Converting 480 three phase to 240 single phase and possible imbalance problems 2

[TempleRob]

Hello All,

Long time lurker, first time poster. ET has been a great resource for the past decade. Thank you everyone for your thoughtful information.

I'm a mechanical vehicle engineer (Temple University, 2013) respectfully requesting a bit of electrical support.
I’ve taken a new role where I’m responsible for both the vehicle fleet and now the maintenance facility. I want to install an off the shelf Snap-On parts washer. It is a 230VAC, single phase, 60 amp machine. I have 480VAC three phase power and I’m looking to utilize a transformer, and/or possibly a converter to step down the voltage and convert it to single phase.

The shop has an 800 amp and 1,000 amp main breaker panels in the power distribution room, and I assume both are fed from the same supply. A power study was previously performed. The 1,000 amp panel was observed to have a peak draw of ~950 amps, but the 800 amp panel had a peak draw of ~225 amps. I will therefore have a “home run” fed to the machine on a dedicated circuit and breaker from the 800 amp panel. Transforming the 480, I expect to consume ~30 amps from the 800 amp panel. I’m looking for the best balance between cost and complexity. My concern is creating an unbalanced load, which may have undesirable heating/performance side effects to motors (HVAC, elevator, large vehicle hoist, etc) and other equipment being fed from the 800 amp panel.

Based on some limited research, the simplest, most reliable, and cheapest option would be to use a 15kVA single phase transformer, connecting two legs of the 480VAC on the input side. I understand there are other transformers (open delta and Scott T) that use all three legs, but still create an imbalance. I also understand it may be possible to use a three phase to single phase converter to avoid a load imbalance, but I suspect cost will increase and reliability will decrease. I planned to write a scope of work for an outside contractor to evaluate the options, propose a solution, and then test/document load imbalance on the 800 amp feed after the job if a transformer is used.

I've attached photos of the panel and load study.

Questions:

1. Since the supply to the 800 amp panel is likely very large compared to the load I need, using a single phase transformer should I be worried about creating a detrimental imbalance for other equipment being fed by both the 800 amp and 1,000 amp panel? Or is the load such a small percentage of overall supply, other equipment won’t effectively “see” the imbalance? Same question would apply for other two transformer types?

2. Would we be better off using a converter? What are the advantages/disadvantages?

3. Are there any other options that I have missed?

Thank you in advance for any thoughts or suggestions that you may have.

https://files.engineering.com/getfi...a0-4659-8104-18eeddb1d24d&file=Load_Study.png

https://files.engineering.com/getfi...45-9484-ca0e2c3f3eff&file=800_amp_panel_1.jpg

https://files.engineering.com/getfi...22-bacc-ea5112f5db69&file=800_amp_panel_2.jpg

https://shop.snapon.com/product/Ele...Heavy-Duty-Automatic-Parts-Washer-(Red)/PBC58

 

[FacEngrPE]

It would be special order but I would ask the factory if they could supply the parts washer in a 3 phase version. See the customer support contact info

https://b2b.snapon.com/customerService

 

[davidbeach]

Way overthinking it, that’s why they make two pole breakers.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[edison123]

Conversion from 3 phase to 1 phase requires a special transformer. Just use two legs of 480 V and use a single phase step-down transformer.

Better yet ask the supplier to fit a 3 phase motor.

Muthu

http://www.edison.co.in

 

[waross]

Current unbalance is often related to voltage unbalance as well as unbalanced loading.
Check the 800 Amp panel for any voltage unbalance.
Connect your 15 KVA transformer to the two highest voltage phases of the 800 Amp panel.
Stop worrying and move on to the next project.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TempleRob]

Thank you all for your insight and time.

FacEngrPE
It is an off the shelf item, I don't think I want to ask for customizations. But, a good thought that I have shared with you and considered.

davidbeach
True, better to overthink than underthing, I think haha. I just don't want to create problems outside my area of expertise. Also, I didn't want to get roasted coming with incomplete information on my inaugural post.

edison123
I interpreted you reply as "does NOT require a special transformer.." Agree.

waross
Agree on I vs V relationship. Great suggestion checking legs preinstall. Unsure if it changes with time depending on what other equipment cycles on and off, but it's better than nothing!

 

[waross]

Uneven loading at the plant down the road may cause uneven voltages at your plant, which in turn will cause uneven currents.
It is often a judgment call as too how much unbalance is too much.
There are a lot of interacting factors that complicate a rigorous analysis.
Uneven currents tend to correct uneven voltages, which in turn affects uneven currents.
The number of three phase motors on-line.
The stiffness of the grid.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TempleRob]

Hi All,

I drafted a scope of work. If you don't mind taking a glance, how does the following technical except look?

3.1 Scope
1. The Contractor shall be responsible for performing the following tasks within the Shop:
a. Prior to any work, the Contractor shall submit a detailed technical and cost proposal for Authority review and approval.
i. The technical proposal shall include a general arrangement drawing, electrical schematic, and bill of materials including OEM part numbers.
ii. The cost proposal shall include a breakdown of labor hours required for each major task, labor markup, and parts markup.
b. Perform a voltage and current imbalance test of the 480VAC main feed supplying the 800 amp and 1,000 amp electrical panels in the power distribution room.
c. Install a dedicated 110 amp circuit breaker in the 800 amp electrical panel. The contractor shall determine if a Wye or Delta configuration is required.
d. Install dedicated conduit, power and ground wiring, and junction boxes from the power distribution room to the equipment location. Conduit may need to penetrate original concrete walls. If concrete walls require penetrations, the contractor shall wet drill the walls to mitigate airborne silica dust. See Section 3.2.
e. Install a transfer switch at the equipment location. The transfer switch shall supply:
i. A GE motor-generator unit, 440VAC three phase, 92 amps.
ii. A 2:1 transformer or three phase to single phase 2:1 power converter supplying a 70 amp single phase circuit breaker powering a Snap-On Parts Washer, 230VAC single phase, 60 amps. If a single phase, or three phase, transformer is utilized, the legs of the transformer shall be connected to the 480VAC three phase supply such that voltage and current imbalance is minimized in the 480VAC three phase supply. If a three phase transformer or three phase to single phase converter is proposed, the Contractor shall provide technical justification demonstrating that a single phase transformer would result in detrimental affects to other motors and equipment supplied by the 800 amp or 1,000 amp electrical panel.

 

[waross]

Or;
Hire a competent contractor or engineer.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TempleRob]

Waross,

The SOW is for an electrical contractor to bid on and perform the work. Neither I, or any of my staff, will be doing the work. I copy and pasted the technical requirements section previously. The general section clarifying it's Contractor work is below.

I'm hoping the technical requirements prevent any reasonably foreseeable issues down the road. If you don't see anything clearly incorrect, I'm inclined to ship it. It's the best I think I can do as a mechE without getting too prescriptive and over my head. Much appreciate your time and support!

1. Summary
1. The REDACCTED (hereby referred to as the Authority) seeks an electrical Contractor to install 480VAC three phase power and associated conduits and grounds, from the 800 amp electrical panel in the power distribution room, to the equipment machine shop, which is located approximately 75 linear feet from the power distribution room. The power shall be connected to a transfer switch. The transfer switch shall power a GE motor generator unit and a transformer, or three phase to single phase converter, which powers a circuit breaker and Snap-On parts washer. Selection of a transformer or converter shall prioritize simplicity, reliability, and cost. All work must adhere to all applicable local, state, and federal codes and regulations. The work shall be completed by a fully insured and licensed electrician. A technical description of the work to be completed is provided in Section 3.1. The Authority shall designate a Project Manager who shall the Contractor's sole point of contact.

 

[waross]

Some comments:

associated conduits and grounds

Cable is often more economical than conduit. The material costs more but this is often more than offset by labour savings.

The power shall be connected to a transfer switch.

Specs of the switch?
Manual, automatic? Cool-down period? Automatic exercise?
Three pole or four pole?
Contactor based or switch/breaker based?
Rating?
You are grossly overthinking this.
Some of your requirements will not meet code.
I have to wonder if you know what a transfer switch is, or what you mean by a transfer switch.

Suggested SOW;
Qualifications.
The contractor must be qualified and licensed to perform work in this jurisdiction.
The contractor will submit to the owner a copy of the electrical permit.
The contractor must submit a copy of the electrical permit, signed off by the AHJ as completed and accepted, before final payment is made.
Contractor will supply information on the qualifications and relevant experience of the senior responsible worker who will be present on site at all times when work is being done.
All work shall be done in accordance with applicable codes.
Power will be supplied and connected to a parts washer and to an MG set.
A breaker suitably sized according to code and feeder conductors suitably sized according to code shall be installed to provide power from the 800 Amp panel to the equipment.
Local safety disconnecting means shall be provided to disconnect the MG set and the parts washer in accordance with OSHA, with the electrical code and with any additional applicable codes..
The parts washer will be powered by a 15 KVA single phase 480 Volt to 240 Volt transformer, connected to the two highest voltage phases in the 800 Amp panel.
Optional:
The transformer shall have voltage adjusting taps. (95%, 97%, 100%, 102% 105% ?)
The contractor may chose between a conduit or armoured cable feeder.
The general layout of the work including wall penetrations shall be walked and agreed upon by an owner's representative and a contractor's representative prior to bidding.




--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LittleInch]

I realise I'm a bit late to this party, but where does your plant currently get its single phase power from?

If this is off the same boards then either uprate it or use different phases to the existing single phase connection.[pre][/pre]

That is thought a pretty big load for a 240v supply.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

This is a comment, not a suggestion:
One way of deriving single phase from three phase often used on smaller generator sets is the zig-zag connection. (Not to be confused with the zig-zag grounding transformer.
This would give true, 120-240 Volts with no phase shift between the 120 Volt legs.
The down side:
Your power factors for a resistive load on the three primary phases will be:
1st phase:- 100%
2nd phase:- 50% leading
3rd phase:- 50% lagging
The load balance seen by the supply system will be better but still not that great and will vary with loading from other loads.

Rather than one 15 KVA, 480:240 Volt transformer the zig-zag connection would require three 7.5 KVA transformers and switching and protection.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TempleRob]

Waross,

Thank you for all your help. I’m doing the best a mechE can to stay out of trouble on this one!

I left the specs of the transfer switch up to the contractor to propose, since I may not select the best, or even correct, option if the SOW is overly prescriptive. But you raise solid points. My understanding is a transfer switch is generally used to connect a load to either a main or redundant power source at any one time, but never both at once. I also believe that it can be used similarly to connect two loads to a power source, one at a time. Is this way, I want to prevent both machines from running simultaneously so the breaker isn’t overloaded and also to mitigate motor imbalance issues on that circuit. If this understanding is incorrect, please let me know why.

What requirement/s is in contradiction with code/s?

Thank you for the scope suggestions.

I prefer to run conduit. All other lines in the shop are run in conduit.

Good idea on local disconnect/LOTO.

I had already included a site walk and post walk meeting for questions in the site conditions section of the scope. Good to know we’re thinking at least a bit similarly.

I haven’t heard of a zig-zag connection (not transformer) before! Interesting. This may be an option for us, but’s a shame the PF is so low on two legs, I hate wasting energy if it can be avoided, with the exception of my C5 corvette, that gets a pass, haha.

Lilttle Inch,

Our single phase supply is inadequate for conversion to 240 with a 60 amp draw, sadly. It is a good idea I can remember for later though. Thank you.

Thank you everyone for your thoughtful suggestions and time. It is greatly appreciated.

 

[waross]

Transfer switch:
For that purpose, a basic, three pole, manual transfer switch will probably be adequate.

Code: (under, not over)
c. Install a dedicated 110 Amp circuit breaker.
E, i. a GE motor generator, 440 Volt(460V?), 92 Amps.
This probably won't start on a 110 Amp breaker.

You may consider adding an appropriately sized lockable breaker for the parts washer to serve as the local LOTO device.

I suggest picking a contractor and then discussing the details with him.
If you have a good result, use him athe next time. As you build a relationship, you will be able to trust his judgement
more and more and spend less time writing detailed scopes of work.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TempleRob]

Waross,

Sir, you are a wealth of incredibly pragmatic info. My kind of engineer (I fixed cars at a Chevy dealer for 7 years before I went back for my BSME, the hand on experience has been invaluable in my career).

The motor is a in a motor generator unit turning a generator that creates (yet another)oddball electrical supply. So, I'm not anticipating anything near stall torque/current. But, that is an excellent point and possible landmine I may have stepped on! I'll add a step that we check the MG unit peak draw and size the CB accordingly. Thank you!

We have a couple contractor we use and trust, and I'm thinking we'll use Snyder Electric for this job.

Yeah, three pole switch, sounds like a plan.

Thank you again! I'll keep you updated on how well (or doesn't) it goes, haha.

Rob