Buck Boost Transformer

[MTW]

In my shop, I use two single Φ 4 winding transformers in an open delta configuration supplied by 240 delta to produce four 3Φ voltages for testing motors and equipment. 120, 200, 240, 480V. The idea came about from being asked to reverse engineer and repair an old hand built motor test stand that the motor shop had acquired from another shop.

I like the idea so much, that I built one for my own use for testing equipment and motors. Their shop supply was 480V, mine was 240V so there were some differences to the connections. What I added to my test board was a bunch of relays and contactors to be able to switch the voltages on the fly. Starting a large motor sitting loose on the floor at full voltage could get it to take off rolling across the shop. Starting at lower voltage and ramping it up on the fly solved that problem. It's the only time I ever come across 3Φ 120V. I thought about adding a 600V option to it since I'm not far from Canada, but it's complex enough as it is, with it's contactors. For 600V the few times I need that, I will drag out another transformer to connect to the mix. Sure is convenient for testing versus connecting up a separate transformer for each thing that need to be tested at a different voltage and repaired.

i supplied the motor shop with a drawing of their old unit, and they built a new larger version for their own use, they had a slotted machine table cast into the floor to bolt their motors down to. A much safer setup for the size motors they fired up. Auto transformers can be a versatile tool.

 

[jim dungar]

Still partially allowed now, via Exception 2 of 215.11 and 210.9:

Exception No. 2: In industrial occupancies, where conditions of maintenance and supervision ensure that only qualified persons service the installation, autotransformers shall be permitted to supply nominal 600-volt loads from nominal 480-volt systems, and 480-volt loads from nominal 600-volt systems, without the connection to a similar grounded conductor.

Cheers, Wayne

Thanks.
Looks like they added this exception, since I last needed this connection some 20+ years ago.

 

[LarryFine]

That would take 2 such transformers to get 480V 2-wire from 240V 2-wire. While as a boost autotransformer you can get the same maximum current at 480V using one such transformer.

I grok. I had your plural in mind when I said it.

 

[winnie]

In my shop, I use two single Φ 4 winding transformers in an open delta configuration supplied by 240 delta to produce four 3Φ voltages for testing motors and equipment. 120, 200, 240, 480V. The idea came about from being asked to reverse engineer and repair an old hand built motor test stand that the motor shop had acquired from another shop.

The best autotransformer setup I've encountered was also at a motor rewind shop.

The owner had taken an old wound rotor induction motor and locked the rotor with a rotary table, so that he could turn a handle and change the angle of the rotor.

He wired the rotor coils in series with the primary supply. By changing the rotor angle he would change the phase of the transformer coupling, which would change the sum voltage (primary supply plus secondary coils), giving him a 3-phase variable voltage for testing.

Jonathan

 

[GoldDigger]

Kind of reminds me of the old motor driven moveable core inductors once used for dimming stage lights. No harmonics, no flickering.

 

[hillbilly1]

Kind of reminds me of the old motor driven moveable core inductors once used for dimming stage lights. No harmonics, no flickering.

Home Depot did that to dim the sales floor HID’s, they had a set of diodes across the brushes that would fail, then start burning the face of the windings. You could tell when it was failing, as the lights would flicker when the taps were changed. After a couple of fires, they put smoke detectors above the cabinets, then eventually bypassed the dimmers. (Peschel dimmers)

 

[winnie]

Home Depot did that to dim the sales floor HID’s, they had a set of diodes across the brushes that would fail, then start burning the face of the windings. You could tell when it was failing, as the lights would flicker when the taps were changed. After a couple of fires, they put smoke detectors above the cabinets, then eventually bypassed the dimmers. (Peschel dimmers)

I don't know the details of a Peschel dimmer, but it sounds like it uses a 'variable autotransformer' or 'variac'. These are toroidal transformer coils with a movable brush which creates a tap somewhere on the coil.

The setup with the wound rotor motor didn't have brushes or a changeable tap; instead it worked by changing the magnetic coupling.

Variac transformers are _much_ more common.

-Jonathan

 

[LarryFine]

I have a 3ph Variac somewhere around here. I believe it's 208v, but I don't remember the capacity.

 

[topgone]

Let me try again. No, starting with a 240V delta, you can't use 240V/32V transformers as boost transformers to get a 480V delta. [Well, you could, but it would take 14 or 16 of them.] With autotransformers you can't rearrange the voltage vectors from a delta to a wye, as the new voltages you get are already referenced to the existing voltages. You'll need 240V/240V boost transformers to get a 480V delta.

Cheers, Wayne

Please remember that the OP install pertains to supplying motor/s with a higher voltage than the voltage available. One doesn't have to bring the neutral to the motors as most motors are wired in delta. We've done 3-phase autotransformer to supply a motor with a higher voltage than what our field station has before. It worked. The supply voltage available was only 240, 3P and we used 3 buck-boost transformers 240/32V. Measured the output voltage to be 471V, 3-phase.
Here's a cut-n-paste from the buck-boost install guide:

 

[winnie]

Please remember that the OP install pertains to supplying motor/s with a higher voltage than the voltage available. One doesn't have to bring the neutral to the motors as most motors are wired in delta. We've done 3-phase autotransformer to supply a motor with a higher voltage than what our field station has before. It worked. The supply voltage available was only 240, 3P and we used 3 buck-boost transformers 240/32V. Measured the output voltage to be 471V, 3-phase.
Here's a cut-n-paste from the buck-boost install guide:

View attachment 2572228

If I understand that connection correctly, it should have given you 305V.

Is there any chance that you used the 120V primary coils in series (neutral to H1, LV supply connected to H2 and H3, H4 output) and ignored the 16/32V coils?

-Jonathan

 

[wwhitney]

The supply voltage available was only 240, 3P and we used 3 buck-boost transformers 240/32V. Measured the output voltage to be 471V, 3-phase.
Here's a cut-n-paste from the buck-boost install guide:

Connecting 240V/32V transformers as in the diagram you posted will not produce 471V 3-phase. It would produce 272V 3-phase.

If you started with 240V L-N, rather than L-L, then it would give you 272V L-N, or 471V L-L, as you say.

Cheers, Wayne

 

[GoldDigger]

I don't know the details of a Peschel dimmer, but it sounds like it uses a 'variable autotransformer' or 'variac'. These are toroidal transformer coils with a movable brush which creates a tap somewhere on the coil.


-Jonathan

The Peschel transformer, unlike the Variac and its kin, does not use a toroidal core. Instead it uses a linear coil on a straight leg of a core. The brushes move over exposed bared wires on one side of the coil.
I suppose one advantage over the highly efficient toroidal coil is that you can build a single core three phase transformer using the Peschel design.

 

[dvcraven052256]

I wish to attached a PDF of the one line diagram for comments. I do not see an option to attached a PDF in the format.

 

[winnie]

Welcome back Original Poster! I hope we didn't chase you off with the tangent discussion of different buck/boost transformers.

You should be able to simply click the 'attach files' button at the bottom of the post editing window. Because you are a new poster you might not have sufficient privileges for this to work.

IMHO if your one-line is simple enough to be uploaded as one of the allowed images, that is a better approach because everyone will see a 'thumbnail' version of the image when reading your post. It is extra work to chase down the PDF file. But if the one-line has lots of details the PDF is better because it contains more information. If you can't yet post images or attachments, you can ask a moderator to help you.

Also adding a reference to my original response to your question, here is a link to a paper about sizing transformers for motor starting:

How is a Transformer selected to feed a motor? | Schneider Electric USA

-Jonathan

 

[dvcraven052256]

Ok figured it out. In essence the attached one is my approach to providing power to (2) lab disperser's/ They are 460V/3ph. I do not know mush about auto transformers as was suggested. I am certainly willing to investigate if that is a better option.

 

[winnie]

The transformer as drawn is a 'step up' isolation transformer, not an 'autotransformer'. IMHO a normal 'delta-wye step up' transformer is better for your application. 'Buck/boost' is slang for certain autotransformer connections, and IMHO you do not want this.

The transformer size is more than adequate for the loads drawn. You can probably get away with a smaller transformer, but at 30 kVA you are so small that the design effort to go smaller is more expensive than the money you will save with the reduced size.

The primary protection for the transformer is smaller than the full load rating of the transformer. There is still enough capacity for the loads shown, but you might have issues with the primary OCPD tripping on transformer inrush.

You appear to have control lines going to the motor downstream of 'frequency inverters'. In general when a VFD controls a motor you want all the control lines to go back to the VFD, and you do not want to have NEMA starters or any contactors between motor and VFD.

Hope this helps.
Jonathan

 

[dvcraven052256]

Thank you I appreciate your comments.

 

[topgone]

Connecting 240V/32V transformers as in the diagram you posted will not produce 471V 3-phase. It would produce 272V 3-phase.

If you started with 240V L-N, rather than L-L, then it would give you 272V L-N, or 471V L-L, as you say.

Cheers, Wayne

Thanks. Took me a long time to search the old Rolodex and find the old fellow's number. Called an verified and you are correct. The source available at that station I mentioned was a lighting transformer, 3-phase 415V (240V L-N).