Three Phase Motor / Inverter / Absolute Basics

[aesmith]

I'm trying to get my head around the basics. The context is the need to run a 2HP motor off single phase 240V. I have no hands on with three phase, only hearsay and some possibly incorrect theory.

Can someone correct me if I'm wrong on the basics, which that a three phase supply is effectively three 240V supplies offset by 120 degrees. So each phase is 240V relative to Neutral, and 415V relative to either of the other phases.

If that's correct then I think it follows that for a motor with three sets of windings, if wired as Delta then each set sees 415V, if wired Star then each will see 240V.

Can anyone confirm, or also point me to a write up if I've over simplified and missed some important points?

 

[jenki]

Well worth getting book 16; Electric Motors by Jim Cox in the Workshop Practice Series.

 

[Martin-Sy]

Can someone correct me if I'm wrong on the basics, which that a three phase supply is effectively three 240V supplies offset by 120 degrees. So each phase is 240V relative to Neutral, and 415V relative to either of the other phases.

Yes, that is correct but I would phrase it the other war around ... a single phase supply is (usually) one phase from a 3 phase system.
In most urban distribution networks there will be a 3 phase cable in the road supplying one of the phases to each domestic property.
The actual phase supplied will vary house to house so that the overall load back at the substation is roughly the same on each phase.

If that's correct then I think it follows that for a motor with three sets of windings, if wired as Delta then each set sees 415V, if wired Star then each will see 240V.

That's not always the case. It is true that a lot (but not all) of 3 phase motors have windings in star topology for 415V operation.
These can usually be reconfigured for operation in delta at 240V.
Often the rating plate will indicate this capability and there will be links to reconfigure it in the terminal box.
In some cases you have to dismantle the motor, find and disconnect the star point then route all the coil connections out to allow them to be connected in delta.

HTH

 

[aesmith]

That's not always the case. It is true that a lot (but not all) of 3 phase motors have windings in star topology for 415V operation.
These can usually be reconfigured for operation in delta at 240V.

Thanks. This is the sort of thing I find hard to follow. If each phase is 240V phase to neutral, then why does a star configuration see 415V? Equally if it's 415V phase to phase, how do you get 240V across a winding in delta? That's the opposite to what I thought I'd worked out.