# A Review of Flywheel Energy Storage System Technologies and Their Applications
## 2 Description of Flywheel Energy Storage System
### Structure and Components of FESS
#### 2.2.2 Electric Machine  
As explained previously, the electrical machine or integrated MG is coupled to the flywheel, to enable the energy conversion and charging process of the flywheel. The machine, acting as a motor, charges the flywheel by accelerating it and drawing electrical energy from the source. The stored energy on the flywheel is extracted by the same machine, acting as a generator, and hence, the flywheel is slowed down during discharge. Common electrical machines used in FESS are the induction machine (IM), permanent magnet machine (PM), and variable reluctant machine (VRM) [18; 23].  
An IM is used for high power applications due to its ruggedness, higher torque, and low cost [18]. The speed limitations, complex control, and higher maintenance requirements are the main problems with IMs [16]. The squirrel cage type can be a less expensive option for slow response applications [20]. A doubly fed induction machine (DFIM) has recently been used in FESS applications, due to its flexible control and lower power conversion rating, allowing mitigated power electronics sizing [16; 20]. An IM is widely used in wind turbine applications to enable the power smoothing of wind generation systems [16; 20].  
A VRM is very robust, and has low idling losses and a wide speed range. It has a simpler control mechanism than IMs, when it comes to high speed operations [16]. On the downside, it has a low power factor and low power density, as well as high torque ripples [16]. Both switched reluctance and synchronous reluctance types are applied in high speed FESS applications [20].  
A PM is the most commonly used machine for FESS because of its higher efficiency, high power density, and low rotor losses [18]. It is widely used in high speed applications due to the speed limitations of IMs, and the torque ripple, vibration, and noise of VRMs. The problem with a PM is its idling losses due to stator eddy current losses, its high price, and its low tensile strength [18]. A brushless dc machine (BLDCM), permanent magnet synchronous machine (PMSM), and Halbach array machine (HAM), are the main types of PM machines used in FESS applications [20]. A comparison between IMs, VRMs, and PMs is presented in Table 1. Regarding the quantification of specific power, the authors in [18] appear to have misreported the published values. In our experience, depending on the rotor speed, the specific power of an IM and VRM would be around half of that of the PM, and not the large difference shown in [18]. In fact, further analysis of the original literature reports that the specific power density of PM synchronous machines is approximately 1.2 kW/kg [37].  
\begin{table}
\begin{tabular}{c l l l} \hline \hline
**Machine** & \multicolumn{2}{c}{**Asynchronous**} & \multicolumn{1}{c}{**Variable Reluctance**} & \multicolumn{1}{c}{**Permanent Magnet Synchronous**} \\ \hline Power & High & Medium and low & Medium and low \\ \hline Specific power & Medium (-0.7 kW/kg) & Medium (-0.7 kW/kg) & High (-1.2 kW/kg) \\ Rotor losses & Copper and iron & Iron due to slots & Very low \\ \hline Spinning losses & Removable by annulling flux & Removable by annulling flux & Non-removable, static flux \\ \hline Efficiency & High (93.4\%) & High (93\%) & Very high (95.5\%) \\ \hline Control & Vector control & Synchronous: Vector Control. & Sinusoidal: Vector control. \\  & & Switched: DSP & Trapezoidal: DSP \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of electrical machines suitable for use in FESS [18; 19].  
To take advantage of both PMs and VRMs, hybrid PM reluctance machines have recently been developed. Other unconventional machine types for FESS are presented in [20] and the latest developments of MG for FESS are discussed in [19].