cnc milling machine feeder calculations

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i have 10 cnc milling machines to wire, what is the rule for calculating the feeder for this group of machines, and they have transformers onboard, does mean seperatly derived system even if it is self contained or do i need to bond to steel?
thank you
thewireman
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
i have 10 cnc milling machines to wire, what is the rule for calculating the feeder for this group of machines, and they have transformers onboard, does mean seperatly derived system even if it is self contained or do i need to bond to steel?
thank you
thewireman


small control transformers can be bonded to the EGC. if thats what you are dealing with they probably already are bonded.

the feeder gets calculated exactly like any other motor feeder. most times there is a nameplate on the machine that will tell you the largest motor plus the FLA required for the whole machine. There is a reason the nameplate is there.

most likely you will end up with a feeder that has 2 or 3 times the ampacity it will ever see.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090315-1034 EST

You must bring an EGC to the machine and this must connect to the machine frame. There should be an equipment grounding lug inside the control cabinet near the main disconnect connections.

You must not isolate the machine from the EGC, and solely use a local ground rod. This is not safe and it is extremely unlikely to clear a ground fault because the impedance thru the ground rod is usually very high compared with what is required to trip a breaker. For example at 120 V and 40 A you need to be below 3 ohms and this won't clear very quickly.

Compare this to DC resistance for 100 ft of #12 which is about 0.16 ohms. #10 at 0.1 ohms, #8 at 0.063 ohms, #6 at 0.039 ohms, and so on. If you want to minimize ground current noise problems, then the EGC may need to be 0000 or larger. The size for the EGC will be determined by the NEC and other factors.

However, I believe isolation on communication circuits is a better approach than just an excessively large EGC.

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the transformer is on the line, meaning there are taps that are used for the voltage setting, mine is tapped 240 volt, the transformer appears to transform the voltage to 200 volts or what ever the motors use, some of these machines are running 200 volt motor flourscent lights etc. so it transforms the voltage to run the machine, not a control transformer,
thanks
wireman
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090315-1721 EST

HAAS CNC machines run a 325 VDC bus and have an input transformer with taps to allow adjustment for low or high input AC voltage. 325/1.414 = approximately 229 V for the AC supply. These machines are essentially a delta load. In addition to this main voltage adjustment transformer there is a control transformer for 120 ouput. If the machines are supplied from 480, then an external stepdown transformer is required. They do all servos and spindle motors off of the 325 VDC bus.

Grounding requirements have to be designed so that no components that an operator might contact will have a high voltage compared to ground for very long. This may be a good reason to add a supplementary ground rod, but that does not eliminate the need for the EGC.

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wawireguy

Senior Member
I hooked up a couple of Haases and a Mori Seiki a couple of years ago.

As far as the EGC the machines will have a connection for the ground that you bring in with your branch circuit. There aren't any derived system concerns.

I think it was the Mori that had a sit on the floor transformer. It was pretty neat. Had connections for just about any type of supply voltage and taps to adjust output voltage to whatever the machine needed. For that we just brought a EGC in with the branch circuit and to the machine with the transformer supply. Per manufacturer specs.

Disconnecting means was a issue if I recall. One of the machines had a after market disconnect that had been mounted to it. The other two machines had factory door type disconnects. The door disco's opened all ungrounded conductors. We used the factory disco's for those two machines. I recall a discussion on this board about that topic. Can't recall much of it.

I didn't do the load calcs for that install. I honestly doubt the company I worked for did much of one either. Since I'm studying for the WA admin test this seems like a good topic for me to follow. Do you just go off the name plate data and size your load at 100% of the manufactures amperage rating? This is an assembly not one motor. Or do you go to 430 and work through that for a calculated service load?

If you treat it as a motor 220.50 points us to 430.24-26 for calculating these loads. This is where it is confusing to me. 430.24 talks about conductor ampacity when 220 is talking about service loads. I'm going to assume you'd use 125% of the FLA and calculate your load off that number.

Wish I was installing the wiring for 10 CNC machines. That was a fun project. Don't forget to have the techs check rotation on the machines. The Mori and Haas guys were pretty sharp and adjusted rotation on their side if there was a issue.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090316-1622 EST

On our HAAS machines. I am not at the shop presently, I believe the nameplate is 40 A. Initially there is a very short current spike to charge the capacitor bank. In an idle state, servos off and the coolant pump off, line current is about 1 A. Servos on and not moving adds a little. However, on the newest machine the Z axis servo supports the weight of the head instead of a counter weight or nitrogen spring, and thus draws more current.

We seldom run heavy cuts and therefore are seldom at full spindle load. However, on our lathe because of the high inertia of the spindle and chuck there can be fairly consistent high loads when facing at constant surface speed where the spindle is constantly accelerating and decelerating.

I believe our machines are fed with #8 wire of not over 75 ft to the main panel. From the main panel to the pole is much heavier and maybe 200 ft.

If your machines are to be run at full power most of the time, then voltage drop might be a concern. Otherwise the smallest wire per the nameplate requirements is probably OK, except for the EGC which may need to be much larger. However, I am happy to sell you isolators to avoid over sizing the EGC.

Where are you located?

.
 
090316-1622 EST

On our HAAS machines. I am not at the shop presently, I believe the nameplate is 40 A. Initially there is a very short current spike to charge the capacitor bank. In an idle state, servos off and the coolant pump off, line current is about 1 A. Servos on and not moving adds a little. However, on the newest machine the Z axis servo supports the weight of the head instead of a counter weight or nitrogen spring, and thus draws more current.

We seldom run heavy cuts and therefore are seldom at full spindle load. However, on our lathe because of the high inertia of the spindle and chuck there can be fairly consistent high loads when facing at constant surface speed where the spindle is constantly accelerating and decelerating.

I believe our machines are fed with #8 wire of not over 75 ft to the main panel. From the main panel to the pole is much heavier and maybe 200 ft.

If your machines are to be run at full power most of the time, then voltage drop might be a concern. Otherwise the smallest wire per the nameplate requirements is probably OK, except for the EGC which may need to be much larger. However, I am happy to sell you isolators to avoid over sizing the EGC.

Where are you located?

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I am in Atlanta, i will gather nameplate and list them, i am confused on the ratings! largest motor 114 amps, fla 72 amps, is the 114 amps lock rotor, or a momentary spike at start, and the xformer contribution to load the computer,the light, and the chip conveyer, the nameplate is vague to me as far as wiring the group! they were running before i came alone and i have solved overloaded feeders in the building, i am considering bringing in 480 volts due to the nature of the system, the 240 is 1200 amps and to upgrade would mean a ton of rework and relocate, where as i can deliver 480 volts without much red tape, coz i am under 2000 amps and adding a 240 service would be special permission, i allready use a 6 disconnect gear so i am maxed there. if i bring 480 i will change over the voltage to each machine 1 at a time with little down time, keeping production running is key. i want to do an accurate calculation to support my reason for proposing the work. the power company highest demand is around 450 amps, but the power meter samples 30 minutes and these machines run much less peak time than 30 minutes, we allso have 2 welding robots and a dozen miller 350 welders, not to mention office hvac computers, 20 hp of air compressor which 10 hp runs allmost constant. i quess i need to be more educated to the cnc machines in general!
thank you
thewireman
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090317-0836 EST

thewireman:

Some general comments:

You can expect the main spindle motor to be the largest load in the machine. Most machines classified as CNC will have an electronically controlled spindle motor in contrast to a simple induction motor directly coupled to the AC line.

Assuming you have an electronically controlled spindle motor, then this generally consists of a rectifier and filter capacitor to provide a DC supply for the motor control. Today motor controls will use solid state switches to convert the DC to the required AC or controlled DC for the motor. These electronic switches are very small relative to the current that they can switch and thus their thermal time constant is very short and overload can only be tolerated for milliseconds.

Because of these short time constant thermal problems the controls are designed to self current limit.

A normal induction motor might have a startup inrush current of 5 to 10 times full load current for many seconds. An electronic motor driver might be designed for a maximum of 2 times the full load rating.

Look in the CNC manual for a discussion on the feeder wire size requirements. The same area of the manual will probable indicate what breaker or fuses should be used on the feeder.

As I indicated before if there is not a need to run the CNC at full load on the spindle motor, then you may not have a voltage drop problem using NEC specs for feeder size based on the machine rating. Otherwise look into the voltage drop and its associated problems. You have to worry about overvoltage on the motor DC bus as well as low voltage.

When a motor is decelerated the energy is fed back to the DC bus and this causes the bus voltage to rise above that defined by the AC input line voltage. When this voltage gets too high a resistor is connected to the DC bus to dissipate some of this energy. In a fully regenerative system the excess energy stored in the DC bus capacitors would be fed back into the AC source. Various combinations of these techniques will be used in different controls. With the simple bleeder resistor method, if on average the DC bus is too high, then the bleeder may be overheated causing the control to fault. This you do not like to happen in the middle of a cut.

In any plants where I have had equipment there was not a voltage stability problem. But these were in areas with good primary power.

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Last edited:

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090317-2023 EST

thewireman:

Following is some information on one of our HAAS machines.

HAAS VF3. Has 20 HP in big letters of the front. This is specmanship.

From the manual:

Internal main circuit breaker in the main disconnect. 40 A for 195 to 260 V input range. Internal transformer with taps is used to supply internally 230 and 115. Note: 230 AC calculates to 325 V DC on the DC bus minus some rectifier drop of about 2 V.

In the installation section.
10-15 HP use a 40 A supply. Under 100 ft #8, and longer use #6. Maximum leg-to-leg 260 V and leg-ground 260 V. This machine has a HAAS 20/15 Vector Drive. Outside nameplate has Full Load 40 A, and Largest Load 30 A. I have no idea what the difference is.

My very rough tests with an Amprobe.

Power up, no servos, no main spindle, and no pump.
0.5 A 0.8 A 1.0 A

Servos on.
0.5 A 1.5 A 1.5 A

Coolant on.
0.5 A 4.2 A 4.2 A

Coolant off, and various spindle speeds with only friction load.
1800 RPM
0.5 A 0.8 A 0.8 A

4500 RPM and HAAS load meter reads 10 %
2.5 A 3.25 A 3.25 A

7500 RPM and load meter is 15%
3.5 A 4.0 A 4.0 A

Extrapolate to 100 % and it might be
23 A 26 A 26 A

As a very crude measure 26 A would look like something near 15 HP assuming 100% efficiency.

The maximum meter scale on this machine is 180%. You may operate above 100 % only for short time periods. The time is inversely related to overload.

Note: the pump is single phase and thus input line phasing is of no importance.

.
 
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