480 - 240V Transformer check. 1

[itsmoked]

I have 480V 3ph (about 40kVA) and need to hook up a 230V machine. We're looking for a transformer and I believe we want a Delta-Delta and not a Delta - Wye. The machine has no neutral connection.

Seems we can find lots of Delta - Wye here about. I should probably hold out for a delta secondary right?




Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

No, go for the delta-wye. Ground the neutral at the transformer and just take the three phase conductors to the machine. That way you have a properly grounded system.

 

[itsmoked]

Well, excellent then. Thanks David.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

Hey David, maybe it's too obvious, or just me, or that you haven't said so, but would it not also be advisable to extend grounding continuity as you've suggested to include the body of the machine?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

I agree with David.
In Canada (And probably the US) grounding the neutral is a code requirement at that voltage.
Look for a 240 Volt secondary. 480:139/240 Volts.
More common is 480:120/208 Volts. That may be a little low foe your machine.
The problem connection is the grounded wye/delta connection. The four wire primary connection locks in the phase angles and voltages and the delta secondary is unable to willingly accept any unbalances.
With the delta/delta the angles and voltages are not constrained and the secondary is free to track primary unbalances.
If you are unable to locate a delta/wye transformer with acceptable voltages, your second choice may be two 480:240 Volt ransformers in an open delta configuration.
Third choice may be a 480:120/240 Volt four wire delta with the center tap grounded.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

If you can find a 480 delta / 138/240Y that would be ideal, but if not a 480 delta / 120/240 delta side tap (more readily available) is the next best option. Just make sure you use breakers that are straight rated and not slash rated. Mark the high leg if you go the 120/240 route.

http://www.practicalmachinist.com/v...egs-120v-output-rpc-system-delta-high-leg.jpg

In the US you could go delta ungrounded, but you need a ground detector and in all honesty its best not to bother.

Some machines like VFDs do not life floating voltages- or one leg elevated- so make sure you double check that if you go the delta route.

 

[cranky108]

I believe you need to carry a ground, but not the neutral. Also the grounded lead can be used for detecting a ground in the machine.

 

[Mbrooke]

Correct, for a straight 230 volt machine. 133/230 would need a neutral.

 

[jraef]

If there are VFDs or servos fed from 3 phase 240V in the machine, absolutely go find the Wye secondary. 3 phase bridge rectifiers in most modern drive systems will have issues with being fed with a delta source. 240 Y transformers are not standard for distribution because of the 139V L-N voltage, but they are often available as "drive isolation transformers".

If there are no 3 phase rectifiers involved, I wouldn't use the non-standard voltage, because Murphy dictates that some future goober will read 240V L-L and ASSume it is 120V to N, then fry something.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Mbrooke]

@Jraef: Exactly what I had in mind. Agree 100%.

 

[prc]

jraef, In such cases the secondary Y neutral is not earthed. That is the standard in solar inverters seen in my area.

 

[waross]

prc: Are the solar inverter systems back feeding a delta:wye transformer?
That is, power in to the wye side and power out the delta side.
That is a special case that often leads to issues in practice.
The issues may be avoided by floating the wye point.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Nice consensus! The machine is mostly VFDs and drive fed motors so we're on the hunt for a wye output.

Indeed 240 wyes are not common. Most of these machines are actually made with a nameplate supply number of 220V. Which gets you into 'trying' 208V. Some are tolerant and don't care but others monitor the voltage and 'tilt' if it drops much below. What happens is when the machine starts a non-VFD'd motor like a coolant pump or hydraulic chiller it causes a brief dip that is enough to cause the already low 208V to dip below the trip and the controller stops on an error which requires human intervention to continue.

Thanks all.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[prc]

jraef,Yes.Power is fed from Y to delta and the neutral of Y is kept floating. In one case where it was earthed by mistake, inverters started failing.
'That is a special case that often leads to issues in practice.
The issues may be avoided by floating the wye point.'- can you explain this a bit more? Why modern inverters find delta source not compatible?

 

[waross]

How many Amps do you need Keith?
Probably the most available transformers will be 480:208/120 Volts.
You can use three three small 120:12 Volt or three 120:16 Volt transformers to boost the line to line voltage to 228 Volts or 236 Volts.
The current rating of the 12 Volt or 16 Volt windings need only be 1.73 times the needed current.
Yes, it's a little kludgey but it may be what is available at a reasonable price.

The other option may be the delta:delta with a four wire 124:240 Volt secondary with a "wild leg and a grounded center tap.
jraef; How do VFDs respond to a high leg system. The voltages to ground will be 120V, 120V and 208 V.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Transformers on transformers. Too kludgey Bill

Kinda sad cuz the place has a ton of 460 drops everywhere but 90% of their machines are 220V. I'd prefer just switching all drops to new machines over to 240 but then the wire is probably too small, but then the conduit probably can't stand the bigger wire, so now you have to yank a bunch of 250 foot runs of conduit off a 40 foot ceiling and replace it all with larger everything.

So the solution seems to be putting a transformer next to each machine so you can heat the local space, trip over the dang things, and have to make more room for them.

Non-optimal - painted gold.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I see some possible code issues.
I don't work with the NEC but there is a lot of similarity with the Canadian Code.
I realize that the Canadian Code does not apply but hopefully someone will supply the NEC equivalent.
There are significant differences between equipment grounding conductors and system neutral grounding conductors.
In Canada if a grounded neutral of a system results in voltages to ground of 150 Volts or less then the neutral must be grounded.
A delta:wye transformer would be considered a separately derived system and would require a system ground. It is doubtful that the equipment bonding conductor will meet code for a system ground. The installation requirements for a system grounding conductor are more stringent than the requirements for an equipment bonding conductor.
The nice fix voltage wise is a two transformer open delta auto-transformer connection.
The voltages to ground will be 277 Volts, 139 Volts and 139 Volts.
The advantages are:
Cheapest.
Smallest.
Does not require a neutral.
Does not require a system ground.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Keith,
As I said, the way to find these transformers that will have a 230V Y secondary is to look for "Drive Isolation Transformers". In the following linked document, see page 5 of the PDF (referred to as page 127), part #DM040JC

http://www.hammondpowersolutions.co...solation-Transformers-Section-4-of-HTP-16.pdf

Pretty much ass of the major transformer mfrs offer similar products, you just need to look in the right places.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Jeff; As a separately derived system, will the NEC require a system ground rather than an equipment ground on the secondary wye point?
In many Canadian plants an additional system grounding conductor would have to be run to remote mounted transformers in addition to an existing equipment grounding conductor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Yes, the secondary Wye point must be grounded. That's another benefit of using a Wye secondary, because for a Delta, it would need to either be corner grounded, or have GFP added.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington