capacitor sizing.

[jamie_sparks]

hi all,
was not too sure where to post this thread so ill put it here for the now. :smilewinkgrin:

just a quick one, at work i have a machine that has shuttles running down it, its an automated picking machine. the shuttles run along 48v dc buzz bars, 48v, 0v, and safety circuit. all shuttles are independent and have 3 collectors fitted which conducts the voltage into the shuttle from the buzz bars.

im wanting to fit a capacitor across + and - on the shuttle's as occasionally the buzz bars/collectors get dirty and stop conducting for a split second, the shuttle then stops and flags up a fault on our system. so i want a capacitor to supply it with the 48v for a split second.

anybody any ideas on what calculation i need to use? pretty sure the shuttles are purely resitive.

thanks in advance.

 

[plugsandsparks]

Wouldn't it be easier to delay the power **** down side with a timer

 

[Knobhead]

Personally I’d be looking at what is causing the dirt build up on the bus-bars.

If it uses carbon bushes as the pick up. The grade is very important, DC uses a courser grade than AC. Brushes are also available with fine abrasive inserts to continually clean the bus-bars.

Have a look for LeCarbon or Morganite on Google. Both will give advise, I’ve had both companies do a site visit and they helped us out greatly.

To keep a drive motor running with a capacitor is going to need a hefty beast even if the motor is small.

 

[jamie_sparks]

its copper graphite collectors on galvanised bus- bars. there not really bushes as such.

tony, could you please explain a little more on your idea as i dont see how thats possible without storing some charge to use.

thankyou for your replys!

 

[Knobhead]

Just to add if there are any solvents used in the process they can cause brushes to degrade and leave traces of the binder used in the bush to adhere to the bus-bar. This looks like a black goo that hardens and forms an insulating glaze.

There’s an awful lot to brush technology.

 

[Knobhead]

Copper shot brushes are normally used for AC.

I’ll have a look for a white paper I have on brush grades and selection.

 

[jamie_sparks]

ok thanks, i am limited to what i can do as the manufacturer of the collectors believes this is the best for our system.

is the capacitor a bad idea then? the biggest fuse on the board of the shuttle is 8A so i didnt think it would be too much of a problem.

thanks again

 

[Knobhead]

View attachment 12047

If you are using this type of track I’ll tell you now they don’t like light loading. I know from bitter experience of them. The track and the brush surface glaze causing bad condution.

The white paper only covers motors. You will find more information on the Morganite site.

 

[jamie_sparks]

no mate, diffrent track to that, thankyou very much for your time so far. i will check that site out now.

 

[jamie_sparks]

just anotheer quick thought, it wouldnt need to run the motor, just keep the memory in the board so it didnt flag the fault up, it soon gets its power back to run the motor so i dont think it would be a problem.

 

[Knobhead]

OK. To be honest your going to have to do a bit of experimenting to find the size as it depends on the load drawn by the control. The control and capacitor will have to be fed through a diode to stop the capacitor discharging through the drive when there is a dip.

It’s getting to be a manageable size now

 

[jamie_sparks]

yes tony. we were going to put a random one on just to see if it worked, then we were hoping to adopt the idea and have the cap pcb mounted and hopefully apply the idea to all of our shuttles.

i will try and find out more information tommorow,

thanks again tony, means alot!

 

[Knobhead]

If you’re going down the capacitor rout have a look at double-layer capacitors. But a word of warning, they can take a heavy charging current. will the bus rail stand it if multiple units are involved?

I used to work on commercial photographic flash units, these required a current regulator when charging the capacitors (some could store 6KJ). Once charged they could discharge a fantastic amount of power, (660V @ 450A for 1/400[SUP]th)[/SUP] of a second. You don’t want power punched out at that rate but at a controlled rate. But be warned large capacitors can whack you with a large lump of power if you short them out.