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Variable frequency drives are very helpful to deal with induction motor working properly with full efficiency as well as long life span. When there are issues with your vfd's it is necessary to make sure that you get repair services. Without proper repair services, the regarding vfd will not be able to deal with functioning of the motor in which it is placed.
Solar panel maintenance costs can vary greatly depending on each solar provider because system installations are often tailored to the unique energy needs of each homeowner.
The inverter used in off grid system directly supply the loads, so there is no such critical situation to control the frequency and the voltage levels. but in case of on grid inverter in order to match with the grid parameters( voltage and frequency) the inverter needs to have controller circuit in it.