Newby question

Discussion in 'General Electronics Chat' started by toofnold, Jul 24, 2007.

  1. toofnold

    Thread Starter New Member

    Jul 23, 2007
    2
    0
    Hello,
    I'm new to the list & fairly new to electronics in general. I have a hobby machine shop and have been involved in building new servo drives for the axis servos on a CNC horz. milling machine for over a year. I understand a little about circuits (very little) and can build using a PCB, list of parts & directions. my mill has fairly large servos and I've been having one heck of a time blowing up drives when applying power to the drives when coming out of E stop. many, many things have been tried to stop the problem without success... my question to the group is double ended:
    first, is it possible to build a device (?) that would be in the circuit between the transformer output, two taps, 100vac each, and the servo power supply that converts the above voltage to 135vdc. the 135vdc feeds the three axis drive amplifiers which control & power the axis servos. the function of this device would to be to prevent the surge thats killing the drive output power sections, specifically the IRFP264N,s and surrounding components. we're dealing with 135vdc / 50 amp servos (85amp locked spindle melt down) on each of the three axis. normal amp loads are more likely 22 amps at the start of a rapid travel move & 5 to 35 amp during feed moves i.e. removing metal with drill bits, end mills, face mills, etc. the wires between the transformer & power supply are 16gauge so I'm guessing 25 amps max on each tap into the power supply? what I envision is: and this needs your input, something that, when the relay is energized, would slow the inrush of voltage so that the dc voltage ramp up from 0 to 135vdc out of the power supply would occur over a time period of ??? 2 seconds or what is recommended ?
    The second part of this question is, if the above is possible? would you people help me design the above device or point me to an existing design that I could build with your help? or?
    sorry for the long post, I'm trying to put forth sufficient information for my query to have meaning when read. hope this isn't too long and/or in the wrong place.
    Thanks
    Paul
     
  2. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    Hi Paul, welcome aboard. First, a few questions: is your AC input & drives single or three-phase? Also, what is/are the transformer KVA rating? Are you using a separate transformer for each drive? Is there a DC bus with filter caps in the drives?

    No. 16 wire is too light for 50A; it's rated for maybe 20A in free air [& 10A in conduit]. I suggest you increase your input & output wiring to no. 8.

    Unfortunately, heavier wiring won't solve your current inrush problem. If your drives have DC busses with caps that charge on startup that could be your problem although that would blow the diodes & not the the output transistors.

    Maybe the computer is calling for max. drive output momentarily on startup. Is the computer PSU [& monitor] wired ahead of the E stop circuit so it's constantly on [except when the main disconnect is off]? If not, try that; the computer may be making wrong decisions when it starts. Use E stop to kill the input & output supply wiring & disable the drives. If the computer is always on & you're having this problem try installing a 130V MOV across the PSU's AC input [assuming it runs on 120V] Also, install 150V or 180V MOVs on the drive transformer secondary terminals.

    If that doesn't help connect an isolation transformer, preferrably a constant-voltage type ahead of the PSU. Sola CVS series are good for electronics applications & size it for about twice the power requirement of the computer PSU to ensure stable output voltage.

    I have one more idea but I wouldn't try it just yet because of the cost & work involved, therefore, I won't go into any detail, just the concept. Use one or more series resistors after the tranny secondaries to limit the current inrush to the drives & allow the voltage to ramp up; parallel these resistors with the normally open contacts from a 50A contactor & connect the contactor coil across the DC bus so that only when it's nearly charged will the contactor allow full voltage on the bus. Allen-Bradley used this concept on one of their servo drives.

    I hope you find something useful here. Regards
     
  3. techroomt

    Senior Member

    May 19, 2004
    198
    1
  4. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    Hi again. There's one more thing I forgot to mention that's vital for reliable operation: grounding. Hopefully you have the control panel & all the electronics connected to the building ground but that may not be sufficient. Drive at least one 8 ft. rod or pipe in the ground as near as possible to the panel & connect it to the panel with at least no. 6 wire & a set screw or "stake on" lug. [To identify it as a ground wire it should be bare, green or identified with green tape on each end.]

    I once worked on a production vertical lathe that had I/O faults at the same point every cycle when 3 size 3 spindle motor starters turned off. Adding an earth ground made about a 30% improvment. [Shielding the I/O racks with screen mesh & installing "dummy" [unused] output modules to load down the data lines in the I/O racks cured the problem the rest of the way.]

    BTW, the Input & output wiring I referred to in my first post was the power input wiring from the transformers & also the drive output wiring to the servo & spindle motors, not wiring to I/O control devices like solenoids, relays, pushbuttons & other switches, etc. Regards.
     
  5. toofnold

    Thread Starter New Member

    Jul 23, 2007
    2
    0
    Hello All,
    wireaddict, My AC is three phase. I'll have to check the KVA, the power supply is based on a toroidal transformer and has three sets of dc leads, Yes there is a series of filter caps (big ins) Although the servos are rated higher, the original drives were rated for a peak current of 32 amperes, 15 amperes average. under voltage 62 volts, over voltage 125 volts. the problem isn't with the computer,
    I can blow a fuse & or drive using the terminal program. I had a computer problem early on and drilled through the floor and set a 6ft. ground rod for just this mill. I've checked & rechecked all the grounds and am sure their OK. the MOV's sound like a good idea. the voltage checks as hi as 137vdc. so looking at mouser's how about 150v MOV's such as # 594-2381-594-51516. ?
    Thanks Paul
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Be a bit careful with MOV voltage ratings. They may be peak voltage, whereas the line voltage is measured as RMS. A 137 VRMS waveform has a peak value of about 192 volts.
     
  7. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    I believe MOVs are rated in RMS volts. We've always used V130L20A MOVs across the AC input on 120V [RMS] devices in GM plants. If your input voltage to the computer gets as high as 137V I'd consider protecting it with a constant voltage transformer. You should also use 150V MOVs so they don't blow up. MOVs are only designed to conduct across momentary voltage spikes, not long-term voltage excursions that exceed the MOV's clamping voltage rating.
     
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