I have never tried to run them "unbalanced", but the electric company
put in a new sub station on my line a couple of years ago, which caused
the inbound single phase 220 to change voltage values (I am on a rural
line). I dont rember the exact alarm number but it alarmed (and it was
electrical supply flagged) as soon as I tried to power up. I had to go
with another tap on the transformer and re-balance the caps. they were
11 and 12 between phases and I ended up with 9 and 14 IIRC (these are
50mf units) to get back in balance. Since that no problems. My CNC's
use almost no "real" 3-phase, nearly everything except the hydraulic,
coolant, and chip auger motors is DC. The spindles use a thyristor to
control them. If your CNC does enough internal conditioning you could
get away with unbalanced 3ph, but most (in my experience anyway) don't,
and you are taking a chance with some very expensive components IMHO.
Caps, even run caps, are a lot cheaper than boards.
Thanks for the information Dave.
Your rotary converter is surely the biggest I've "encountered". Thats a
40 HP idler with 23 capacitors of 50 microfarads each for balancing. Wow,
thats a nice set up.
No problem Jerry, I could never have got the thing done without a bunch
of help from members of this group. It has been a few years ago but if
you go to google groups and search for me on this group and the term "3
phase power" power you can still see the origional thread where I
decided to upgrade from 30hp, it has some very good info. on this
subject and it was pretty much all correct. Just tryin to pass it along
The unit I purchased was supposed to be balanced out for 5-7hp. It's
got 4 caps, here's the info from what I posted before. Well I don't
know which is which but there are 2 smaller ones that have
540-648 uF 125 VAC that look like they are wired in series and 2 larger
that say 70UF 240VAC on them looks like wired in parallel.
Motor run caps should have no problem as long as they are rated for the
voltage. It sounds as though your buddy was using electrolytic motor
starting caps and voltage was applied to them too long and/or too often
without enough cool down time.
Motor run caps or power factor caps are rated for across the power line
You may already know about the negative results of unbalanced 3-phase
voltages, concerning increased motor temperature and reduced performance.
A NEMA pocket reference guide I have states that operation of 3PH motors
above a 5% voltage imbalance is not recommended.
The example in the booklet shows an average unbalance of 2.3% for phase
voltages of 220, 214 and 210V (the average being 215V and the maximum
deviation from the average being 5V).
I don't have any 3PH or CNC equipment to comment about real-world effects of
line voltage imbalance.
I'm guilty of not being real detailed in my research on 3 phase motors
that run from single phase. I sure didnt measure the temperature rise of
the 3 phase motor when running from single phase. I did build a little dyno
that loaded a 3 phase motor then ran alot of plots of its HP for various
idlers and capacitor balancing. This data made me think I knew all I
needed to know about 3 phase moptors fed single phase. Now that I think
about it, I still have alot to learn. But, for alot of home-hobby
porposes, a 3 phase motor can provide its full name plate rated HP even when
supplied single phase, with *no* idler. Thats about the limit of
"imbalance", and certainly way beyond the 5% indicated to be max allowable.
The 3 phase motor must be supplied some sort of power to the third wire of
the motor to start it spinning prior to being heavily loaded.
The motor's RPM, at full name plate HP load will be measureably lower than
it will have with 3 phase power supplied to it.
I'm certain that a lot of folks have benefited from the many RCM discussions
of using 3PH motors in home shops.. I certainly have, even though I don't
have any 3PH machines.
Your acquired 3PH testing experience has definitely provided a good level of
working knowledge for the inexperienced, like me.
I've worked with conveniently available 3PH in industrial maintenance,
although it was mainly limited to equipment installations and motor