Star or Delta

Hi I am fortunate to have a spare 240v single phase to 240v three phase and a 240v single phase to 440v three phase in my workshop. After buying a high speed milling head I need to use one or the other to power the motor which can be wired star or delta.

Are there any advantages or disadvantages with using 240v or 440v three phase on an induction motor (2 pole, 375W)?

Thanks

Barry

Be careful, normally motors that are dual voltage must be connected in star to run off a 415V supply and should be connected in delta to run from a 240V supply. The output power will be the same as the phase current is 1.732 times greater when connected in delta on the lower voltage (240V) supply.

HTH Dave

Probably not, I thought star was for 440V and delta for 240V, no doubt the experts will enlighten us.

STAR = 415v

Delta = 240. Noel.

Maybe wrong but it's my understanding 415 is the better option

H

I'm not sure what the question is really, it's a bit garbled. But for the avoidance of doubt and for about the 10th time, as far as a 3 phase supply is concerned, if you have a 240V phase to neutral supply, the phase to phase voltage is 415V. "240v" and "415v" 3 phase are THE SAME THING. 415 = sqrt(3) x 240.

Although not explicit I assume it is implicit that all voltages are phase to phase. In theory running 415V star or 240V delta shouldn't make a difference. There might be a small advantage in using star as the lower currents might result in lower resistive losses.

Andrew

Small motors are often wound for 415V star and 230V delta. When motors get large enough for star delta starting to be useful they will be wound for 415V delta and around 690V in star. A motor large enough to require star delta starting is unlikely to be encountered in the average home workshop. Always read the plate on the motor to be sure of what you have. A plate that only states 415V may not have the star point readily available and require some delicate digging to access it and rewire in delta. I am sure there will be a small motor somewhere that is wound for 415V delta which will be unsuitable for a typical single phase input VFD although single phase input with 415v output VFD units are available.

Mike

For clarity I have 240V single phase in my workshop and have two options for running a 2 pole 375W motor.

The motor is capable of either running 415V star or 240V delta

(1) I have a spare 240V single phase to 240V VFD, and

(2) I have a spare 240V single phase to 415V VFD

My question was which of the two options (1 and 2 above) is optimal? From what has been said here and on the web there is little difference.

Thanks for replying

Barry

Barry - I suspect that you already understand the star/delta 415/240 issue. My feeling (without specific evidence!) is that if you have "real" 3-phase supply then the star/415 connection would be better as currents are lower so slightly lower cable loss - but this is unlikely to be of any real significance for such a small motor. However, a 240V in/out VFD will use the incoming supply directly via a rectifier/capacitor arrangement where the 240/415 in/out VFD will need an extra voltage step-up stage. Therefore I would expect the overall system efficiency to be slightly less in this case. But this is all a bit theoretical and the differences will be barely measurable.

So, the "engineering" thinking - in this case, use the 240 in/out VFD with the motor delta-connected. This leaves the other VFD free for the day you have a motor without the delta option!

Nealeb has answered the question very well - and my thoughts exactly ! Noel.

Using some (any) conversion from single to three phase will involve some energy loss. That is for certain, definite and unavoidable. Therefore direct from the mains is always cheaper to run.

If a VFD, there will be some reduction in the sine wave purity. Unavoidable, however minimal.

VFDs always produce some energy losses in the form of mains/radio interference. These can be minimised, of course, with suitable filters.

A VFD may need a different circuit breaker, due to unbalanced input/output currents or other losses.

Overall simpler, if the three phase mains is available and already being paid for, to utilise that supply with the motor in star connection (if that is appropriate for that motor).

Edited to add:  The VFD option, if that is what it is, would likely afford a deal of programming - such as variable speed, soft start, etc

Edited By not done it yet on 29/09/2022 10:11:31

A 240 to 415 VFD is much more expensive than a 240 to 240 one. I suggest saving the 240 to 415 one for when you get a piece of equipment that has a star only motor. Even if you have raw 415 available there will be some time when the speed control on that extra machine is desirable.

My understanding is star is better than delta for load-balancing reasons when many motors are starting, stopping and drawing current more or less randomly. Most likely in a factory or perhaps a largish workshop with several employees. High voltage also reduces resistive losses in the distribution wiring and there's lots of that in a factory, which becomes important if greedy equipment other than motors are connected - big ovens, welders, etc.

For small scale use the advantage lies with delta motors, but I doubt the difference is detectable in a small workshop. If I had both high and low voltage 3-phase I wouldn't bother changing motors from star or delta, I'd just plug them in to the appropriate supply.

Industrial best practice is worth knowing about but it's not always useful in a home workshop. There's an enormous difference between a hobbyist using a few simple machines in his shed and large-scale manufacturing! The latter has to optimise almost everything, and are likely to take professional advice from electrical engineers and others when configuring production: in that scenario a few percent shaved off the electricity bill is big money. Home workshops are much less fussy - we tend to do low intensity general-purpose work, when it suits us, and don't work against the clock to make a profit. It's not worth chasing micro-efficiencies, unless of course, it's done for interest or learning!

Dave

You don't mention the sizes of the VFDs. The motor is small at 375W so use the smaller of the two VFDs. If both the same do as Bazyle says and use the 240 output one and save the 415V for when you don't have the option.

Robert G8RPI.

What are these high and low voltage supplies If you mean 415 or 240 then they are the same supply!!! It just depends on how you measure them.

I would suspect the 240>415V is a converter, not a VFD. A converter will be either static or rotary. Running a rotary converter obviously means running another motor, for a start. On top of that, I believe the rotary converter requires ‘changing over’ every time a motor may need to be reversed (it seems that that of doubleboost is like that?).

Static converters are a compromise on motor size, so must only be most efficient for one power output?

Personally, I would be changing any motor from star to delta and running a 240V output VFD in most circumstances - unless I had a 415V grid supply.

Edited By not done it yet on 29/09/2022 13:44:37

I mean, you do see both 240v per phase (commonly called 415V) and 400V per phase (commonly called 690V) three phase supplies in fairly common use...

I always felt that the line to line voltage of the "normal" three phase being similar sounding to the phase voltage of the the "added beans" supply, added an unfortunate element of confusion which wasn't really helpful and meant you had to be just a bit more specific in your language when talking about installations with both systems in them...

In any case I doubt OP would have a supply like that mind, and even if they did it would still be "Low Voltage" (i.e. 50V < P.D. < 1000V) just a non-standard one.

That's down to his particular setup, rather than RPC's in general; if you watch any of Keith Rucker's videos you'll see him frequently reversing motors on a three phase supply that comes from an RPC.

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A well made RPC, using a modern motor, with the phases balanced and tuned to the desired range of load outputs, should be highly efficient too, with about 95% efficiency at the power rating they're tuned for, and up to about 90% across the full range of possible power outputs (smaller RPC's will see a drop in efficiency, commensurate with the drops in efficiency smaller electric motors have).

The off-load power consumption of a RPC is directly related to how good a motor was used as the autotransformer, I know the old rule of thumb is for the no-load power consumption being 30-40% of full load, but with modern IE2 & IE3 motors the reality should actually be much lower... Like 15% (for a 0.75kW load) going down to 6% (for a load >30kW).

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Which fits my experience the 11kW motor in my RPC consumes about 8-9% of full-load at no-load (which is actually slightly better than the manufacturer's databook value), combined with the power factor of 0.88, and the new price cap price of £0.52/kWh that means it would cost me about £0.40 for each hour it's left idling...

That's not nothing, but it's also sufficiently small that it would need to run on idle for several thousand hours before it cost me as much as buying 7 VFD's for all the 3-phase motors in the workshop, and I have the option to turn it off when it's not actively in use.

The OP clearly states in his second post that he has two VFDs