Civil Engineering Reference
In-Depth Information
robust, reliable, and a low-cost machine. For this reason, it is widely used
in numerous applications in the industry.
The working principle of the induction machine can be seen as the trans-
former. The high voltage coil on the stator is excited and the low voltage
coil on the rotor is shorted on itself. The power from one to the other can
flow in either direction. The theory of operation of the transformer, therefore,
holds true when modified to account for the relative motion between the
stator and the rotor. This motion is expressed in terms of the slip of the rotor
relative to the synchronously rotating magnetic field.
6.2.3
Rotor Speed and Slip
The slip of the rotor is defined as the ratio of the speed of the rotating
magnetic field sweeping past the rotor and the synchronous speed of the
stator magnetic field. That is,
NN
N
−
s
=
s
r
(6-4)
s
where s
= slip of the rotor
N
= synchronous speed = 60·f/p
s
N
= rotor speed.
r
The slip is generally considered positive in the motoring operation. In the
generator mode, the slip would therefore be negative. In both the motoring
mode and the generating mode, higher rotor slips induce higher current in
the rotor and higher electromechanical power conversion. In both modes,
the value of the slip is generally a few to several percent. Higher slips result
in greater electrical loss, which must be effectively dissipated from the rotor
to keep the operating temperature below the allowable limit.
The heat is removed from the machine by the fan blades attached to one
end-ring of the rotor. The fan is enclosed in a shroud at the end. The forced
air travels axially along the machine exterior, which has fins to increase the
dissipation area.
Figure 6-4
is an exterior view of a 150 kW induction machine
showing the end shroud and the cooling fins running axially.
Figure 6-5
is a
cutaway view of the machine interior of a 2 MW induction machine.
The induction generator feeding the 60 or 50 Hz grid must run at speed
higher than 3,600 rpm in a two-pole design, 1,800 rpm in a four-pole design,
and 1,200 rpm in a six-pole design. The wind turbine speed, on the other hand,
varies from a few hundred rpm in the kW range machines to a few tens of
rpm in the MW rage machines. The wind turbine therefore must interface the
generator via a mechanical gear. Since this somewhat degrades the efficiency
and reliability, many small stand-alone plants operate with custom designed
generators operating at lower speed without the mechanical gear.
