Single Phase vs. Three phase
Single Phase vs. Three phase
(OP)
I have two machine tools in my little machine shop. A milling machine wired 230 3 phase with a phase converter, and a lathe wired 220 single phase. I want to add another milling machine but it is wired 220 single phase. Or I can purchase a more expensive mill wired 230 3 phase. My question is, which is the most economical from a power usage standpoint, & from a power to the spindle (performance) standpoint etc. I know my lathe seams to stall at higher rpms when introduced to heavy depth of cut and feed rates. If this is normal of a 220 volt single phase motor, then I have answered my own question.





RE: Single Phase vs. Three phase
For a given kW, single phase will draw greater current (but only in two wires) vs. a 3 phase motor which may or may not require different size of wires/ switch etc., but it does not make a difference in kWh (energy) consumption.
It may turn out the the 3 phase machine is higher rated in kW and may have more power...but that is different.
RE: Single Phase vs. Three phase
Power-wise, a three-phase motor usually (always) has a higher efficiency than a single phase motor. So it is cheaper in the long run to use a three-phase motor.
The reason that the single phase motor is inferior is that the "fake phase" never is at 120 degrees from the other phases (as in a true three-phase system), but at an angle that could be as small as 40 - 60 degrees. So the resulting rotating vector is always weaker in a single phase motor and results in more slip (=more losses). The stalling that you have noticed is also a result of this.
RE: Single Phase vs. Three phase
RE: Single Phase vs. Three phase
How large a motor are we talking about here? How long and how often is it going to run?
RE: Single Phase vs. Three phase
RE: Single Phase vs. Three phase
Just an additional thought which is nothing new to the illustrious gentlemen who have already posted, but which may be a new idea to you:
If you purchase the 3-phase machine, you will very probably have the option to retrofit a variable speed drive to the machine at a later date. They are becoming quite cheap in the sizes you are considering, and you should get better performance from your machine than you get from the phase converter, and almost certainly better than you will get from a single-phase motor. Get the motor checked by someone who understands motors, as some motor winding connections are not suitable for use with a VSD.
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RE: Single Phase vs. Three phase
RE: Single Phase vs. Three phase
RE: Single Phase vs. Three phase
any efficiency gains are incidental and likely to be marginal in any event. You will make quite a saving when it comes time to replace the motor as three phase motors are maybe half the price of the equivalent single phase unit