VFD braking

I want to add tapping ability (need to stop and reverse fast) to my CNC mill with a VFD on the spindle. Its a 5 hp Hitachi SJ100.

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Pages 5-8 to 5-10 discuss adding a brake resistor. If I understand correctly I can put 2 resistors in parallel for more braking. This unit calls out a 35 ohm resistor. Two in parallel would be 17.5 ohms. Am I reading these tables right?

I seen a 500 watt 16 ohm power resistor on eBay. Close enough or do you have to be right on?

Karl

Reply to
Karl Townsend
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Did the book say you can put two 35's in parallel ? The VFD might not be able to handle 17.5 ohms.

Wattage increases in parallel - it dissipates quickly.

If the final amount or the lowest amount is 35 then two 70's or 3 105's.....

Mart> I want to add tapping ability (need to stop and reverse fast) to my CNC mill

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Reply to
Martin H. Eastburn

The tables specify 35 ohms as the LOWEST recommended value.

Two 16 ohm in SERIES 32 ohms is within 10% of this value which should be close enough.However, if you want to play safe, use three 16 ohm in series.

Have a good look at section 3-16. If you only want a fairly small amount of braking the DC injection facility can do this without any external resistor.

The external resistor system dumps the mechanical stored energy into the resistor.

The DC injection system dumps it back into the motor - mainly into the squirrel cage rotor. Only about 10% appears as extra dissipation in the windings.

Jim

Reply to
pentagrid

Sheesh! Talk about a confusing chart.

It looks like you have 4 classes of this base VFD model: 200 and 400 volt versions, each of which can be set up with or without the drive circuits for an external resister.

If you need to use the external "Breaking Unit" which has a resistor on the end, it's quite unclear what the resister value is.

If you want to try it out > I want to add tapping ability (need to stop and reverse fast) to my CNC mill

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Reply to
RoyJ

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A 230 V VFD has a 340 V DC bus voltage. A 35 Ohm resistor would draw about 10 A. The 17.5 Ohm would be about 20 A.

340 V/ 16 Ohms = 21.25 A, sounds well within reason. (340 ^ 2) / 16 = 7225 W, so this resistor needs to have a REAL good surge rating. Those big, hollow-tube vitreous enamel resistors generally qualify. Mount it vertically so convection can help cool it.

With only about 20 A worth of energy disposal, you can't reverse this spindle like plug reversing on real 230 V 3-phase mains. You should still be able to stop the spindle in well under a second, though. You can reduce the decel time until the drive faults out and then increase it a little.

Jon

Reply to
Jon Elson

Thanks, Jon. I'll give it a try.

And Roy, you make me feel better. I kept reading it different every time.

Karl

Reply to
Karl Townsend

Reply to
RoyJ

[ ... ]

But -- the hot resistance is higher than the cold resistance. Which is your 40 ohms? Measured while cold? I calculate about 57 ohms hot from your 1KW at 240V. This means that a series of stops in quick succession (like tapping a lot of holes in one workpiece) may reduce the braking as the elements get hotter.

What kind of tapping do you intend to do on the CNC machine? Rigid tapping, which requires the spindle to feed down at precisely the thread feed rate?

If not that -- why not go for a tapping head which releases when the feed stops so the tap freewheels, and then reverses the tap when you start to withdraw -- all while keeping the spindle rotating in the forward direction. I use one for tapping in a drill press, and it can be used in a milling machine as well. There are versions designed for CNC machines which can even be handled by tool-change robots.

Enjoy, DoN.

Reply to
DoN. Nichols

Don,

What resistance wire are you assuming. For a stove element I would have assumed a FeCr or FeCrAL wire and my Kanthal data shows a resistance increase with temperature of 4% to 5% at about 500C depending on composition, the maximum resistance increase is given as 8% for some wire but at temperatures around 1300C which isn't likely to be desirable.

Reply to
David Billington

Mistake, when I wrote FeCr I meant NiCr such as Kanthal Nikrothal wire. FeCrAl would be the likes of Kanthal A1 or similar.

Reply to
David Billington
[ ... ]

Well ... I was thinking in terms of elements which reach red heat -- including ballasts in old tube (valve for the UK contingent) radios without transformers. They would have multiple tube filaments in series, and then a ballast, which would look like a metal or galss cased tube, but which would simply contain a heating element wound on insulators like mica sheets. The purpose was to keep the current fairly constant (keeping the tubes at the right level of emission and reducing the chances of early failure) while the power voltage fluctuated. This was in the period where some locations had 110 VAC power, some 115 VAC, and some 117 VAC. This was before we got to the current 120 VAC standard. But also there were wider swings in the voltage in those days.

Anyway -- it depended on the resistance element having a fairly high positive temperature coefficient. Exposed wire hotplate kind of elements are similar in that the positive temperature coefficient both stabilizes the temperature somewhat with voltage changes and reduces the chances of an early burnout.

Just for the fun of it -- take an unused 100 Watt incandescent lamp, measure the resistance cold, and then calculate the needed resistance to draw 100 Watts from a 120 VAC line.

Granted, the filament in the lamp goes well beyond the red hot temperature which heaters usually reach.

An 8% change in resistance probably is not that bad for the application being considered here. I rather expected a higher temperature coefficient.

Enjoy, DoN.

Reply to
DoN. Nichols

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