Page 179 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

166 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
®

TABLE 7.6
Comparison of a Commercial Permanent Magnet
Generator and the Ferrite-Based Design
Machine Characteristics PM Off Shelf Ferrite
Turbine power 6.8 kW 5 kW
Maximum torque 1049 N m 1000 N m
Efficiency 79% 77%
Total mass 82 kg 132 kg
External radius 795 mm 240 mm
Air-gap radius 689 mm 208 mm
Active length 55 mm 476 mm
Source: Ojeda, J. et al., IEEE Trans. Ind. Appl., 48(6), 1808–1816, 2012.

electrical power. Therefore, the copper losses of the exciter do not exist and the absence of the
mechanical commutator and brushes as well as slip rings means low mechanical friction losses. An
additional advantage is its compactness.
The recent introduction of high-energy density magnets (rare-earth magnets) has allowed the
achievement of extremely high flux densities in the PMSG. Therefore, a rotor winding is not
required. These in turn allow the generator to have a small, light, and rugged structure. As there is
no current circulation in the rotor to create a magnetic field, the rotor of a PMSG does not heat up.
The only heat production is on the stator, which is easier to cool down than the rotor because it is on
the periphery of the generator and the stator.
The absence of brushes, mechanical commutators, and slip rings suppresses the need for regular
maintenance and also the risk of failure in these elements. They have very long-lasting winding insu-
lation, bearing, and magnet life length. Since no noise is associated with the mechanical contacts
and also the drive converter switching frequency can be above 20 kHz, the system might produce
only inaudible noise for human beings.
When compared with the conventional ones for low wind speed, PMSGs have the following
advantages: (1) no speed multiplier or gears since there may be multiple permanent or electromag-
nets in the rotor for more current production; (2) few maintenance services because of its simplified
mechanical design; (3) easy mechanical interface; (4) cost optimization; (5) highest power-to-
weight ratio in a direct-drive system; (6) location of a moving magnetic field being generated in the
center of the field; (7) more precise operations since a microprocessor controls the generator/motor
electrical output and current instead of mechanical brushes; (8) higher efficiency for the brushless
generation of electrical current and digitally controllable flexible adjustment of the generator speed
with less friction, fewer moving components, less heat, and reduced electrical noise; and (9) can be
kept cooler and thus have a longer life since the PMs are located on the rotor.
The PMSG has some disadvantages related to: (1) their high cost of PMs and (2) their commer-
cial availability. The cost of higher energy density magnets prohibits their use in applications where
initial cost is the major concern. Another disadvantage is the field-weakening operation for the
PMSG machine, which is somewhat difficult due to the use of PMs, but usually the field-weakening
mode is not a concern for wind power generators. Any accidental speed increase might damage the
power electronic components above the converter rating, especially for generator-drive applica-
tions. In addition, surface-mounted PM generators cannot reach high speeds because of their limited
mechanical strength of the assembly between the rotor yoke and the PMs. Finally, the demagne-
tization of the PM is possible by a large opposing magnetomotive force and high temperatures.