Wednesday, May 14, 2014

Variable Frequency Drive (VFD) disadvantages

Although there are various advantages of VFD's but still there are some disadvantages which must be considered while selecting a VFD.

1. Very High Initial Cost 
Cost is the main factor for using VFD’S as compared to other speed controls. Cost of VFD’S is usually high which obstacles in installation for the same in industry although it saves energy but initial cost is very high. It has been found that payback period for the cost of VFD is around about 1- 2.5 years depending upon application where it is used and amount of energy saving during process.

2. Maintenance and Troubleshooting requires Special Skills and costly 
Starters like star- Delta Starters and DOL starters maintenance is very easy and very much cost effective . A Thorough inspection of the above starters is enough to see whether and why devices fail. VFDs, like any solid state device, require special troubleshooting practical and theory knowledge. Special trained personal is required for the maintenance and troubleshooting. Also if any part get failed then repair cost for the same is very high. Also it’s easy to repair the VFD’s for everyone as special skills required for the same.
  1. High Harmonics:-
It has been observed in VFD’S that O/P waveform generated by VFD’s is non linear which will create harmonics the motors. These harmonics will leads to create heat in motor. It has been found that VFDs create between 5 to 8% more heating in a motor when compared to that same motor running on a sinusoidal waveform from the power line. This is the reason inverter rated motors are used for the VFD applications. Inverter rated motors are nothing but motors with insulation class F.

4. Lower Speed might leads t Overheating 

Induction motors which require constant torque there is a possibility that the motor will overheat during low speed operation. As constant torque loads will draw the same current regardless of the motor operating speed. VFD’S will reduce motor frequency in order to decrease motor speed which will leads to lower speed of motor but same torque requirement will leads to overheating of motor. If the motor produces a high level of heat while operating at low speed, overheating results.
It has been found that a fully loaded motor with Class B insulation running at 50% rated speed on a constant torque load will not overheat. If the motor is run below 50% speed continuously, it will overheat.
If motors are used having Class F insulation then the speed of a fully loaded motor may be decreased to approximately 20% speed without overheating. The lower the continuous operating speed below the motor overheating point, the more the motor and VFD must be derated