Environmental Engineering Reference
In-Depth Information
envelope, and the speed axis. Useful operation ceases when the machine enters
second breakdown. This last bit of terminology may not be as widespread as first
breakdown (i.e. corner point or base speed) is for the region where constant torque
transitions into constant power.
In the second breakdown region different processes begin to dominate the
electric machine's ability to produce torque, and these processes are technology
dependent. We saw that first breakdown is dependent on ac drive system power
supply and machine design in that its corner point is defined as that speed at which
the machines internal voltage approximates the input dc supply voltage. During
field weakening this internal voltage effect is mitigated so that current can continue
to be injected into the machine at rated value - hence constant power. Now, when
the electric machine reaches the final limits of holding constant power the power
begins to break down. In an asynchronous machine second breakdown is reached
when the slip parameter increases from rated slip to breakdown slip. Torque is at its
breakdown limit (~
V
s
=w
L
leakage
, where the parameters are supply voltage and
leakage inductance) and the machine slip is held constant at its breakdown value
(again, typically 250% of rated). Beyond this second breakdown speed, the current
and power drop reciprocally with speed and the torque drops as 1/speed.
2
In a permanent magnet synchronous machine of any variety, the second
breakdown speed occurs when the injected current can no longer be held constant
while field weakening is in progress. This occurs in a surface magnet machine or to
some extent in an inset magnet machine when the angle control of the injected
currents exceeds about 30
. For an interior magnet machine, the second breakdown
point is much further out in field weakening and occurs when the
d
-axis, or magnet
axis current, is no longer able to hold the machine internal voltage constant because
it has reached the rated value of input current; that is, there is no longer any
component of input current left to develop torque. When this condition occurs the
machine is completely out of torque. In a variable reluctance, or switched reluc-
tance, machine the conditions of second breakdown are somewhat similar to the
permanent magnet machine. During constant torque operation of the variable
reluctance machine, the current dwell angle is controlled while operating at fixed
advance angle. During constant power operation, the dwell is fixed as the advance
angle of current is progressively shifted ahead in time. When the advance angle is
no longer capable of being advanced, the machine enters second breakdown and
power drops reciprocally with speed.
We can say that operating at peak torque on the capability curve is not the case
in hybrid propulsion M/G design practice. True, the electric machine retains some
overdrive capacity, but in general the electric machine is designed for operation at
near its maximum capability during intermittent use (10-30 s). It is the limitation of
the power electronics that determines the envelope of the M/G capability. The
semiconductor power driver stage has no provision for overdrive conditions. The
semiconductor devices have thermal time constants of milliseconds so that a 10 s
overdrive condition in reality is steady state for power electronics. Therefore, the
intermittent operation envelope shown in Figure 4.15 represents the limit of the
