106
Figure 74 shows the Phase A stator current in each of the three motors during the momentary
service interruption. The current flow between the three motors is clearly seen while the
breaker is open.
As the power rating of the resistive load increases, rate of decay of back-emf also decreases.
This is because the resistive load absorbs power from the energy stored in the motor during
the momentary service interruption. The lower the magnitude of the back-emf at the instant
of reconnection, the lower the magnitude of the negative torque transient produced, as shown
in Fig 32 of section 3.3.5.1. Therefore, in a practical application of a contactor ride-through
device, the loads that remain connected to the supply during the interruption will draw power
from the induction motors that also remain connected to the supply. This in turn leads to a
lower magnitude torque transient at the moment of reconnection. If the load is very large, or
there is a fault at the supply, the energy stored in the rotor may be transferred to the system
very rapidly. In the case of a fault, a high magnitude negative torque transient can develop
due to some or all of the motors being forced to supply a fault. However, previous literature
shows that even a fault at the motor terminals will not produce negative shaft torque
transients greater than the shaft torque transients produced during a momentary service
interruption.
Additional tests on the motor system model indicated that it is possible for the system to
"stall" if the line impedance is high and/or too many motors are allowed to ride-though the
momentary service interruption. At the moment the power supply voltage returns, if the
power demand on the system caused by many motors re-accelerating is high, the supply
voltage will collapse and the motors will never re-accelerate.