VFD to feed a 3-phase isolation transformer(?)

I am still trying to figure out an optimal method to supply power to a Tree J-325 CNC mill (230V +/- 5%) and the rest of my shop as well (2 BPorts, Clausing lathe, tool and cutter grinder I currently have a 10HP phase converter which is giving me about 241V on two legs and 267V on the third leg, which is possibly the reason my Tree is not working at the moment.

I am wondering if I could employ a 10HP VFD with a fixed output in place of the rotary phase converter, with a 3-phase "isolation" transformer between the VFD and the loads, and be able to use the native switching that is present on the machines without destroying the VFD in short order. I'm guessing "no", but I don't know why it isn't feasible.

Have you looked into purpose built static phase converters? Basically just VFDs with better filters and DOL starting capacity.

That's how my workshop gets its 415V 3 phase. The transformer (after a sine wave filter to keep the electrical noise down) is wired star-star with the secondary providing a neutral that is grounded.

I suspect a 10hp drive is large enough to direct online start a 2 hp motor. certainly a 1.5hp motor. if other motors are running then the available buffer is reduced. the drive should trip on overcurrent without any risk of blowing up.

Ground the output neutral of the isolation transformer, (not the input neutral if it has one) and then add on the order of 30-50uF capacitors line to line after the transformer. (or 100uf line to neutral) --you can experiment with adding more actually to increase the available start current because motors are about 50% reactive current when starting.


The leakage inductance of the transformer will be enough to handle the pwm to sine wave conversion.. but you may not be happy with its internal losses when used as such, so use a watt meter to measure the no load draw of the VFD and then compare to what it is after connecting the capacitors.

If the power draw is rather high, try adding a line load reactor between the vfd and the transformer.
Currently there is MTE Corp RL-03502 0.800MH 35A Ith= 52.5A 600V LINE REACTOR 3PH LOAD TRANSFORMER on ebay for 70$ free shipping.

specfab said:

... I currently have a 10HP phase converter which is giving me about 241V on two legs and 267V on the third leg,

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What kind of converter is this? If a rotary converter you probably can reduce the output voltages without having to resort to any other equipment. The 241 line to line
is the utility pair, sounds like. Consider a buck transformer to bring that down to 230 volts.

jim rozen said:

What kind of converter is this? If a rotary converter you probably can reduce the output voltages without having to resort to any other equipment. The 241 line to line
is the utility pair, sounds like. Consider a buck transformer to bring that down to 230 volts.

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its likely running hot and just needs some capacitors removed.

johansen said:

its likely running hot and just needs some capacitors removed.

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Tuning capacitors won't affect the utility lines. The manufactured leg, of course, another matter. I'd start with the first and move on to the next after that.

Thanks for the comments and suggestions. My main issue is that I'm looking for the "easiest" way to get my power under control, without suffering an indefinite period of downtime, hence my interest in any ways to replace/add on while avoiding disabling. The cap change-out/removal came up in some previous discussion in another thread, and it just feels like one of those projects that might take much longer than I want to, with a lot of dicking around and testing, and no guarantee of success if there is something else amiss to start with.

Anyway, it is starting to feel like I need to plan a day to be down and work on the phase converter then...

specfab said:

The cap change-out/removal came up in some previous discussion in another thread, and it just feels like one of those projects that might take much longer than I want to, with a lot of dicking around and testing, and no guarantee of success if there is something else amiss to start with.

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nah, should take all of 10 minutes to disconnect some of the capacitors and measure the voltage.

take the cover off the panel and take a photo of the caps and post it.

It would be of interest to me to see a listing of commercial VFDs that could output power to anything other than a three phase induction motor without faulting.

I've got several VFDs from a variety of makers, and Non of them will "drive a transformer".

Call me innocent.

Regarding Bill's comment about only one motor, The several TOSVERT VFDs in my shop clearly show multi motor drive arrangements in the manuals. The sum of which should be within the VFD ratings, and all run simultaneously.

With that, I frequently "scab on" the suds pump to the VFD powering the MAZAK 16 inch lathe. The load change isn't even noticed. But the pump hates it when I throw the apron lever into reverse! ;-)

N.B. The VFD is vastly oversized for the lathe motor.

johansen said:

nah, should take all of 10 minutes to disconnect some of the capacitors and measure the voltage.

take the cover off the panel and take a photo of the caps and post it.

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When I was running a RPC to power lath mill and SG in the shop, I eventually "gave up" on balance caps. Took 'em all off line, 'cause when the motors were actually called on to do the work, the balance seemed pretty darn good.
(multiple amp meters on the panel confirmed. I could care less about Volts, It's amps that does the work, and the damage.)

Balancing for no load seems sort of stupid.

eta!

When running the mill today off the RPC, I always turn on the table feed motor, even if it's not being used.

The more, the merrier!

CalG said:

It would be of interest to me to see a listing of commercial VFDs that could output power to anything other than a three phase induction motor without faulting.

I've got several VFDs from a variety of makers, and Non of them will "drive a transformer".

Click to expand...

Danfoss VLT 5000 series specifically state 'unlimited switching on output' and output protected against short circuits.

Teco Speecon 7200 series do not state this, but my supplier provided one guaranteed by them to be suitable for feeding an entire workshop.

Those are two instanced known personally to me.

For an invertor output stage, a transformer is just another inductive load that very slightly modifies the apparent characteristics of the motors behind it.

Is a phase perfect really that expensive that you would rather buy and wire in an oversized vfd, a filter, and a transformer to have not quite a guarantee of running the machine without tripping? And tolerating that VFD whine?

As far as balancing an RPC for variable loads, you really can't. The most stable option is no caps at all, and you can sort of optimize for particular loads with caps. But measure current, not voltage, because caps change the phase angle. My old RPC read out both output voltage and output current (as well as the current contributed by each idler) and the results were not at all intuitive.