Electric bill with phase converters - How much is yours?

[Matt@RFR]

I know there's a few guys here that have their machines running on phase converters. My utility bill each month is between $500-$600 and I only run the machine after work and on the weekends. Maybe 40 hours max or so per week. There's been many weeks that I didn't even start the thing too. The machine is an '07 VF-2ss running on a 50hp American Rotary phase converter.

The majority of my work is prototyping and very small runs (max of 10 parts on average) so the machine is sitting idle quite a bit, and the biggest cut I take for the most part has the spindle at 50%.

I took some amp readings today and will share the results. I'm no electrician, so hopefully I did everything correctly. The way I measured amperage is with a clamp-on amp probe clamped onto one of the wires coming from the main panel into the phase converter.

At idle, it pulls 14A per leg.
Starting spindle from 0 to 12k rpm pulls 140A per leg (200% spindle load).
12k rpm steady state pulls 18A per leg (3% spindle load).
12k rpm, 50% spindle load cut pulls 27A (50% spindle load).

I used this formula to come up with Killowatts:
Amps per leg x 2 legs x voltage (240) /1000 = killowatts.

We get charged roughly $0.20 per kwh. So worst case scenario, if I roughed something out at 54A for 40 hours/week that would cost me $415 over and above our regular house usage. That works out to taking up $11 of my hourly rate. Not bad, but I don't use the mill like that. A big chunk of its time is sitting idle while messing with programs, changing offsets, etc. That, and the majority of the tooling I run is 1/4" and smaller, down to very small endmills. I'll see 3-5% spindle loads for hours on end.

What are your thoughts? Is this in line with what everyone is seeing?

[gar]

080323-1604 EST USA

Matt@RFR:

First, you doubled your current, and second you do not know your power factor. Also you are assuming the current is a sinewave.

Your cost sounds high.

Let us assume your $0.20 cost per KWH, then $600 = 3000 KWH. Assume your house base average load is 1.5 KW and a 30 day month, then 30 * 24 * 1.5 = 720 + 360 = 1080 KWH. This leaves 1920 for your shop. Divide by 160 and you have an average load of 12 KW or 12000/746 = 16 HP. Does not seem to compute.

Even if you assumed the idle load of the phase converter was resistive that is a load of 14 * 240 = 3.36 KVA . Actually it is probably about 2 KW of power, or slightly under 3 HP.

Use your service entrance watt-hour meter to measure the actual power input at idle load and then at your 50% machine load. Also try to determine the average base load for everything except your shop.

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[Donkey Hotey]

I second the idea about checking the meter. In fact, I tracked usage on my house many years ago. I read the meter daily and tracked it in Excel. It was an interesting exercise in learning just how much power a refrigerator costs per month (!!!) or a waterbed heater (!!!!).

Take a reading before you leave in the morning, mark the time. Take another reading when you get home and mark the time. You can figure out roughly what your home's parasitic load looks like from that. Then fire up the shop and do a 'typical' evening of work. That'll give you some indication of the real power consumption.

Somewhere in there, you're running a good sized compressor to keep up with the spindle. With that kind of money tied up, you probably treated yourself to a bunch of big florescent lights. Do you have a TV running out there while you're working? A gas furnace (fan)? An electric heater? They all sound insignificant but they add up quickly. Especially if you've got a family and the rest of your house is still running 'normally' while you're outside (other TVs, lights, cooking, etc).

After all of that, your bill does still sound high. How often do they really come out and read your meter? Has this been a couple of months? One of the reasons I started tracking usage was they misread one of the dials on my bill and sent me a huge bill one month, then didn't read it for two more (estimated). It caught up on the fourth month but that didn't matter: I had to pay it.

[davereagan]

50hp sounds like major overkill. I don't think you often use more than 15hp on that mill. For reference, 15 hp will cut over 20 cubic inches per minute of mild steel with a good face mill. Example: .15" depth of cut, 2" width of cut, 65 inches per minute.

[gar]

Another perspective.

If you think your phase converter is the cause of high electric bills, then it has to get very hot.

A phase converter is a relatively small package and suppose from my above assumptions that you take 1/2 of the 12 KW figure or 6 KW and dissipate this within your converter package and imagine how hot it will get. You can judge this by what would happen if you put 6 1 KW heaters in side the converter package.

A phase converter only supplies 1/3 of the power to the 3 phase load, the rest comes directly from the single phase lines. View the load as a three phase Y connected resistor bank and you can see why. If a 50 HP converter is only supplying 50/3 HP and if the converter is 75% efficient, then the losses in the converter at full load would be about 50/(3*4) or about 4.16 HP or 3.1 KW. But you will never run at full load. A converter will not be as efficient at light load as full load, but my guess is that you might expect that a 1 KW average load from power loss in your converter. Thus 160 hours is 160 KWH or $32 per month for your converter. These are wild guesses but illustrate an analysis method.

Cross checking this with your 14 A at idle and 240 V we have 3.36 KVA and I do not think my 1 KW figure for average dissipation is too far off.

(edit) Get a wattmeter and an appropriate current transfomer and measure actual power input to your phase converter. Maybe better get an electronic wattmeter that uses a clamp on Hall device current sensor. Note: ordinary transformer type current transfomers are a serious problem if the secondary is not connected to a low impedance. With the secondary unloaded it can produce a destructive voltage. (end edit)

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[bob1112]

When we tested converters, it seemed they pulled between .2-.3A/HP of the slave motor. This could be 10-15A just sitting there on the single phase side. Rotary converters are pretty efficient "when" loaded between 20-90%. The SS can easily pull 10A of three phase just sitting from the servos being fired up and all the fans and such running. The SS also does not have a counter balance and has to hold more power on the Z servo to keep it in position. I can't remember what they do under power down.

Our other shop has 2 of them and they love power but even with 2 of those and three others, the bill does not exceed 600. I would guess the reason you have a 50HP is because of the amperage?? I know the breaker in the SS is 100A! They really do not need it. You could sneak by with a 25HP converter on that machine for what you are doing.

We almost had to run the SS machines with converters so this is interesting. The only thing I can recommend if you only need control functions, you can add a circuit to only power the 120V control of the machine until you are ready to fire up the rest of it. You may also try testing with the estop on to see if that can drop consumption a bit.

If you wanted to back up to a 25HP converter, just find a slave motor and retune the caps a bit. My self or others here could walk you through it. I am interested to see what happens here because this could be a scenario for me soon. I have a lower HP Haas on a converter right now but need to get an SS.

[gar]

080324-1906 EST USA

Our 20 HP VF-0E uses the Z servo for counter balance on Z.

With servos off the input lines read about 1 A for the pair supplying the single phase load and the wild leg reads near 0. With servos on and no motion the said 2 legs are about 1.2 to 1.3 A and the wild leg is about 0.8 A.

Not very much power consumption.

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[bob1112]

Wow, that is much lower than ours. That might mean he has a problem with the converter slave motor or mill. Maybe a meter issue??? Never heard of it but 600 bucks sure seems high. I did notice he said 20cents/kw. I think ours is around 9 cents here.


Gar, just curious what meter you used for that test? Hell, my old machine has three cooling fans in it that pull more than that! Now I have to go test my VF1.

[Matt@RFR]

Thanks for the help guys. What I'm going to do is track my electicity useage via the meter and see if that brings any light to the subject. I think that will take a couple weeks to get a good average, so I'll update this thread when I feel I have something decent to post.

The reason I have a 50HP phase converter is simple: When I bought this mill, I had literally never touched a CNC machine before and didn't know exactly what I was getting into. I had no customers (it was bought for another business venture that failed) so I didn't know how I was giong to use it, and there's allways the possibility of getting a CNC lathe at some point. It made sense at the time, anyway.

And just to clarify, I never refered to all this as a problem. I just wanted to get an idea of whether my electicity bill was inline with what others were paying in similar situations. Ofcourse, if there IS a problem I'd love to get it solved because these electricity bills are a much bigger portion of my shop rate than I initially figured.

Originally Posted by Viper Our other shop has 2 of them and they love power but even with 2 of those and three others, the bill does not exceed 600.

Do these machines run on a phase converter?

[gar]

080324-2019 EST USA

Matt@RFR:

We think your bill is excessive and therefore we think you have a problem.
But obviously part of it is the CA high cost of electricity.

I have assumed this is at your home and you have some other average base load that is part of your cost.

A good way to evaluate your base average load would be to record your watt-hour meter reading on a day that starts a number of days, maybe a week, when the shop will not be used, and record at the end and see the difference. Divide this by the number of days of the measurement, and multiply by the number of days in a billing period.

Do additional measurements when the shop is running.

If you keep all other loads constant and monitor the meter rate, seconds per rotation of the disk, or disk rotations in a longer time, then you can measure short time average power. Next keep the base load at the same level and run your phase converter only and see what the increase is. Then load the machine with a heavy cut and again do the measurement.

Note: the rate of rotation of the meter disk is a measure of power, and the integral of its rotation is watt-hours. It may have a calibration constant on it, or you may need to calibrate with an electric heater.


viper6383:

The SS may keep fans running because you can expect that in use more power will be dissipated in the cabinet. My mentioned measurements were with the coolant pump off. I will have to check Z load when in this state. However, I do not expect your fans are very high power.

My meter was an Amprobe, about 50 yrs old. I have a Hall device 3 phase power meter, but it is too much trouble to remove the wires to run an actual power test. I also have a Fluke Hall device clip-on probe. It generally correlates with the Amprobe.

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[bob1112]

Matt, Sorry, the SSs are on Delta 240V 3ph. I am headed out now to see what the VF1 pulls at steady state. I use a Fluke 336 for this and it has yet to lie to me. I would agree that something may be using some serious power like your wife left the hair dryer on for a month! I would really want to find the source of that draw if my bill hit half of that here. You may not have a problem but you will need to get a food meter and a pad of paper to figure it out.

[little bubba]

No Haas, but RPCs, a 10hp and a 20hp. The 10hp runs a 5hp machine and a 2hp machine and stays on at least 100 hours a week, a lot of times it runs 24/7. The 20hp runs a 15hp Fadal and a 25hp at 30minutes, mazak. Both can't run at the same time. The 20hp is off as often as it can be.

At around 15 cents per KWH, we are running about $400-$500 a month. Add in a 5hp compressor, air dryer, metal halide lights, heater fan, frig, stand up freezer, 4 computers that never get turned off, mercury vapor light that is on 24/7 outside.

I don't think its too bad.

On the 50hp converter. I think you are at a little bit of overkill. With a 20hp converter, on the Fadal, I was comfortably running at 18+ cubic inches of 1018 a minute. I hit 23 cubic inches, but the spindle started to bog.

For what its worth, the Mazak lathe has a tough time with the 20hp converter. We can only run it in low gear(1360 rpm max), and had to set the spindle ramping for 5 seconds instead of 0.3 seconds.

We just borrowed a 25hp converter so we can run both big machines at once. 3 phase is coming soon, also shouldn't be a problem running all 3 converters at once, 400amps of 220 coming in, used to be a welding shop.