# what about...



## discomic (Jan 18, 2012)

...feeding a machine with much less power supply than officially required (e.g. 3-phase feeding max 2.2 KW against 4.4(spindle) + 1.1(axles) KW, being satisfied with proportionally lower cutting performances?
And, in the case on a CNC controlled machine, any possible electronic conflict feeding separately (and with no phase-matching) 3-phase section and single phase one?

Presently, I run a manual milling machine with 3 3-phase motors of 8.5 KW total maximum power absorption, via a 2.2KW inverter single phase input-3phase output, just needing sometimes a "reset" at the moment of uncoupling.

Thank You.


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## Tony Wells (Jan 18, 2012)

What is likely to happen is you will see a "sag" in voltage if your demand is greater than your supply. Motors won't perform at their rated power, and running on a lower voltage due to the sag, they will get hotter than normal, if run very long. In general, it's not a good idea.

As far as the single phase issue goes, every CNC machine I've worked on uses a stepdown transformer to drop the operating voltage to low voltage anyway, and it is single phase. There should be no problem using an independent supply for the control, as long as it is drawn from the same building source. The synch between the axis drives and the spindle encoder should not be a problem, but if it is, you may try switching to another leg, because by design, it wold probably be synchronized within the control to one of the phases brought in one one leg of the main power.


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## Starlight Tools (Jan 18, 2012)

This issue may have different aspects to it depending on if it is 3 Phase power supplied from the utilities or is it manufactured by a VFD or RPC.

Either way, by trying to operate a machine with motors that are 3 or more times the current draw than the supply is capable you will do a few things.  

The current draw of a motor spikes at start up (13 x the FLA (full load amperage) that is stamped on the motor) , drops to a lower value in idle ( approx 1/2 the FLA) then rises under load til it reaches the FLA.  If the input supply cannot handle these current loads, you will have problems.

The wiring in a circuit has to be sized to handle the highest current load that a circuit will see plus a safety factor.  If you are drawing too much current, the wires will overheat and could cause a fire, or arcing in a connection which will increase the current draw.  Increased resistance in the supply will lower the voltage reaching the machine.

The breakers and/or fuses to the circuit are there to protect the wiring.  If you are trying to draw too much current the breakers will be tripping constantly.  They are only designed to take so many trips without being damaged, so you may end up replacing breakers.  If the breaker is defective and does not trip for higher amperage, you could overheat wiring and start a fire.

Now if this unit is being fed single phase input to an inverter which then puts out 3 phase output, and if you are trying to draw higher current than the unit is designed for he inverter will try to comply with the increased load but you will overheat and blow components in the inverter.

Also as Tony mentioned, if the unit tries to draw increased amperage over what its supply can handle, the input voltage will be dragged down and you will end up running the unit in "brown-out mode"  this reduced voltage can further damage the motors.

Now having said this, you may find that if you operate the machine, starting each motor seperately, you will be able to keep it drawing under the 2.2 KW that the inverter is rated for.  Next if you take light cuts and keep the motors from using high amperage you might be able to get away with it.  Bearing in mind that the first time you stall a cutter in a crash that the currrent will spike enough to fry the inverter circuits.

Some machines, like Core Drills come complete with an Ammeter so that the operator can watch the power used by the drill motor and try to0 prevent burning out the motor. 

I would recommend you set up some way of monitoring this machine and see just how much power it is drawing as it goes through its paces.  Once you have established this you may be able to set the operating parameters so that you can keep the power load low enough to operate it safely.

Walter


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## discomic (Jan 19, 2012)

Thank You Tony and Walter for Your observations.
Due to the language and to the fact I'm not too deep in electrical matters, I'm afraid I caught only part of them.
What I would like to underline is. 
1.       Full home power supply (single phase 3.3 KW) comes through supplier electronic breaker, constantly monitoring absorption and instantaneously cutting the feed, if this exceeds 4 KW (nearly 18 A).
2.       3-phase motor feeding to miller comes via inverter whose working logic, as far as I know, is  
1.        frequency rump up (at starting) from 0 to 50 Hz (displayed), keeping constant tension (220V X 3-phases), controlling current output level to fit to motor mechanical load. For this reason, I don't really think to have 13 times rated current spikes. Most likely, current level will remain to inverter rated maximum, all transient long, due to machine mechanical inertia.
2.       Steady state operation (at full 50 Hz frequency), constantly adapting current output to machining instantaneous requirement (of course within inverter power supply capability). In this phase, if power request exceeds inverter possibility, frequency (not tension) decreases and, consequently, spindle and moving speeds.
3.       frequency rump down at shutdown, when inverter acts like a brake, always trying to control (via frequency) motor speed against inertia (and thus many times failing, resulting in overflow message, automatic uncoupling and reset need).
4.       final continuous current pulse (when not in overflow) to firmly stop the motor.
3.       I'm aware this is not rated use for inverter whose manual clearly tells not to exceed unit capacity, but I have no alternative and, on other hand, it's very well protected against over-currents. Moreover, machine use factor (percentage of working time over total) is, on average, near to 0. The biggest limitations are on working speed an cutting flow. It's nearly 8 years I operate this way my FRITZ-WERNER ISO50 milling machine (something like 5 tons of cast iron, 1200 mm longitudinal travel -I'll send You pictures, and I annex electric scheme I made-) and since 2009 my "new" lathe with a 4 KW motor.
Now, my doubts are about the possibility to extend the same logic to a CNC machine, whose high power need comes from high operation speed rather than from cutting torque.
If so, my intention is to divide machine feeding between 3-phase supply from inverter to (uncontrolled) spindle motor and direct single-phase supply to CNC section and axles motors.
I would like to understand, for stepper or brushless motors, which form must have their power supply, to evaluate whether I’dd be able to feed them.
Thank You again.
Michele


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## DJKnutsen (Jan 19, 2012)

*Re: what about... 3 phase power considerations....*

I don't claim to be a expert on VFD's, but one unique fact about AC motors is that they are an inductive load. That means that if you could see the sine-wave of both the current and voltage at the same time... they are not in sync  (E-L-I).... the more inductive the load, the more out of phase they get.... the measure of this effect is expressed as the Power Factor (PF)... So a good PF of 100 would mean that both voltage and current were tracking together (in sync..) and the load was resistive only. Another unique aspect of AC motors is that they well try hard to produce rated horsepower.... as the voltage and current waves diverge, (and voltage is controlled..) the current draw will go up drastically to attain the needed power. I've been told that rotary phase converters and VFDs have a positive PF... which is good as it makes things efficient.... but when you drive multiple motors from the same source, at the same time, the reflected impact on the power source can confuse the electronics and have a very undesirable effect.... Should you consider individual VFDs for each motor that has to run simultaneously...??? Often, in industrial applications especially... they will add capacitors in parallel with a motor circuit to help correct for a poor PF.  This is also where the supply wiring becomes a factor... a long run of undersized wire can produce a significant voltage loss at the motor end... and you'll see a much higher resultant current. So.... keep the VFD close to the motor for simplicity sake... These are just some things to consider...   Cheers, Dave


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## Starlight Tools (Jan 19, 2012)

Michele

If this is a factory made machine, the inverter will have been sized for the motor loads and the three motors will be operating at the same speed.  I do not see a potentiometer so I am guessing this is not a VFD.

The Inverter does control the start up current with its soft start feature which helps get larger motors up to speed without the higher current spikes.

If this is a shop made machine, where you have built it up and added the inverter, with one on hand, then you have two options, Try it and possible burnt out something or get a bigger inverter that is sized for the unit.

If you go ahead and try it, set your CNC programming parameters on the light side.  Take lots of light passes at slower speed rates.  Avoid putting the motors under "load"

If you have any way to monitor the current, like an ammeter do so and slowly increase the parameters watching to keep the laod within "your max ratings."

Note:  I can not tell you whether you should or should not do something, I will however err to the side of safety.

Walter


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## discomic (Jan 20, 2012)

Hi  Walter,
this morning I took some pictures of my milling machine (and of my lathe, too), and here they are.
This miller was probably born in 60's.
I bought it nearly 8 years ago.
It worked, as usual, with simple 3-phase direct supply.
I have, as already told, single phase 3.3KW domestic supply.
So I totally dismantled original electric installation, and designed and put up the one annexed to my previous post.
I adopted the biggest available inverter size with 1-phase input and 3-phase output: just 2.2 KW. Up today I never fried anything, so, from my point of view, the experiment was successful.
By the way, it is a velocity controlling device, as it has variable frequency capability (I don't remember lower limit), via on board keypad or external potentiometer; but I don't use this feature as the machine itself has lots of mechanical speeds.

Now I would like to change the miller with a used, newer (1984) one (I'm in love with a MAHO MH 600 P, the rigth size and architecture for me).
This machine is a sort of hybrid between a manual machine (like the older) and a CNC machine.
In fact it is equipped with a TC control Heidenhain TNC 135.
As far as I understood, the machine has an ordinary spindle motor (4.4KW rated) feeding spindle via lots of gear ratios and single motor drive that can move one axle per time, while the other 2 are kept locked by hydraulic clamps.
This kind of control does not work, presently.
So, I think there would be 2 operating possibilies: totally downgrade to visualized manual operation (electrical assisted), or, on the opposite, upgrade it to a real CNC.
While I'm avare of mechanical impact of such a change (first of all, adoption of ball recirculation screws), I have no competence at all for electronic block.
It would be great for me to learn the foundamentals of block scheme. That is. What parts should I purchase, how should theese parts be connected and feeded (keeping in mind my utility limitations). I know there are several generations of CNC since 80's, I'dd like to understand what would be the best choice for me, in terms of fitting to that machine, expected performances (I could define my machining "prototyping", where speed is not important, while precision is much more), and, of course, wich costs I would need to face.
I have not already bought this machine, so I should evaluate all this in advance.
The conclution of all this thinking, could also be that it's not a convenient way to follow, rather being preferable to look for an old, same features, but full CNC machine.
I think Your suggestions are very helpful, and, however the story will end, I' will have learned something.
Thank You.

_Michele_


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## Starlight Tools (Jan 20, 2012)

Michele

What has me puzzled is your 3.3KW Domestic Supply. that is a bit less than 15A at 240V

What is the size of the main breaker in your electrical panel?  How many amps at what voltage?  Generally in North America we see anywhere from 100 A at 240 V (24KW) to 200 A at 240V (48KW)

This is then broken down into seperate circuits.  Most households use 15A at 110V, but we can also run 20 A at 110V, 30A at 110V etc.  Then we have the 220V circuits, 15A, 20A, 30A, 40A, 50A etc.  These circuits are wired with appropriate sized wire for the breaker installed, 14Ga for 15A, 12 Ga for 20A, 10 Ga for 30A, 8 Ga for 40A etc.

For my workshop I generally use a minimum of 20A circuits for both the 110V (2.4KW) and 220V (4.8KW) plugs for hand tool use.

My mill is wired to a 220V 50A circuit. 

My lathe is run off the RPC and is run on a 220V 3 PH 40A circuit.

So the basic question is, is there any way you can increase the breaker in your main circuit panel to get more than the 3.3KW?

I noticed that the mill you are looking at is 380V 3 Ph.  

Now since the other machine is running 220V you will need to be able to increase your voltage to 380V.  Will this be possible for you?  Can you consider installing a (RPC) Rotary Phase Converter?

Overall the new machine you are looking at looks like a nice mill and would be worth your while to get going.

Walter


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## discomic (Jan 23, 2012)

starlight_tools said:


> Michele
> 
> What has me puzzled is your 3.3KW Domestic Supply. that is a bit less than 15A at 240V
> 16 A at 220 V, temporarily (1hr) increasable to 18A. And I can't have different supply (for cost reasons).
> ...





_Michele
_


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## Starlight Tools (Jan 23, 2012)

Michele

Ga is Gauge and stands for a measurement of wire diameter. As the number gets smaller, the diameter gets larger Here is a link to a Conversion Chart http://www.engineeringtoolbox.com/awg-wire-gauge-d_731.html

As for your machine dilemma, the only way I can explain it is by using an example.

Lets say that you have a creek that delivers 4 litres of water per hour. The water is pouring over a 1 meter diameter paddle wheel so that it rotates. With nothing attached to the paddle wheel, it will turn, but as you add more load to the paddle wheel you will reach a point where the water can no longer turn the wheel and it will stall under the load. If you balance your load to the water supply, it can keep the paddle wheel turning and you will be able to at least do some work with it. Now you replace the load on this paddle wheel with one that is 2 twice as heavy. The water flow may be enough to get the paddle to turn, but the increased resistance may also be enough the the paddle just stalls and the water runs off without getting the paddle wheel to turn.

Your electrical supply is too small for the motor load in your your current machine, but by reducing the load, taking lighter cuts at lower feed rates, etc you have been able to keep the system balanced and running. Now you want to hook up a machine that is almost twice as big. From what I can tell, if you transfer your system over from the schematic you supplied to the new machine, it "may" opporate the same way by reducing the "work load", providing you have enough current to get the "paddle wheel" moving. It may stall out and not work. 

My concern is that you may end up loading the motors to the point that they cause the voltage to drop, which will try to increase the amperage to cover and will cause the motors to overheat and burn out, but you will probably only be able to determine this by trial and error.

Walter


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## discomic (Jan 24, 2012)

_Now you want to hook up a machine that is almost twice as big.

_Everything You explained me is clear, and I thank You to help me confirming/clarifying my ideas.
Your only statement I can't understand is the one quoted above.
"New" spindle motor is 4.4 KW against 10Hp "old one".
The rest of power absorption is in movement feeding, whose speed, and thus power, could be choosed from nearly 0 up to 1m/min.

If I am to remove this second motor, replacing it with 3 different stepper or brushless motors, feeded via proper, independent (from spindle) supply, for total, let's say 1-1.5 KW (I don't know how to evaluate the minimum size to fit to given machine size), being satisfied with lower motion speed than machine itself could accept, everything controlled by a Personal Computer.
This is the bet.
And I'm looking for information about how to do it.
Have a nice day.

_Michele
_


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## Starlight Tools (Jan 24, 2012)

> feeding a machine with much less power supply than officially required (e.g. 3-phase feeding max 2.2 KW against 4.4(spindle) + 1.1(axles) KW, being satisfied with proportionally lower cutting performances?



Michele

sorry had not caught that, I was taking that you were upping the motor HP from your old manual set up.  

As for steppers, never played with them, so somebody else will need to help you with those.

Walter


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## BRIAN (Jan 24, 2012)

Hi Michele

I live in Sicily and have the same problems, thats why I run a mini workshop; 

even then, I have to be careful when sharing the load with the Lady of the house.

I would love to have larger machines but it just seems impossible with the supply limit,

 so I will be very interested to see how you manage your system.

Regards, Brian.


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## discomic (Jan 26, 2012)

Hi, Brian.
Sicily in Italy, is it?
And what is an "English chap" doing, in Sicily?
If You go back with the posts, You'll find the electrical scheme I presently use to feed a very big milling machine.
Of course, nothing can output more then You input in it, but it's, as far as I know, the best way to manage that kind of problems.
For sure You'll have to use one tool per time (don't forget about sicilian landlady... It could be far more dangerous than a poor electrical supply).

If You have any specific question You think I could answer to, don't esitate.

Bye.

_Michele

_


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