Transformers and Drives - sizing and harmonics

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SUMMARY

When sizing transformers for three-phase drives or variable frequency drives (VFDs) used with induction motors, it is essential to consider that the transformer's power rating may need to be approximately 40% higher than that of the drive to mitigate harmonics and other potential issues. This recommendation stems from the variability in the quality of VFDs and the filtering methods employed. While there is no universally accepted standard for this sizing, the 40% increase serves as a precautionary measure based on historical practices rather than empirical engineering analysis.

PREREQUISITES
  • Understanding of three-phase power systems
  • Familiarity with variable frequency drives (VFDs)
  • Knowledge of transformer sizing principles
  • Awareness of harmonic distortion and its effects on electrical systems
NEXT STEPS
  • Research the IEEE 519 standard for harmonic control in electrical systems
  • Explore transformer sizing guidelines specific to VFD applications
  • Investigate filtering techniques for reducing harmonics in VFD systems
  • Learn about the impact of transformer quality on system performance
USEFUL FOR

Electrical engineers, power system designers, and professionals involved in the specification and installation of transformers and VFDs for industrial applications.

elcraft
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Hello,

Is it true that when using transformers and three-phase drives/VFDs for induction motors, the transformer's power needs to be much higher (e.g. +40%) than the drive's in order to avoid increased harmonics (or even other problems)?

If you know any official standard, paper or recommendation regarding the matter of sizing transformers vs drives, please share.

Thank you.
 
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It is a very generalized question - there is a pretty big range in quality of VFD and the types / amount of filtering that can be applied. -- So the "needs" question could be "may need" - However, every installation or application is different.
Regardless 40% seems like a lot, but depending on who you got your info from - they may have learned that spec'ing a 40% larger transformer is insurance against problems, and has worked in the past without looking at the causes or doing real engineering.
 
So there is no paradigm with reliable recommendations or relevant discoveries?
 

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