Phase Converters for CNC-Equipped Shops

If you need 3-phase power for a CNC machine as well as other equipment, you should consider your phase converter choice carefully. The wrong setup could damage your equipment. July 9, 2007

We need three phase to run CNC, edge bander, dust collection, air, and panel saw. It's $30,000.00 for three phase, so we are thinking of going with 400amp single phase with 30hp converter for router and two 40hp converters feeding into a three phase panel to supply the rest, including the vacuum pump on the router. Will this work, and how high do you think the power bill will be?

Forum Responses
(CNC Forum)
From contributor C:
Why not just get a 100-150 HP motor with a soft-start or VFD and be done with it? You can buy big used motors like that cheap from an industrial recycling center or used electrical house. It won't kill your electric bill if you start it properly, especially since it will have no load on it. That way you'll have sufficient, decent 3P power.

Bolt it to the floor close to the main panel to keep the wire cost down, and install another 3P panel or buss duct or whatever you're doing after the motor. Start it in the morning, shut it down when you leave...

I don't remember the loss running a rotary, but go bigger than you need, cause you just might buy some more toys in a year or two. I would think you can put that system together for less than $5000 pretty easily. You can have 3P power where you want/need it, and it's a whole lot easier than trying to link up multiple converters.

From contributor B:
I can't speak to the bander and panel saw, but you should do a bit of investigating with the router company regarding the use of a manufactured 3 phase 3rd leg for the CNC. The spindle should be fine, but the other electronics could be an issue. This applies to the bander and panel saw too, as 3 phase motors work fine on a manufactured 3rd leg but if those have electronic components in the 3 phase circuitry, there might be an issue.

If the only 3 phase you need on the CNC is for the spindle, then you can purchase a single phase input frequency drive that will output the 3 phase for you. You'll need one of those for speed control on the CNC anyway, so getting one with single phase input versus 3 phase only input is a pretty easy solution.

As I understand these, though, you'll need to double up on the hp capacity of the inverter in order to take advantage of the full hp of the spindle. This is the system I use (single phase shop), but with a 10hp capacity frequency drive on a 10hp spindle. This limits my real hp at the spindle to around 5hp. This, however, is more than adequate for our uses of the CNC.

From contributor D:
You are on the right track with the two converters. I did something similar a year ago.
When using a converter for a CNC, you should not run any other equipment on that converter. Starting and stopping of other machines will wreak havoc with the CNC controller and if there are any problems, you can bet the warranty won't cover it.

What I did was have 800 amps single phase brought in and I run 1-30 hp CNC balanced converter just for the CNC. Then I have 1-50 hp balanced converter for the rest of the shop, 2-10 hp vac pumps, 10 hp dust collector, 2 head wide belt (20 and 25 hp), 10 hp chop saw, edge bander, beam saw, etc.

When I did this I was stuck in the same bind as you - power company wanted $30,000+ to get me three phase. They were thinking closer to $60,000. I ended up having the power co. put in a new pole and transformer for $950. Bought two new balanced converters, 30 hp - $1500.00, 50 hp - $2500.00, and had my electricians install a new panel and hook everything up for another $14000.00. Yes, it does seem like a lot of money, though much cheaper than 3 phase and/or moving the shop to grow. I did go a bit bigger on the incoming service at the time, though it leaves me plenty of room to grow without ever having to upgrade my service. After doing all this power upgrade, my power bill actually went down quite a bit.

When I bought my converters, I got them through American Rotary. They know their stuff and the price is right. They will help you get everything sized correctly.

From contributor J:
American Rotary builds a great converter and the service is wonderful. I have two currently and they work flawlessly. Directions were also so good that install could be done by idiot (me). They are provided with a soft start which is inside the starter box. No, I don't work for them - just feel they deserve praise for what they did for me.

From contributor I:
I too am real happy with American Rotary. They have several choices for CNC applications and combination loads. Since they have both rotary and digital, AR will best be able to recommend what you really need.

From contributor B:
Here are a few more things to consider, as well as another phase converter company to consider.

1) If you truly need those size converters (2 x 40 hp and 1 x 30 hp), then the 400 amp service really is not big enough. For a properly sized system, a 25 hp converter requires a 135 amp branch circuit, therefore, a 400 amp service is good for about 75 hp of total connected load.

2) The advice to use multiple smaller converters is an excellent one, particularly with respect to voltage balance of the manufactured third phase. Rotary converters are generally designed to significantly boost the manufactured phase during lightly loaded conditions so that when you fully load the converter, the voltage on the third phase sags towards the voltage of the utility supplied phases and the net output is close to balanced. The problem with this is that if you try to run a single small machine on a very large converter, there is a very real chance that boosted third 3rd phase will cause the thermal overloads on the small motor to trip. We have seen this when a 1/2 hp spindle sander was operated on a 25 hp converter.

3) If your shop has extended periods of time when no machines are operating, then the droning on of the phase converter can be quite annoying as well as wasteful (a 25 hp converter use about 2000 watts running with no load).

Our company, TCWTechnologies, offers a fully automatic phase converter system, Intelli-Phase, that detects when you want to operate a machine and runs the phase converter only when three phase power is required. Additionally, since the Intelli-Phase knows when you are running a machine, it can control your central dust collector with its integrated dust collection controller.

From contributor W:
More advice than you bargained for, I am sure… But there are some things to point out.

In reality, the 3 converters equaling 110 HP are likely to run about half, or maybe two thirds, of that in actual motor loads, which is a max of about 75 HP (actual motor loads). This assumes pulling full HP on that 75 HP – which isn’t likely. You are more likely to pull about 75%, which is about 150 three-phase amps, which is equal to 260 single-phase amps. In determining your service, most places double the three-phase current, so figure on 300 single-phase amps. So, a 400 amp service should be fine to run 2-40HP and 1-30 HP converters.

Where it might be a problem is starting the converters unless the converters have a soft-start rating. GENTEC (American Rotary) is known for their soft-start and you would probably want to consider this for this type of application.

The advice that contributor B gives about running smaller motors is excellent. Most phase converters do not account for this very common situation in a shop – where you want to run a very small motor on a large converter – and there is definitely a danger of ruining your smaller motor. At the risk of sounding like I work for American Rotary… They balance their converters within 5% from a 0% load condition. Since they balance from 0% load, it doesn’t matter how small a motor you use with any size converter. I have a 1/2 HP Baldor grinder running on a 40HP American Rotary converter.

Lastly, if you measure the current in the line using a clamp-on type amp meter, you will calculate a power consumption of about 10-15%. What you don't see is the fact that about 95% of that current is reactive and re-circulates back into the line. So you only end up paying for about 5% of that current. The real charge is when you constantly start and stop the idler motor. This takes real current and is what drives up the bill. This is why most converter companies will suggest leaving the converter run if you will be using it every half hour or so. GENTEC/American Rotary has several timers and a remote start already built into their converters that can detect the machine's need for power. They then close a contactor to provide the power to the machine.

This sounds similar to TCW's automatic converter. GENTEC calls it an Autolink system, and their prices are hard to beat. It is in their manual, but I have not seen it on their website. It can use a series of timers to control a dust collection system etc. Pretty cool, but again, you don't want to start and stop the converter unless you like high power bills or a have a good soft start converter.

I think another good point made by another poster was about American Rotary's selection of phase converters. They have everything, so there is no bias on what they recommend.

From the original questioner:
Thanks to all for their help. As it turns out, we were able to get the power company to give us three phase for $15000, so in the long run this will be better but has taken a lot of time.