Buck Boost Transformers

[JHHAMIII]

I have a 3 phase 480/277 VAC feeder that I have connected to three (3) 1.5 KVA 240/480 to 24/48 VAC Buck Boost Transformers connected in a WYE configuration to get close to a 380/220VAC WYE system. The output voltage right now is L1-L2, 407 VAC, L2-L3 405 VAC and L1-L3 is 406 VAC, L1-Gnd is 236.5 VAC, L2-Gnd is 235.9 VAC, and L3-Gnd is 236.7 VAC. The WYE of the Buck Boost transformers is H1-H3 from each transformer connected together and I am reading 147.5 VAC from the un-grounded WYE (H1-H3) of all three (3) Buck Boost transformers connected together for the WYE to the grounded WYE 480/277 VAC feeder connected to the Buck Boost transformers. I want to ground the WYE of the Buck Boost Transformers so that I have zero Volts from the 480/277 VAC grounded WYE to the grounded WYE of the Buck Boost Transformers. This should give me close to a 221.6 VAC/383.8 VAC WYE system form the Buck Boost transformers. Can you confirm that I need to ground the WYE of the Buck Boost Transformers?
We are the Electrical Contractor installing the transformers to connect to a 380 50 HZ piece of equipment with 220 VAC controls connected from one phase to ground. We have told the customer that the motors on the system will run 20% faster from the 60HZ system, which they do not have a problem with.
They have recently been installed and we are about to connect and test the equipment by connecting it to either the ungrounded WYE from the Buck Boost Transformers or the grounded WYE of the Buck Boost Transformers. The 406 VAC average voltage from the ungrounded WYE of the Buck Boost transformers is within the +/- 10% voltage tolerance of the 380 VAC motors as that is from to 342 to 418 VAC.
The simplified question is it correct to ground the WYE of three (3) Buck Boost transformers connected in a three (3) phase WYE?

 

[GoldDigger]

You seem to have a serious problem with your understanding of the buck-boost operation.
It works by creating the equivalent of an autotransformer which raises the voltage of whatever source is connected to its primary. The low voltage winding of each transformer is connected in series with the high voltage end of the primary source. It is in series in phase (additive) to make a boost configuration and in series out of phase (subtractive) to create a buck configuration. Neither end of the secondary winding can be connected either to ground or to any other transformer which is being used with a different primary source, including another phase of the same supply.
Connecting the secondary windings of the three transformers to make a wye will not allow you to connect more than one of them to the top of its corresponding primary phase. Either of the configurations you propose will create a near bolted short and let all the magic smoke out.

 

[drcampbell]

This really calls for a consultation with an engineer who's familiar with the inner design of the machine.

Motors that were originally designed for 380 volts, 50 Hertz will run 20% faster on 60 Hz, but they also ought to be fed with 20% more voltage. (which just happens to be ~460v) Buck transformers for the entire machine and all three phases is probably the wrong approach.

It's likely that only the 220-volt controls need a buck transformer, and even that might be unnecessary if there's a 277- or 480-volt tap available on the control transformer, or it's a switching power supply with wide input-voltage tolerance.

But don't ask the Internet. We don't have enough detail to provide the correct answer. Consult an engineer.

 

[kwired]

This really calls for a consultation with an engineer who's familiar with the inner design of the machine.

Motors that were originally designed for 380 volts, 50 Hertz will run 20% faster on 60 Hz, but they also ought to be fed with 20% more voltage. (which just happens to be ~460v) Buck transformers for the entire machine and all three phases is probably the wrong approach.

It's likely that only the 220-volt controls need a buck transformer, and even that might be unnecessary if there's a 277- or 480-volt tap available on the control transformer, or it's a switching power supply with wide input-voltage tolerance.

But don't ask the Internet. We don't have enough detail to provide the correct answer. Consult an engineer.

I agree, if the load can handle the increased speed all that really needs changed is the control transformer.

Motors that were designed marginally to driven load will likely be overloaded at the higher speed. Machines that have mechanical speed controls maybe will be fine but won't have same speed ranges as originally intended.

 

[jim dungar]

Can you confirm that I need to ground the WYE of the Buck Boost Transformers?

As the others have said: Do not ground the Wye point of the autotransformer. Bad things will happen.
Among other problems you will be violating the NEC by bonding the 480Y/277V neutral to ground at other than its source. The output of an autotransformer is not a separately derived system.

 

[GoldDigger]

As the others have said: Do not ground the Wye point of the autotransformer. Bad things will happen.
Among other problems you will be violating the NEC by bonding the 480Y/277V neutral to ground at other than its source. The output of an autotransformer is not a separately derived system.

IMHO it is more serious than this, Jim. There should not even BE a wye point of the buck (secondary) windings.

 

[gar]

191120-1113 EST

JHHAMIII:

Nothing in your post seems to correlate or make sense.

Apparently you have a wye power source with line to its neutral being 277 V. Thus, the initial way to look at this is to use three buck transformers with 277 V primaries with output tap at 220 V if you really want 220 line to neutral. Your 480 to 48 would produce an output of 277-28 = 239, and at reduced kVA.

Apparently what you are doing is connecting a buck transformer line to line with 480 as the input and subtracting 48 gives you 432 V. If you take each of these three new points, and consider them to be three new lines, then the voltages are line to line 410.1 V, and line to neutral of the source of 236.8 V. These voltages you get from drawing the phasor diagrams. Note also there is a phase shift. These values are close to what you measured, and therefore this is likely your circuit. Your buck transformers connected in this fashion have no neutral of their own. Those transformers alone have no neutral. Their circuit neutral is the neutral of the source wye neutral. Those buck transformers are three separate isolated transformers floating off in space until you connect them to the source.

Individually each buck transformer is a single phase transformer, and there is no such thing as that transformer having a wye point.

You do really need to figure out what voltage you want to drive the motor at, and whether the motor can do the desired work under those condition. This has already been mentioned.

.

 

[mike_kilroy]

"We are the Electrical Contractor installing the transformers to connect to a 380 50 HZ piece of equipment with 220 VAC controls connected from one phase to ground"

As the others have clearly stated, you should not be making these decisions. You or your customer needs to contact the manufacturer of that equipment and get an official drawing how to hook it up. You are leaving yourself wide open to a lawsuit making decisions you are not qualified to make.

 

[jim dungar]

IMHO it is more serious than this, Jim. There should not even BE a wye point of the buck (secondary) windings.

If (3) buck-boost transformers are wired in a WYE connection there physically will be a wye point, however it should be left floating.

 

[gar]

191120-1535 EST

jim dungar:

An autotransformer has no neutral. It is important to define what a neutral is.

An autotransformer is simply a load. It is a transformer with one continuous winding, meaning it has DC continuity between any two or more taps on the winding.

This site crashed what I was writing. The site is still junk software.

If two taps on an autotransformer are connected to a wye source from a hot line to the wye neutral, then you can produce a new hot line in the system at some other voltage relative to neutral. This autotransformer does not have a neutral, but it does have one primary terminal connected to the original wye system neutral.

Put two more identical autotransformers into the wye system. One on each of the other phases, and you have created a new wye source which uses the original wye source neutral as its neutral. If you do not tie the common point of the three autotransformers together, then you have a new wye source that is unstable, and very load dependent.

.

 

[jim dungar]

191120-1535 EST
An autotransformer has no neutral. It is important to define what a neutral is.

An autotransformer is simply a load.

I am fully aware of what an autotransformer is. You need to expand your vocabulary.
First, you are describing a single type of autotransformer. You are neglecting the type that is created by the connection of two individual windings (i.e. an isolation transformer) connected in series.
Second, you are not following the NEC 450.3 definition of a 'transformer' as two or more transformers operating as a unit, like 3 single-phase devices connected into a wye configuration.(also see the definition of a neutral point in Article 100)
And lets not forget 450.5(A) which specifically describes 3-phase 4-wire autotransformers.

 

[gar]

191120-1745 EST

In my last post the last sentence did not say what I was trying to say.

From the previous post. --- "If you do not tie the common point of the three autotransformers together, then you have a new wye source that is unstable, and very load dependent."

This should have said --- If you tie the common point of the three autotransformers together, this is not a neutral, but do not tie that common point to the original wye neutral, then you have a new wye source, but not with stable voltages.

That common point will jump all around with loading, and as such so will the new line voltages relative to the original wye neutral.

Hope this is clearer.

.

 

[gar]

191120-1937 EST

jim dungar:

From my earlier post: --- "An autotransformer is simply a load. It is a transformer with one continuous winding, meaning it has DC continuity between any two or more taps on the winding."

This statement includes what otherwise might be called an isolation transformer where the windings are not connected together. But once you wire two or more windings in series on a common core transformer, then you have created what can be called an autotransformer. Whether or not it is an autotransformer will depend upon use.

.

 

[topgone]

@JHHAMIII ,
You ground the neutral to have a reference point, stability of your voltages to ground, and if your device needed a reference point. That aside, if your supplied device is a motor, it would be happy being supplied with 380 X (60/50) = 456V at 60 Hz. The things is, it just runs faster and may overload.
Hope that helps.

 

[oldsparky52]

The simplified question is it correct to ground the WYE of three (3) Buck Boost transformers connected in a three (3) phase WYE?

I would connect it exactly as the manufacturer's drawing.

 

[jim dungar]

@JHHAMIII ,
You ground the neutral to have a reference point, stability of your voltages to ground, and if your device needed a reference point.

The neutral point in a wye connected autotransformer should never be connected to ground. The autotransformer output is not a separately derived system, the neutral point is common to both the high and low voltages. Because the neutral conductor is common it has already been connected to ground 'upstream'. Connecting it to ground at the autotransformer output would create a second N-G which would be an NEC violation.

 

[winnie]

The low voltage winding of each transformer is connected in series with the high voltage end of the primary source. It is in series in phase (additive) to make a boost configuration and in series out of phase (subtractive) to create a buck configuration.

A small nit: a common configuration is to have the coils connected in an additive configuration, with the supply connected to the higher voltage tap of the series windings, in order to reduce the output voltage ('buck'). The OP, for example, could place the 240V coil in series with the 48V coil to get a configuration that would be a max volts per turn at 288V. This would be a good match for their 277V supply.

Neither end of the secondary winding can be connected either to ground or to any other transformer which is being used with a different primary source, including another phase of the same supply.
Connecting the secondary windings of the three transformers to make a wye will not allow you to connect more than one of them to the top of its corresponding primary phase. Either of the configurations you propose will create a near bolted short and let all the magic smoke out.

I believe that if you have 3 separate single phase autotransformer arrangements, each connected L-N, then you will have a perfectly fine wye arrangement, with changed output voltages and the neutral carried from the primary source. In this case the neutral would be common to all of the transformers.

-Jon

 

[winnie]

The neutral point in a wye connected autotransformer should never be connected to ground. The autotransformer output is not a separately derived system, the neutral point is common to both the high and low voltages. Because the neutral conductor is common it has already been connected to ground 'upstream'. Connecting it to ground at the autotransformer output would create a second N-G which would be an NEC violation.

This!!

To be perfectly clear: the neutral point of the wye autotransformer _is_ connected to the supply neutral, which _is_ a 'grounded conductor'. So the neutral point of the wye autotransformer is _indirectly_ connected to ground...but you do _not_ create a _new_ ground connection.

-Jon

 

[kwired]

A small nit: a common configuration is to have the coils connected in an additive configuration, with the supply connected to the higher voltage tap of the series windings, in order to reduce the output voltage ('buck'). The OP, for example, could place the 240V coil in series with the 48V coil to get a configuration that would be a max volts per turn at 288V. This would be a good match for their 277V supply.



I believe that if you have 3 separate single phase autotransformer arrangements, each connected L-N, then you will have a perfectly fine wye arrangement, with changed output voltages and the neutral carried from the primary source. In this case the neutral would be common to all of the transformers.

-Jon

Which is exactly what one does with single phase only autotransformer lighting ballast and similar applications. supply grounded conductor and "output" grounded conductor are tied together in making it work and for all practical purposes are considered one system conductror. You do not ground again at this autotransformer - it is already grounded back at the source.

If you have any transformer arrangement with multiple taps (single or poly phase)and a grounded conductor as part of that arrangement, you can not ground any of the other taps or you will have objectionable current, and very likely at fault current levels in many cases.