Long Acceleration Time (Lat) - Eaton EMR-3MP0 Installation, Operation And Maintenance Manual

4.2.15

Long Acceleration Time (LAT)

When the LAT function is enabled, the »LAT« timer is used to set a time interval during
which the motor is permitted to accelerate a high-inertia load, which is longer than the
locked-rotor time. This function can be (and usually should be) set to »inactive«. If the
thermal-model accumulator bucket fills to 100% during the long acceleration time, it is
limited to that value and the thermal trip is held off until the LAT timer expires. By then,
the thermal bucket level must have decreased (thermal model cooled) below 100% or the
motor trips.
The LAT function should be used but not limited only on motors with a zero-speed switch
(a normally-closed contact that opens when the motor actually begins to spin). Connect
the zero-speed switch contact to one of the protective device Digital Inputs. The Zero-
Speed Switch function must be enabled (ZSS ON). The protective device requires the
zero-speed switch to open within LRT/2 (one-half of locked-rotor time) after a start, or the
motor is tripped by the ZSS function. This protects a completely stalled motor from being
damaged when the LAT timer blocks the locked-rotor thermal trip.
The long acceleration time (LAT) function can block the critical LRC-LRT rotor thermal
protection during a start and destroy the motor. Turn LAT OFF unless absolutely needed
and the motor's suitability for this starting duty has been confirmed. Use only with zero
speed switch function ZSS ON and switch input connected to protect a stalled motor.
The user can temporarily defeat the I2t thermal protection limit after a start by setting a
Long Acceleration Time delay. This can be a dangerous setting that blocks thermal
tripping and holds the bucket at a 100% level if the load takes a long time to reach
running speed. An example is a motor spinning a large centrifuge. In using LAT, the User
can take advantage of the partial cooling from airflow produced by the motor spinning at
below-normal speed, as compared to unfanned heating of a locked rotor. The motor must
be rated for this severe starting duty. Also, the User must ensure that the motor actually
has begun to spin well before the locked-rotor time has expired. This is accomplished by
connecting a zero-speed switch to a Digital Input and turning on ZSS function. The zero-
speed switch is a contact that is closed when the motor is at rest, and opens as the motor
begins to spin, usually at 5-10% of running speed. If ZSS is set to ON and the protective
device relay does not sense the contact open in one-half the locked-rotor time setting, it
trips the motor.
Turn OFF LAT unless the application specifically demands it. Use a zero speed switch with
LAT. Using an LAT setting greater than locked rotor time without a zero speed switch
temporarily defeats thermal protection and damages the motor if the rotor actually is
locked.
If »LAT« is used, check the settings of transition time »TRNT« and jam start delay to be
sure they are coordinated with the prolonged starting cycle.
EMR-3MP0
CAUTION!
WARNING!
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4 Protective Elements
4.2 Motor Starting and Control Module
151