The values for the R-L line impedance model are the values that are estimated above. The
system model therefore is similar to connecting the three motors and a resistive load to the
power supply in the lab.
1kW R Load
Figure 70 Block diagram of a system used to study momentary service interruption
5.2 SYSTEM SIMULATIONS
Fig 71 shows the results of simulations conducted on the system described in Fig 70. When
the system is at a steady state condition, the circuit breaker is opened and then re-closed a
short time later. The length of the interruption duration was varied from 1 to 100 cycles, in
0.01 cycle increments. The maximum current amplitude, minimum shaft torque, maximum
shaft torque, and minimum speed were all recorded for each interruption duration. These
points are plotted in Fig 71 as a function of interruption duration.
The interruption duration that causes the worst case torque transient for all three motors is 7.9
cycles. It is not surprising that all three motors experience the worst case torque transient at
the same interruption duration. The back-emf of all three motors must be equal at any instant
in time since the motors are connected in parallel. Therefore, when the circuit breaker re-
closes, the back-emf of all three motors will be at the same phase angle. However, for a
system of various size motors that are connected to various different loads, some motors will
try to slow down faster than others. In order for the back-emf to remain equal across all
motors, the motors that try to slow down will be accelerated by the other motors. Some of
the motors are forced to become generators while other motors continue acting as a motor.
This is seen by examining the electromagnetic torque produced by the motor.