Environmental Engineering Reference
In-Depth Information
when operating above n s . The difference is the ''slip'' which is usually kept to a
few percent of the synchronous speed, partly because the slip determines one of
the main losses, the so-called I 2 R losses dissipated as heat in the rotor. Slip usually
increases with decreasing motor size because the rotor resistance is higher for the
thinner wire of the cage and hence needs a greater fraction of the rated voltage to
induce the required cage current. This is one reason why the efficiencies of small
induction motors are usually lower than comparable PMGs, see Table 1.4 . There
are also I 2 R and iron losses in the stator, and smaller amounts of windage and
friction losses. Windage refers to the movement of air, either directly by the rotor,
or by a fan attached to the generator for cooling.
The synchronous speed is set by the number of poles according to
n s ¼ 120f = N P
ð 11 : 1 Þ
where the frequency f is either the grid frequency (or the output frequency of a
variable frequency drive) for a motor or the rotor frequency for a wind turbine. N P
is the number of poles which is less than 12 for commonly available induction
motors. Usually the efficiency decreases with increasing N P —see Table 1.4
probably because more space is needed for the copper wires, which reduces the
cross-sectional area of stator iron and the magnetic coupling. This increases the
leakage inductance which does not contribute to the generation of electrical power.
In other words, the ratio of copper space to iron space increases which makes
poorer use of the existing iron material.
Because readily-available IGs have fewer poles than PMGs, they rotate more
rapidly and therefore usually require a gearbox for small wind turbine applications.
The word ''usually'' was carefully chosen because blade frequency increases with
decreasing turbine size, so it is possible to design a direct drive wind turbine with
an induction generator of say 8-12 poles with a rated power of between 1 and
2 kW. As far as the author knows this has not been done, possibly because these
generators are relatively heavy and inefficient.
IGs are only moderately more complex than PMGs. Their rotors have windings
rather than magnets, but these windings are shorted and so, like PMGs, do not need
brushes or commutators. A variation is a doubly fed induction generator, DFIG,
Fig. 11.4 Torque-speed
curve for induction machines
1
0.5
generator
0
motor
-0.5
-1
0
0.5
1.0
1.5
2
(Shaft speed)/(Synchronous speed)
 
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