If the present total load and its harmonic profile is known, the transformer capacity may be calculated with some additional capacity for future expansion. The K of such a K-rated transformer may be taken at a higher value to account for any possible future harmonic load without increasing its KVA. But for an over sized transformer, even any additional normal load would likely make it seriously overloaded as it is feeding its existing harmonic load because its de-rating to account for the existing harmonic load may be lost due to increased load on it.
VFD on step-up transformer problem
- Thread starter tom kanzler
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mbrooke
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Are K rated units just transformers with a de-rated kva?
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No. K-rated transfomers are designed to take into account the effects of harmonic currents such as higher temperature class for winding insulation etc., whereas conventional transformers are not.
How?
petersonra
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I don't see how his response even gave an answer, so I don't see how it could be "wrong".
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ATSman
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Good one, Digger!It is a general principle: Sahib's answers are wrong until proven otherwise.
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I'll say it again.
Having a transformer with a higher K-factor does NOT increase the capacity of the transformer. It also does NOT mitigate harmonics. All it does is SURVIVE better in a high harmonics environment. Higher K-factor rated transformers have their place and should be used where they are required. THIS WAS NOT ONE OF THOSE APPLICATIONS.
This is a step UP transformer to feed ONE machine that apparently has a VFD on it, no future growth, no neutral loading issues, NONE OF THAT. He got a VOLTAGE DROP on the secondary (in this case the 480V side) that caused his control system to fault, and increasing his transformer size cured the problem, indicating that the nature of his load profile was such that it was overloading that 300kVA transformer and causing the voltage drop. A 300kVA transformer is rated for 360A max, his loading per the machine data was 354A peak, it was just too close to the bone, a slight voltage drop on the 208V side (which with a 354A load is a LIKELY outcome on a 208V service) and you don't even have 360A capacity on that transformer. 400kVA was the way to go, and what do you know? It solved the problem!!!
Bringing those other issues into the fray has caused nothing but distraction and confusion.
Having a transformer with a higher K-factor does NOT increase the capacity of the transformer. It also does NOT mitigate harmonics. All it does is SURVIVE better in a high harmonics environment. Higher K-factor rated transformers have their place and should be used where they are required. THIS WAS NOT ONE OF THOSE APPLICATIONS.
This is a step UP transformer to feed ONE machine that apparently has a VFD on it, no future growth, no neutral loading issues, NONE OF THAT. He got a VOLTAGE DROP on the secondary (in this case the 480V side) that caused his control system to fault, and increasing his transformer size cured the problem, indicating that the nature of his load profile was such that it was overloading that 300kVA transformer and causing the voltage drop. A 300kVA transformer is rated for 360A max, his loading per the machine data was 354A peak, it was just too close to the bone, a slight voltage drop on the 208V side (which with a 354A load is a LIKELY outcome on a 208V service) and you don't even have 360A capacity on that transformer. 400kVA was the way to go, and what do you know? It solved the problem!!!
Bringing those other issues into the fray has caused nothing but distraction and confusion.
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Bob, read what he has posted in this thread.
He seems to have zero understanding of the differences between a kva and K rating.
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As is generally the case.
mbrooke
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Well, wait,jerks,
Sahib, don't let them get to you. Its not easy learning English as a second language latter in life so I understand
mbrooke
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You are assuming the VFD would have faulted. It could be programmed to respond to the voltage drop by reducing speed and thus load. The drive might ride through it, but not the rest of the system. Happens more than you might think.
But if input voltage variation is not within tolerable limit, the VFD will shut down to prevent any fault.
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- Electrical Engineer
No, you assume that. Most good have the ability to continue operations under adverse conditions.
At the cost of damage to motor and/or associated control equipment? Sorry, it does not make sense.
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Again, you are ASSuming.
A VFD is like an entirely new power source for the motor. All it needs from the line side is raw energy to convert to DC, everything else from that point on going to the motor is fabricated by the VFD, and the VFD can fabricate it to be whatever is needed. So for example if I have a 600V supply but I have a 480V motor, I can program the VFD to put out 480V at 60Hz and the drive will do it. I can't make a 480V supply run a 600V motor at 60Hz and full power, but I can make it run the motor at 48Hz at full power, or run at 60Hz at 80% power. It's all just a matter of programming your output. So let's say I have a 480V drive and the line voltage drops to 375V under full load. I can tell the drive to run at up to 375/480= 78%, so about 47Hz, and still get full torque at that speed. On a crane, it might only be told to run at 30Hz, maybe less, so the drive has no trouble at all keeping that motor running. But I picked 375V because that is below the threshold of OTHER control components working correctly, such as anything with a coil. So the SYSTEM might trip, but the VFD itself might not.
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