VFD Drives have been supplied on almost every application that uses an AC motor. When the VFDs were originally designed in the 1980s, energy reduction was the main reasons for using the VFD on centrifugal fans and pumps. Mechanical load flow control was found to use significantly more energy than a VFD controlled fan or pump.
Figure 1. VFDs replace Inlet & Outlet Dampers and Throttling Valves
As the variable frequency drive VFD reliability increased, production related applications of VFDs started to occur to provide for a better motor speed control of the operation of the motor. Over several years of installations, the users of VFDs found several other benefits that they were not aware of. Some of these benefits ended up being quite substantial reducing system downtime, reduced production waste and extend life between maintenance shutdowns. These new benefits then became the main reason for installing variable torque drives in production equipment. Some of the other benefits are:
There is really no application of an AC Motor that is not suitable for VFD operation. The VFD is the most expensive method for starting the motor. If any of the many benefits that the VFD provides for a given application justifies the upfront cost, then the VFD application is acceptable.
A user had a gearbox on a fixed speed application that has been wearing out over several years of operation. The teeth on the gears were becoming brittle and started to chip during each start. The gear box required an overhaul; however the new set of gears had a six month delivery time frame. The user believed that the gearbox would fail before the new gears would arrive and cause a shutdown of their production for an extended time frame. The user purchased a VFD to provide for a mechanical soft start to reduce the across the line starting stress of the gears and reduce further damage until the replacement gear components arrived. After the gears were replaced, the user then decided to leave the VFD in place to potentially delay or prevent a future gear box failure.
A fixed speed centrifugal centrifuge had been operating successfully for several years until one day the indusction motor failed. The motor was replaced with a motor sized on the original motors nameplate speed and HP. The purchaser provided a standard motor from a local motor distributor and the motor overheated upon the first start. The new motor did not produce enough constant torque to accelerate the high inertia load before it overheated and tripped out. The user was not aware that the original 75HP motor was oversized by the OEM to 250% to handle the high inertia starting time of the centrifuge. The motor really was a 200HP motor frame rated for 75HP continuous duty. The user had a VFD for a different application and temporary installed it until the proper replacement motor arrived. Upon the first start the user found that there was no significant heating in the motor and the motor full load current draw was 15 amperes lower that the original motor. The user then decided that the VFD should remain on this application because of the soft starting of the VFD and the energy saving of not using a motor 250% oversized for the application. Another benefit showed up later when the centrifuge became unbalanced and the vibration monitor caused a shutdown. The fast stopping parameter of the VFD was able to stop the high inertia of the centrifuge in about 2 minutes whereas it would take 20 minutes to stop during the coast to stop on fixed. The faster stopping significantly reduced the time of the high unbalance causing the centrifuge to violently shake reducing equipment damage and personnel safety risk.
There have been several customers that inherited a VFD on what they perceived to be a fixed speed application only and questioned why the consultant made the mistake of specifying the VFD for this application. I quickly told the customer that I did not know what the actual reason the consultant chose to install the VFD, but it may be:
All motors will see a benefit when being used on a VFD. The benefit(s) are sometimes not very obvious.
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