3 phase inverters

inverters

Dave,

I set one up to drive a compressor this way via an autotransformer kindly supplied by Transwave (very helpful people). It gave good service for a couple of years then I got a better compressor, and the inverter / transformer are now driving my neighbours two post car lift very happily. I am not aware of any emc problems - the post office haven't been round with the detector van I believe a very few inverters actually monitor each phase and will complain if one is seen as missing, but I've never seen this personally.

Does anyone know how the 240 to 415 v single to three phase inverters popping up on ebay do to raise the voltage? They look like normal units, and there is an ebay 'note' saying they are not CE marked and implying that they are modified standard units, but how????

AWEM

... snipped

Did you just parallel the inputs or did you make the other mods I mentioned?

Dave

A related question. I use a phase converter with idler motor, it's worked well for a number of years but struggles to start the motor on my big TOS (about 8hp), lots of complaints from the contactors, but never seems short of power once it's going. My guess is that it'll start more readily with an inverter on the converter output, is that the case? I'm about to change the lathe for another with very similar sized motor, but if it has a higher inertia there might be a real problem starting it direct.

Cheers Tim Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

successfully

transformer

All I did was bung 415v single phase across L1 & L2 and it ran sweet as a nut. (not that I've ever seen a nut running !)

AWEM

Tim, If you follow a phase converter (I'm assuming a transwave type transformer plus switched capacitor banks) with anything other than a motor load the extra phase won't be generated as it relies on the motor inductance. It is unlikely to go 'bang'or letout the built in smoke but the inverter will be running on essentially a single phase input just as the OP wrote.

I'm assuming you are running on 250 ac input and need the converter to make

440ac.?

Yes the inverter will make starting a breeze as you will be able to program a gentle speed ramp up to cope with the inertia and also to ramp down gently. Having run machines on an inverter I can't image going back to stepped pulley speed changing again.

regards

Bob

It's with an idler motor (commonly described as a 'rotary converter' though it isn't really), 240 1 phase in, 440 3-phase out.

I do have an inverter on my mill spindles, never used one on a lathe. I've got a 10hp 440 to 440 inverter which I had other plans for, but it struck me this might be a good alternative use for it.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Andrew Mawson wrote: ... snipped

My only guess is that they've re-wired the input stage to be a voltage doubler - actually not a bad idea and easy to do if the resultant ripple doesn't cause problems and if there's space for the extra capacitor.

Anyone got a surplus 1.5kW-3.0kW 415v (3ph in/out) inverter that they'd like to convert into beer tokens so I can play?

Dave

inverters

voltage

Dave, yes I wondered if that was the case, but that would give you 480 ish volts - do these things regulate the o/p voltage or just switch what they are presented with having rectified the input ?

AWEM

They use pulse width modulation so the control function would be capable of effective amplitude modulation, but I don't know whether they actually regulate. They chop the DC bus so wouldn't be able to output a higher voltage than the input.

A voltage doubling rectifier would give you half wave rectification so the DC bus ripple would be 50Hz (rather than 300Hz with 3ph input - or do they use half wave so it's 150Hz?); there'd be so much droop that the mean might even be too low and some extra bus capacitance be needed. Also, the ripple current through the bus capacitor might be a bit high.

There's an excellent write-up on the standard inverter techniques here:

Dave (still looking for a 415V inverter to experiment with)

Single phase drive usually works fine. A bit depends on how generous is the size of the tank capacitor in your particular VFD inverter. The substantial increase in capacitor ripple current can overheat this component. 50% power derating is sometimes suggested but this is generally overkill. The main tank capacitor is the only item that is significantly affected and most VFDs will be happy with 100% short term rating and 70 - 80% long term.

The power for the VFD logic circuits is usually drawn from only one pair of the 3 phase inputs so you'll need to experiment to find which pair to use. The third wire can be left open or tied to one of the other inputs.The internal 3 phase rectifier is usually generously rated and can deliver full rated power with the single phase input using only four of its six diodes.

One point to watch is to be generous with the rating of the

230 to 415V auto because the the peaky nature of the input current of the VFD increases the heating of the auto windings.

All VFDs generate quite a bit of RFI But it's fairly low frequency stuff and usually not too bothersome. Smaller units mostly have built in RFI filtering. Others, particularly larger units, have separate (and expensive!) filter boxes.

If you're in an RFI sensitive environment be careful with the VFD to motor wiring - screened twisted triple runs may be necessary.

Jim

Thanks Jim, it sounds like you've already done some of the investigations I was planning. I was considering trying to do away with the transformer by using a voltage doubling rectifier and some large external DC bus caps - ripple current will be the issue but it could be more convenient than a transformer; have you played around with this?

Dave

I've not actually used the voltage doubling trick in this context but there's no doubt that it will work. The peak rectified output voltage will be a bit excessive and being mean with capacitor values to reduce the average DC level doesn't help because the peak and transient peak levels remain.

I think, to be safe, you should precede or follow the voltage doubler with some form of voltage reduction. A lamp dimmer, or a duty cycle buck reducer or an auto transformer springs to mind.

The transformer route is attractively simple and only needs to handle about 10% of the load power. An old 24/28V power transformer with the secondary bucking the supply voltage would do nicely.

I think you're tackling a pretty common problem and the results should be interesting - keep us informed.

Jim

snipped-for-privacy@coxvictor.wanadoo.co.uk wrote: ... snipped

I've only looked at the input specs for the Mitsubishi range; they have an operating spec of 380-480 with a permissible range of 325-528; the output can be set at any voltage below the input.

I'd thought of using a phase control circuit on the input but it would need to be a special because of the currents involved (especially the inrush) - fairly straightforward but it would add to the EMC problems. However, I hadn't thought of a bucking transformer (no typo ;-) ) and that's a very interesting idea Jim which may even slightly help reduce the conducted EMI from the VFD - the downside is that the secondary would need to be rated at 10-15A.

I'll certainly do that; but first I need to get my hands on a cheap 415V VFD to experiment with (you can never find a cynical trader when you need one).

Dave

Try Gavin Oseman goseman [at] f2s.com

01684 574966 Malvern, Worcs. UK I have seen him advertise several inverters a month at when I was looking for one. I bought a newton tesla setup in the end, as I neede a new motor anyway, and the lathe it went on was free, so i could (almost) justify it to make the thing work ;) std discalmer, No connection, and not even a customer in this case.

hth

Dave

For testing... 240 x 240 / 110 = 523... I could not possibly recommend running a power tool 110 V transformer "backwards" to provide a power source... :-)

Regards Jonathan.

Well I'm glad you couldn't recommend that because it won't work 8-), put

240V into the 110 secondary of one of these transformers and you will saturate the core and present virtually a short circuit to the mains. The MCB or fuse should operate in time to avoid damage, if the installation is sound of course...

However, you can use them as an auto transformer which is what's in most phase converters etc to keep the size down, but you'll only get a nominal

230+110=340V which isn't really enough. Of course if you're mains is as high as mine you get around the 380V mark which may well be.

Greg

Some of the power tool transformers ground the centre tap of the low voltage winding so you'll need to be sure that this is isolated before adding it to the primary voltage.

Using TWO of these transformer on my local supply you get

240 + 120 + 60 = 420V which is a pretty interesting number!

The first transformer is fully utilised, the second runs at

50% so the total output using 1KW transformers is 3.5 KW. A pair of paralleled "first" transformers and one second transformer yields 7KW

An alternative trick is to change the base frequency (this is the frequency that the VFD expects to be the nameplate motor supply frequency). This is normally set to 50Hz but most VFD's include a facility for changing this.

If you accept 240 + 120 = 360V and change the base frequency to 360/415 x 50 = 43Hz the VFD will automatically provide the correct flux drive for a 360V 43HZ motor. Full torque will be delivered but both the normal full load speed and HP will be 360/415 = 87% of the motor nameplate rating.

Jim

Yes that's right, in fact they all should, though the wire from the centre tap to earth is usually readily accessible and they are always an isolating transformer so long as you can disconnect this wire.

Other safety issues to consider are that the secondary should be connected to the mains neutral, never the live as the earthed neutral system we have means that this would produce a higher voltage to earth and thus increase the danger from a shock. Also the yellow sockets must be completely disconnected and on no account used as an output at this voltage.

One tip if you're doing this, get a metal cased transformer as these can have the transformer removed easily and fitted in your own enclosure, the plastic ones are 'potted' so you're stuck with the case.

Oh and don't forget that 'tool rated' means intermittently rated so these have to be derated, there are continuously rated ones around but they are less common.

Indeed 8-), again both secondaries should be in series with the mains neutral.

Greg

I've got a 110V tool transformer, an older one whose metal case had rotted away. It's marked 1.5 KVA, by its size I reckon that's less optimistic than the current yellow box type. I threw away the metal case a few days ago, couldn't chuck the transformer away until I'd tripped over it a few more times. Free to whoever can make use of it, strictly collect & strictly no guarantees.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service