Do you really need to worry about inrush tripping the breaker? 1) If the DC supply is the old linear type, it will have a very large filter capacitor and supply peak loads without a large AC draw. The trick is to turn on the supply and then the motor a second later. 2) Most breakers have a time constant and will delay tripping for some milliseconds. 3) The better solution is a bigger breaker and larger wire.
On Wed, Jul 28, 2021 at 11:20 AM Gene Heskett <[email protected]> wrote: > On Wednesday 28 July 2021 12:57:30 Earl Weaver wrote: > > > **I took some voltage readings on the field windings F1 and F2. > > This is on the edge of my experience with electrical testing, > > so you may need to coach me with getting the readings that you need. > > > > Motor stopped: > > From Ground to F1 = +107.9 - Ground to F2 = -107.8 > > Leads on F1 and F2 = 215.0 > > > > Motor @ 50 RPM : > > From Ground to F1 = +107.5 - Ground to F2 = -107.4 > > Leads on F1 and F2 = 214.4 > > > > Motor stalled: > > From Ground to F1 = +105.7 - Ground to F2 = -105.5 > > > > I have noticed if the main power disconnect switch is let on the > > machine for a length of time, the motor will feel hot to the touch. > > Is this normal? The F1 and F2 wires show voltage anytime the main > > power disconnect switch is on. > > > > It appears to me that this is the original motor supplied with the > > lathe. > > > Since that motor is shown with a max field currant of 3 amps, and you > have nominally 215 volts available, the resistance of that winding s/b > in the 72 ohm area. Can you disconnect it and measure that ohmage? > > Also as a side comment, I disable all power to the machine when motion is > off, making linuxcnc running AND motion (F2) on as the power switch for > the whole thing, but I get cute with time delays, so the turn on is > current limited for about 4 seconds by a big high wattage resistor > allowing a much smaller circuit breaker since the inrush at turn-on is > limited. And in your case if the field winding is good, add an off delay > for the field circuit AC power src using $11 HOYMC SSR's which you can > control from a std bob card or whatever. Delay long enough that the > instantly turned off AC power to the armature supply has decayed to > where the spin-up by shutting down the field supply is no longer a > problem. > > That is why they didn't turn it off at all in the first place. > > But its also a 645 watt load on the power bill anytime the breaker is on, > and that 645 watts is the major source of the motors heating when on. A > couple rough service 120 volt light bulbs of lower wattage, in series > added across the 215 dc to the armature will hasten the demise of that > power to a safer level by acting as bleeders, and tell you how long you > need to delay the field shutdown. Say 10 seconds after the bulbs have > gone dark, if that's not enough, and the motor still jerks, add another > 5 or 10 to the delay time via the setp in the hal file that sets that. > Or use higher wattage bulbs for a quicker drain. Get back to me if you > need to when that time comes. > > You could get even cuter and turn the light bulbs off with a relay when > the motion is enabled. No delays in that circuit needed, and the bulbs > should come on and then fade with the F2 that toggles it off. > > And leave LinuxCNC running, or make sure the machine is dead before > stopping it. I tally that stuff with pyvcp leds in my axis gui. > > > **Earl Weaver > > Email: [email protected] <mailto:[email protected]> > > > > On 7/28/2021 9:47 AM, Gene Heskett wrote: > > > On Wednesday 28 July 2021 05:33:46 andy pugh wrote: > > >> On Wed, 28 Jul 2021 at 00:46, Gene Heskett <[email protected]> > > > > > > wrote: > > >>>> Yes, this is an approx. 1984 machine. > > >>> > > >>> That new? I would have guessed 1934, the year I was born. That > > >>> means of making a variable speed motor is ancient tech because its > > >>> not very efficient. > > >> > > >> I think you are maybe thinking of the Ward Leonard set, as used on > > >> the early Monarch 10EE, where an AC motor ran a DC generator which, > > >> in turn ran a DC motor. > > >> That is pretty old-tech, but a DC motor with solid state drive is > > >> hardly old-school at all (only the use of field coils is slightly > > >> outdated at that motor size) > > > > > > Even with my broadcast history, big motors in the 10 to 25 HP range > > > have always been std 3 phase. I've only read about using the control > > > of speed via the field currant applied, and here I probably need to > > > mention just for the record that loss of field leaves the motor > > > running on residual magnetism in that old steel, and if unloaded, > > > will spin up till it explodes. If loaded, it will trip every breaker > > > back to the fuses on the pole.\ > > > > > > And having some experience with that, I would not bet more than 50 > > > cents that they would actually break the circuit, I've been witness > > > on two occasions when it did not, the ceramic tube containing that > > > fuse had blown so many times that it was metal plated on the inside > > > and failed to open the 7200 volt line, eventually throwing hot > > > pieces into the grass below the pole, causing fires we had to stomp > > > out. We also had to replace 4 750mcm cables from the substation cans > > > on the pole, into and thru the weather head, and on into the > > > distribution bus inside the building. That building, which preceded > > > the NEC by a couple decades, had no entrance breaker, and an open > > > transmitter door by a curious new operator crowbared a 4000 lb plate > > > transformer, and a GE AK-225 breaker feeding that transformer failed > > > to open from old age, which led to the 7.5 amp fuse wires in the > > > poles flag switches to try and open. > > > > > > Thank $DIETY we had a spare transformer so we were only off the air > > > something under 3 days. Just one of the BTDT's over my nearly 40 > > > years in broadcast engineering. > > > > > >>> An AC motor and a vfd should cut the energy bill in half > > >>> compared to that. > > >> > > >> But will have less low-speed torque. Bear in mind that the lathe is > > >> geared for 2500rpm and was probably running at 250rpm (steel part > > >> 8" dia, carbide tooling) > > > > > > I had assumed a change in the gearing to address that. Probablt with > > > stuff from Tractor Supply. At 2500 revs, throwing swarf, I'd want a > > > metal wall between that swarf and me. That stuff would be start a > > > fire hot. > > > > > >> A VFD would be down at 5Hz to achieve that. > > > > > > Which I am doing on my ancient Sheldon. With torque enough to cut a > > > 10 thou chip with a carbide tool. At 100 revs, the vfd is at 16HZ. > > > Bring in the backgear and I can turn the house around. > > > > > >> I think that a DC motor is appropriate here, but probably a PM > > >> servo motor would be better. > > > > > > I won't argue that. Both of my brushed PMDC motors are getting long > > > in the tooth and running fine on the OEM brushes. And don't have a > > > minimum speed other than enough to overcome friction. And the motors > > > FLA torque is available down to zero speed. Temp rise limits with > > > time at low fan speeds will eventually wave a hand at you of course. > > > Generally the job is done by then, but its a consideration not to be > > > ignored. > > > > > >> Is this the motor originally supplied with the lathe? > > > > > > I suspect it is, but Earl would have to answer that. > > > > > > Cheers, Gene Heskett > > > > _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > Cheers, Gene Heskett > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > If we desire respect for the law, we must first make the law respectable. > - Louis D. Brandeis > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
