3 phase questions

[Alan Turner]

In our new building we havde three phase power, which is great, but it is 208 volts. It seems that some of the motors that this is destined to run are 220 volts, some 230, and some 240.

Help!!!!

We were told that a $4000 transformer would kick the voltage from 208 to 240. Or we could try to get the electric company to change the incoming power, but I don't know if they would, or what the cost, if any, might be. Is this necessary? If most of the motors are either 220 or 230, is there a cost effective way to transform some of the power to 240? What is safe for these motors in terms of voltage variation. If we kick it to 240, either way, will that still be OK for the lower voltage motors?

I do not understand either 3 phase power, or the voltage ranges, and any help or guidance would be much appreciated. I have talked to both electricians, and electrical engineers, but can't seem to understand what they are saying well enough to develop a cogent a plan of action.

Thanks in advance.

[Larry Reimer]

Alan, if you've already talked to electricians and engineers I doubt I can help, but that won't stop me from putting in my two cents worth.

A 208 volt three phase service will provide you with 120 volts from a single phase to ground. If you increase that 208 volts you will no longer have that 120 volts (it will increase also)

A motor will draw the watts it's rated for. That means if you lower the voltage to a motor you will increase the amperage draw (the watts used will stay the same). Increase the amperage and you will increase the heat of the motor, that increased heat may shorten the motor's life. Electric motors are typically designed to take a ten per cent variance in incoming voltage with no ill effects.

If it were my shop I would not change the incoming voltage. I wouldn't even bother with changing the voltage to a certain motor or two. I would hook that motor up to the 208 volts and run it. The motor that's rated at 240 volts and only has 208 volts coming to it will run warmer. That MAY shorten it's life, but there's a lot of other variables in that too (running at full load all the time?, dirty environment?, bearing condition?...)

[John Hart]

I'm inclined to agree with Larry...Back in my aerospace engineering days, we worked regularly with 3-phase motors because most aircraft used that or DC. Variations on aircraft power were fairly wide and aircraft specifications required that we test over a wide range. At the ratings you are describing, I would expect an increase in heat generation, the failure, if any, would be premature bearing failure. Now that doesn't mean you can't compensate by using a better grease in motor bearings or by adding additional air flow over the motor to dissipate that heat, especially if you intend to run it for extended periods (hours).

But the bottom line is that you're not that far from standard tolerances of the motor design anyway. Keep the machine clean and I doubt you'll have a problem.

[Juan Rivera]

I also agree with Larry but I'll add my 2 cents. First I would check the available voltage at the sites (receptacles) at which these machines will be located. You'll notice that the available voltage can vary on an average -/+ 5%. Most transformers have taps (connection points) that allow the installer to connect the service -/+ 2.5% of the available voltage (some even more). The last thing I would check is with manuacturer of the motor and and ask if there would be a problem running these motors under the "available voltage". You should ask for the technical department when you call the manufacturers since the customer service reps are rarely qualified to answer these questions.

Good luck!

[Ralph Barhorst]

Alan,

I was an engineer before I retired and I designed many 3 phase power supplies. I agree with the other responses.

From my experience, I would say that you will not have a problem if the actual incoming voltage is at least 208 volts. Have your electrician check the taps on the transformer and boost the volts if it is less than 208. Don't boost it too high as that will also boost the 120 volts.

Most motors are designed for a certain "duty cycle" at a "rated Amps". The motor is tested at this duty cycle and load to ensure that the motor will not overheat is use.

If the motor is in a machine that will run all of the time then the duty cycle must be 100%. It could be rated at say 50% which means is runs 5 minutes out of 10. However, the motor is also rated to run at its "rated load" during that time. For simplification, the rated load is the "Amps" rating of the motor.

If you are using these motors in woodworking, there is a good chance that you are not running at the rated amps unless you are cutting very thick material and you are not running at rated "duty cycle" unless you are cutting this thick material piece after piece without stopping. It would be impossible for this to happen if you are manually feeding the lumber. At least I know that I could not do it. :D Also, many motors have a thermal overload that trips if the motor is overheated. As long as you have not tripped the overload, you will be fine.

[markus shaffer]

Alan,

Just to put your mind at ease, I am runnning three separate machines on 208 volt three phase power with no problems. While I can't give you any technical advice, I can say that all of my machines have run fine at 208 volts. Two of these (jointer and feeder) often run at the same time if that means anything.

-Markus

[Chris Rosenberger]

You need to check your motors. Alot of 3 phase motors are rated to run on 208 to 230 volts. Out of the 15 three phase motors in my shop, 2 are rated at 230 volts only. Both of those are imports.

[Alan Turner]

Thanks, guys. So, to summarize (really to see if I understand your guidance). I should bbe OK since these will not be used in a continuous duty setting, at least up to a 230 volt motor. And, I will check the voltage. But, will it also be OK at 240 volts?

[John Hart]

Yup-sir-ee

[Timo Christ]

Hello,

there is another option to run your motors: use a VFD or Variable Frequency Drive (= Variable Speed Drive). This is an electronic device which can change the voltage and frequency of the power feed to your motors. In this way, you can control the RPM of your motor easily.
VFD cost depends on output power, but it will be less than 4000$ unless you have a very big motor to run..
You'd need one for each motor.

Regards
Timo

[Alan Turner]

Timo,
The 4K off the cuff estimate was for a transformer which would handle the enitre 800 amp service for the enitre building.

[John Hart]

My two cents for what it is worth...The variable speed drives are a device that takes the available power and re-creates the frequency through the use of SCRs...Silicon Controlled Rectifiers. The SCR is a power component that provides a high power output to the motor but can be controlled so that the frequency of the signal is variable. This allows you to control the speed of the motor. Because your application is more suited to a stable, non-variable frequency, I would think this is an unnessary expense.... Although, it would make your shop very high-tech! :)

I have tons of experience with running 3 phase motors under full load 24 hours a day, from -65 degrees to 150 degrees F, and from sea level to 30,000 feet, over a full range of voltage levels. These tests were designed to put motors through the most extreme possibilities. The worse conditions were at low voltage and high ambient temperatures... But still, you just don't have the duty cycle that would hurt this motors. If you were uncomfortable, you could always attach a low cost temperature gauge to the motor housings and give them a rest if they got too hot....like maybe 160-170 F.

[lou sansone]

hi alan

all of my shop is 208 3 phase and I have not had any problem with running any of my motors, up to 20 hp. most motors will tollerate 10% off of the name plate voltage, so 208 is within that range.
lou