On Sun, 20 Dec 2009 12:49:56 -0500, Joseph Gwinn
wrote:

In article ,
Gunner Asch wrote:

On Sat, 19 Dec 2009 23:34:47 -0500, GeoLane at PTD dot NET GeoLane at
PTD dot NET wrote:

On Sat, 19 Dec 2009 15:27:15 -0800, Gunner Asch
wrote:

As for me...whenever I use a VFD...it will have a ramp up and a ramp
down time..and the ramp down time will be just about as fast as I can
get it.

I can set the VFD I use for my Bridgeport to stop in 1 second. Would
it be hard on the VFD to come to a quick stop like that? Sould you
have an external add-on resistor if you're going to be doing that? My
VFD is rated at 3 HP and the motor is 2 HP.

RWL


It really SHOULD have an external resistor. Its a slush pond that all
the nasty stuff gets sent to when you slam on the brakes. And if you
brake to a stop a lot..it really needs to be what the manual calls for,
given some margin for farting around with surplus goodies.

If you can find something appropriate..you really should install it.
The BP spindle is a fairly massive group of steel chunks and needs a
braking resistor to stop it quickly.

A good wirewound works pretty well, and is adjustable...and are often
damned cheap.

Yes. However, if you use a wirewound resistor not intended as a braking
resistor, there are a few things to ensu

1. If the open-tube vitrified type (which is what adjustable units
usually are), the resistor must be mounted in a grounded metal enclosure
such that no fire will result should the resistor achieve red heat.
This can happen, and will not damage an open-tube vitrified wirewound
resistor if it doesn't happen too often. Be sure to use heater wire to
connect to the resistor. Unless the resistor is too large for such
overheating to occur.

2. The resistor mounting enclosure must prevent accidental contact with
the terminals, as their normal voltage is around 350 volts DC.

3. Entry of and shorting by metal chips must be prevented. Mounting up
high helps a lot.

4. The resistor must be physically large enough to absorb the stored
energy of all that rotating metal without burnout, as no real heat
dissipation can occur over the one-second stopping time. The energy
from stopping is stored in the resistor as heat, which is only later and
slowly transferred to the atmosphere.

For metalworkers, none of this is hard to do, but I wanted to be sure
that the issues are understood. The easiest option is probably a large
aluminum-clad boltdown power resistor. These are available on the
surplus market for small dollars, as are the big open-tube vitrified
units.


Joe Gwinn


Very well stated Joe. I tend to simplify things perhaps too much for
some readers.

Gunner


"First Law of Leftist Debate
The more you present a leftist with factual evidence
that is counter to his preconceived world view and the
more difficult it becomes for him to refute it without
losing face the chance of him calling you a racist, bigot,
homophobe approaches infinity.

This is despite the thread you are in having not mentioned
race or sexual preference in any way that is relevant to
the subject." Grey Ghost