I'll be installing a machine with a 240V 3-phase system. The loads consist of motor loads controlled by position servo drives. I have a 75kVA Delta-Delta isolating transformer on hand, which has a primary voltage of 480V delta and a secondary voltage of 240V delta. If I connect this transformer, is it considered an ungrounded system? If so, would I not need to install ground fault indication? However, if it's considered a grounded system, which I personally can't see how it is, could someone thoroughly explain how it is grounded?
Is a Delta-Delta isolation transformer considered an ungrounded system?
- Thread starter JohannW
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- retired electrician
The transformer secondary is always an ungrounded system unless you decide to make a grounded system. For a delta, you would make it a grounded system by grounding one phase, typically B phase. If you do that the conductor connected to that phase must be identified as required in 200.6.
If you leave it ungrounded, you need to provide ground fault indication.
Note some types of drives have line to ground filters that might not like 240 volts to ground.
If you leave it ungrounded, you need to provide ground fault indication.
Note some types of drives have line to ground filters that might not like 240 volts to ground.
texie
Senior Member
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- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
In my view, the OP is wanting to step down in voltage, so the delta/delta xformer he has is probably a very poor choice for a number of reasons. If he really needs 240 he could use 480 delta to 240 delta with a center tap secondary for grounding. If 208 volt is OK, then he could just use an off the shelf 480 delta X 208Y.
On re reading his post I'm not sure what exactly he already has. Maybe it is a 480 X 240 center tap which would be fine if he really wants 240 volt.
On re reading his post I'm not sure what exactly he already has. Maybe it is a 480 X 240 center tap which would be fine if he really wants 240 volt.
This is the transformer I have. I feel a delta-wye isolating transformer is the best for my application but delivery times are 10 weeks out. I want to hook it up temporarily with this transformer but I want a solidly grounded system. If i understand correctly, if i ground x6 i will have a solidly grounded system?
Last edited:
texie
Senior Member
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- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
This xformer will give you a solidly grounded 240 volt system. It is what is known as a 240 volt 3 phase 4 wire secondary because it has a center tap which you ground/bond. There will be 240 between phases, 120 to neutral on A and C phase and 208 to neutral on the B phase.
What voltage do you need? Is it really 240 or is it 208?
Also, you keep referring to this as an isolating transformer and while a all xformers of this type are isolating by definition, this sounds like just a voltage step down.
LarryFine
Master Electrician Electric Contractor Richmond VA
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Welcome to the forum.
Strange that it's X6 instead of X0, but yes, that's your bonding point, and you'll have a high-leg delta.
As long as your load only cares about L-L voltages, and not L-G voltages, you'll be golden.
Strange that it's X6 instead of X0, but yes, that's your bonding point, and you'll have a high-leg delta.
As long as your load only cares about L-L voltages, and not L-G voltages, you'll be golden.
I need 240 3 phase for motor loads. No neutral loads located
I’m thinking it’s not XO because not all phases have the same voltage to X6. X1 and X3 will be 120V to X6(ground) whereas X2 will be 208V to X6(ground). I’m thinking X0 is reserved for wye systems where all three phases have the same voltage to ground
texie
Senior Member
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- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
yes.
texie
Senior Member
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- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
Your xformer is fine for this if connected correctly and will be a grounded system.
MD Automation
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- Maryland
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You say the load consists of servo drives. You should take the time to read up on any instructions for those drives (or the DC power supply for them) that may involve removing a jumper if the AC Mains supplying power to the front end (charging the DC bus) is ungrounded / corner grounded / or a High Leg Delta.
There is often a set of MOV surge suppressors present in the servo power supply front end, and they are Wye connected and grounded at the center. They are expecting to be supplied by an AC Mains that is also Wye - with symmetrical voltages to ground. If the AC Mains is not Wye, various manufacturers will want you to remove a jumper, disconnecting the MOV suppressors. Sometimes a manufacturer will not allow anything feeding their equipment other than a Wye.
Can't hurt to check ahead of time.
There is often a set of MOV surge suppressors present in the servo power supply front end, and they are Wye connected and grounded at the center. They are expecting to be supplied by an AC Mains that is also Wye - with symmetrical voltages to ground. If the AC Mains is not Wye, various manufacturers will want you to remove a jumper, disconnecting the MOV suppressors. Sometimes a manufacturer will not allow anything feeding their equipment other than a Wye.
Can't hurt to check ahead of time.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
@MD Automation knows servo drives. Take the caution above to heart. The motors only care about L-L voltage, but the MOVs in the drive care about L-G voltage.
Jon
Jon
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- Wisconsin
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- PE (Retired) - Power Systems
X0 is for neutral points
X6 is typical for the center tapped coil of a delta transformer built on a common core.
kwired
Electron manager
- Location
- NE Nebraska
I believe center tap of one side of a delta is commonly labeled X4 instead of X0. The X6 did make me go what the...when I first seen it.
kwired
Electron manager
- Location
- NE Nebraska
Why X6, I always thought it was X4?
kwired
Electron manager
- Location
- NE Nebraska
Getting back to OP, your secondary is ungrounded until you ground something.
This particular unit could be used for:
Ungrounded system
Corner grounded system - ground any ONE corner
Mid point of one phase grounded system - will result in opposite corner being high leg with 208 volts to ground.
The first two you simply don't utilize the X6 point at all and leave it capped.
Either grounded system is "solidly grounded"
What is not solidly grounded systems includes ungrounded systems and impedance grounded systems but impedance grounded is used on wye systems in similar fashion as ungrounded and monitoring systems are used with delta systems.
This particular unit could be used for:
Ungrounded system
Corner grounded system - ground any ONE corner
Mid point of one phase grounded system - will result in opposite corner being high leg with 208 volts to ground.
The first two you simply don't utilize the X6 point at all and leave it capped.
Either grounded system is "solidly grounded"
What is not solidly grounded systems includes ungrounded systems and impedance grounded systems but impedance grounded is used on wye systems in similar fashion as ungrounded and monitoring systems are used with delta systems.
MD Automation
Senior Member
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- Maryland
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- Engineer
Just a quick follow-up to illustrate how one manufacturer does not allow anything other than a solidly grounded Wye to power their Servo DC Bus power supply. A few years back I did a project with a cool Schneider multi-axis PAC Controller - and the following chart is snipped from a manual for their Lexium LXM 62 Servo Power Supply. The TN, TT & IT designations are IEC naming conventions for various AC Mains types. And you can see where they indicate ONLY a grounded Wye is to be used as AC mains.
Not trying to send anyone into a panic. If you powered this device with a Hi Leg Delta, would it work for 5 minutes? Very likely. Would it work for 5 weeks? Probably. Would I do this for testing in my shop if I had no other choice? Absolutely (and have done so). But if that device fails, it's my fault and it's on me to replace it. A warranty claim might be hard to follow thru on. Would I do it with customers equipment on site? I would not.
Every Mains transient and spike is going to hit the DC Bus front end filtering and suppression devices and, if not connected they way the manufacturer expects, they may have a shorter life than you want.
You say the transformer you want is 10 weeks out? Did you look at the (more) common 480D -> 208Y transformer? You could adjust primary taps to get the secondary a bit higher. A 5% tap would get you to 218 and a 10% tap, if available, could get you to 228. Do you really need 230 AC Mains? I assume this equipment might be from overseas?
If the motor are servo driven, having a slightly lower AC Mains can be OK. It will mean a slightly lower DC bus voltage, which in turn usually means a slightly lower max rpm (especially for PM motors). But if the motors are never run at max rpm, it does not matter. Just thinking out loud , to ask if the equipment supplier really needs 230V? Of course they want it, but do they need it?
Not trying to send anyone into a panic. If you powered this device with a Hi Leg Delta, would it work for 5 minutes? Very likely. Would it work for 5 weeks? Probably. Would I do this for testing in my shop if I had no other choice? Absolutely (and have done so). But if that device fails, it's my fault and it's on me to replace it. A warranty claim might be hard to follow thru on. Would I do it with customers equipment on site? I would not.
Every Mains transient and spike is going to hit the DC Bus front end filtering and suppression devices and, if not connected they way the manufacturer expects, they may have a shorter life than you want.
You say the transformer you want is 10 weeks out? Did you look at the (more) common 480D -> 208Y transformer? You could adjust primary taps to get the secondary a bit higher. A 5% tap would get you to 218 and a 10% tap, if available, could get you to 228. Do you really need 230 AC Mains? I assume this equipment might be from overseas?
If the motor are servo driven, having a slightly lower AC Mains can be OK. It will mean a slightly lower DC bus voltage, which in turn usually means a slightly lower max rpm (especially for PM motors). But if the motors are never run at max rpm, it does not matter. Just thinking out loud , to ask if the equipment supplier really needs 230V? Of course they want it, but do they need it?
Thank you all for the informative discussions; your input has been incredibly valuable. I wanted to share an update: I reached out to the transformer manufacturer, and they provided crucial clarification regarding the X6 terminal. Contrary to what I initially thought, it's not meant for grounding the system. Instead, it's designated for small 120V loads, as long as they don't exceed 5% of the transformer's rated kVA.
Given this insight, it appears that my best course of action is to procure a properly specified delta-wye transformer. Fortunately, there are used options available, and I'm planning to invest in one. If a newer model becomes available later on, I'll replace it accordingly. I believe it's worth the extra investment to ensure we get it right.
Given this insight, it appears that my best course of action is to procure a properly specified delta-wye transformer. Fortunately, there are used options available, and I'm planning to invest in one. If a newer model becomes available later on, I'll replace it accordingly. I believe it's worth the extra investment to ensure we get it right.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Under US NEC, if you are supplying 120V loads using the X6 terminal, it must _also_ be the grounded terminal. CEC probably has the same requirement for grounding.
The delta-wye transformer is a much much better choice.
-Jonathan