I.L. 17562
Page 99
PR 0.3 Effective 8/99
For a complete supply interruption, the nominal ride-through rating is 30 cycles. The relay will
typically continue to operate for 25 to 50 power cycles, depending on power supply loading
variables:
• Number of output relays picked up.
• Current being delivered by the 4-20 mA analog transducer output port.
• Type of PONI communications module powered by the MP-3000, if any.
• Electrical URTD communications (using recommended optical communications lowers this
burden slightly).
The one load which may depress the MP-3000 ride-through times well below 25 cycles is the use
of a Cutler-Hammer Ethernet PONI (EPONI) for communications – particularly the EPONI with
optical fiber communications. In this case, ride-through could fall to 10-13 cycles. However, this
is easily remedied by powering the EPONI from its own optional external power transformer.
Then, the MP-3000 ride-through will return to its 30 to 50 cycle range.
In applying this ride-through capability, consider the external factors:
• Typical utility feeder fault tripping and reclosing systems on overhead medium- or high-
voltage feeder lines up to 161 kV cause interruptions of 15 to 20 cycles, especially during
thunderstorms.
• For higher or extra-high voltage supply circuits to large plants, interruptions may be longer,
up to 30 cycles. Determine if the power system has special mitigating features like multiple
well-separated sources, or local paralleled generation.
• Conventional motor contactors will drop out in 3 to 5 cycles, stopping the motor.
• Special delayed-dropout contactors are available which store energy, and will ride through
interruptions of 15 to 20 cycles.
• Latching contactors or motor breakers may also be used – they trip only by active unlatching
or trip coil energization.
The MP-3000 will declare a motor stop after about 20 cycles of no current. The only way to
extend this time is to assign the MOTOR STOP function to one of the discrete inputs, and
connect an external stop-indicating contact (see Section 5.6.1).
If the ac supply returns after the stop declaration, but before the MP-3000 microprocessor shuts
down, the relay can go through another start cycle and return to the RUN state without ever
tripping the motor. However, this on-the-fly restart declaration won’t happen if the relay has
antibackspin or other jogging timer blocking in effect – the relay will trip at the moment it
declares a stop.
WARNING
Beware of reenergizing many larger motors after an interruption of over 20 cycles but less than
several seconds. The induced rotor current decays slowly, yet the rotor slows its rotation and
develops a large angular difference from the disconnected stator field. When the stator is
reenergized after, say, 30 or 40 cycles, the sudden torque shock can break a shaft key or do
more serious mechanical damage. Use a contactor with an appropriate dropout delay.
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