EMR-5000
IM02602012E
Zero Speed Switch (ZSS ON or OFF)
ZSS enables the function that verifies if the motor begins to physically spin after a start. It requires a zero-speed switch
on the motor, which is closed at rest and opens as the rotor reaches (5%-10%) its normal speed. Connect the zero-
speed switch contact to one of the protective device Discrete Inputs. If the contact fails to open within LRT/2 (one-half
of locked-rotor time) after a start, the relay trips with a zero-speed switch trip message.
This protection is always useful, but is essential if the Long Acceleration Time (LAT) function setting is used.
With ZSS being enabled and being mapped to one of the digital inputs, the protective device checks the ZSS input
status at the very moment it sees a start - it wants to sense the initially closed zero-speed switch, which opens shortly
thereafter as the motor spins. If it fails to find the closed contact, it trips immediately. Check the wiring and contact for
problems.
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 OFF. 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 Discrete 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 Discrete 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.
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