Frequency inverter disadvantages

Due to industrial standards and regulations there are not many disadvantages to frequency inverters. Most high quality frequency inverters comply with all these standards and regulations. Manufacturers improve their products with ongoing research and development programs. To eliminate any disadvantages, frequency inverter manufacturers will also advise users of how to install and operate their units.

Audible Noise
Various stages of the switching frequency produce audible noise from the motor. Although this is not harmful to the motor, in most instances, the sound is not acceptable. The acoustic noise is unpleasant and irritating in quiet offices, hospitals and other similar environments. To overcome this problem, most frequency inverters switching frequency could be increased to a higher value, which will eliminate the noise problem, but this will introduce Harmonics. Therefore proper design of an installation should be done before using inverters.

RFI
Radio Frequency Interferences generated by frequency inverters can be very problematic by introducing faults on other equipment in close vicinity of the installed unit. Most frequency inverters can be expected to meet the immunity requirements of the CENELEC generic standard EN50082-2.

Harmonics
Frequency inverters, like most other electronic equipment, do not draw their current as a smooth sinusoid. The supply current waveform is generally referred to in terms of the harmonics of the supply frequency, which it contains. The harmonic current causes harmonic voltage to be experienced by other equipment connected to the same supply. Because harmonic voltage can cause disturbance or stress to other electrical equipment connected to the same supply system, there are regulations in place to control it. If installations contain a high proportion of frequency inverters and/or other power electronic equipment such as UPS, then they may have to be shown to satisfy the supply authorities' harmonic guidelines before permission to connect is granted. As well as obeying regulations, users of frequency inverters need to ensure that the harmonic levels within their own plant are not excessive.

Some of the practical problems, which may arise from excessive harmonic levels, are:

• Poor power factor, i.e. high current for a given power
• Interference to equipment, which is sensitive to voltage waveform
• Excessive heating of neutral conductors (single-phase loads only)
• Excessive heating of induction motors
• High acoustic noise from transformers, bus bars, switchgear etc.
• Abnormal heating of transformers and associated equipment
• Damage to power factor correction capacitors

An important property of harmonics is that they tend to be cumulative on a power system, i.e. the contributions of the various harmonic sources add up to some degree. This is different from other high-frequency electromagnetic compatibility (EMC) effects, which are generally localized and not significantly cumulative. It is important to differentiate harmonics from high-frequency EMC effects, which tend to cause interference to sensitive data and measuring circuits by stray coupling paths. With few exceptions, if harmonics cause disturbance it is through direct electrical connection and not through stray paths. Screening is rarely a remedial measure for harmonic problems.

Knowledge
The biggest disadvantage is to understand frequency inverters and their ability to improve our lives and of course the unwillingness to change, because, "My motors worked for the last 20 years with Direct On Line starters, why should I complicate my life and let something, that I do not understand, and is very expensive, control my process?  Even worse, now I need to employ a highly qualified Technical Engineer to maintain my plant". These remarks, and by not reading the instructions manual, create a bad image of a product that improves life for all of us.