Special couplings solve VFD vibration problem
9 Mar 2007
The Scherbius control system was installed on two induced-draught fans at one of the company's cement works on the Japanese island of Honshu. It was intended to cut energy costs and to increase efficiencies by allowing the fans to be operated at optimum speed.
After installation, however, Mitsubishi engineers encountered high levels of vibratory torque at speeds close to the desired 1,000rpm operating speed. Until a solution to the problem could be found, and to avoid any possibility of damage to the motor, engineers were forced to introduced a barred speed range between 890 and 940rpm.
The problem lay in the characteristics of the Scherbius control system, which is essentially a type of variable frequency drive (VFD). The system uses electronic switching techniques to digitally create different frequencies from the supply frequency to control the motor speed.
Each switching event will cause a small torque pulse in the motor, which in effect, is a twist of the drive shaft. If the frequency of the switching coincides with the natural frequency of the drive system it will result in resonance, which can cause catastrophic fatigue failure.
To solve the problem, Renold Hi-Tec's engineers first conducted a transient analysis of the complete drive train to simulate the system's natural frequencies and levels of vibratory torque that were measured on-site. The results were as expected and showed that as soon as the desired operating speed was approached, high reversing torques would impose keyway stresses three times higher than with the original control system.
The calculations were then repeated with one of Renold Hi-Tec's PM60 rubber-in-compression couplings instead of the original gear-type coupling. By careful selection of the rubber hardness and damping characteristics of the rubber elements, Renold engineers managed to shift the resonance points away from running speed and reduce vibratory amplitudes to an acceptable level.
After installing the new couplings, on-site measurements were again taken and the results showed that keyway stresses at resonance had reduced by a factor of four, higher than was predicted by the calculation. The barred speed range was subsequently lifted and the fans allowed to operate at optimum speed to recover the maximum power savings from the control system.