A bit over-designed power.....


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    Default A bit over-designed power.....

    I have been using my machine for the last 3 years, no big issues (very long build log here). Lately I got a hold of a 240V Watt meter (like the 120V Kill-A-Watt gadgets) and measured the power consumption of my machine. I have the steppers (three very quick 460 in-oz NEMA34 for x an y and a slow fat 1600 in-oz NEMA34 for z) with Gecko203Vs on an analog 72V/20A supply as well as a Chinese 2.2kW spindle with Hitachi VFD on 240V power.

    I was surprised how little power the machine actually needs:

    - Idle (spindle off but VFD powered, steppers in stand-still with half current) only 0.76 Amps total. That is only 182 Watts! I guess I have a green machine
    - Spindle on at 13,000 rpm but no load: adds about 0.9 amps
    - Machine moving at average 300 ipm in x/y circles - no clear indication, does not change much the load. I suspect at that speed the steppers can't take up more current. May be different at lower speeds.
    - Spindle cutting with 1/4" bit and 1/4" deep at 300 ipm in poplar: hard to say (fluctuating) but adds maybe 1 amp or 240 watts
    - Spindle cutting with a 3/4" router bit 4mm deep in poplar at 300 ipm: adds only 4-5 amps or 1200 watts.

    Just some data for your pleasure. But I know at least my stepper power supply is seriously over-specified and the 2.2kW spindle is more than up to the jobs I normally do.

    I did have the spindle bog down occasionally when diving a large bit (say 1.5") into the surface at high speed without ramp. But I never had my 20 amp circuits for the machine trigger the breaker.

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    Default Re: A bit over-designed power.....

    Yeah, I was suprised too about this.
    I got a clamp on amp meter to figure out what kind of circuits I would need when designing electricals for my shed.
    I can't remember exactly, but depending on what kind of cuts are going on, 90% of the time the whole system draws less than 2 amps.
    Dust collection is another story, I think mine draws 10 amps continuous.



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    Default Re: A bit over-designed power.....

    You probably cannot measure the power used by the stepper drives.
    Normal volt meters wont measure it.
    Its turned on and off about 25.000 time per second, its how the current is controlled.

    Stepper drives will use about 20-50W (to 100W), at all times.
    So, not a lot.

    At 220V mains (in eu), 3 steppers x 50 watts == 150 W, ==> 0.7 amps.

    I use ac brushless servos, these days.
    The big ones are 750 W, 220V (mains), about 1 amp each in light use, 3 amps peak.
    So about 2 kW total use, only when moving heavy masses.
    A heavy mass is a T&C grinder, at 400 kg, for example.
    Or a lathes carriage, at 200 kg.

    Avg use of servos is about 0.1 x or 10% of theoretical, or about 300W, or 1.5 amps at 220V mains.
    The same applies to industrial CNC VMCs, full power is mostly used to accelerate and decelerate the spindle.
    Most cutting loads are 10% to 1% of max possible.



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    Default Re: A bit over-designed power.....

    Quote Originally Posted by JerryBurks View Post
    I have been using my machine for the last 3 years, no big issues (very long build log here). Lately I got a hold of a 240V Watt meter (like the 120V Kill-A-Watt gadgets) and measured the power consumption of my machine. I have the steppers (three very quick 460 in-oz NEMA34 for x an y and a slow fat 1600 in-oz NEMA34 for z) with Gecko203Vs on an analog 72V/20A supply as well as a Chinese 2.2kW spindle with Hitachi VFD on 240V power.

    I was surprised how little power the machine actually needs:

    - Idle (spindle off but VFD powered, steppers in stand-still with half current) only 0.76 Amps total. That is only 182 Watts! I guess I have a green machine
    - Spindle on at 13,000 rpm but no load: adds about 0.9 amps
    - Machine moving at average 300 ipm in x/y circles - no clear indication, does not change much the load. I suspect at that speed the steppers can't take up more current. May be different at lower speeds.
    - Spindle cutting with 1/4" bit and 1/4" deep at 300 ipm in poplar: hard to say (fluctuating) but adds maybe 1 amp or 240 watts
    - Spindle cutting with a 3/4" router bit 4mm deep in poplar at 300 ipm: adds only 4-5 amps or 1200 watts.

    Just some data for your pleasure. But I know at least my stepper power supply is seriously over-specified and the 2.2kW spindle is more than up to the jobs I normally do.

    I did have the spindle bog down occasionally when diving a large bit (say 1.5") into the surface at high speed without ramp. But I never had my 20 amp circuits for the machine trigger the breaker.
    In my machine shop at work there's a HAAS VF-2, there's a nice little meter on the control panel that will tell you the power consumed by the spindle. Running it in an R&D machine shop (not production feed/speed rates) I've rarely seen it exceed 2kW of power drawn cutting aluminum stock. I guess in a production machine shop grinding out steel plates as fast as possible you might be able to exceed that 22kW capability, but for lower speed CNC work it doesn't quite draw what you think it would.

    That's encouraging about your power draw for your machine too - I've been worried about this as I build up my CNC and tripping breakers. I'll be servo'ing stepper motors on my machine, so hopefully they consume even less power.



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    Default Re: A bit over-designed power.....

    "You probably cannot measure the power used by the stepper drives.
    Normal volt meters wont measure it.
    Its turned on and off about 25.000 time per second, its how the current is controlled."


    The peak power number is of little use. The capacitors in the power supply so their job and the input power is probably accurate.

    Don



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    Default Re: A bit over-designed power.....

    Quote Originally Posted by DonKes View Post
    ........ the input power is probably accurate.....
    I believe so. There may be some power factor involved since the 72V analog supply goes through a huge toroidal transformer but the low amperage tells me that I am using only a fraction of the power supply capability. I should have probably run the test with a slow speed instead of 300ipm (about 500 rpm for the steppers) to see the maximum power consumption. At least I am sure now that the power supply is not a limiting factor.



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    Default Re: A bit over-designed power.....

    I think the idea is that when the stepper motor slows down, it is back feeding this braking power into the power supply. This appears to be the biggest reason for suggesting to use the large analog power supply vs. a switcher and some caps.

    I have wondered more than once if the driver power supply could be replaced with something much simpler and a sub C or other battery pack.



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    Default Re: A bit over-designed power.....

    I have a 3 axis cnc router that has two 10 hp heads (which I often use simultaneously), 2kw servos on each axis, and a 5hp rotary vein vacuum pump, all powered off the same line in. Setting this up in my detached garage-turned-shop, I expected that it would take a ton of power, so I bought a 30hp phase converter to supply the needed 3 phase. As it turns out, I rarely get much over 30 amps of power consumption, and the majority of that is simply the vacuum pump (19.2-19.6 amps usually). The machine itself draws only about 3.6 amps (240v single phase before phase converter) per spindle head during operation, and only about 5 amps for the servos if all three are going at the same time.

    What's kind of funny (not to my power bill), is that the machine uses almost the same amount of power when it's running as it does when it's not running at all if you throw out the vacuum pump consumption, since the phase converter seems to consume about the same amount of power as it supplies in that situation. I plan to update my servos to 2kw single phase 220/240v AC servos, as well as updating the rest of the machine to single phase as well. I think I'll actually use less power even though 3 phase is normally more efficient just by eliminating the current I draw while the machine is idle in between cuts.

    Overall, I think these machines are pretty efficient with power compared to what the sum total of their max currents would lead you to think.



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    Default Re: A bit over-designed power.....

    This really shouldn't surprise anybody even handheld routers are seldom used at full capacity. A two horsepower router would give a guy running it a serious workout if used at full capacity for any length of time. In the case of a CNC machine it would take some aggressive machining in wood to load it fully.

    I like to use a contractors table saw here as an example. Those are often rated 15 amps at 120 VAC. It is very hard to load those saws down to trip a breaker when cutting plywood. Run some tough hardwood through the machine and your chance go up dramatically. Keep the feed rate moderate and you have no problem.

    The thing with manual machining is that the operator gets real feed back to how the cut is going. You don't get this on a CNC machine except maybe via sound. This is where a power or Amp meter comes in handy as you get some idea what your motor is producing power wise. Some sort of power feed back (spindle power indication) to the operator would be of value for any CNC machine stressed in a production environment.



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A bit over-designed power.....

A bit over-designed power.....

A bit over-designed power.....