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Three phase, three wire / four wire

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Scarr

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Hi all,

Got a question from a friend, he has a new milling machine that requires three phase, he's got a three phase supply with three wires RED/YELLOW/BLUE

He has connected these color to color in the cabinet on the miller but the miller runs slowly. There is a fourth wire in the cabinet that has been cut off (BLACK), he has tried to earth this without any change in performance.

Any ideas how he can determine what is the problem?

Thx
 
The fourth wire is neutral, it's not required for a completely balanced three phase load.

Perhaps he has two of the lives connected the wrong way round?, I've never had much dealings with three phase, so don't know what effect it would have - although I do know that if we lose one phase at work the goods lift motor just sits and buzzes!.
 
If you swap two phases over on an induction/sync machine it will just go backwards.

However, if it is a newer one imploying power-electronics (ie take 3ph, rectify it and use a power inverter to drive output machine) swapping a phase over would make no difference to the recification aspect (since it is phase-indiscriminate)


If it was a phase-dropped however, as Nigel pointed out IF it is a induction machine it would effectivly be in a stall (sync machines behave a bit better)

For power-electronic based drive you would get horrendous output torque ripple and you would end up shredding rather then milling and even nice hard carbine bits will shatter.


What I suspect is you have an induction machine based millar (Power-electronic ones are rare unless at high-end full produciton based).
Now the speed of the induction machine is proportional to the supply freq.

This millar wasn't bought from a 60Hz region and now trying to power from 50Hz was it?


Or what is the power requirement of the Millar with respect to the power output of your main 3ph supply. IF you are saturating you isolation XFMR you could be artifically imposing a V/f effect that would not be optimum
 
gerty said:
Could it be the machine is connected to 208v and needs to operate from 480v?

induction machine speed not related to voltage unless getting towards its power limits
 
Styx ,agreed, and it could be a combination of low voltage and 50/60 hz problem. We won't know any more till the OP gets back ... :D
 
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