Gene Heskett wrote: > On Thursday 01 November 2007, Kirk Wallace wrote: >>> Kirk Wallace wrote: >>>> What is the best way to drive a 4hp, 90VDC, 40A spindle motor on a >>>> Hardinge CHNC? Some of the large DC motors I have seen, look like >>>> universal motors which would operate on AC or DC. Does anyone know if >>>> the CHNC spindle motor could work on AC? I'm thinking, a few dimmer >>>> switches in parallel and a stepper to turn the dials ;). I wonder if it >>>> would be cheaper to go with a 3-phase motor and VFD. Thanks. >>> No, these motors are NOT universal motors. You could rectify >>> the mains to drive the motor, but then it would draw 40 A from >>> the line. You should be able to find an SCR drive for this. >>> You can't parallel dimmers, they can't be trusted to give the >>> same pulse width. >> I was joking about the dimmers. So far I have found this: >> >> http://www.baldor.com/products/detail.asp?1=1&page=1&catalogonly=1&catalog=B >> C155&product=DC+Controls&family=One+Way%7Cvw%5FDCControls%5FOneWay&voltage=1 >> 80 >> >> but this puts out 180V and I need 90V. Plus it seems that this kind of >> controller does not interface with a computer well at all. It's >> beginning to look like DC lathe spindle drives are a specialty item, >> which means, hard to find and big bucks to buy or repair. >> >>> If it is a standard-frame motor, then you >>> could go the 3-phase and VFD route. That may not give you the >>> range of speeds and torque required. >> The newer VFD's are much better with this aren't they? >> >>> If you want smooth, >>> controlled reversing, then you need a DC servo drive. If you >>> don't need a complete 4-quadrant servo drive, then the SCR DC >>> motor drives may be the way to go. Baldor and plenty of others >>> make these, they show up on eBay all the time. >>> >>> Jon > > Hey guys, even the lowly board out of a Harbor Freight 47xx8 micromill, > except > for the current rating, makes a truly excellent vsc when combined with a > PMDX-106. > > This board, with a much larger bridge rectifier replacing its puny 4 amp > device, and about 6 of the mosfets it uses in parallel with smallish current > sharing R's in series with the src's of each of the devices could probably do > that just fine. The speed control is very stiff with this unit, so stiff > that if I'm doing something heavy, I have to rig an ammeter in series with > the motor in order to see how close I am to the 'red line'. Otherwise it > just blows the fuse with no detectable motor slowdown first. > > The only problem that I could foresee might be related to this boards ability > to drive that much gate capacitance of the paralleled mosfets and still > achieve good switching speeds. It is something I have not played with > personally, so one might approach this by adding one device at a time & > watching the heating. Running correctly, the mosfets shouldn't heat more > than 10F if screwed to a good heat sink and delivering 50% of their rated > current. In my own case, that would be at least 10 amps, but I didn't yet > replace the puny bridge rectifier either. Mine is currently in a closed > plastic box with the PMDX-106 and has run that way for half a day at a time > without the box getting noticeably warm. The direction relays make as much > heat as the rest of the circuitry. >
I would NOT attempt to beef up a MOSFET H-bridge to run a 4HP 40A DC motor, unless you are fairly experienced with power electronics. There are lots of things that you can ignore at 4A but will bite you at 40A. If this was 180V DC instead of 90, I would go farther and say flat-out that an SCR based drive would make more sense than an H-bridge, but the low voltage changes things just a little. Some of the issues that need to be considered to decide between an SCR based drive and a MOSFET H-bridge: Torque ripple - H-bridge will be best, a three phase SCR drive will be good, a single phase SCR drive will produce some ripple. Whether that will affect your part finish or cause vibration will depend on the motor and the machine. Motor insulation life: The H-bridge produces fast-rising voltage edges which may or may not stress the motor winding insulation. Older motors are more likely to have problems. On the other hand, 90V is so low (I'm used to 480V stuff) that it is probably a non-issue. Input issues: Kirk didn't say what his available line voltage is. Four HP is a LOT for 120V single phase, regardless of the drive technology. On 240V single phase its not so bad. Three phase is ideal of course. SCR drives do not have large internal capacitors that need to be charged on startup - H-bridge drives do. In fact the dealing with the caps and their inrush current is one of more difficult challenges if you scale up a small H-bridge drive. Ken Lerman wrote: > Someone on some list suggested that DC motors have much better > torque/horsepower curves than a corresponding 3ph motor with a > VFD. That's another reason to stick with the DC motor. Agreed 110%. If the original motor was designed for variable speed spindle duty (which seems the case), it will outperform just about any standard AC motor plus VFD combination. A lot depends on how much time and effort you want to put into it, and how much performance you want to get out of it. That original motor is probably a gem, and with the _right_ drive can deliver excellent speed torque performance as well as low vibration at lots of other nice stuff. But driving the DC motor will be a bit of a project. SCR drives are robust from a power electronics perspective but require complex control. H-bridges are simple to control but require better power electronics design. Either way, you need to understand the motor and how Hardinge used it - I bet they are doing field weakening to extend the constant-power region of the speed-torque curve. The AC motor plus VFD approach is easier, as long as the AC motor is mechanically compatible with the lathe. But you won't get the same performance, especially at low speeds. Regards, John Kasunich ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
