vfd?
- Thread starter jimbo123
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- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Your truncated SMS style text has me very confused. Are you posting from a cell phone?
It looks like you are saying you have a 600V shielded isolation transformer dropping you down to 480V for a VFD that is feeding a 460V motor. Then you also have a lreactor in front of the VFD and another on the output.
But I don't quite understand your question here, are you asking why you need the transformer? If that's it, the reason would be that if you have a 600V line and a 460V motor. Even if you limit the output of the VFD to 460V for the motor, the VFD would need to be a 600V rated VFD and that means fewer suppliers to choose from, which then means a higher price for the VFD. If it's a few years old, it may have been even worse, at one point there were very few suppliers who made real 600V rated VFDs (as opposed to 480V versions that were "tested" at 600V and didn't smoke yet).
Yes i was posting from a cell phone and did a poor job explaining by rushing and having fat fingers.
Here it goes. We have a 480volt vfd controlling a 460v motor. Circuit is fed from a shielded transformer with a primary rating of 600v and the secondary rated at 347volts. This trans has 480volts feed and the output is to the vfd that is rated 460v. Am i missing something ? all i did was look at the name plate of the trans and the vfd . The vfd has input reactor and a output reactor and i was thinking why go throught the trans with the 480 and not straight to the vfd.
Is this a common rating for a trans ?
Sorry for any confusing on my part.
Here it goes. We have a 480volt vfd controlling a 460v motor. Circuit is fed from a shielded transformer with a primary rating of 600v and the secondary rated at 347volts. This trans has 480volts feed and the output is to the vfd that is rated 460v. Am i missing something ? all i did was look at the name plate of the trans and the vfd . The vfd has input reactor and a output reactor and i was thinking why go throught the trans with the 480 and not straight to the vfd.
Is this a common rating for a trans ?
Sorry for any confusing on my part.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
No problem on the text, I hate those tiny keyboards too. Fat thumbs :thumbsdown: (Brachydactyly Type D)
OK I think I got it. So you have a 480V feed and a 480V drive to a 460V motor, but the drive power is being fed through a shielded transformer and the nameplate probably says something like "Primary: 600V Delta (or a triangle symbol), Secondary: 600Y347V" right?
Here's what I think is going on. First off, if all that transformer is expected to do is provide a Delta-Wye connection and a shield, that means it is just a 1:1 transformer. So it may be RATED for 600/600Y347, but it really isn't transfoming the 3 phase voltage to 347, that would be the phase-neutral (single phase) voltage. Line to line would not be different, it's just a 1:1 transformer that creates a Wye on the secondary. If you used it on 480 you would get 480/480Y277, so still 1:1 as far as ratios go. There is no problem with using a transformer at any voltage LOWER than it is rated for, just not higher. Hopefully that's the case and this will clear that up for you. Delta-Wye 1:1 transformers are actually somewhat common for VFD installations for a number of reasons. The Shielded part is probably not germaine though, see below.
So is the transformer necessary? Maybe. It could be related to what your line supply source is. For example you have a grounded delta source from your utility. Many VFDs don't like that for a number of reasons that I won't go into here, but read this other thread for a common one. So if that's the case, the transformer may have been added just to create that grounded Wye service and avoid the issues of the VFD not being sublect to a gounded Delta service. There are other potential reasons as well, you really have no way of knowing. So don't assume it's unnecessary unless you can talk to the person that specified it, or if the VFD supplier will certify that it isn't needed (i.e. it may be a relic of a previous installation).
Back to the shielding: a while ago it was very common to specify shielded isolation transformers ahead of both AC and DC drives, because the power conversion sections of them were very noisy (electrically). The shielding was to protect OTHER equipment on the line from what the drives were doing. Long ago we went away from those kinds of front-ends on drives, but the concept lingers on. Like I said it doesn't hurt anything and it may be that the transformer they needed just came that way, but the shielding itself is probably not that important any more.
OK I think I got it. So you have a 480V feed and a 480V drive to a 460V motor, but the drive power is being fed through a shielded transformer and the nameplate probably says something like "Primary: 600V Delta (or a triangle symbol), Secondary: 600Y347V" right?
Here's what I think is going on. First off, if all that transformer is expected to do is provide a Delta-Wye connection and a shield, that means it is just a 1:1 transformer. So it may be RATED for 600/600Y347, but it really isn't transfoming the 3 phase voltage to 347, that would be the phase-neutral (single phase) voltage. Line to line would not be different, it's just a 1:1 transformer that creates a Wye on the secondary. If you used it on 480 you would get 480/480Y277, so still 1:1 as far as ratios go. There is no problem with using a transformer at any voltage LOWER than it is rated for, just not higher. Hopefully that's the case and this will clear that up for you. Delta-Wye 1:1 transformers are actually somewhat common for VFD installations for a number of reasons. The Shielded part is probably not germaine though, see below.
So is the transformer necessary? Maybe. It could be related to what your line supply source is. For example you have a grounded delta source from your utility. Many VFDs don't like that for a number of reasons that I won't go into here, but read this other thread for a common one. So if that's the case, the transformer may have been added just to create that grounded Wye service and avoid the issues of the VFD not being sublect to a gounded Delta service. There are other potential reasons as well, you really have no way of knowing. So don't assume it's unnecessary unless you can talk to the person that specified it, or if the VFD supplier will certify that it isn't needed (i.e. it may be a relic of a previous installation).
Back to the shielding: a while ago it was very common to specify shielded isolation transformers ahead of both AC and DC drives, because the power conversion sections of them were very noisy (electrically). The shielding was to protect OTHER equipment on the line from what the drives were doing. Long ago we went away from those kinds of front-ends on drives, but the concept lingers on. Like I said it doesn't hurt anything and it may be that the transformer they needed just came that way, but the shielding itself is probably not that important any more.
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- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Probably just what they had readily available at the time and since it would work fine, they decided to go with it rather than wait. Happens a lot.
robbietan
Senior Member
- Location
- Antipolo City
or it could be that the supply is an ungrounded source and that transformer is there to provide a grounded service for the vfd's computer controls
weressl
Esteemed Member
Drives in current production from all major manufacturers have addressed this issue and is no longer a problem. (BTW ground reference is NOT for 'vfd's computer controls'.)
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