How can we limit inrush current for large CNC machines?

Discussion in 'General Electronics Chat' started by TSmith, Jan 20, 2015.

  1. TSmith

    Thread Starter Member

    Sep 21, 2010
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    We have a shop with several CNC milling machines. The shop has a 200 amp supply panel. We are adding more machines. At the moment when 2 machines start up at the some time, the draw is about 185 amps. We have no control of the startup time of the machines. They are constantly stopping and starting up throughout the day at random. 200 amps is ample to run all the machines. I think on average the machines are using about 50 amps when all of them are running. The problem is going to be with the amount of inrush current when 3 or more machines start up at the same time. Are there any commercial devices we can put inline between the machine and the panel, like a very large capacitor, to reduce the brief transient load on the panel to prevent it from tripping?
     
    Last edited: Jan 20, 2015
  2. profbuxton

    Member

    Feb 21, 2014
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    Firstly, I assume these machines are connected to an AC supply so a capacitor will not work. You may be able to provide a "soft start" device for each machine to hold the start up current to some limit. These are used in some variable speed drives and large motors to control start up current. Don't know if you can buy off the shelf add_ons for your application.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    CNC spindles are the cause a high initial inrush, 200 amps does not appear to be a large enough capacity for 'several CNC milling machine tools' depending on the supply voltage.
    It sounds like you have a low capacity supply for a large machine shop?
    Max.
     
  4. TSmith

    Thread Starter Member

    Sep 21, 2010
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    Unfortunately, we have to work with what we have. It's a 200 Amp 208 VAC 3 phase supply.
     
  5. Little Ghostman

    Member

    Jan 1, 2014
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    I dont know if this helps or not and keep in mind I am in the UK so the voltage is higher here. When we lived on the farm we had 3 phase and alot of equipment, sometimes it would trip the main board if the self start stuff started at the same time, my dad got around it with a device that would not let the machine start if the overall current was too high.
    I will try and find it online for you, it had something like you get on clamp meters that went around the cable near the main switch and the machine plugged into the device itself (if that makes sense).
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Surely not entirely at random? What commands a machine to start? Couldn't you just command them to start in a known sequence or under known conditions?
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    why cant you adjust the drives for less startup current
    ? most real cnc equipment driives are that way.
    adding a soft start to the motors without telling the machine about the delay could lead to other problems.
     
  8. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    As alfacliff says, most CNC machines have variable-speed spindles which are driven through vector-controlled variable frequency drives. If this is your case, all you'd have to do is adjust the startup parameters of each machine so as to minimize inrush current.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Newer CNC machine spindles the drive accel/decel parameters would have to be changed, older types had pot adjustment.
    There is no way that I have heard of doing it through the CNC part program?
    For a 208 supply rated at only 200 amps for several machines seems to be pushing it, my residential 240v supply is 200 amps.
    Max.
     
  10. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Neither have I... I've never heard of an EIA/ISO, or G-code for that...
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The only way I could see, is if the drives have a up-to-speed confirmation when a M3/M4 is issued, and try issuing a series of incremental steps for the M3/M4 for very high S values.
    Rather than going from 0-6000rpm in one shot!
    Extreme situations call for extreme measures!;)
    Max.
     
  12. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    So you could, for instance, call twelve 500 RPM increments until your 6,000 RPM target is reached?... not a bad idea... now let's hope his machine's memory is large enough for all the updated programs to fit in...
    Come to think of it... it's a very good idea... maybe you don't need twelve steps to reach 6,000 RPM, since the maximum inrush current might present itself when starting from zero, maybe going from 500 to 6,000 does not demand that much current...
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The systems and drives would need the up-to-speed wait for it to work, otherwise it would just roll straight through unless a wait time was included for those systems that do not have it.?
    Max.
     
  14. cmartinez

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    Jan 17, 2007
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    I thought there was a standard G04 code for that...
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes pretty standard, but to use it where the up to speed is absent.
    Max.
     
  16. Rocket.Man

    New Member

    Jan 1, 2012
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    I have a machine shop at home in the back yard. I ran into this problem about 10 years ago. This is an easy fix. The easiest way to fix the problem is to use an old Lincoln stick welder for a variable current limiting transformer. Connect the Lincoln welder in series with one of the main wires to your CNC mill. All the power going to the mill must travel through the transformer inside the welder. The welder has 2 welding cables, ground and the wire you weld with, connect the ground to the other wire to make a short circuit. Turn the welder down as low as it will go = 40 amps on a 225 amp lincoln welder. Start your CNC machine, use an amp clamp to check start up current. With low start up current motors on the CNC machine will start up slow. Turn the switch on the welder up to the next setting = 60 amps then turn on your CNC to see how it does and how much start up current is. Change the switch on the welder until you find the correct setting for the start up current you need for each machine. You will need current limiting for each machine.

    I was pulling over 400 amps of start up current on a 100 amp breaker with 240 volts AC. I adjusted it down to 95 amps to start up and it worked for years.

    I had another 60 amp brakers that I adjusted to 59 amps for start up and that worked for many years too.

    You may need to build a resistor for the welder. Instead of clamping the welder ground cable to the welding cable clamp both cables to a 240 volt hot water heater element. Hot water heaters come in several watt ratings, you might need 3 or 4 of these. The hot water heater elements act like large power resistors for the transformer inside the welder. Now instead of the welding cables being a short circuit it has a large power resistor. Put the heater element in a 5 gallon oil tank to keep them cool. The resistors will give you better current control. The welder selector switch might increase current 10 amps at a time, 40, 50, 60, 70, 80, 90 amps each time. The resistor will let you change current to any number you like. Assume you need 57 amps use the resistors to drop current 3 amps. Now the welder selector will give you 37, 47, 57, 67, 77, 87 etc.

    Lincoln welders are cheap on craigslist. You can build this with a 3 phase transformer but you will not have the selector switch like the welder and your current limiting will be totally dependent on the resistors.

    [​IMG]
     
    Last edited: Jan 27, 2015
    cmartinez likes this.
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    In that case - the only option may be a diesel generator set in one of the parking spaces.

    Quite a few companies buy these to keep production going when there is a power outage - a quality used set might be available from an industrial plant broker.
     
  18. dl324

    Distinguished Member

    Mar 30, 2015
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    Don't know of any commercial products, but do a search for inrush limiters. I've designed them in several circuits. It's just a negative TC thermistor. With the currents you're talking about, final resistance could be important.

    HTH,
    Dennis
     
    Last edited: Apr 2, 2015
  19. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Then you will have to live with the problems..
    More power is always an option..

    Is that problem based on financial reasons only?
     
  20. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    You could bypass the inrush limiter a few seconds after startup using an extra contactor, that way the limiter will only absorb energy at startup and will not heat any further afterwards.
     
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