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Introduction to Electric Motors
Published in Wei Tong, Mechanical Design and Manufacturing of Electric Motors, 2022
Cogging torque originates from the interaction between the rotor-mounted PMs and the stator teeth, which produces reluctance variations depending on the rotor position [1.21]. In a motor design, it is highly desirable to reduce cogging torque because it can result in vibration, speed ripple and noise, particularly at light loads and low speeds. As depicted in Figure 1.4, when the rotor rotates, the magnets attached to the rotor successively pass through the stator teeth and slot openings, resulting in the periodic variations of the magnetic field. During the process, the PM rotor tends to lock onto the position where the permeability reaches the largest. When the rotor deviates from this equilibrium position, tangential forces are produced between the magnets and stator teeth, either to return the rotor back to the old equilibrium position or to push the rotor to the next equilibrium position, leading to cogging torque.
Brushless DC Drives
Published in Ali Emadi, Handbook of Automotive Power Electronics and Motor Drives, 2017
Cogging torque is apparent at low speeds and is due to the natural attraction and repulsion of the stator teeth by the magnets. Some may prefer to think in terms of reluctance torque; i.e., cogging torque is due to the flux of the magnets attempting to seek out the least reluctance path by moving the rotor or stator toward magnet-tooth alignments [6]. A typical reluctance torque profile is a function of the angular position, the rotor within the slotted stator, and is periodic with a period of ∫ pole pitch [12]. Cogging torque is overcome by various machine design methods. The most commonly used machine design method to reduce the effects of cogging torque includes skewing either the magnets or the stator slots by one slot pitch; however, this increases the distortion of the back-EMF waveform, and may also increase axial forces.
Permanent Magnets and Machines
Published in R. Krishnan, Permanent Magnet Synchronous and Brushless DC Motor Drives, 2017
An electromagnetic torque exists in PM machines even when there is no excitation of the stator windings. This arises due to the interaction between the rotor PM field and the stator teeth. As the rotor magnet is approaching or leaving a tooth, the field around it is changing, resulting in a change of coenergy in the air space between the magnet and tooth. The change in coenergy results in a net torque. This torque is sometimes referred to as detent or cogging torque in the literature. The cogging torque may also be thought of as a reluctance torque due to the reluctance variation presented by the tooth and slot to the magnet current source. Its period is the same as the slot pitch in general during most of the measurements but it is not the case, which is derived later. The cogging torque is alternating and almost symmetric about its angular axis.
Experimental Investigation of Magnet Grouping Technique in Reduction of Cogging Torque and Total Harmonic Distortions in the Axial Flux PM Generator for Direct Drive Wind Turbine
Published in Electric Power Components and Systems, 2023
The cogging torque is generally caused by the interaction between rotor poles and stator slots. This phenomenon is torque fluctuation. That is generated by the magnets’, which create rotor poles, high magnetizing power attracts the stator’s iron core and release the stator slots as well. Thus, it is generated when the EASM generator is not running. The general formulation of the cogging torque is given in Equation 1. In the equation 1; and are the cogging torque, air gap’s magnetic flux, rotor’s angular value and the reluctance value of air gap, respectively.
An optimal selection of slot/pole combination and its influence on energy efficient PMSM for submersible water pumping applications
Published in International Journal of Ambient Energy, 2023
Anand Mouttouvelou, Vinod Balakrishna, Sundaram Maruthachalam, Suresh Muthusamy, Hitesh Panchal, Meenakumari Ramachandran, Vennila Ammasi
Cogging torque is a caused by the interface between the rotor magnets and stator slotting. Wide applications are sensitive to torque ripple, with power electronics drive it is possible to produce smooth torque, if their sources are conventional AC and DC. The cogging torque can be expressed as shown in Equation (4) (Tangudu and Jahns 2011). where k-order of the harmonic; Q is the LCM (number of stator slots and number of rotor poles); is the coefficient of each harmonics and is the angle (mechanical degrees).
Study of a Spoke-Type Ferrite Structure as an Alternative to Surface-Mounted NdFeB PMSGs: A Performance Comparison Based on Getting the Same Efficiency
Published in Electric Power Components and Systems, 2023
As can be seen from Eq. (9), the cogging torque is the interaction of the magnets, which are the source of the air-gap flux, and the variable reluctance caused by the stator teeth and slots. The periodic variation of the reluctance caused by the slots and teeth forming the stator core also causes the cogging torque to change periodically. As is known, periodic waves can be expressed with the Fourier series by Eq. (10).