10 hp inverter

kwired

Electron manager
Location
NE Nebraska
This is a substantial research and discovery. I can find a transformer for 200$ whereas rotary phase converter (RPC) costs 2,5k$ .
so far I told customer a price for RPC and he replied ‘too expensive’

Will this aeropoane ‘fly’?
What exactly are you finding (presuming new) for $200?

I'm thinking to build your own open delta out of two single phase transformers you probably needing at least 2 7.5 kVA minimum possibly bigger if I missed some the loads this is just for the two motors you mentioned in OP.

I think those alone probably going to cost you around $1500-2000, and you will get either 240 corner ground delta or 120/240 high leg delta as a secondary voltage - if that is something your equipment can take for input volts. But keep in mind a RPC will have two output lines that are the same as input volts but the third leg will not be a steady voltage either. The open delta will have significantly less operating and idle losses than a RPC.
 

darekelec

Senior Member
Location
nyc
I can get a mint used 20 kVA delta wye transformer for 200$ with local pickup.
This machine will run twice a day for half an hour so I’m not concerned about losses.

I don’t wanna be a laboratory mouse.

This is an interesting experiment though.
 

kwired

Electron manager
Location
NE Nebraska
I can get a mint used 20 kVA delta wye transformer for 200$ with local pickup.
This machine will run twice a day for half an hour so I’m not concerned about losses.

I don’t wanna be a laboratory mouse.

This is an interesting experiment though.
Not sure how well delta-wye on single core will work compared to using 2 units/separate cores, and I think that was discussed a little earlier in the thread. Generally a no-no to connect the neutral when backfeeding a delta-wye single core unit you get high circulating currents that can let the smoke out of the windings.
 

darekelec

Senior Member
Location
nyc
I wanted to ask somebody to explain in more simple words idea of delta wye transformer. I understand now it’s a no-no.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
As kwired just mentioned, MD Automation did testing with a reverse fed delta-wye (i.e., a wye-delta) and was able to create a third phase. He found significant idle currents and an elevated harmonic distortion level. I believe this was at least partly due to the common 3-leg core's inability to handle "zero sequence" flux, which I believe was occuring because the transformer is not being provided with symmetrical 3-phase excitation (which it would in normal usage).
I think building a wye-delta from three individual single phase transformers could eliminate this specific issue, and it would allow you to have a somewhat arbitrary secondary voltage. But there is still an issue that reverse fed transformers typically have significantly higher inrush current when they are powered up.

Transformers such as the 5kVA used ones at the links below are available which can be configured for a 120V primary and 120V secondary. With those you could wire two of them as shown in the following thread and not have to reverse feed them:
https://forums.mikeholt.com/threads/1-ph-to-3-ph-converter.2546600/#post-2546705

Here are some examples I found of used transformers suitable for 120V/120V:

https://www.ebay.com/itm/274559051157?hash=item3fecfea595:g:81YAAOSwvBVfsIsp

https://www.ebay.com/itm/172832361871?hash=item283d9c8d8f:g:ZfEAAOSwyc1ZnEnR

https://www.ebay.com/itm/264753706626?epid=18038988402&hash=item3da48cf682:g:3RoAAOSwR~Fe2VB5
 

kwired

Electron manager
Location
NE Nebraska
As kwired just mentioned, MD Automation did testing with a reverse fed delta-wye (i.e., a wye-delta) and was able to create a third phase. He found significant idle currents and an elevated harmonic distortion level. I believe this was at least partly due to the common 3-leg core's inability to handle "zero sequence" flux, which I believe was occuring because the transformer is not being provided with symmetrical 3-phase excitation (which it would in normal usage).
I think building a wye-delta from three individual single phase transformers could eliminate this specific issue, and it would allow you to have a somewhat arbitrary secondary voltage. But there is still an issue that reverse fed transformers typically have significantly higher inrush current when they are powered up.

Transformers such as the 5kVA used ones at the links below are available which can be configured for a 120V primary and 120V secondary. With those you could wire two of them as shown in the following thread and not have to reverse feed them:
https://forums.mikeholt.com/threads/1-ph-to-3-ph-converter.2546600/#post-2546705

Here are some examples I found of used transformers suitable for 120V/120V:

https://www.ebay.com/itm/274559051157?hash=item3fecfea595:g:81YAAOSwvBVfsIsp

https://www.ebay.com/itm/172832361871?hash=item283d9c8d8f:g:ZfEAAOSwyc1ZnEnR

https://www.ebay.com/itm/264753706626?epid=18038988402&hash=item3da48cf682:g:3RoAAOSwR~Fe2VB5
One tricky thing about this is it possibly a little harder to find transformer with 120 volt primary other than in the below ~5 kVA ratings.

Not that you can't get one but they maybe not so common and end up being somewhat of a special order.

The bulk of what is readily available over 2 or 3 kVA probably are 240/480 primary.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
One tricky thing about this is it possibly a little harder to find transformer with 120 volt primary other than in the below ~5 kVA ratings.

Not that you can't get one but they maybe not so common and end up being somewhat of a special order.

The bulk of what is readily available over 2 or 3 kVA probably are 240/480 primary.

I agree, and it is certainly understandable why there would be little demand in the marketplace for a transformer over 5 kVA that would support 120V on the primary.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Not sure how well delta-wye on single core will work compared to using 2 units/separate cores, and I think that was discussed a little earlier in the thread. Generally a no-no to connect the neutral when backfeeding a delta-wye single core unit you get high circulating currents that can let the smoke out of the windings.
In this case when backfeeding a delta-wye from a 120V/208V 3-wire source, one of the L-N windings of the wye (call it L3-N) is not being driven and forced to a specific voltage, but instead it's providing its own voltage developed by the transformer to a load. Effectively the L1-N and L2-N voltages establish the voltage across what can be thought of as an open delta formed by the two corresponding secondary windings. The voltage across that open delta is then transformed back to create the L3-N voltage on what would normally be a driven winding on the primary, but instead it provides a voltage on L3 that's created by the transformer.

Current will circulate in the delta but it will be the load current drawn on L3 times the primary-to-secondary turns ratio. This is different than the possibly high circulating currents that occur when a wye-delta is driven from a 4-wire source where the neutral is connected to center point of the wye and there is imbalance on the L-N input voltages.

Like I said above, I don't recommend that a 3-phase delta-wye unit be used based on the findings of MD Automation.
 

kwired

Electron manager
Location
NE Nebraska
In this case when backfeeding a delta-wye from a 120V/208V 3-wire source, one of the L-N windings of the wye (call it L3-N) is not being driven and forced to a specific voltage, but instead it's providing its own voltage developed by the transformer to a load. Effectively the L1-N and L2-N voltages establish the voltage across what can be thought of as an open delta formed by the two corresponding secondary windings. The voltage across that open delta is then transformed back to create the L3-N voltage on what would normally be a driven winding on the primary, but instead it provides a voltage on L3 that's created by the transformer.

Current will circulate in the delta but it will be the load current drawn on L3 times the primary-to-secondary turns ratio. This is different than the possibly high circulating currents that occur when a wye-delta is driven from a 4-wire source where the neutral is connected to center point of the wye and there is imbalance on the L-N input voltages.

Like I said above, I don't recommend that a 3-phase delta-wye unit be used based on the findings of MD Automation.
Makes sense, and I'm pretty sure I've seen wye-delta that lost a primary phase before and the secondary still works, as long as not loaded too heavily and ends up taking out additional overcurrent protection. I did know it works that way on separate core transformer banks. Lose an input phase and you essentially left with open delta.
 
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