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.
http://www.hitachi.us/supportingdocs...ok_NB585XF.pdf

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

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.....

Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
http://lufkinced.com/


Karl Townsend wrote:
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.
http://www.hitachi.us/supportingdocs...ok_NB585XF.pdf

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




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On Fri, 1 Aug 2008 21:55:43 -0500, "Karl Townsend"
wrote:

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.
http://www.hitachi.us/supportingdocs...ok_NB585XF.pdf

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

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

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 on the quick and cheap, a standard stove
element runs around 40 ohms and is rated above 1000 watts at 240 volts.
Run two in parallel for 20 ohms. I've been getting these for $1 each at
my appliance recycling place. The resistance is not particularly fussy,
the VFD is just looking for a place to dump the heat. An advantage of
the stove elements is that you will never have to worry about
overheating them.

Karl Townsend wrote:
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.
http://www.hitachi.us/supportingdocs...ok_NB585XF.pdf

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

Karl Townsend wrote:
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.
http://www.hitachi.us/supportingdocs...ok_NB585XF.pdf

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?
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.

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

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

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

Karl

If it makes you feel any better, my Haas TL-1 has the stove element
sitting in a little cage on top of the controller cabinet. If we ran it
hard enough we could use it as a coffee warmer.

Karl Townsend wrote:
Thanks, Jon. I'll give it a try.

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

Karl

On 2008-08-02, RoyJ wrote:
Sheesh! Talk about a confusing chart.

[ ... ]

If you want to try it out on the quick and cheap, a standard stove
element runs around 40 ohms and is rated above 1000 watts at 240 volts.
Run two in parallel for 20 ohms. I've been getting these for $1 each at
my appliance recycling place. The resistance is not particularly fussy,
the VFD is just looking for a place to dump the heat. An advantage of
the stove elements is that you will never have to worry about
overheating them.

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.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

DoN. Nichols wrote:
On 2008-08-02, RoyJ wrote:

Sheesh! Talk about a confusing chart.


[ ... ]


If you want to try it out on the quick and cheap, a standard stove
element runs around 40 ohms and is rated above 1000 watts at 240 volts.
Run two in parallel for 20 ohms. I've been getting these for $1 each at
my appliance recycling place. The resistance is not particularly fussy,
the VFD is just looking for a place to dump the heat. An advantage of
the stove elements is that you will never have to worry about
overheating them.


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.

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.

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.

David Billington wrote:
DoN. Nichols wrote:
On 2008-08-02, RoyJ wrote:

Sheesh! Talk about a confusing chart.


[ ... ]


If you want to try it out on the quick and cheap, a standard stove
element runs around 40 ohms and is rated above 1000 watts at 240
volts. Run two in parallel for 20 ohms. I've been getting these for
$1 each at my appliance recycling place. The resistance is not
particularly fussy, the VFD is just looking for a place to dump the
heat. An advantage of the stove elements is that you will never have
to worry about overheating them.


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.

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.

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

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.

On 2008-08-04, David Billington wrote:
DoN. Nichols wrote:

[ ... ]

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.

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.

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.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---