What is the relationship between VFD input and output current?
Introduction
Variable frequency drive (VFD) is often seen as a black box. This black box converts fix voltage and frequency supplied from the grid into variable voltage and variable frequency delivered to the motor. But what is the relationship between VFD input and output current?
Relationship between input and output current
Some people believe that the VFD input current and VFD output current are almost the same. The input current is eventually marginally higher due to internal losses in the VFD. That sounds plausible, right? Well, as will be explained, this simplification may be a bit off up to completely wrong. The error depends on the VFD topology, system parameters and operating (load) point.
Basic principles
In order to understand the relationship between input and output current of the VFD, it is useful to mention basic principles of the frequency conversion:
- VFD transfers the active power from the grid side to the motor side. The power losses of a medium voltage VFD typically range 1-1.5% of the throughput power at nominal load. Thus, the input active power is marginally higher than the output active power delivered to the motor.
- Amount of reactive power on the grid side and the motor side is generally different and there is no direct link between them. Reactive power is not transferred from the grid side to the motor side or vice versa!
- The relationship between input and output current heavily depends on the corresponding voltage level. Grid nominal voltage and motor nominal voltage can generally be different.
- Motor voltage below the field weakening point is proportional to the motor speed. For the majority of motors the field weakening point is identical to the rated point. Thus, at 30% of the rated speed the motor terminal voltage is 30% of the rated motor voltage. So even if the grid nominal voltage and motor nominal voltage happen to be the same, for any motor speed below the nominal speed the input and output voltages are not equal.
- The VFD input current significantly depends on the front end. DFE drive normally has completely different input current than an AFE drive. Not even mentioning VFDs with an integrated transformer.
Why the input current may be completely different than the output current?
Summarizing above points 1-5 we have following factors affecting the input and output current:
- Input power factor ≠ Output power factor
- Input nominal voltage ≠ Output nominal voltage
- Number input phases ≠ Number output phases
- Motor actual voltage ≠ Motor nominal voltage
Practical examples
In this section we look at several specific examples of medium voltage VFDs and their input / output current.
Case A: 3-level inverter with 12-pulse diode rectifier (DFE) and external input transformer
Case B: Cascaded H-bridge (multi-level inverter) with integrated input transformer
Case C: 3-level inverter with 24-pulse rectifier and integrated input transformer
Case D: 5-level inverter with 36-pulse diode rectifier and external input transformer
Case E: 3-level inverter with an active front end (AFE) rectifier and external input transformer
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When is the VFD input current similar as the inverter output current?
There are particular cases where the input and output current is similar.
- AFE drive with synchronous motor at or above the nominal speed
- DFE drive with integrated transformer with grid voltage equal the motor nominal voltage
An asynchronous motor can reach as high power factor as 0.93 or 0.94. Such motor drive, when utilizing AFE type of VFD, can have input current similar to the output current. It is assumed that the VFD active front end is practically the same hardware as the VFD inverter thus having the same nominal voltages. However, keep in mind that above is valid only for the nominal output voltage. At low motor speed the output voltage is proportionally reduced as well while input voltage remains constant.
More detailed examples
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References
[1] Inverter current higher than input current – perpetuum mobile?, MB Drive Services, December 2020, available online, https://mb-drive-services.com/inverter-current-higher-than-input-current-perpetuum-mobile/
[2] ABB Medium Voltage AC Drives, https://new.abb.com/drives/medium-voltage-ac-drives
