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Variable Frequency Drive Selection Guide

Variable frequency drives are commonly used in industrial control, how to choose suitable VFDs for these motor has attracted many attentions. The VFD shall be selected according to working environment on the site, controlled objects, range of speed requirements, steady-state speed accuracy, torque requirements and on-site wiring. A balance should be made between the production technology and economy.

Selection of the VFD is based on the working current performance curve of variable frequency drive, including the load current curve. Qualitatively speaking, the VFD shall be choosed from the perspective of voltage matching, current matching, torque matching. As a result, the user shall have a thorough understanding of the electricity consumption on site, including the level of voltage, waveform and quality of voltage, so as to ensure that the VFD can work normally. Then, the user shall also have a certain understanding of the load. The performance curve of load determines the application method of VFD, the rated current of common centrifugal pump VFD is in accordance with that of the motor, but the deep-well pump requires a larger current.

According to the engineering experience, the VFD shall well fit with the motor power and a residual shall be reserved, so the VFD with a larger specification shall be selected. If the motor used on site needs to start and brake frequently, the brake resistance must be installed, its size shall be determined according to the power. If the equipment is required for aging testing, four quadrant products can be considered to effectively reduce power loss. If there is a separate DC current in the field, the pure inverters can be used to save investment. Moreover, a reactor must be added at the input end of high-power VFD, so as to improve the power quality of input equipment and its power factor.

When driving a high speed motor with VFD, the higher harmonics increases due to low reactance of the high speed motor, as a result, the output current increases. Therefore, the VFD used for high speed motor should have a larger capacity than that of ordinary motor. In case the VFD has to run with long cable, measures should be taken to restrain the influence of long cable on ground coupling capacitance, so as to avoid insufficient of VFD capacity. Therefore, the capacity of VFD shall be a slight larger or a reactor shall be installed at the output of VFD. For some special application occasions, such as high temperature, high altitude, capacity of VFD would reduce, so a larger capacity shall be selected.

In some cases where the precision is strictly required, the VFD should be selected after a series of tests for the motor. Some requirements would even put forward to the rotary encoder of the motor. In a word, it all depends on actual conditions. The motor and VFD shall be selected according to actual conditions on site.

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