[WallaceLau]

Hello guys - any electrician out there who can help answer this?

Just bought a Tormach PCNC 1100 (used), trying to wire it up and realized my office has industrial 3-phase power (as oppose to residential split-phase 120/240v). On residential power, if you connect the two "hot" legs, because their phase is 180 degrees offset you get double the voltage (240v). But on 3-phase, since the phase offset is 120 degrees, you get 208v (120v x square-root of 3). We also did not have a "wild" leg, meaning any combination (black+red, black+blue, red+blue) all reads 208v, and any legs against neutral (black+white, blue+white, red+white) reads 120v.

The question is, if I run the PCNC1100 from two of the legs that reads 208v, will it shorten the life of the machine? I am not that much concerned about voltage, the documentation says "200-240v single phase", so 208v is sufficient plus worse case scenario a buck-boost transformer is only like $200 installed. However, since the two phases are not 180 degree offset, I am wondering if I still get perfect sine-wave, or more importantly would it have detrimental effects on the mill over a long period of time.

Just so you know, I contacted Tormach and they didn't have a clue what 3-phase power is... I was told all I needed is a buck/boost transformer, however the fact that they have never heard of 208v left me a bit skeptical. A 2nd opinion from qualified electrical engineer or electrician would be wonderful!!

Thanks,



Wallace

[SCzEngrgGroup]

If the machines data plate indicates 200-240VAC is OK, then you'll be fine. The waveform between any two legs of a 3-phase line will still be a perfect sine wave.

Regards,
Ray L.

[byellin]

The buck boost recommendation is well founded. The machine will work without damage to it at 208, but the VFD that is in the machine does not generate rated torque without a boost to higher voltage. I believe most buck boosts will get up to 239 from 208. We run two 1100s at commercial locations with buck boost transformers. It's worth the small expense.

[WallaceLau]

I may have answered my own question... from this post:
explain 120/208v - Page 4 - Electrician Talk - Professional Electrical Contractors Forum
(which is part of this thread: explain 120/208v - Electrician Talk - Professional Electrical Contractors Forum)

It seems the combined sine wave drawn from the voltage differentiation on any 2 phases of a 208v 3-phase power is indeed a pure sine wave. Meaning if I run two of the legs I will effectively have a single phase 208v, pure sine-wave power. Anyone care to kindly confirm?

Cheers,



Wallace

[WallaceLau]

If the machines data plate indicates 200-240VAC is OK, then you'll be fine. The waveform between any two legs of a 3-phase line will still be a perfect sine wave.

Regards,
Ray L.

Excellent, thank you for the confirmation!

The buck boost recommendation is well founded. The machine will work without damage to it at 208, but the VFD that is in the machine does not generate rated torque without a boost to higher voltage. I believe most buck boosts will get up to 239 from 208. We run two 1100s at commercial locations with buck boost transformers. It's worth the small expense.

Ok that's good to know, this way I can start running some tests and get the machine setup before the buck/boost transformer arrives! One question - if I calibrate the spindle speed now, would it change once we boost the voltage to 240v?



Thanks guys - I am loving this forum!

[thefu]

that is correct.

[Al_The_Man]

Using two phases of a three phase supply (without reference to a star neutral) IS effectively single phase.

On residential power, if you connect the two "hot" legs, because their phase is 180 degrees offset you get double the voltage (240v). But on 3-phase, since the phase offset is 120 degrees, you get 208v (120v x square-root of 3).
Wallace

True, 208v, but when using 2 phases without the Neutral reference, the difference is 180° also.
Al.

[SCzEngrgGroup]

The buck boost recommendation is well founded. The machine will work without damage to it at 208, but the VFD that is in the machine does not generate rated torque without a boost to higher voltage. I believe most buck boosts will get up to 239 from 208. We run two 1100s at commercial locations with buck boost transformers. It's worth the small expense.

Torque comes from current, not voltage. Reduced voltage will, if anything, reduce maximum RPM.

Regards,
Ray L.

[Al_The_Man]

Torque comes from current, not voltage. Reduced voltage will, if anything, reduce maximum RPM.

Regards,
Ray L.

The only thing I would question there is that with an induction motor the RPM is a result of frequency, because this is produced artificially in the VFD, the max frequency will be the same.
In this case if the voltage is lower the wattage capability will be down.
Very similar to running a 3ph motor direct off of 3ph, if the motor is rated at 220 and it is run off of 208, it will still run up to the Same Frequency, just that the slip frequency will be affected slightly for the same load.
Al.

[MichaelHenry]

For what it is worth, Tormach appears to sell a transformer for this purpose:

32554 - Buck-Boost Transformer

$115 doesn't seem too bad to me, for a little peace of mind.

Mike

[WallaceLau]

Great and thanks for all the reply! I learn something new everyday. :-)

By the way, Mike, thanks for the link! I know Tormach offers a transformer but I didn't think their price would be any good, turns out I was wrong! McMaster wants $136 for the same 500VAC unit, plus tax and shipping. I guess I will be sending more business to Tormach...

For now I will start running the mill and get everything setup (leveling, gib adjustment, etc). May have to buy a gauge to adjust tram... and the real fun should begin once the transformer is installed. Wohooo!



W.