The 3 phase question

[Steve GarnettParticipant @stevegarnett62550]

Posted by John Haine on 17/07/2012 09:32:56:

Actually, you don't even need to feed all the phase inputs, just one, though at high powers the input diodes might suffer.

That's why he did it that way. The reason for this device was that it was intended for use with a Hardinge capstan lathe, and these are dual-speed star-wired. I pointed out that switching the motor speed from high to low under power wasn't going to be a very clever idea – it's a good way to blow an inverter apart in a fraction of a second; this sequence of operations needs to be prevented by a safety switch. Dunno what the current status of this project is, though – he's currently working on a less ambitious project completely rebuilding a flat.

[JonParticipant @jon]

Posted by John Olsen on 16/07/2012 22:45:53:

It seems to me Jon that there is something wrong with that particular inverter. 4kW is about 5hp, you should be able to get a decent cut with that before it starts to struggle. Are you sure it is not set up to current limit at too low a value? What should happen is that the setup will work fine and maintain speed until you reach the current limit. You don't need to oversize the inverter to allow for any starting surge, since there is none. The inverter soft starts the motor by ramping up the voltage and frequency.

Its not just that smaller 4kw inverter its also a 4kw Transwave rotary convertor. Symptoms are both the same.

No soft start, its plug and play just the way 3ph 400V+ would be installed to premises.

Its not only powering the motor up but also have to bear in mind the additional load from belts/pulleys, gearbox and shear weight of chuck fitted at time. Even my current 7.5kw inverter with a small 5 1/4" chuck set on 2500rpm takes at least 10 to 15 secs, on 540 rpm its instant, bang.

I do have something not right some where this morning blew another 3ph coolant pump, its only been on 6 weeks. Since pump was running whilst attempting to start and stop lathe 70 times this morning, reckon it could be surge.

Duly ordered a single phase pump to get me out of the proverbial, jobs got to go out to Ozzy by next Tuesday.

[KWILParticipant @kwil]

All I can say is I have a 2.2kw 3ph motor on the Harrison M300 and a matching 3.9kva inverter. Absolutely no problems with control, running or lathe operations and use. The 2hp Bridgeport uses 2.8kva inverter with similarly no problems.

[JonParticipant @jon]

Above is on an M300 but not dual voltage.

A mate has just sent me this link, it seems i may have underestimated the startup, it could be anything from 3 to 10 times the motor rating so 22.5kw (30hp)! Also why last lathe 1.125kw (1.5hp) 240v blew a 13A fuse 1 in every 3 startups with no load.

http://wiki.answers.com/Q/Why_is_the_starting_current_of_an_3phase_induction_motor_higher_than_its_rated_current

[John OlsenParticipant @johnolsen79199]

When you start a three phase motor by directly switching it on line, it will take a large starting surge. This is because the armature will act as a shorted secondary on a transformer. Larger motors usually have more sophisticated arrangements for starting to avoid this, eg they will have a wound rotor instead of a squirrel cage, and they will switch resistance into the rotor circuit for starting. Incidently like any transformer, when you switch it online there is a random chance of hitting a point in the cycle which will give larger surge, which is why things will randomly blow fuses.

However if you use it with a VFD there should be no starting surge. This is because the inverter will start out at low voltage and low frequency and ramp the motor up to speed. The time taken to do this is usually adjustable but should not be shortened too much. It may need to be longer with loads with a lot of inertia. You should never switch between the motor and an inverter, the control should always be from the inverter, and any isolation switching should be upstream from that.

You don't usually need to size an inverter much larger than the motor, although it does no harm and can be useful if you later want to use the VFD on a bigger machine. You can set things like the current limit to suit the smaller motor and tweak it again later.

Steve, the possible problem with feeding single phase to a three phase inverter is that the peak current through the diodes will increase, as will the ripple current through the capacitor. There will be some margin in the design, but there is no way to be sure unless it fails. So you takes your chances…..Parallelling all the diodes should help them, but does not help the capacitor.

John

[KWILParticipant @kwil]

Jon,

A 10 second start ramp time on the M300 is more than adequate, even with a very heavy 260mm 4 jaw chuck. Have you been controlling via the inverter as John Olsen points out or are you just dropping the lathe onto the inverter through it's own switch gear?

[JonParticipant @jon]

As said before "No soft start, its plug and play just the way 3ph 400V+ would be installed to premises." Existing control panel untouched, inverter wired direct to main on/off rotary, retaining all functions.

An inverter is a means of changing DC to AC, misrepresented as a VFD.

At the time if i could have got the remotist of help from Direct Drives, the second inverter 4kw 400V would have been used as a VFD. Months passed lathe needed pronto as other Harrison was shot no time to figure the rewiring. Motor ran ok but never tried under load with 3 sec soft start which would have been tolerable.

Might get that 4kw inverter back if i can work out the lathe functions, should do but prob is where to wire in to the inverter, manual absolutely uselss blind you with science 100 odd pages but no ref to inputs. Or buy a Machine Mart motor and another cheap inverter 220v where you can get help.

I couldnt live with a 10sec soft start for higher revs, job could have been finished by then.

Currently using a 7.5kw 3ph 400V inverter 'Plug and play' meant for running whole workshops. Only problem i get is a live to neutral unbalance so trips the main RCCD at anything starting lathe up past 540rpm.

What sort of cuts can you put on power feed running at 220v 3ph?

[Steve GarnettParticipant @stevegarnett62550]

Posted by John Olsen on 17/07/2012 21:58:13:

Steve, the possible problem with feeding single phase to a three phase inverter is that the peak current through the diodes will increase, as will the ripple current through the capacitor. There will be some margin in the design, but there is no way to be sure unless it fails. So you takes your chances…..Parallelling all the diodes should help them, but does not help the capacitor.

Whilst all that's true if you load the inverter heavily (and I'd suggested adding an additional capacitor anyway, as the particular inverter allows for it) it wasn't really an issue as this particular inverter was completely over-specced for the job – by about 50%.

Most halfway decent inverters allow for short-term overloads, often up to 200% or so. The diodes they use on the input side are pretty hefty anyway – they have to be!

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