Page 5-4
MPS Motor Protection System Rev. 6-F-022117
Protective Functions
5.2.2 LOCKED-ROTOR TIMES
In all cases, values for T
H
and T
C
should be obtained
from the motor manufacturer. The following information
is provided to assist in selecting values for T
H
and T
C
only
if manufacturer data is not available.
Heater-style overload elements are available as
Class 10, Class 20, or Class 30. Class 20 is recommended
for general applications, Class 10 is used for motors with
short locked-rotor time capability, and Class 30 is used in
high-inertia applications to allow additional accelerating
time where motors are within Class-30 performance
requirements. These overloads can be replicated by
setting T
C
= 10, 20, or 30 s; T
H
= 0.1 s; and Locked-Rotor
Current = 6.00 x FLA.
An induction motor built to the NEMA MG 1 standard
is capable of:
• two starts in succession (coasting to rest between
starts) with the motor initially at ambient temperature
(cold start)
• one start with the motor initially at a temperature not
exceeding its rated-load operating temperature (hot
start).
Since the connected load has a direct influence on
motor heating during a start, NEMA MG 1 defines the
load torque and the load inertia (Wk
2
) for these starts as a
function of the motor’s rated power and synchronous
speed. To satisfy the cold-start requirement, a start must
not use more than 50% thermal capacity. To satisfy the
hot-start requirement, used thermal capacity at steady
state must be less than 50%.
If the thermal model in the MPS has the correct value
of T
C
and if Used I
2
t increases by 50% during a start, the
load is equal to the NEMA-defined load and two starts
from cold will be permitted. If Used I
2
t increases by more
than 50% during a start, the load is greater than the
NEMA-defined load and two starts from cold should not
be permitted — a delay is required between starts. The
appropriate delay can be obtained by enabling I
2
t Start
Inhibit and setting the I
2
t Inhibit Level equal to 100%
minus the I
2
t used during a start (a slightly lower level is
recommended to allow for supply and load variations). If
Used I
2
t increases by less than 50% during a start, the
load is less than the NEMA-defined load and two starts
from cold will be permitted.
The magnitude of T
H
relative to T
C
determines if a hot
start will be permitted if I
2
t Start Inhibit is enabled and the
I
2
t Inhibit Level is set as described above. If Used I
2
t
increases by 50% or less during a start, a hot start will be
permitted if T
H
is equal to or greater than 50% of T
C
.
Increasing T
H
above 50% of T
C
is not recommended
unless specific information is available with respect to T
H
.
5.2.3 EMERGENCY THERMAL RESET
OPI Menu: Messages | Emerg I2t Reset | Reset I2t Memory
Emergency Thermal Reset (ETR) sets Used I
2
t to 0%,
resets starts-per-hour variables, and disables PTC and
RTD temperature trips. Program access (password) is
required. Disabled-temperature protection is indicated by
t
°
Disabled by ETR in the Status Messages display. If
PTC or RTD temperature protection is not enabled, t
°
Disabled by ETR will not be displayed. Stator RTD or
PTC trips are reset when ETR is performed regardless of
measured temperatures. Temperature protection must be
re-enabled in the Messages | Emerg I
2
t Reset | Reenable
Temp menu, or by cycling supply voltage.
Temperature alarms and sensor verification remain
enabled during ETR.
NOTE: Temperature protection is not automatically re-
enabled after an Emergency Thermal Reset.
5.3 OVERCURRENT
OPI Menu: Setup | Protection | Overcurrent
Overcurrent protection is based on the largest
fundamental-frequency component (DFT) of the three
phase currents. An alarm-level setting is not provided.
When enabled, overcurrent protection is active at all
times. It is not bypassed during a start.
Trip Level ............................. 1.00 to 15.00 x CT-Primary
Rating (I
p
)
Trip Delay (TD) .................... 0.00 to 10.00 s
(See Tables 5.1 and 5.2)
Protection .............................. Enable/Disable Trip1, 2, 3
Measurement Method ........... DFT
TABLE 5.1 TRIP TIME
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