Woodward MRM4, Manual

Page: 270 / 460

Protective Elements
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 (digital 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 Digital 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 » 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.
Page 270 EN MRM4 01/11
I
0, 3 - FLA
Case 1 – Always Low Trip
Case 2 – Always High Trip
Case 3 – ZSS too early
Trip
Case 4 – ZSS to late
Trip
Case 5 – ZSS correct
OK
LAT Timer/2
Power On
t