• Anubhav Gupta

Myths, selection and usage of Variable Frequency Drives

While investing in a Variable Frequency Drive a process owner needs to decide which VFD he/she should go in for, what are his needs from the VFD apart from changing the speed of motor.

As a precaution almost all the process owners consider their process as heavy duty as it is certainly important for their production and that’s where one of the biggest myth comes into picture…

The Heavy Duty application

While selecting a VFD it is notable to mention if the process will overshoot the current drawn beyond the rated capacity of motor or VFD. An application falls in range of heavy duty if the overload requirements are upto 50% of rated current for 60 seconds.

Now when we have a VFD with this capacity customers confuse the HD rating as an offence. It is worthwhile to note that while considering Heavy Duty application a VFD derates itself to a step below but that does not mean it is a wrong selection. It is important to understand which models have Normal Duty + Heavy Duty provision and which models have Light Duty and Normal Duty provisions. For special cases there are now VFDs with Heavy Duty and Super Heavy duty operation provisions.

The next myth is all VFDs are same…

While almost all VFDs contain similar components i.e. Bridge Rectifier, Soft Charging Unit, DC Bus capacitor bank and Output Inverter section they all differ in how the inverter switches, the reliability of components, and efficiency of thermal dissipation.

Sizing a VFD

A VFD should never be decided on the basis of the KW or HP rating of motor, the actual factor to be considered are Full Load Amps of Motor, requirement of converting single phase input to three phase output, Service Amps, ambient temperatures, operational altitudes, and not to be forgotten the modes of communication required.

Significance of VFDs

These days Variable Frequency Drives have found their usage for reasons such as:

1. Energy Savings

2. Increased Reliability

3. Speed Variations

4. Soft Starting

5. Extended Machine life and lower maintenance

6. High Power Factor

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