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3.3 COMPREHENSIVE EXPERIMENTAL RESULTS
The most significant results from the preliminary testing are that the motor loading does not
effect the magnitude of the worst case current and shaft torque transients and that the load
inertia does have a significant effect on the magnitude of the worst case current and shaft
torque transients. Based on these findings, the comprehensive testing will include three
different load inertias for each of the three motors tested and all of the tests will be conducted
at 100% load.
3.3.1 Electrical Configuration
The electrical test configuration for the comprehensive testing is similar to the electrical test
configuration for the preliminary testing shown in Fig 29. 480V
rms
three-phase power is fed
to three variacs, which are used for voltage control. The variacs can be adjusted
independently from 0 600V
rms
and are used to balance the three-phase supply voltage. An
isolation transformer is used to isolate the power system ground, but the impedance of the
variacs and the transformer is not large enough to affect the transient current magnitude. The
fused disconnect and the NEMA 5 starter is used only for fault current protection, overload
protection of the motor, and a disconnecting means. Control of the motor during testing is
achieved by the power disturbance generator, which is necessary to precisely vary the
momentary service interruption duration.
3.3.2 Mechanical Configuration
Figure 29 shows the mechanical test configuration. Torsionally stiff couplings are used on
all shaft couplings in order to minimize shaft torque damping. A 1024 pulse-per-revolution
(ppr) speed encoder is fixed to the torque transducer to capture shaft speed. The motor is
coupled to a free-floating torque transducer, which is coupled to a custom built variable
inertia flywheel device. The variable inertia flywheel device is used to adjust the load inertia
and is coupled to a water brake power absorption unit. The torque of the power absorption
unit varies approximately as the square of the speed.
The variable inertia flywheel device consists of a shaft-hub assembly that allows for
fastening two flywheels to a hub. When the Flywheels 1 and 2 are parked in the positions
shown in Fig 29, they are not attached to the shaft or hub and do not contribute to the load