How do you size a transformer for a VFD load and avoid overheating or harmonics issues?

1 Why transformer sizing matters

Transformers are electrical devices that step up or step down the voltage level of an alternating current (AC) power supply, making them essential for matching the voltage requirements of different loads and sources in a power system. However, not all transformers are suitable for VFD loads, which have different characteristics than linear loads. If you use a transformer that is too small, under-rated, or not designed for VFD applications, you may experience issues such as overheating and insulation damage due to increased losses and thermal stress caused by harmonic currents and voltage distortion, reduced efficiency and performance due to reduced transformer impedance and increased voltage drop caused by power factor and reactive power demand, and nuisance tripping and false alarms due to increased inrush current and magnetizing current caused by switching frequency and soft start feature.

2 How to size a transformer for a VFD load

When sizing a transformer for a VFD load, there are several factors to consider, such as the rated power and voltage of the VFD and motor, the harmonic distortion level of the VFD, the impedance and reactance of the transformer, and the cooling and protection methods of the transformer. The transformer should be able to supply enough power and voltage to the VFD without exceeding its nameplate ratings or derating factors. Additionally, it should be able to handle the harmonic currents and voltages generated by the VFD without exceeding its thermal rating or causing excessive losses or distortion. Furthermore, it should have a high enough impedance and reactance to limit the short-circuit current and voltage drop caused by the VFD. Lastly, it should have adequate cooling and protection devices to prevent overheating and damage due to the VFD's effects. The IEEE 519 standard, ANSI C57.110 standard, and IEEE C57.12.01 standard can be used for guidance in determining acceptable values for these factors, or you can use manufacturer’s recommendations.

3 How to avoid overheating issues

One of the main risks of using a transformer for a VFD load is overheating, which can reduce its lifespan and reliability. To avoid this, you should use a transformer that is designed for VFD applications or has a higher K-factor rating. This rating is a measure of the transformer's ability to withstand harmonic currents without overheating. Additionally, look for a higher temperature rise rating or lower ambient temperature rating, as this indicates the maximum temperature increase and operating temperature the transformer can endure. Lastly, make sure you use adequate ventilation or forced cooling to dissipate heat and prevent overheating. This can be done with natural ventilation, fans, pumps, or heat exchangers.

5 Here’s what else to consider

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