3 phase CNC

Thread Starter

wberminio

Joined Feb 4, 2008
2
Hi
I'm installing a new CNC router system in my wood shop.
I have a 3ph 4HP spindle that draws 17 amps/230v
Also 16hp blower 43amps/230v

One leg is 216v to ground/the other 2 are 120v each. Is this normal?
Are all legs to be almost =.Is this safe for my equipment?

Thanks for your response

Erminio
 

John Luciani

Joined Apr 3, 2007
477
You should have a licensed electrician determine the
problem.

All of the phases should have equal voltage. For the phases
that have a low voltage you may be drawing too much current
which could cause overheating.

The situation does not sound safe for either you or your
equipment.

(* jcl *)
 

Electra Guy

Joined Feb 4, 2008
17
Sounds like what you may have is a three phase 208/120 Vac power source.
On this type of transformer one leg of phase will read 208 to ground while both other phases will read 120 to ground.
 

Thread Starter

wberminio

Joined Feb 4, 2008
2
I had the power company come out.They checked the line voltage and said that
the voltage was normal.My electrician was also looking into it.I just wanted to an expert opinion.

Thanks for your help

Erminio
 

subtech

Joined Nov 21, 2006
123
It sounds like what you have is called a delta (maybe with 3 transformers, can also be done with only two) connected secondary with the X2(neutral) bushing of one transformer earthed or grounded.
This results in a three phase, 4 wire, 120/230 volt service. There will be 230 volts between any two of the three phases.
From two of the three phases to ground you will read 120 Volts.
From the third phase to ground you should measure approx. 208 Volts.
This connection is quite common for customers who require small amounts of both single phase, and three phase power.
If you are interested, an important check for you to make is the voltage at your machine, with it running normally. If the measured voltage is within +/- 5% of the machines rated voltage,(look on the nameplate, or in the literature that came with your device) then you've no worries.
Typically, motor damage occurs from being operated with voltages that are too low, (excessive current drawn) or voltages that are imbalanced. (which causes negative sequence currents to flow and can cause excessive heating)

Hope this helps
 

John Luciani

Joined Apr 3, 2007
477
Did the power company check the power inside your shop or outside?

I would be curious to know what type of three phase power you have.
I have had many 208/120 three phase drops installed. Eech one had balanced
phases -- 120V between any line and neutral, 208V between two phases.


(* jcl *)
 

gerty

Joined Aug 30, 2007
1,248
When I went to school (circa 1965) for residential wiring we were told about a 'wild leg' on 3 phase wiring, and being a residential wiring class we never got any further than talk about it.The main reason the instructor brought it up was to be careful to never wire an outlet or lights on that phase. Seems like having such a service would cause an imbalance , and cut down on the usasble slots in the service panel.I have done a fair amout of 3 phase work and have never seen this in practice.
Just my 2cents worth...
 

John Luciani

Joined Apr 3, 2007
477
When I went to school (circa 1965) for residential wiring we were told about a 'wild leg' on 3 phase wiring, and being a residential wiring class we never got any further than talk about it.The main reason the instructor brought it up was to be careful to never wire an outlet or lights on that phase. Seems like having such a service would cause an imbalance , and cut down on the usasble slots in the service panel.I have done a fair amout of 3 phase work and have never seen this in practice.
Just my 2cents worth...
That's interesting. Thanks.

I have never heard of a "wild leg". Googling yielded this from 3phasepower.org

Another system commonly seen in the USA is to have a delta connected secondary on the step down transformer with a center tap on one of the windings supplying the ground and neutral. This allows for 240V 3 phase as well as three different single phase voltages (120V between two of the phases and the neutral, 208V between the third phase (sometimes known as a wild leg) and neutral and 240V between any two phases) to be made available from the same supply.

(* jcl *)
 

zekeman

Joined Jan 17, 2008
1
quote"
I would be curious to know what type of three phase power you have.
I have had many 208/120 three phase drops installed. Eech one had balanced
phases -- 120V between any line and neutral, 208V between two phases."

FYI
Your 208/120 3 phase is simply 4 wire WYE connected. His is delta with a common between two legs, i.e. centertapped on transformer between two of the legs. Then the third leg to the common is 208 , called the wild leg ;the line to line is 240
 

John Luciani

Joined Apr 3, 2007
477
FYI
Your 208/120 3 phase is simply 4 wire WYE connected. His is delta with a common between two legs, i.e. centertapped on transformer between two of the legs. Then the third leg to the common is 208 , called the wild leg ;the line to line is 240
Thanks for the information. I googled around last night and found a delta diagram
that had a centertap between the two legs.

Is this a common configuration? Why would you use a delta with a common rather
than a WYE?

(* jcl *)
 
This phase of power 208/120 is currently used in one of the places I work at. I will tell you for a fact that the "Wild" leg does happen. On this system phase 1 & 2 are 120 to ground. Phase 3 is 208 to ground, but all three are 240 phase to phase. It REALLY freaked me out the first time I saw it.:)
 
Top