On Sunday 04 January 2009, Jon Elson wrote:
>Gene Heskett wrote:
>> On Sunday 04 January 2009, Jon Elson wrote:
>>> Gene Heskett wrote:
>>>> Interesting. The length of the radial portion of the bearing must be
>>>> fairly short then?
>>>
>>> The radial bearing sections are about 3/4" long, I think. These are
>>> combo hydrostatic/hydrodynamic bearings.
>>> I once had a hose blow off while drilling a hole, and waited until the
>>> drill was out of the hole to pause EMC and shut off the motor.
>>> It seemed to spin down pretty normally without the bearing air.
>>
>> I'd have assumed the 'hydrodynamic' would have held it properly even
>> without pressure at that rpms, allowing contact and drag only when it had
>> slowed considerably.
>
>Yes, exactly.
>
>> [...]
>>
>>
>> I wonder if a triplet of stepdown transformers would more properly match
>> the vfd's current abilities to that particular motors requirements? A 2/1
>> ratio might be worth a try just for grins. That might require a minimum
>> speed setting to prevent core saturation in the iron though, as allowing
>> that would be pretty hard on the vfd. With typical iron from Herbach &
>> Rademan, not less than 50 hz for a low limit should prevent that.
>
>Yes, but they'd have to be designed for a wide frequency range. If I
>could get the VFD to go to 1000 Hz, then I'd need 200 V RMS output, so
>you couldn't use a step down. Limited to 400 Hz, I can't go over 80 V
>RMS, so I suppose I could, but what I have works. I put a fan on the
>VFD.
Always a good idea. I have a pair on my xylotex board, of the 3.5" 12 volt
stuff in pc psu's, but running on 18 volts. Square cross section box long
enough for a fan in each end and the board in the middle, one sucking and one
pushing. They sing right well at that voltage and I've only nuked 1 of 4 in 5
years now.
I considered putting a fan in the box the repaired VSD and PMDX-106 are in,
but I've walked out forgetting to kill it so often, and never have found the
box warming up even when I leave it doing something that will take the rest
of the night or more, and never found it the least bit warm the next day.
>I doubt 4 A RMS will hurt a VFD rated for 3.3 A continuous.
With good cooling it might do more than that.
>I did
>put an inductor in series with each motor leg to reduce the ripple current.
I wonder if that's the correct answer.. That would choke the high speed
torque faster than big power resistors according to my thinking. The idea
is, if I understand the problem, that excessive curents will flow at the
lower frequencies, heating the VFD. Using big resistors (200 watters) would
tend to move the heat from the VFD to the resistors at the expense of a
little low speed torque when running at lower speeds. With the right sized
capacitors across the resistors, one could get a very flat torque curve from
10k to 50k.
If you have the drawings on the VFD, I don't imagine getting it up above 400
would be a very difficult task. How much voltage is available for its output
stage?
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
The tree of research must from time to time be refreshed with the blood
of bean counters.
-- Alan Kay
Chuckle. I've nearly always figured they were the most expendable item in the
payroll myself. :) Some of my battles with them to do it right, so I only
had to do it once, have achieved the status of legend in the trail behind me.
------------------------------------------------------------------------------
_______________________________________________
Emc-users mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/emc-users
