230 1ph -> Transformer 460v -> 460v 3ph VFD -> 460v motor questions.

Hi guys. I have a few questions about a design to use an inexpensively purchased Ebay 460v/3ph VFD controlling a 230/460v 3ph regen blower. I only have access to 1ph power here. I'd like to start with the transformer sizing. My 10hp 3ph 230/460v 60hz motor is rated at about 8kVA. My plan (based on research done from this forum) is as follows:

230v/1ph/60hz 30amp circuit --> Unknown transformer 230v/460v 1ph (step-up from 230v to 460v) --> 460 VFD (Japanese 3ph/3ph 20hp) --> 460v 3ph blower motor.

My first question, of which there will likely be many... Do I need to match what the motor is rated for with the transformer at 8kVA, or do I need to look for something in the 20kVA range because I'm doubling the voltage? I'm thinking a standard 10Kva transformer will work. This is the only piece of gear off this circuit. The blower will be used for both vacuuming and blowing small grains up into a tank, some of which could be wet and sticky putting a definite load on the blower. Mostly it will be run at like 50% - 75%. -Above I didn't account for the service factor of 1.15 with the 8kVA statement. I guess I need to.

I'll have some wiring questions after I get all the parts together, but this really looks like such a great way to go, instead of adding in a RFC setup. Love the idea of this design.

If the blower is a fan load you can use a 3phase 10kva transformer after the vfd. Program the vfd for linear volts per hz with no boost volts which will saturate the transformer at startup.

If that is not an option you will need a 10kva 240v:240v isolation transformer configured as an autotransformer to double the voltage, or you will need a 15 minimum or 20 kva 240:480v isolation transformer. The reason why is because the "power factor" of a single phase rectifier is about 60%

If however you are only running the motor for several hours a day you can get away with a smaller transformer.

OK. I hadn't read any of this in some other threads on here. Put the VFD before the Transformer... I guess I need to go back and read some more threads. Some of them are totally overwhelming.

Yeah, it'll be run about 2-3 hours a day.

The motor at full load of 13.6 amps is 11.2KVA. i would recommend a 15Kva transformer to feed the vfd. since its only 2-3 hours and it sounds like not at full load. as i mentioned the "power factor" of the single phase vfd rectifier is about 60%.

anyhow, sorry, i forget to see that you already have a 480v vfd you intend to use so no you can't put the transformer after the vfd.

problem with single phase is you need to double the size of the VFD because the capacitors and the diodes can't handle full load on single phase. did you buy a 20hp vfd?

Joh... I did not, but was originally planning (based on a 50% derate I had read about) to get a 20hp 460v VFD, and there's tons of them for like $100 on Ebay (slight exaggeration). Correct me if I'm not reading you right, but I could do as originally planned, but boost the transformer to 15-20kVA and boost a 460 VFD to like 30hp?

Above you said "as I mentioned the "power factor" of the single phase VFD rectifier is about 60%". I planned on a 3ph/3ph rectifier. In that statement did you just mean that I would be feeding it single phase? I won't have any single phase VFDs anywhere in this proposed design.

One additional thing to mention is that 90% of the time this will be run for a couple of hours at 75% load, but I'd like to design this setup so that it could run full tilt for 12hours if it was needed to. Not sure when that would be, but vaguely possible if something odd came up.

You only have single phase 240vac available. that means you're going to be supplying your 480v vfd with single phase, and that's why the vfd needs to be twice the size of the motor you're driving.


because of the bad power factor drawn by the vfd from the single phase supply: the transformer in front of the vfd needs to be twice the size of the motor as well. (if its run continuously at full load)


the advantage of an isolation transformer reconnected as an auto transformer is that a 10kw 240vac isolation transformer can deliver 20kw of 480vac when its connected as an auto transformer.

given that you are on a 120v/240v supply you can get 20KVA worth of 480v power from (2) 120:120vac 5KVA isolation transformers, you may be able to find a pair of them cheaper than you could a 20KVA 240:480v single phase transformer. (note that a 3 phase 20kva 240:480v transformer won't get you 20KVA single phase power, but in your case such a transformer would work given intermittent operation.

Great. That clears that up. I specified a 20hp VFD for a 10hp motor in my original statement. I'll look for a 25hp unit, can't hurt.

What about the available amps vs. the size of the transformer? I could go about 50amps max (leaving headroom for other equipment). But that combination only provides 12kVA. I'd need at least a 60amp circuit feeding the transformer to get ~15Kva correct? Does a 15kVA transformer absolutely require 15kVA input (no more, no less), or is that just the maximum?

At 75% load your 10hp motor will draw about 8KW which means the VFD will draw 16.6 amps from a 480vac single phase supply.. however that 16.6 amps is the real amps, the 60% power factor means you will be drawing 28 amps RMS from the 480vac single phase supply.

28 amps at 480v is close to 14KVA which is inline with my recommendation of finding a 15 KVA transformer.


yes you need a 60 amp breaker minimum to feed the 240:480v transformer. the details of the transformer don't really matter all that much as far as the math goes.. it matters with regard to grounding the secondary.


a pair of 120v isolation transformers configured as auto transformers will deliver 480vac, each line is 240vac to ground.

A 240:480v transformer may have a delta or y output or if its single phase its just a single secondary which may or may not be center tapped. so the code would require it to be grounded which means your 480v vfd's rectifier and mov protection circuitry may be strained. if you can find a 240v:480v center tapped single phase transformer, then its electrically equivalent to a pair of 120v isolation transformers for your application.


you might find it cheaper to buy a 240v 15 or 20 hp VFD. if it can't handle single phase get a rotary phase converter running. its probably cheaper to find a 5 or 10 hp motor than it is to find a 20KVA transformer.

MaineShop said:

What about the available amps vs. the size of the transformer? I could go about 50amps max (leaving headroom for other equipment). But that combination only provides 12kVA. I'd need at least a 60amp circuit feeding the transformer to get ~15Kva correct? Does a 15kVA transformer absolutely require 15kVA input (no more, no less), or is that just the maximum?

Click to expand...

15KVA = 1Φ, 62.5A @240V or 31.25A @ 480V. For the transformer feeder circuit, the overcurrent should be sized at 125% of capacity, to avoid nuisance tripping when energizing. They have a short but large surge current similar to motors. 62.5A x 1.25 = 78.12A, next size up permitted 80A.

This permits the full capacity output, and avoids tripping when energizing. Won't need that much under reduced load, but needed to function properly.

To get a 1Φ 15KVA unit, the simple and least expensive way is to use a standard 1Φ 7.5KVA 240/480 - 120/240V 4 winding used unit, such as one of these. Connected in a autotransformer arrangement.

https://www.ebay.com/sch/i.html?_from=R40&_nkw=7.5KVA+240%2F480+-+120%2F240&_sacat=0&_sop=15




This standard arrangement will produce a unbalanced voltage to ground, from each output leg.
But it can be modified to produce a balanced output to ground, with a slight modification, If you decide to use this method. For a VFD load, you will want a balanced output with respect to ground.

SAF Ω

I believe the solution is to buy a 1ph->3ph 230v VFD rated for 12-15hp motor and be done with it, or install a RFC as mentioned, and go 3ph->3ph VFD. As much as I'd like to be able to spec this gear myself, I don't have time to get a degree in electrical engineering for a VFD install. I appreciate the input.