Thread: Running larger machines on single phase

Interested in your experiences running larger machines on single phase - and what you've found to be the practical limits in terms of HP or particular functionality.

Situation is this - no mains 3ph anywhere near my place, just LV 2ph on the pole. Currently have a 64A-1ph supply to the property, and good 32A supply to the shed.
Theoretically 32A is roughly 7.5kW (10HP), but I'm well aware that startup current etc can be LOTS higher depending on inertia and machine characteristics. Too much voltage drop from that can cause all sorts of control weirdness too, with contactors dropping out etc.
Soft starting using VFD's or using clutched machines that only spin up part of the machine on start can be used to reduce that start current requirement.

I'm looking at getting a slightly larger lathe, but they typically have 10 or 15hp motors. It would be a clutched spindle style.
My understanding is that as long as I don't need to use the full HP capability of the motor, (and can soft start the main motor) - that its possible to run a larger motor that what your supply can theoretically power.
ie I should be able to run a 15HP machine on my theoretical 10HP supply - but if I load it up and try and consume 12HP the supply won't cope and the main supply breaker will trip.

Is that basically correct, and if so - does it work in practice?
Anyone actually doing it?

Steve

To clarify, are you looking at getting a VFD with a 240 volt single phase input and running a 15HP motor in 3 phase delta (240 volt) configuration?
I thought this type of VFD was limited to smaller motors?

If you can find a suitable VFD, the specifications will give you the maximum input and output load ratings.
You would also need to look carefully at the motor before you purchase it. Many larger 3 phase motors only operate on 415 volt delta and cannot be run on 240 volts.

Another option may be a rotary phase converter, however it will be expensive. You would also need to check the load ratings very carefully
before purchase.

Cheers,
Paul

Thanks Paul.
I already have numerous 3ph machines running off VFDs and/or RPC, so I'm familiar with the motor voltage star/delta restrictions.
Basically what I'd be doing is running a 15hp motor on a 10hp inverter (7.5kw so readily available).

Steve

I haven't seen VFDs more than about 5HP available for single phase operation.

Maybe I’m missing something but from a quick look on eBay there seems to be plenty.

Here’s a screenshot from one of the Conon Motor BD600 ones



Steve

A couple of random thoughts on this

• motors are not 100% efficient, so one concern I would have with a large motor is how much 'overhead' power it would draw before doing useful work
• Fuses, circuit breakers etc cope with brief power draw over their rating at start up, but you would have to study the manuals to workout whether you are good or bad
• If you were to run a 15 hp motor on a supply capable of delivering 10hp continuous, a power meter would be a must to avoid nuisiance tripping

Not a sparky, just stuff I've picked up over the years.

Michael

Thanks Michael - that’s the sort of experience I was looking for.

A power meter is a great suggestion, and something I noticed on a machine I looked at recently.

Good point on the efficiency aspect too. I had a brief look into commercial RPC’s yesterday, and noted that for a 10hp model the suggested max size machine load/motor was 8hp.

Steve

I don't have any direct experience with VFD's, but a couple of things spring to mind. Does your 32A single phase supply to the shed need to run lights or other loads apart from the proposed VFD and motor? I notice that the 7.5 Kw VFD you used as an example in an earlier post states 39A input current, based on your 32A maximum, that reduces the 7.5Kw by a factor of 1.218, suggesting the theoretical 10Hp capacity would drop to around 8.2Hp. If those theoretical numbers hold true, then you are horribly close to only drawing half the potential output of your 15Hp motor which seems a bit of a waste. Having said that, I've probably done stranger things before.
I ran a few numbers into Fz Wizard and came up with the following scenario. Turning 4140 bar, 200mm diameter with a 4mm depth of cut and .3mm/rev feed using carbide tooling at 160 RPM (100 odd M/min) gives a figure of 6.8 Hp which is far from useless and still leaves 1.4Hp in reserve to account for transmission losses.
I wonder what the efficiency of electric motors is like when only run at partial loads?.

Thanks Karl.
I’d forgotten that FS Wizard has the power requirement - good suggestion.

Yes, my 32A supply does need to run lights, but they are LEDs do only a couple of hundred watts total.
It’s a valid consideration though, and I was discussing with someone along similar lines the other day about plasma cutters - ie that they require a compressor running at the same time.
We ran 16mm2 cable the 30m from main switchboard to shed, so I’m pretty sure I’ve got the option to move up to a 40A breaker if I need to - as the supply from the street is 64A.

Motor efficiency at different loads is something I hadn’t thought of.
I’ll look into that further, as it may be better to re-motor that run a big motor at half power.

Steve

Just did a quick bit of research into motor and VFD efficiency. What a rabbit hole!!!

Here's a few generalisations I noted:
Typical modern 3ph induction motors around the 10-15hp range are just under 90% efficient.
That efficiency seems to be pretty linear from full load down to around 30% and then drops steeply.
Using a VFD to drive the motor can improve power factor (practically I equate that to efficiency) compared to driving the motor directly from supply.

A 3ph 30kva diesel generator (yes, Chinese) can be had for ~$10k, uses 8L/hr at 100% load so roughly 40cents/kwh at current fuel prices.

Wonder if they'd miss the nice new 45kw Perkins/Stamford gennie from work......

Steve

Originally Posted by BobL

I haven't seen VFDs more than about 5HP available for single phase operation.

Quick look on Ecogoos website suggests they go to at least 22kw, 220v single in to 380 3 phase out.

22KW Frequency converter single phase 220v input 3phase 380v outputecogoo

Originally Posted by OxxAndBert

We ran 16mm2 cable the 30m from main switchboard to shed, so I’m pretty sure I’ve got the option to move up to a 40A breaker if I need to - as the supply from the street is 64A.

What, exactly, is the wiring situation to the shed (cable type and how is it installed)?

Regular white 2C+E (TPS) in 16mm2 could be rated from 34A up to 114A depending on just how its installed...

If it's running 30m, my guess is that it's run almost entirely in conduit underground, which most likely makes it good for a max of 86A (if you had the supply for it!)...

50A breaker would allow you to reach the full 15hp, and shouldn't trip on startup if using the VFD to ramp it.... Plus leaves you at least 13A in the house for the wife to boil the kettle etc. In reality probably more available in the house most of the time, unless you plan on productions runs making toothpicks out of 4" stock....

(I haven't actually got round to checking what the Graziano pulls compared to dataplate figures under various cutting loads yet, and won't be able to for at least a couple of months because reasons.)

Cable is SDI, about half the run through the roof of the house and the other half drops down the wall in external conduit and then underground to the shed.

Steve

Originally Posted by OxxAndBert

Cable is SDI, about half the run through the roof of the house and the other half drops down the wall in external conduit and then underground to the shed.

Steve

PM sent.

Hi Steve, wanting to send a PM, but your inbox is FULL.
Regards,
Kryn

Sorry Kryn - should be OK now.

Steve