Grounding delta delta transformers on CNC machines

[alblock]

We have several CNC machines that are ungrounded. Most have 480/240 delta delta transformers sitting right on the machine. We have been told by Mitsubishi, Fanuc, and other machine tool technicians that is part of a problem we have been having with NC units going bad. I will order or machine copper bars for dummy fuses. Can I just bond the incoming B phase at the disconnect switch and call it a day?

I know I have to run a GEC, SSBJ, EGC, What other requirements do I need to do to be 2014 NEC code compliant? I made a rough diagram using drawio.
Thank You.

 

[LarryFine]

Seems to me you should bond the B phase at the service as you would a neutral, keep it isolated like a neutral after that, and bond the machines to an EGC bonded back at the service.

 

[Joethemechanic]

Your 480 bus is corner grounded on B isn't it? Transformer at each machine would be a SDS. Machine and transformer came from Mitsubishi as listed? Or is trans field installed?

Mitsubishi means "Three Diamonds" Tuna and machine tools from the same company

 

[MTW]

You need transformers with a wye secondary and ground the center of the wye.

Corner grounding a delta secondary, will most likely cause more equipment damage than you currently are having. The MOV surge protectors in your drives will not like one or two legs of a higher voltage to ground. They are normally built to be applied on wye input systems.

Check your machine specifications, what is the allowed voltage input range of the equipment?

 

[don_resqcapt19]

Because of the issues MTW mentioned in post 4, I would think about replacing the delta/delta transformer with a delta/wye drive isolation transformer with a 240Y/139 volt secondary.

 

[MTW]

I have in the past used a standard 480 delta primary/ 208 wye secondary if you get the models that have four 2 1/2% taps you can adjust the 208 wye output closer to 220V which is what the machines are usually designed to run on. Thats why I suggested that you check the specifications for the equipment.

I have used this method for some Mitsubishi 100HP bridge mills and others and they have performed excellent for 10 years, the customer tells me. Some of the newer models come with a multi tap primary transformer that eliminates the need for the customer to provide the correct type externally.

But I assure you feeding them a corner grounded delta with unequal voltages with respect to ground is a recipe for bigger trouble than ungrounded.

The problem with ungrounded is wild voltage swings with respect to earth/ ground being that the supply system is not referenced to ground.

Making an unequal reference to ground just makes the problem worse. It can readily cause the spindle drives to go bad in a hurry, when they are expecting a uniform voltage to ground.

Which is what a corner grounded is. Unequal voltages to ground.

 

[Lara jain]

It appears that bonding the B phase at the service, treating it similar to a neutral, and then isolating it, would be a suitable approach. Additionally, bonding the machines to an equipment grounding conductor (EGC) that is bonded back at the service is recommended. This helps ensure proper grounding and electrical safety for the FEMA generators. It's essential to follow OSHA guidelines and consult with a qualified electrician for specific requirements in your situation.

 

[winnie]

Bonding the B phase will create a grounded system and is appropriate from a safety perspective.

But it doesn't solve the equipment damage issues.

Another possible approach is to add a grounding autotransformer which derives the required neutral and then grounding that derived neutral.

Jon

 

[alblock]

Your 480 bus is corner grounded on B isn't it? Transformer at each machine would be a SDS. Machine and transformer came from Mitsubishi as listed? Or is trans field installed?

Mitsubishi means "Three Diamonds" Tuna and machine tools from the same company

The bus is 480v wye 3 phase. The transformers are field installed. Most are 45kva delta delta. I knew about the three diamonds. I looked up Mitsubishi tuna and was surprised at how much tuna they control.

 

[alblock]

You need transformers with a wye secondary and ground the center of the wye.

Corner grounding a delta secondary, will most likely cause more equipment damage than you currently are having. The MOV surge protectors in your drives will not like one or two legs of a higher voltage to ground. They are normally built to be applied on wye input systems.

Check your machine specifications, what is the allowed voltage input range of the equipment?

That's what I didn't want to hear. Most have 230v on the nameplate. A couple say 220v but we have the taps adjusted to accommodate.

 

[alblock]

Because of the issues MTW mentioned in post 4, I would think about replacing the delta/delta transformer with a delta/wye drive isolation transformer with a 240Y/139 volt secondary.

Yeah now if I can find a place to trade.

 

[alblock]

I have in the past used a standard 480 delta primary/ 208 wye secondary if you get the models that have four 2 1/2% taps you can adjust the 208 wye output closer to 220V which is what the machines are usually designed to run on. Thats why I suggested that you check the specifications for the equipment.

I have used this method for some Mitsubishi 100HP bridge mills and others and they have performed excellent for 10 years, the customer tells me. Some of the newer models come with a multi tap primary transformer that eliminates the need for the customer to provide the correct type externally.

But I assure you feeding them a corner grounded delta with unequal voltages with respect to ground is a recipe for bigger trouble than ungrounded.

The problem with ungrounded is wild voltage swings with respect to earth/ ground being that the supply system is not referenced to ground.

Making an unequal reference to ground just makes the problem worse. It can readily cause the spindle drives to go bad in a hurry, when they are expecting a uniform voltage to ground.

Which is what a corner grounded is. Unequal voltages to ground.

Ok, thanks for the info. I'm just trying to comply with the NEC. It's either buy ground fault detectors or switch to delta/wye transformers.

 

[alblock]

Bonding the B phase will create a grounded system and is appropriate from a safety perspective.

But it doesn't solve the equipment damage issues.

Another possible approach is to add a grounding autotransformer which derives the required neutral and then grounding that derived neutral.

Jon

Do you know what size autotransformer needed for a 45kva delta/delta?

 

[Joethemechanic]

If your bus comes from a 480 WYE, that would make the B phase feeding the transformer ungrounded and unfused

 

[jim dungar]

If your bus comes from a 480 WYE, that would make the B phase feeding the transformer ungrounded and unfused

This is an isolation transformer, the configuration of the primary source B phase conductor would not influence the secondary side at all.

 

[Joethemechanic]

This is an isolation transformer, the configuration of the primary source B phase conductor would not influence the secondary side at all.

The source for the bus is 480 WYE bonded to ground in the center. There is a piece of busbar or something on B phase of the disco

 

[winnie]

The source for the bus is 480 WYE bonded to ground in the center. There is a piece of busbar or something on B phase of the disco

I misread the original post.

You are correct. The OP shows removing the B fuse on the _primary_ of the transformer. This would be appropriate if the source were ground B, but would do _nothing_ to change the grounding of the secondary.

-Jon

 

[alblock]

If your bus comes from a 480 WYE, that would make the B phase feeding the transformer ungrounded and unfused

Power goes to a delta/delta 480v/240v transformer sitting on the CNC machine first. At the machine disconnect switch I was going to bond B phase and use a dummy fuse at the disconnect suppling power to the transformer. It's seems like that's gonna cause more problems than just leaving them ungrounded so I'm probably not going to go this route.

 

[MTW]

Ok, thanks for the info. I'm just trying to comply with the NEC. It's either buy ground fault detectors or switch to delta/wye transformers.

Ground fault detectors might solve your NEC compliance problem, but won’t do anything for your equipment reliability problems.

 

[winnie]

Do you know what size autotransformer needed for a 45kva delta/delta?

I'd suggest contacting Hammond for their recommendation.

See the below for the information that they need to size a transformer for you:

https://documents.hammondpowersolutions.com/documents/Product%20Info/Technical_Articles/HPS-Grounding-Transformers.pdf

As I understand things, there are only two significant loads placed on a grounding transformer. 1) The continuous 'charging current' of the system, caused by capacitive coupling between the phases and ground. For your small system, my bet is that this will be dominated by the high frequency capacitive coupling of inverter PWM through the motor frame. This might be up to a couple of amps but is probably smaller. 2) The short duration fault current which flows until a breaker trips.

The short duration rating of a transformer is much larger than its continuous rating, so the rating of a grounding transformer for a 45kVA system with normal OCPD is probably only a couple of kVA. But again contact Hammond or another grounding transformer supplier.

IMHO your _best_ option is to get the correct delta:wye transformers in the first place. It is unlikely that the grounding transformers will be cheaper; they will certainly be more subject to installation error. However if you find that replacing the existing delta:delta transformers is problematic for whatever reason, the grounding transformer is a useful backup approach to consider.

-Jon