Identifying Torsional Resonance Problems Associated with VFD Driven Fans

C3 - Motors

The use of Variable Frequency Drives (VFDs) has become very common for controlling fans even for large horsepower motors. Controlling the inrush of electrical current during the start of an AC induction motor, the ability to accurately control the fan speed and the potential for significant energy savings are common elements justifying the application of VFDs for fan control. The initial cost of a VFD is often recovered quickly by productivity improvements and reduced electrical costs.
Due to the ease with which VFDs can be applied, they are often installed without due consideration for their potential negative impact. Three basic questions generally need to be answered; i) Can the predicted energy savings actually be obtained? ii) Can the aerodynamic operating points be obtained in a manner that does not compromise system aerodynamic stability? iii) What is the potential that the fan can suffer mechanical problems if controlled on variable speed? This technical paper addresses the latter question and specifically mechanical resonance of torsional natural frequencies caused by motor ‘air gap torque harmonics’ that may lead to shaft and coupling failures.
The potential effect the electronics and electrical switching that takes place in a variable frequency drives (VFD) on the voltage and current harmonic feedback into the electrical supply is generally understood and there are well documented standards for control and regulation of those harmonics such as IEEE519-1992. However, VFD electrical harmonics in the output electrical feed to the driven equipment is not so regulated. Mechanical torsional fluctuations caused by ‘air gap torque harmonics’ associated with the VFD fundamental and carrier frequencies have resulted in catastrophic failures of fan shafts and couplings of variable speed fans using variable frequency drives.
This paper will provide illustrated examples of field measured torsional pulsations that led to shaft and coupling failures and identify the correlation with potential excitation due to "air gap torque harmonics".