Question
We have just acquired property to expand our business. Three phase power is available from the power company. Apparently the way the local utility charges is peak for the month, so the minute we start a hard starting machine, like a wide belt sander, that is what we are billed at for the month. It seems that ganging phase converters might be the way to go. Any comments?
Forum Responses
(CNC Forum)
From contributor L:
It is commercial power. Is the single phase priced out different? There is no demand on the single phase when it is in a commercial area. I don't know, just asking. I have 3PH power at my shop and the demand charge is determined by the highest power consumption in a 15 minute segment.
As others have said, it is not the momentary startup surge that spikes your demand charge, but rather the average current draw over a given period of time (15 minutes I believe here in CT).
Rotary converters are not magic wizards that make free electricity. It takes single phase 220v to spin the phase converter motor. That is being drawn from the grid. The 3rd leg being generated could be considered free, except for the fact that you are paying to spin the converter.
You also have the two legs from the grid going to each 3-phase motor. So since you are now spinning your machine motor with the single phase portion of the power system, and spinning the rotary converter motor, you are actually going to be using more power than if you were using straight 3-phase from the grid, at least theoretically.
To counter this whole argument, though, I'll add that I've had one or two people tell me their power usage and electric costs went down when they were using a rotary converter. They felt it had something to do with the rotary converter back feeding into the grid. Personally I don't see how this is possible.
Our single phase electric bill is anywhere from 1,500 to 2,000 a month and we are only a 3 man shop. If I could get 3 phase power here I definitely would.
On a commercial 3-phase service, the power is often cheaper per watt hour than a residential service. This is often due to a higher rate paid. As you use more electricity, you pay a penalty. In a commercial shop they want to sell more power to you and often the higher rates are not applied for higher use.
In terms of feeding power back to the grid, this is correct with some converters as when the motor slows it is using regenerative braking. This power needs to go someplace and it will go back into the grid. Much like how electric cars charge their batteries when slowing down. I am not sure if most meters will show the power going back into the grid. Obviously you had to use power to get the motor started and running, so it is not really generating a net gain as if you used a solar panel, but the power from the motor slowing will have to go some place (the motor does need to use a dynamic brake).
I use a digital phase converter which uses about 750 watts of power when idle, far less than a rotary converter. My machines do not know the difference between my generated 3-phase or power delivered by the utility. All of my phases are within a few volts of each other. The converter corrects for power factor errors, so the amp readings are much lower. I used to use a rotary converter and had issues starting a large shaper; it just did not have the backbone to generate the power. The digital converter runs my CNC machine perfectly. Since I am pulling less amps, the wires do not have to be as large as with a rotary converter.
When you say "digital" converter, I assume you are referring to a static converter. It is my understanding that these send a surge to the motor to get it started, but then the motor runs on single phase 220v. This reduces the power output of the motor by approximately 1/3. There is a lot of misinformation out there on this topic so I cannot state for 100% certain that this is the case.
I ran my 15hp Timesaver on a 10hp rotary converter quite successfully for years. Never a problem starting or running, as long as I didn't overload the sanding capacity of the machine.
I was always told that the generated 3rd leg from a rotary converter needed to be on one specific leg of a machine. I don't know for sure why this is the case, but I theorize that it's because you wouldn't want the generated leg on the coils and electronics of the machine circuitry. Perhaps your machine gave startup problems because of this? Did you pay attention to which input leg on the machine had the rotary converter generated 3rd leg?
With the Phase Perfect all the legs are the same voltage, and it does not matter which leg is used for the electronics; there is never more than 1-2% difference in voltage. This is often better than 3-phase from a utility - as the legs are often not balanced due to single phase loads taken from 1 or 2 of the legs, thus resulting in lower voltage.
I had 2 Kay Phasemaster converters for running my shop equipment (before CNC) and could not get my Martin shaper into the second speed - it has a 2 speed motor and is very hard on the power to get to the second speed, almost like shifting a car from 1st to 4th gear. I ran a widebelt sander and other machines without issue on the rotary converters.
One of my main concerns with a rotary converter is that it is sized to a particular load, and if this load is not there, the voltage is very high from the generated phase. This is why rotary converters have a minimum motor size.
After changing to the Phase Perfect, my machines started faster and quieter. I have a 30 hp model.
I had the power company come out and try to help figure out how to beat the demand meter (I even tried to get another meter set), and the only way you can help it is to turn large motors on, and let them come up to speed before turning the next one on. My electric bill is about $900.00, and right at half of that is the demand charge. If I had 3 phase available, I would not have the big start up load with the 60 hp converter, so that would reduce the bill some.
They also explained that the demand charge is because I do not use a consistent amount of electric. If I used a small amount or a large amount all the time, I would be a better customer for them, but when you hit them with a large load for a little bit and then it tapers off, they have to purchase that power on the spot market, which is a lot more expensive for them.
