life is the result. If you were doing 1 or 2 braking cycles per day, this might make sense, but it is severely limited.

Dynamic Braking using a VFD and braking resistors. Better choice because the kinetic energy is moved into the resistors and away from the motor and drive. Getting resistors that will last for very long can be challenging however, and the long term cost of ownership must be considered against the best option listed below.

Dynamic Braking using a Fully Regenerative VFD. This option uses a more expensive (most expensive) VFD that is essentially two inverse parallel VFDs. Instead of the front-end being a simple diode brige converter, the front end is actually another inverter, set to pump the kinetic energy back into the line at the fixed frequency of the line. This means that no matter how often you want to brake, there is no energy needing to be dissipated in your process equipment. it also provides some energy recovery, assuming you have other loads running when you are braking. Although significantly the most expensive hardware as far as initial cost is concerned, it is the most reliable and will provide the lowest long term cost of ownership.

regen drives would be your best,but most expensive,as mentioned solution,If you switched to a dc motor and regen.type controler you would save even more enrgy than an ac drive. Replaced a 150hp ac moto run by a soft starter with dc injection braking on a large wood chipper,with a 150 dc with a 12 scr drive. Cheaper to start chipper,basicaly free to stop it,overall better performance,speed feed back controled rim speed within .5% at any load.Stopping time cut in 1/2,only draw back motor did have more mainteance.

Hi,

if acceleration time equals deceleration time why not brake with an external resistor?

Faster will/may be a big problem as previous post states.

We usually brake 3KW DC motors and 2KW asynchronous motors with a switched resistance. (Disconnect power supply and connect the resistor.)

Cooling may be a problem - we once did a high power (9KW) resistor by a stainless steel stripe, this was really good unless melting started so we immersed this into purified water. Titanium is also a good choice as it can be anodised for insulation.

Oxide coating by heating and subsequent epoxy or PAI or PI coating (as class H or C insulation is strongly recommended if in contact with any fluid.

Simplest and cheapest resistor that works perfect in air: light bulbs (many if 18KW), electric heaters 9x2KW?

Best and safest: follow previous post and ask the manufacturer.

Unconventional: make a small gap between rotor and stator and inject some water: terrible fast and efficient (may be too fast so you get pieces until experience is accumulated.)

RHABE

I strongly agree with Ed.I maintained and repaired Sharples and then De Laval centrifuges for 32 years.You do not stop large masses of rotating metal quickly.I do not know what kind,size or speed of centrifuge you are talking about.It could weigh ounces to tons.They can often be stopped most safely by running water through it correctly.

Caution.Centrifuges have disintegrated and killed people.

I do not even like to be around one running as I knew that it could kill me at any time.Alfred

You've covered some great ideas. I think a VFD will probably work best. It also allows you to reduce starting heat being introduced into the motor. If you start/ stop this motor much more frequently you're going to exceed the long term heat buildup in it. Does it have a "K rotor" or anything else to allow for slow starts? Do you know it's maximum starts per day? I know most large continuous centrifuges even with K rotor motors are usualy rated for less than 4 or 5 starts per day.

As per my opinion, its better to give time to stop. may be quick stopping will create damage to other functional parts.

Thanks everybody for your valuable comments. Sorry about the delay in getting back, I was away for a few days. You have certianly given me food for thought. The "Green" idea is interesting Sapper. JRaef, you have some very good suggestions, do you know where I can get more info on the Dynamic Braking using a Fully Regenerative VFD ?. I am afraid that I do not have much information on the original manufacturer of this centrifuge, it is an old one which was origionaly hydraulic and it had an electric motor with a VFD fitted. It is quite a large centrifuge as you can imagine having an 18.5kw motor. I will try to find out more info.

Once again thank you all

It's simple, do NOTHING!

you are still only going to run it a few times a day. It will be done winding down and at rest when you need it again. Have you looked at the cost of stopping and starting this 3 or 4 times a day?

Yes....it seems that might be the best option from a cost point of view.....Thanks all

Why not just shorting the run time and use that 20 to 30 mins of free spin as an advantage.

Thanks dadw5boys that seems like a good idea, I will check if they are not doing that already.