Active Control of Rotor Vibrations in Electrical Machines
The imbalance of the rotor mass and the magnetic fields of the motor excite forces that vary depending on the motor drive speed. These forces are problematic for a motor control design. The forces generate mechanical resonances that vary depending on the motor drive speed. At certain point these resonances occur at same frequency as the natural resonance frequency of the motor. These natural frequencies are called critical speeds. If a motor is run at its critical speed the amplitude of the vibration force increases at every cycle leading to several problems. The vibration causes wearing in the rotor bearings, thus decreasing the lifetime of the motor and increasing the maintenance efforts. The vibration may also cause break ups in the elements surrounding the motor; the heavier the motor the bigger the forces excited to the motor surroundings. Also the noise increases and in some applications the problem is with the torque, which varies according to the rotor air-gap that also changes as the rotor vibrates.
Of course the motor can be run in speed regions, where no vibration force is excited. These regions are usually very limited due to the harmonics of the natural frequencies. One way to overcome these kinds of problems is to increase the size of the rotor thus causing the critical speed to increase. This isn’t a very cost efficient method as most of the costs of a motor come from the material costs. Therefore for larger motors the costs become higher. The control applications for motor-driven systems must also take these forbidden speed regions into consideration in the design phase, making the control design much more complicated. The application area for a single type of motor is also limited due to the motor speed demands.
The research aims to find a way to compensate the vibration force with opposite force, thus leading to a non-vibrating system that can be run at any speed range (within the motor’s mechanical limits) without vibration forces building up. The compensation force of the actuator is created with supplementary windings in the stator slots. The actuator is voltage controlled, and an increase of the current in the windings excites the control force. The control problem is quite complicated as the rotor is rotating at variable speed and the actuator is in the stator cage, which is stationary. So the actuator force has to be controlled so that it has the right amplitude and right phase compared to the vibration force excited on the rotor.
Several different control methods are to be implemented, some of these are:
- Model Predictive control
- Convergent control
- Repetitive Control
Our research partners are:
- Helsinki University of Technology:
- VTT Industrial Systems
- University of Sheffield,
Our research is funded by The Academy of Finland.
- Keywords: Electrical machines; electromechanical interactions; rotor dynamics; critical speeds; magnetic forces; meachanical vibrations; active control; repetitive control
- Duration: 2006-2009
- Research area: Power Electronics & Electric Machines