UFC 3 -520-01
June 10, 2002
motor starting. The engine -generator can be overloaded if several motors
simultaneously have high inrush currents. In these cases, the loads must be
sequenced onto the generator. Time delays are usually used on less critical loads to
assure that the most important loads are connected first. Load shedding ASDs with
subsequent restart might be necessary to avoid damaging the ASD during the period of
ATS power transfer.
13-5.6.2 The standard method used to avoid the potential problems with motor loads is
to include a time delay in the transfer circuit to allow the residual motor voltage to decay
to a safe level before allowing connection to the alternate source. When a running
induction motor is disconnected from its power source, the motor generates residual
internal voltage until the stored residual magnetism dissipates due to losses in the iron
and as the motor loses speed. Figure 13-3 shows the typical voltage decay rate for
different size motors.
Figure 13-3. Induction Motor Open Circuit Voltage Decay
300 Hp (223,800 watts)
200 Hp (149,200 watts)
100 Hp (74,600 watts)
50 Hp (37,300 watts)
Induction Motor Open Circuit Voltage Decay (seconds)
13-5.6.3 The motor voltage should be below 0.25 per unit to allow a safe connection to
the transferred source. Although only 1 to 2 seconds is necessary for voltage to decay
to this level, the transfer should be delayed by 5 to 10 seconds to be conservative. This
provides adequate margin for several motors operating from a common source. If this
long of a time delay is not desired because of other loads, the motor loads should be
separately disconnected by the automatic transfer equipment and then restarted in a
sequenced manner after the transfer has been made.
13-5.6.4 Some transfer switches use a phase angle monitor in an attempt to avoid the
motor voltage decay problem. The rate of change of frequency associated with motor
loads is complex and varying during tra nsfer. Phase angle monitors should not be used
because they are not guaranteed to achieve an in-phase transfer.