Buck/boost use on rotary phase converter

[hillbilly1]

I have a customer that has a piece of equipment that specs 240 volts max, with 230 volts preferred. Problem is the utility is high because they have too much load on the transformer, and long runs to the other end of complex. This is the second unit in, so the voltage is at 245. The phase converter is pumping out 260-280 on the generated leg unloaded. I’m bucking it 12 volts, and the transformer manufacturer is saying to use an open delta with two transformers. I would be more comfortable using a wye, but don’t know how the phase converter would react because a neutral would be connected. The manufacturers instructions does not give a delta connection, which I would be more comfortable with. Anybody had this problem before?

 

[synchro]

The phase converter is pumping out 260-280 on the generated leg unloaded.

Say that L1, L2 are from the single phase supply and L3 is the output of the phase converter. Are you saying, for example, that L3-L1 is 260V and L3-L2 is 280V? Or are L3-L1 and L3-L2 about the same voltage but it varies over 260 to 280V with loading, etc.?

Does the piece of equipment require a neutral?

 

[hillbilly1]

The generated leg is higher, but all three are too high with the other two being 245 volts. Read an article from Square D, and they advised against load side buck/boost, so I think I will do a single line side buck/boost before it hits the converter. The biggest problem is it will take a 2000 va vs two 750 va. And no, the equipment does not require a neutral.

 

[electrofelon]

I guess you already have the phase converter, but maybe a single phase to three phase vfd would be a better option? It could make three phase and any voltage you want

 

[hillbilly1]

I guess you already have the phase converter, but maybe a single phase to three phase vfd would be a better option? It could make three phase and any voltage you want

The manufacturer called for this phase converter model and size, it’s a brand new CNC machine (which hasn’t arrived yet) customer didn’t give me the details until after I got the job. His bank locked me into no change orders (won’t do that again) at least I put it in the contract he was supplying the converters. When I got the specs on it, and found out it required < 240 volts, preferably 220, a good profit job turned into a poor profit job. I was trying to avoid having to buy the bigger transformer.

 

[winnie]

I’m bucking it 12 volts, and the transformer manufacturer is saying to use an open delta with two transformers. I would be more comfortable using a wye, but don’t know how the phase converter would react because a neutral would be connected.

A wye 'autotranformer' generally requires a neutral from the supply to function correctly. But the rotary phase converter usually doesn't supply this neutral.

The original single phase neutral is _not_ the three phase neutral; rather it is more like the grounded conductor of a high leg delta system. If you were to connect this single phase neutral to a balanced wye autotransformer you would smoke the system quite quickly.

When you say 'the manufacturer called for this phase converter model and size' do you mean the CNC manufacturer or the phase converter manufacturer? A CNC machine very likely has lots of VFD or servo drives, and these tend to prefer balanced wye sources.

(NB: I could imagine a rotary phase converter that does maintain a common neutral, by essentially synthesizing _two_ legs rather than one. I don't think any suppliers actually do this. I can also imagine an unbalanced wye buck-boost setup looking like an inverted T that would buck the high leg delta down to a lower voltage high leg. I don't think any of these hypothetical concepts are actually supported by any manufacturers, so don't go down that rabbit hole. Keep things simple and common.)

-Jon

 

[electrofelon]

A wye 'autotranformer' generally requires a neutral from the supply to function correctly. But the rotary phase converter usually doesn't supply this neutral.

The original single phase neutral is _not_ the three phase neutral; rather it is more like the grounded conductor of a high leg delta system. If you were to connect this single phase neutral to a balanced wye autotransformer you would smoke the system quite quickly.

When you say 'the manufacturer called for this phase converter model and size' do you mean the CNC manufacturer or the phase converter manufacturer? A CNC machine very likely has lots of VFD or servo drives, and these tend to prefer balanced wye sources.

(NB: I could imagine a rotary phase converter that does maintain a common neutral, by essentially synthesizing _two_ legs rather than one. I don't think any suppliers actually do this. I can also imagine an unbalanced wye buck-boost setup looking like an inverted T that would buck the high leg delta down to a lower voltage high leg. I don't think any of these hypothetical concepts are actually supported by any manufacturers, so don't go down that rabbit hole. Keep things simple and common.)

-Jon

Jon, so just as an academic question, how do you see these options for cases where the machine wants a balanced supply:

1. A delta->wye isolation transformer after the phase converter
2. A single phase to 3 phase VFD
3. Something like one of these phase converters (not sure if they provide a wye output, they still use the term "generated leg")

Phase Perfect | 3 Phase Power Anywhere With Our Digital Phase Converter | Variable Frequency Drives and Digital Phase Converters | Phase Technologies

www.phasetechnologies.com

 

[synchro]

Apparently it's common to have the generated leg intentionally run at a higher voltage unloaded so that it doesn't drop down too low when it's loaded.
The values of the capacitors in the rotary converter that go between the generated leg and the single phase input legs will affect the generated voltage. If these capacitor values can be modified, it may be possible to get a better voltage balance when using a specific load.

 

[ptonsparky]

Line to line voltages are generally pretty close. That is all you need to worry about. The motor could care less what the voltages are to ground.

I've used BB on equipment that utilizes the neutral. Don't use the generated phase. The instructions will tell you that. I never used it for control xfmr primary either.

 

[hillbilly1]

The

A wye 'autotranformer' generally requires a neutral from the supply to function correctly. But the rotary phase converter usually doesn't supply this neutral.

The original single phase neutral is _not_ the three phase neutral; rather it is more like the grounded conductor of a high leg delta system. If you were to connect this single phase neutral to a balanced wye autotransformer you would smoke the system quite quickly.

When you say 'the manufacturer called for this phase converter model and size' do you mean the CNC manufacturer or the phase converter manufacturer? A CNC machine very likely has lots of VFD or servo drives, and these tend to prefer balanced wye sources.

(NB: I could imagine a rotary phase converter that does maintain a common neutral, by essentially synthesizing _two_ legs rather than one. I don't think any suppliers actually do this. I can also imagine an unbalanced wye buck-boost setup looking like an inverted T that would buck the high leg delta down to a lower voltage high leg. I don't think any of these hypothetical concepts are actually supported by any manufacturers, so don't go down that rabbit hole. Keep things simple and common.)

-Jon

The CNC manufacturer gave a specific model in their literature.

 

[winnie]

Jon, so just as an academic question, how do you see these options for cases where the machine wants a balanced supply:

1. A delta->wye isolation transformer after the phase converter
2. A single phase to 3 phase VFD
3. Something like one of these phase converters (not sure if they provide a wye output, they still use the term "generated leg")

Phase Perfect | 3 Phase Power Anywhere With Our Digital Phase Converter | Variable Frequency Drives and Digital Phase Converters | Phase Technologies

www.phasetechnologies.com

If you are specifically using a standard rotary phase converter, then IMHO 1 is the best approach to a wye output. 2 is a non starter because VFDs are intended for running motors, not an arbitrary 3 phase load.
3 is basically an inverter type system with appropriate filtering and control be able to run arbitrary loads, but this is an entirely different beast from a rotary phase converter. The electronics is similar to a VFD, but fixed frequency, fixed voltage, and with lots more filtering.

The topology for a switching based phase converter could be designed to synthesize all three phases and have a wye output, or synthesize only a single additional phase. If you only add a single additional phase, then you are back to the high leg case.

If a system is designed with lots of VFD or servo drives with internal DC busses, my preference would be to go to the machine manufacturer and see if there were any way to provide _DC_ power to the system, and put the money into a single phase rectifier with sufficient filtering.

-Jon

 

[winnie]

The CNC manufacturer gave a specific model in their literature.

Then I would adjust the single phase input voltage to be inline with the manufacturer limits, and not worry about the high derived phase voltage until you've tested it under load.

It is probably overkill, but I would probably arrange the buck converter to maintain balanced single phase line-neutral voltage. Using a standard 120/240 : 12/24V single phase 'buck-boost' transformer you can arrange to buck 245V to 223V while keeping L-N voltage symmetric.

-Jon

 

[synchro]

Jon, so just as an academic question, how do you see these options for cases where the machine wants a balanced supply:

1. A delta->wye isolation transformer after the phase converter

The delta-wye transformer will give you equal L-N output voltages if the L-L input voltages are equal. But if the L-L inputs are unbalanced then the output L-N voltages will also be unbalanced.

 

[hillbilly1]

Then I would adjust the single phase input voltage to be inline with the manufacturer limits, and not worry about the high derived phase voltage until you've tested it under load.

It is probably overkill, but I would probably arrange the buck converter to maintain balanced single phase line-neutral voltage. Using a standard 120/240 : 12/24V single phase 'buck-boost' transformer you can arrange to buck 245V to 223V while keeping L-N voltage symmetric.

-Jon

That’s what I decided to do, the problem is, 750 va buck boosts are easily obtained, but the 2000 va is a 60 day lead time. The machine is supposed to come in March, so
I’m hoping it will be late. Everything else he has ordered has come in late.

 

[winnie]

What are the power requirements of the machine? What sort of 750VA buck boosts can you get quickly?

If you have _2_ 750 VA 120/240V : 12/24V transformers, you can get 62.5A at 222V single phase.

-Jon

 

[hillbilly1]

What are the power requirements of the machine? What sort of 750VA buck boosts can you get quickly?

If you have _2_ 750 VA 120/240V : 12/24V transformers, you can get 62.5A at 222V single phase.

-Jon

On the three phase side the 750 va would work, but a single 750 on the single phase input side would be too small. Machine draws 40 amp three phase, so the single phase side would be much higher. I could probably get away with a 1000 or 1500, but I went ahead and bucked it 16 volts to be on the safe side.
Converter takes a 125 amp minimum breaker.

 

[synchro]

What are the power requirements of the machine? What sort of 750VA buck boosts can you get quickly?

If you have _2_ 750 VA 120/240V : 12/24V transformers, you can get 62.5A at 222V single phase.

-Jon

On the three phase side the 750 va would work, but a single 750 on the single phase input side would be too small. Machine draws 40 amp three phase, so the single phase side would be much higher. I could probably get away with a 1000 or 1500, but I went ahead and bucked it 16 volts to be on the safe side.
Converter takes a 125 amp minimum breaker.

I believe Jon was saying that you might use two buck boosts, where each of the two single phase line voltages has its own buck boost to reduce its voltage by about 12V.

There are 1kVA buck boosts that seem to be readily available. The Acme T111683 is good for 83A when its low voltage windings are in parallel for 12V.

https://www.zoro.com/acme-electric-transformer-120240vac-1224vac-1kva-t111683/i/G0399917/

 

[hillbilly1]

I believe Jon was saying that you might use two buck boosts, where each of the two single phase line voltages has its own buck boost to reduce its voltage by about 12V.

There are 1kVA buck boosts that seem to be readily available. The Acme T111683 is good for 83A when its low voltage windings are in parallel for 12V.

https://www.zoro.com/acme-electric-transformer-120240vac-1224vac-1kva-t111683/i/G0399917/

Parallel two? If I use one on each line, it would be a 24 volt drop.

 

[synchro]

Parallel two? If I use one on each line, it would be a 24 volt drop.

The two separate 12V windings that are inside of each transformer would be in parallel to remain at 12V, but it could provide twice the current as when configured in series for 24V.

Yes, the total drop would be about 24V.
You had mentioned that 220V was preferable, and so 245V - 24V = 221V.

When I got the specs on it, and found out it required < 240 volts, preferably 220, a good profit job turned into a poor profit job.

 

[hillbilly1]

The two separate 12V windings that are inside of each transformer would be in parallel to remain at 12V, but it could provide twice the current as when configured in series for 24V.

Yes, the total drop would be about 24V.
You had mentioned that 220V was preferable, and so 245V - 24V = 221V.

Ok, I misunderstood you when you said 12 volts, but on each leg, which would be a 24 volt drop.