Thread: 3 Phase Power

I'm not electrician, but my understanding is that 3 phase has more "grunt" than single phase. i.e. if you need more than 15amps from a circuit, you need to go three phase.

Generally, the more "industrial" the machinery, the more likely it is to require three phase.

Someone correct me if I am wrong.

3 phase is essentially 3 lots of single phase in parallel, but you prob already guessed that. Because it is alternating current, and the three phases do not peak as the same time you can wire each winding in the motor across two of the phases. This is 'Delta' wiring and the winding sees 415V between the two phases (also referred to as phase-to-phase voltage). This gives the motor the highest tourqe but the highest inrush current so a star-delta starter is used to start the motor in a star configuration then swith it across to delta after about 10-30sec.
'Star' wiring is phase-to-neutral, so the motor winding only sees 240V, hence the lower inrush, but the tourqe is also lower.

You also get direction control of your motor with 3 phase but not with single phase. Great care is taken to make sure a single phase motor always spins the same way. In 3 phase The ordering of the phases in relation to the windings on the motor gives you the direction. If it is incorrect you just swap any pair (not neutral - it stays were it is - ALWAYS.

This brings me to VSD's. Variable speed drives basically work on frequency not voltage. the speed control is achieved by sending full voltage to the motor but lower than 50hz. Details sketchy but the range is something like 0-60hz to give you a little over speed depending on the drive and motor. There is tuning that works around voltage control but the principle is changing the frequency. To change direction the VSD internally swaps two of the phases for you.

If you are looking at serious machines over 3hp, 3ph is generally your only option. Goes for other industrial equipment like MIG welders too.

Wongdai - I too am not an electrician (not that that stopped me from wiring 3 phase machines in a passed life) but you are correct.

Hope this helps, and of course never play with things you don't understand without a grownup ready with a fire extinguisher.

Funky,

Base load electricity is generated in three phases and normally reticulated that way. This involves 4 wires, one for each of the three phase actives, and a reference line (Common) termed the Neutral. For maximum efficiency, the phases should be balanced, i.e. the load current drawn in each phase should be similar to that in the others. Individual phases have a sinusoidal waveform, and the phases are offset by +/- 120 degrees (1/3rd of a cycle).

For moderate residential use, houses are connected sequentially to an individual phase and Neutral, i.e. if your place is connected to phase A, then the neighbours on one side are most likely connected to phase B and those on the other side are probably on phase C. The upper limit on supply capacity for single phase services is generally in the 80 to 100A range.

For heavy duty domestic, commercial and industrial, premises are connected for three phase and neutral. Then single phase loads can be connected between individual phases and neutral, while three phase loads connect to all three.

I am not sure how your maths and spreadsheet skills are, but to understand the concept, try calculating and plotting the following equations in a spreadsheet.

Va = 240 x sq root (2) x sin (50t)
Vb = 240 x sq root (2) x sin (50t + (2pi/3)
Vc = 240 x sq root (2) x sin 50 t - (2pi/3)

over a time range of 0 to 60ms.

This should give three conplete cycles of sine wave, with a peak value of 240 x sq root (2), plus two similar waveforms offset by +/- 120 deg (2pi/3 radians). The sq root (2) factor converts the RMS volage level to peak levels for the plot.

Now, have the spreadsheet calculate the differences between the phases

V1 = Va -Vb
V2 = Va -Vc and
V3 = Vb-Vc

These difference voltages represent the normal connection for the generation and consumption of 3 phase power, and are also 59Hz sinewaves with 120 degree offsets between phases. However the magnitude is now approx 415 x sq root (2).


As a generality;

1. A general purpose 10A outlet can run 1 motor of 2HP max or multiple motors totalling not more than 2Hp.
2. A dedicated 15A outlet can run a single motor of 3HP max.
3. Beyond these levels, or for multiple units, three phase is normally required.
4. Three phase is more expensive to connect due to the requirement for more cable, breakers, consumption meters etc.
5. Three phase motors have better starting torque than equivalent power single phase motors, and can often fit into a smaller case.
6. It is possible to convert single to 3 phase using inverters or rotary converters. These devices are often cheaper to install than reticulated 3 phase power, but are less efficient and an require significant input current and dedicated wiring.

Hope this gives you some insight into this issue. Looking a the long term, I would be tempted to go 3 phase if the combined motor power exceeds about 5 HP, but this means that you have to also find something to convert single to 3 phase initially.

Both the Combination Machine and the Bandsaw I wanna get have 4hp motors

Thanks for the input

Originally Posted by malb

1. A dedicated 15A outlet can run a single motor of 3HP max.

Not questioning your electricityness, but I have a 5hp air con running on a 15amp single phase circuit, and it seems to work fine.

Originally Posted by funkychicken

What's the difference between 3 and 1 phase power? Apart from the different voltage

See I'm going to get some Hammer Woodworking machines and I can get them in 1 or 3 phase.
But I'm wondering whether I should build my shed here and then get the machines with 1 phase configuration (Because where I am now has no 3 phase power)
In a few years I'm planning on buying my own land and building a house and shed on that. So if I can get 3 phase power there and 3 phase has a significant advantage over 1phase then I get the machines in three phase.

That's why I need to know the difference between 1 and 3 phase



Confusing eh

while there are technical advantages with 3 phase,

Consider your situation:
Its not confusing,
you don't have 3 phase now.
you don't know if your new place will have 3 phase

Be safe and go for single phase machinery as it can work anywhere.
btw you are not limited to 3hp with single phase, you can have a high power electric supply, though the cost will be more than a basic domestic supply.

Originally Posted by Wongdai

Not questioning your electricityness, but I have a 5hp air con running on a 15amp single phase circuit, and it seems to work fine.

Is it 5hp of electric input though? My 7kW Aircon actually only takes 2.1kW of electric power to run the compressor and fans etc. Power of the crystal does the rest (well that, conduction and expanding gas)

If it was the only thing on the circuit, it would just survive on 15A anyway. (5hp*745/240=15.5A)...

Originally Posted by Robomanic

Is it 5hp of electric input though? My 7kW Aircon actually only takes 2.1kW of electric power to run the compressor and fans etc. Power of the crystal does the rest (well that, conduction and expanding gas)

If it was the only thing on the circuit, it would just survive on 15A anyway. (5hp*745/240=15.5A)...

And that doesn't take power factor into account, so you can typically add 20% on top, plus of course those lovely start currents etc.

Good point - forgot about those.

Funky

in really simple terms,
3 phase means 4 wires from the street to the house – 3 active plus 1 neutral.
a typical house will usually be wired with 2 phases,
houses with ducted air conditioning might be wired with 3 phases
houses which use gas (or wood) for heating and cooking, especially older houes in original suburbs, are usually only wired with a single phase.

domestic 3 phase wiring costs a lot more than single phase wiring because you need more conductors in a wire run.

at the machine level, you can run a 20A single phase motor (~6hp) provided the motor is hard wired back to the switch board – i.e. only the one item on the circuit. I've lived in houses where the electric stove was so wired.
you can also get 15A power points, which again need a dedicated fuse, but can run a ~4.5hp motor

big single phase motors typically cost a lot more than similar sized 3 phase motors – why doesn't really matter.

In your situation, talk to the Hammer folks, but I'd suggest buying single phase now, so you can use it, and upgrade to 3 phase in the future if you
a) need to, and
b) can afford to



ian

Thanks guys, that cleared it all up

With the larger single phase stuff recon a soft start would be worth investigating, due to the nature of motors drawing higher current (about 6 x run current) at startup.

Too bad most domestic oven makers seem to have given up on the idea of gas ovens, cause that dedicated 32 amp fuse in the board looks really good. Of couse the appriopriate sized wiring would need to be run and one would dedicate it to one outlet.

Just my 2 bobs worth.

Originally Posted by funkychicken

Thanks guys, that cleared it all up

Suuuuure.

This thread got me thinking - I have a separate meter and massive 70A contactor on my switchboard for the coil floor heating. Didn't recognize any of it as 3 phase but maybe is has to be. I wonder what it would take to.... Maybe I should talk to my electrician friend.