Single Phase vs. Three phase

[reamer]

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.

 

[rbulsara]

It does not matter for power (kWh) usage if the motor is single phase or 3 phase. It will consume the energy at the rate of its rating (KW or HP).

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.

 

[Skogsgurra]

I would like to add that a single phase motor (usually a three-phase motor with a capacitor to produce a "fake" third phase) has less starting torque and also less overload capability than a true three-phase motor. If there is a choice and if you have three phases available then I think that the three-phase motor is the obvious and best choice.

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.

 

[aolalde]

I agree with skogsgurra; On the long run the three phase motor consumes less electrical energy to produce the same mechanical output (Horse power).

 

[rbulsara]

I agree too that in theory 3 phase motors are more efficient, but I am not sure it is a factor when considering a tool machine in a "little" shop.

How large a motor are we talking about here? How long and how often is it going to run?

 

[reamer]

Thanks for all of you help gentlemen. In answer to rbulsara's question, the motors typically are 2 horse power on the size mill I am looking to purchase. This milling machines motor will experience some load when machining steel.

 

[ScottyUK]

Hi Reamer,

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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I'm getting a great education!

 

[panelman]

Infact, you can probably use a drive to convert single phase to three phase for a small motor like you need

 

[jclough]

I suspect that any efficiency gains you get by using a three phase motor over a single phase motor will be lost in the inverter or converter. One may want to consider the cost of the phase converter also. Long story short...If you aren't supplied with three phase power from the utility, the single phase motor will be cheaper in the long run.

 

[panelman]

IMHO the point of using a drive to convert single to three phase is that you get more torque thus allowing deeper cuts and higher feed rates…you also get the advantage of variable speed

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