Juha Orivuori

Attenuation of Harmonic Rotor Vibration in a Cage Rotor Induction Machine by a Self-Bearing Force Actuator

In this paper, attenuation of flexural rotor vibration in electrical machines is considered. In order to generate force on the machine rotor, an electromagnetic actuator based on self-bearing machine working principle is examined. A control method for attenuating harmonic rotor vibration components is applied in a 30 kW two-pole cage induction machine. The machine is equipped with a four-pole supplementary winding for generation of lateral force on the rotor. Experimental results for the two-pole induction motor are presented. The main contribution of this paper is to apply a control method, specially designed for compensating harmonic excitations, by using a built-in electromagnetic actuator in an induction machine.

Parallel Disturbance Force Compensator for Electrical Machines

Abstract The objective of the research is to diminish unwanted forces generated by rotation and unbalanced rotor mass on the rotor of an electrical machine. These forces, dependent on rotational speed, cause vibration that, when occurring in the machines natural frequency, causes severe problems. Extra windings are built in the stator of the machine, and they are supplied with current to create an opposite force to the vibration. The main task is to develop a new controller to the system, in order to continuously provide the needed voltage input to the new actuator. The system was first modeled for finite element model (FEM) software, and based on FEM simulations a simplified state-space model was identified. Separate models for the rotor mechanics and for the actuator were created for convenience. Input to the actuator model was voltage given by the controller, and the output was the compensating force to the rotor. The rotor model mapped total input force of rotor to displacement, vibration. There was an internal feedback from rotor displacement to actuator, which was taken into account in the actuator model. Because the source of vibration is well known, the problem was attacked at the very source. A compensator was designed for balancing the forces in the rotor. The forces were not measured and remained thus unknown, but they could be estimated. The adaptive compensator was designed so that other controllers can be used parallel, without having to make any changes to the compensator.

Robust Attenuation of Frequency Varying Disturbances

Systems described by differential equations with time-periodic coefficients have a long history in mathematical physics. Applications cover a wide area of systems ranging from helicopter blades, rotor-bearing systems, mechanics of structures, stability of structures influenced by periodic loads, applications in robotics and micro-electromechanical systems etc. (Rao, 2000; Sinha, 2005). Processes characterized by linear time-invariant or time-varying dynamics corrupted by sinusoidal output disturbance belong to this class of systems. Robust and adaptive analysis and synthesis techniques can be used to design suitable controllers, which fulfill the desired disturbance attenuation and other performance characteristics of the closed-loop system. Despite of the fact that LTP (Linear Time Periodic) system theory has been under research for years (Deskmuhk & Sinha, 2004; Montagnier et al., 2004) the analysis on LTPs with experimental data has been seriously considered only recently (Allen, 2007). The importance of new innovative ideas and products is of utmost importance in modern industrial society. In order to design more accurate and more economical products the importance of model-based control, involving increasingly accurate identification schemes and more effective control methods, have become fully recognized in industrial applications. An example of the processes related to the topic is vibration control in electrical machines, in which several research groups are currently working. Active vibration control has many applications in various industrial areas, and the need to generate effective but relatively cheap solutions is enormous. The example of electrical machines considered concerns the dampening of rotor vibrations in the so-called critical speed declared by the first flexural rotor bending resonance. In addition, the electromagnetic fields in the air-gap between rotor and stator may couple with the mechanic vibration modes, leading to rotordynamic instability. The vibration caused by this resonance is so considerable that large motors often have to be driven below the critical speed. Smaller motors can be driven also in super-critical speeds, but they have to be accelerated fast over the critical speed. Active vibration control would make it possible to use the motor in its whole operation range freely, according to specified needs given by the load process. Introducing characteristics of this kind for the electric drives of the future would be a major technological break-through, a good example of an innovative technological development. 13