3 phase questions

[Rod Sheridan]

Matt, auto-transformers continue the ground reference, they are not isolating transformers.

That is why they are used for that function.

The alternative is wye secondary with the neutral point grounded.....Rod

[Chip Lindley]

Please allow me to jump in with a question about my 3-phase. I have run a home shop for years off of a 240v rotary converter Now I have acquired a nice Delta 16-inch (long arm) radial arm saw ( but it is 200 volt/not 240 volt) I hesitate to consider running the 200 on 240. Just my luck it will cook the windings.

A 240 volt, 5-hp 3-phase motor pulls 12 amps. The 200-volt 5-hp pulls 15 amps. What kind of money am I looking at for a step-up transformer with sufficent amperage for a 5-hp 3-phase motor? All input is appreciated.

[Matt Mattingley]

Please allow me to jump in with a question about my 3-phase. I have run a home shop for years off of a 240v rotary converter Now I have acquired a nice Delta 16-inch (long arm) radial arm saw ( but it is 200 volt/not 240 volt) I hesitate to consider running the 200 on 240. Just my luck it will cook the windings.

A 240 volt, 5-hp 3-phase motor pulls 12 amps. The 200-volt 5-hp pulls 15 amps. What kind of money am I looking at for a step-up transformer with sufficent amperage for a 5-hp 3-phase motor? All input is appreciated.

I’m not exactly sure. I’ve never seen a name plate that has 240 V three-phase.

I have seen 208-230 and I’ve also seen 208, I’ve also seen 200, i’ve also seen 220 and 230. But never 240V. If you already have an RPC, my thoughts are are you will want to have a step down transformer. Do you mind sharing with us what your actual voltages are coming off your RPC during this one machine load? What size is your RPC? And how many amps is feeding it a single phase? If you use the VFD, you can restrict its out put voltage in a linear form through pulse wave modulation.

[Julie Moriarty]

Chip, I suggest taking your question to Mike Holt's forum. If they can't definitively answer your questions, no one can.

[Jeff Bartley]

Except that Mike Holts forum is exclusive to electrical contractors! I read this thread, and have the same question as Chip, so I registered on mike holts forum. First post I put up got locked and I was sent a link stating they only support those in the electrical industry.

Kind of a bummer! Reminds me of bad jobsites where all the different trades are uncooperative with one another! I don't like jobsites like that.

Chip, I suggest taking your question to Mike Holt's forum. If they can't definitively answer your questions, no one can.

[Darcy Warner]

I’m not exactly sure. I’ve never seen a name plate that has 240 V three-phase.

I have seen 208-230 and I’ve also seen 208, I’ve also seen 200, i’ve also seen 220 and 230. But never 240V. If you already have an RPC, my thoughts are are you will want to have a step down transformer. Do you mind sharing with us what your actual voltages are coming off your RPC during this one machine load? What size is your RPC? And how many amps is feeding it a single phase? If you use the VFD, you can restrict its out put voltage in a linear form through pulse wave modulation.

Any motor made in the last 10 years is so that is dual voltage will usually say 240/480. Also suitable for use on 208. 200v motors are basically made for 208v services.


That motor on 240v will probably last a long time with moderate use, 6+ hours a day, not so much.

[Chip Lindley]

Thank you all for your interest in my question about my 200v 3-phase Delta radial arm saw being run in the home shop with commercial ARCO Type R, 7.5hp RPC. The RPC is on a dicatated 40A single-phase circuit. I am not right at my shop to measure the amp draw, but my largest 3ph machine is 5 hp (12 amps) on that curcuit. I hesitate to run the 200v saw on 240v since the nameplate does not state any voltage other than 200v at 15 amps. (sure, I could run it, but it would be in the overheating range, I am sure) Like my Daddy used to say, "It doesn't take much more to go 1st Class."

Attachment 404384

I need a suitable 3 to 5 KVA buck-boost transformer to reduce 240v 3 phase to 200v (208v probably) 3- phase. I know they are available new, in the $400+ range. Hopefully I can do a lot better than that . Thanks for any help out there.

[David Kumm]

Chip, I think the size of the buck transformers can be smaller. I used two 1 kva single phase transformers to boost my 240 to 275 for my 7.5 hp Robinson ET/E. They were even a tad oversized. I think I paid less than $100 total for both. Dave

[Frank Pratt]

Chip, I think the size of the buck transformers can be smaller. I used two 1 kva single phase transformers to boost my 240 to 275 for my 7.5 hp Robinson ET/E. They were even a tad oversized. I think I paid less than $100 total for both. Dave

Both buck and boost transformers need to be sized to the load. There is no reduction for a buck transformer. Whether increasing or decreasing the voltage, the transformer still needs to be able to handle the load of the motor.

[David Kumm]

Two single phase Buck Boost transformers wired open delta need to be sized .5 kw or to be extra safe at .75kw to handle a 15-18 amp 208v motor. They do not need to be sized at 3-5 kw. Dave

[Frank Pratt]

Two single phase Buck Boost transformers wired open delta need to be sized .5 kw or to be extra safe at .75kw to handle a 15-18 amp 208v motor. They do not need to be sized at 3-5 kw. Dave

Dave, I must be missing something here. His RAS motor uses a 5190 VA at full load, so event a 5 kVA buck transformer is technically too small, but the saw is not going to be running continuously so he could get away with it. Although when doing a really tough cut there may be voltage drop issues at full load. How do you calculate a .5 kVA?

[David Kumm]

Frank, I used the calculator supplied by various buck boost transformer manufacturers. The RPC supplies 240v delta so two transformers only need to be sized to reduce the voltage to 208 which reduces the size needed. I'm above my pay grade here but I got the same info from others when I needed to boost voltage for my 7.5 hp motor. The sizing depends on how large the voltage adjustment is and whether you use two or three transformers. I calculated using two and wiring them open delta. Using three would reduce the size so .5 kva each would be more than enough. Dave

[Frank Pratt]

Frank, I used the calculator supplied by various buck boost transformer manufacturers. The RPC supplies 240v delta so two transformers only need to be sized to reduce the voltage to 208 which reduces the size needed. I'm above my pay grade here but I got the same info from others when I needed to boost voltage for my 7.5 hp motor. The sizing depends on how large the voltage adjustment is and whether you use two or three transformers. I calculated using two and wiring them open delta. Using three would reduce the size so .5 kva each would be more than enough. Dave

I'm an electrician, though I may be forgetting something (that seems to happen more frequently lately), I'm confident I've got this right. If using single phase transformers, you can go with an open delta configuration, but the sum of the capacity for the 2 transformers still must at least equal the total motor load. So for a 5 kVA motor, you could use 2 transformers of 2.5 kVA each, or 3 transformers of 1.67 kVA each. Or use a 3 phase transformer of 5 kVA. The bottom line is that the VA capacity still needs to at least match the motor.

How much the voltage is being change has no effect on sizing. Even if you needed to change only 1V, you still need the same capacity. Changing up or down has no bearing on this. If you try running a 7.5 HP motor on 2 transformers of .75 kVA each, The motor will only develop about 2 HP maximum. That's still enough to cut, buy nowhere near 7.5 HP

When sizing the transformers, the kVA rating the calculators show may vary depending on the voltage change, but that's just a function of the transformer models available.

The laws of physics dictates that if your load needs 5 kVA to do its job, then the transformer feeding the load must have a capacity of at least 5 kVA. No getting around that.

Autotransformers are the thing to use for this because they are cheaper and physically smaller than an isolation type transformer.

[David Kumm]

I deleted my prior post so posterity doesn't rely on my info. Dave

[David L Morse]

Frank, hopefully someone will chime in....

Ok, I'll try. Let's keep it simple and just look at a single phase autotransformer. Also, I'll use nice round numbers to keep the arithmetic from being a distraction.

Consider the following:

Autotransformer.jpg

An autotransformer has it's primary and secondary connected in series. For stepdown the input is applied to the series connection and the output taken from the secondary. In this example our turns ratio is 1:5 so an applied 240V divides to 40V on the primary and 200V on the secondary (1:5 = 40:200).

For a load of 2.4kVA we have to have 2.4kVA in (neglecting transformer losses). So 2.4kVA/240V = 10A in. The 10A flows into the primary so the primary sees 10A x 40V = 0.4kVA.

The secondary output current is one fifth of the primary current (turn ratio) or 2A. Secondary kVA = 2A x 200V = 0.4kVA.

The output current is the sum of the input current and the secondary current (because of the series connection) or 12A. 12A x 200V = 2.4kVA.

In general, the required transformer kVA is the output kVA times the ratio of the difference voltage to the series applied voltage, in this case 40V/240V x 2.4kVA = 0.4kVA.

And that's why:

Autotransformers are the thing to use for this because they are cheaper and physically smaller than an isolation type transformer.