VSD Supply Transformer Selection

[raar]

Have a grinder machine with an approximately 1HP motor with a Mitsubishi Model FR-D720-1.5K drive. Per the machine manual (English), the product requires 230 VAC, 3-Phase, 3-wire, 60Hz and appears to be originally designed to IEC standards, but also sold in North America. We've already had a 3rd party FEB inspect for NFPA 79 compliance and in-process of determining related corrective actions.

Our building distribution is 480 VAC, 3-Phase, 3-wire, 60 HZ, and desire to install an equipment / isolation transformer for this machine, with no intention on providing that voltage for other loads using an existing 30A disconnect, to transformer (to be selected and installed), then to another disconnect (to be installed) for the machine with appropriate protection via disconnect fusing. The cabling distances are approximately ~20 or less feet from existing disconnect to proposed mounting location for transformer and about 20-40 feet to the machine from there. The drive is installed on the machine and has 20 feet or less cable run to its motor.

I desire to provide a grounded separately derived system for this machine, so believe my options are a either appropriately sized delta-wye or delta-delta with a high leg delta in order to minimize the number of items for our installers. Finding an off the shelf, listed, 3-phase, 60Hz, 480:240/120 VAC (delta-delta) or 480:240Y VAC (delta-wye), 7.5KVA, K-Factor (4, or 13) transformer, 115C rise hasn't yielded many results. Need a relatively quick solution (part arrival within 2-3 weeks from order), so believe I have to find available off the shelf. For energy reasons I'd prefer not to install a substantially larger transformer if possible, but am open to suggestions.

Yesterday I did find this transformer that is defined as a Drive Isolation Transformer, DTGA72S, but doesn't seem to specify a K-Rating. When I view the image for that transformer it appears to just list "General Purpose". Although described as 460:230 VAC, it has a 5% above and below tap on primary, so I believe from a voltage standpoint it could work. Somewhat recently we ran a couple hour power test at a busy time of day and voltages stayed at 480 VAC with little deviation (+/- 1 VAC).

Would you consider this transformer for this application? If not, why? Any other suggestions?

Thank you for any assistance related to this.

Attachment links: Transformer Catalog - Drive Spec Sheet

 

[itsmoked]

1) If only the motor is 3phase you could probably supply only single phase to that small a drive.

2) The transformer suggested is more than adequate for the task. It's probably about 5X larger than needed since it's 7.5kVA and the drive is only 1.5kW.

Don't forget to solidly ground the secondary neutral point of your separately derived source.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[raar]

Thank you for the quick reply @itsmoked.

1) Yes the motor is 3-phase, however the drive specs list 3-phase input as a requirement.
2) Understand, there are some controls loads, however I believe the small motor load is still larger. The controls are still WIP due to some problems found during inspection.

Regarding the grounding the neutral, I'm going to verify the structural steel column(s) are connected to the concrete-encased electrode by an approved means, and so long as that is the case the intention is to verify article 250 requirements of NFPA 70, and so long as acceptable, solidly bond the secondary neutral point to the structural steel column.

 

[waross]

I thought so too Keith.
Then I found the instruction manual.
Mitsubishi want a 4 KVA transformer to feed that drive.
For three phase that may be the next available size.

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

 

[itsmoked]

!!! Interesting Bill. I couldn't get my hands on the manual and one of those manual sites was sooo frikken painful I gave up. It's a bit of a shame as you have to wire that transformer up for 7.5kVA just so it doesn't trip the breaker on start up. Then you'll have to protect the drive with more OCP because the source is bigger than needed. But to get the job done raar might as well grab it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Link
Page 290.

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

 

[jraef]

That is the right transformer for the task (not commenting on the size however); the secondary in this case is 230Y133, which is what you want for the VFD, because it will want a solidly grounded Y secondary (neither the drive nor the transformer care about the primary side so long as you give it the rated village). So DEFINITELY ground the X0 terminal on the transformer.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

133 huh... ?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

It's just what you end up with for a 230V Y (230/1.732 = 133). It's not a voltage that is used, it's just to give the VFD a solidly grounded voltage reference on all 3 phases.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

I used to find 120/208 Volt systems on fishing and lobster boats running at 127/220 Volts because the untrained ships engineer "knew" that it had to be 220 Volts.
The solution?
Do nothing.
That avoids the possibility that something will be damaged when someone tries to set the voltage back to 220 Volts.
133V? What Jeff says.
Question:
Does the lower impedance of the larger transformer reduce the amount of harmonics passed back to the system?
Could specifying an oversized transformer be a sneaky way of offloading harmonic mitigation onto the end user or do I have this backwards?

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

 

[itsmoked]

Ah of 230V.. I've only ever heard it at normal winding voltages e.g.240V.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Does the lower impedance of the larger transformer reduce the amount of harmonics passed back to the system?
Could specifying an oversized transformer be a sneaky way of offloading harmonic mitigation onto the end user or do I have this backwards?

If the non-linear load (in this case the VFD) is on a larger transformer, it is in theory a smaller percentage of the total demand distortion (TDD), ASSuming there are other loads on that transformer. If it's the only load, the lower impedance does not help and compared to one that is properly sized, would offer LESS mitigation. ADDING impedance helps, hence adding Line Reactors. A typical Drive Isolation Transformer, sized to the drive load, has about the same harmonic mitigating effect as a 3% line reactor; in other words some, but usually not enough. It will take the I-THD from 80%+ down to around 30-35%. But the goal is >5%.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[FacEngrPE]

Either oversize transformer to allow it to handle the harmonic losses or use a K rated transformer (

https://www.eaton.com/us/en-us/cata...ols-systems/k-factor-rated-transformers.html.

Neither is intended to act as a harmonic filter ), although they could be part of one. Line reactors are good.
Fred

 

[waross]

Thank you for the information.

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

 

[raar]

Thanks again everyone for the replies.

I did look at the K-rated Eaton's, the smallest they had in a single can is 9kVA, and the secondary voltage would be at 208 VAC line to line. Originally I was leaning towards a 208Y/120 VAC since that provides an 8 volt margin to the minimum drive voltage, where as a 240 VAC is at max nominal voltage for the drive. The grinder product manual just says 230 VAC, which should be the machine's utilization voltage. In this application, there will be little line loss, so the drive will probably see at or just under 240 VAC RMS most of the time.

Knowing that there will be harmonics, and that the machine doesn't have line reactors installed, my plan is to take some initial measurements and get a feel for how bad the THD via Fluke power quality meter (has ability to measure harmonics also) on primary and secondary side of transformer (line to line), then also measure temperatures to make sure the installation doesn't have any problems on commissioning. Unfortunately the drive and controls enclosure has little room for installation of line reactors, so if they are needed, we are going to install another enclosure to house them. My understanding is that line reactors installed on the input of drives should be as near the drives as practical.

The 7.5kVA is nearly double required by the drive (4kVA) at just above the machines required voltage. The controls shouldn't use more than 1-2A max (AC high side) since they are being modified to a 24VDC limited energy supply (<4 Amp dc). Since it's that much larger, and is identified as a drive isolated transformer, I was thinking that harmonic currents shouldn't be a problem for the transformer. Understand there may be some affects reflected back across to the primary of the transformer, if that measures less than 5% THD, it should be fine correct?

Does any of the above raise any red flags? If so, what would you all do differently?