Shaded-Pole Induction Motors, used mainly in household and low power applications, are renowned for their simplicity, low cost and robustness. They are also, however, notorious for their low effciency and low starting torque.
To counteract some of these shortcomings, finite element simulation can be applied to improve some of the critical parameters of such motors. This is extremely useful since, in contrast to other motor types, there is no accepted standard for the equivalent circuit of such motors which is not easy to derive.
This article briefly demonstrates the potential of CST EM STUDIO® to simulate shaded-pole and similar single-phase motors. Due to the strong non-linear magnetic fields and eddy current effects in the shaded poles and rotor cages, the transient LT solver with motion is applied....
Figure 1 shows the CST EMS model of a typical shaded-pole motor of asymmetrical form which consists of a single winding (stranded coil), two-shading rings (auxiliary windings), rotor with embedded squirrel cage and stator. It is assumed that both the rotor and stator, with non-linear BH characteristics, are laminated. Eddy current currents cannot arise in this model.
The stranded coil is excited with a 50 Hz alternating current. Without the presence of the shading rings, the motor is not self-starting. Eddy currents are induced in the rings which in turn introduce a phase-shift in the magnetic fields of both poles. This leads to a 2-phase rotational field and hence rotational motion.
Figure 2: Variation of magnetic flux density, B with time during start-up
For efficiency purposes, the motor can be simulated in 2D. The workflow in CST EMS entails the creation of a 3D model and the specification of the desired cut-plane in the model. A 2D cartesian mesh is then created.
The transient LT solver in CST EMS delivers results such as the torque versus speed or, as shown in figure 2, the magnetic flux density as a function of time. In this case, the start-up behaviour of the motor can be investigated. This is facilitated by solving the equation of motion at each time-step. The inertia was specified in this example but other parameters such as the damping constant, spring constant and external torque can be specified.
The field animation shows the motion of the rotor from standstill to synchronous speed. Critical to the design of such motors is the magnetic field saturation which arises due to the non-linear steel characteristics, the geometry and the applied stator current.
The poor efficiency of such motors is an inherent design characteristic and is not the focus in this article. On the other hand, simulation enables the designer to test new single-phase motor concepts - something which is difficult, if not impossibe, to carry out analytically.