Optimizing Permanent Magnet Linear Generators: Design Insights

Motors and Generators Permanent magnets
By Kousseil Ben Ahmed | 22/10/2021

PERMANENT MAGNET LINEAR GENERATOR 

A permanent magnet linear generator (PMLG) is simply a generator made up of a stator, translator, and air gap.  The stator is usually made up of copper windings and laminations which aim at minimizing the eddy current losses.  The translator consists of permanent magnets that can be either rings, cylinders, or rectangles and made of rare earth material.  Depending on the required force and the air gap magnetic field, the magnets can be arranged radially, axially, or in Halbach fashion.  Numerous research papers have reported that the PMLG has several distinct advantages including high efficiency, i.e., >90%, small air gap, and a small size.  In addition, PMLG is environmentally friendly as it emits no CO2 and consumes no fossil fuel.  However, there are several challenges and hurdles that must be overcome including demagnetization of the permanent magnets due to temperature rise, high cogging force which can produce noise and vibration, and the high cost of the rare-earth magnets.  


A SolidWorks model of a PMLG 


LESSONS LEARNED FROM EMS SIMULATION 

To study the above-mentioned design challenges, the popular EMWorks’ virtual prototyping software EMS is used to find answers and workarounds, including the following: 

  • The cogging force heavily depends on the ferromagnetic material of the stator core and the speed of the translator.  Among the various ferromagnetic cores studied, the Somaloy gives the least cogging force.  The higher is the speed of the translator the higher the cogging force.
  • The Back-EMF is directly proportional to the speed of the translator.
  • Upon comparing the permanent magnets, the Halbach yields better output power when compared to axial and radial arrangements.  Furthermore, comparing the axial and radial arrangements, the former don’t need a back iron, where the latter does in order to achieve better performance.   
  • A thermal analysis must always be performed to make sure that the magnets don’t heat up to the extent that get demagnetized.   
  • The PMLG is an excellent candidate for power generation in ocean using the motion caused by the waves.
  • The smaller the airgap is the higher is the back EMF and the cogging force.  Hence the tradeoff between cogging and power must be carefully studied.
  • Although the generator produces AC voltage and current, the frequency and amplitude of the voltage changes with the speed and this make it essential to use power electronic converters to maximize the power utilization. It is worthwhile to mention that even with a constant speed, the magnitude of back EMF will not be constant. Therefore, power converter efficiency should be considered for the determination of efficiency of the whole system 


Cross section plot of the magnetic flux density using EMS 



Cogging force versus time using EMS 



Back EMF of the 12 coils in the PMLG using EMS 

Conclusion

Optimizing a Permanent Magnet Linear Generator involves navigating challenges like demagnetization risks, cogging force management, and the cost of materials. EMS simulation provides invaluable insights, revealing that material choice, translator speed, magnet arrangement, and thermal management are pivotal. These findings not only guide effective PMLG design but also underscore the importance of comprehensive simulation in achieving high efficiency and environmental sustainability in renewable energy technologies.