Question:
Which costs (for the plant operator) result from the reactive power consumption of a drive system in operation on the inverter in comparison to the direct mains operation of the motor?
Answer:
- Inverters and all drive controllers and supply modules with uncontrolled rectifier bridge at the mains input and with dc bus consume beside the active power only reactive power of the upper harmonic wave and hardly any reactive power of the fundamental wave from the mains.
- With motor directly connected to the mains (DOL = Direct On Line) there is only the reactive power of the fundamental wave but no reactive power of the upper harmonic wave.
- According to EN 62053-23:2003 / IEC 62053 standard the reactive energy counter record only the reactive power of the fundamental wave, i. e. no harmonic waves. Therefore the inverters (servo and frequency inverter) do not contribute to the rise of the fundamental reactive power, which are measured in industrial plants over separate energy consumption meters and about separate rates are settled of the energy provider.
- A centralised compensation of the reactive power of the fundamental wave (as usual in industrial energy distribution systems) is not required for frequency inverters. Nevertheless, it is applied with DOL.
- The costs to be paid for the reactive power of the fundamental wave result from the energy provider rate especially existing for such cases. Usually, the customer pays only the reactive power of the fundamental wave (kVAr) exceeding a certain value (depends on the provider contract). Each case must be evaluated individually when comparing the costs of the reactive power consumption between DOL and the inverter operation. It is necessary to clarify the following factors:
- Is the reactive power compensated in a centralised way with DOL (yes or no)?
- How does the contract with the provider look like in terms of the reactive power (costs for kVar, which quantity is invoiced)?
- How high is the proportion of investment costs for a DOL drives compensation system?
Source: Local energy provider
Search terms: Calculation, Cos phi, Cosphi, Distortion reactive power, Measurement, Power factor, Public network supplier
Further information of the disturbance to the power supply see in the specialist book Variable speed drives in the automation, chapter 6.
Source of information: Description_SpecialistBook_VariableSpeedDrives.pdf
