Littelfuse MPS - 5 Protective Functions; General; Overload; Thermal Model

Page 5-1
MPS Motor Protection System Rev. 6-F-022117
Protective Functions
5. PROTECTIVE FUNCTIONS
5.1 GENERAL
The MPS measures true RMS, peak, and fundamental-
frequency values of current and voltage. Fundamental-
frequency values (magnitude and phase angle) are obtained
using Discrete-Fourier Transform (DFT) filtering that
rejects dc and harmonics. The type of measurement used
for a protective function is indicated in each section.
Unless otherwise indicated, protective functions have a
programmable definite-time characteristic. Each protective
function can be assigned a trip action that defines the
output contact(s) used. Except for overload protection,
which has auto-reset available, MPS trips are latched.
Trips are logged.
Trip-action selections are:
• Disable
• Trip1
(1)
• Trip2
(2)
• Trip3
(2)
• Trip1 and Trip2
• Trip1 and Trip3
• Trip1 and Trip2 and Trip3
• Trip2 and Trip3
(1)
Initiates a starter stop. See Section 6.
(2)
Does not initiate a starter stop. See Section 6.
Most protection functions can be assigned an alarm
action. Alarms auto-reset and are not logged.
Alarm-action selections are:
• Disable
• Alarm1
• Alarm2
• Alarm3
• Alarm1 and Alarm2
• Alarm1 and Alarm3
• Alarm1 and Alarm2 and Alarm3
• Alarm2 and Alarm3
To operate output contacts, trip and alarm actions must
be assigned to output relays using the Setup | Relay Outputs
menu. See Section 4.2.7.
NOTE: When starter functions are used, only set points
with a Trip Action that includes Trip1 will cause the starter
to stop when a trip occurs.
When enabled, Jam, Power-Factor, Undercurrent, and
Underpower protection are not active during a start and are
active in the Run mode. The Run mode is initiated when
motor current is between 1.5 x (CT-Primary Rating/FLA)
and 125% FLA for the duration of the setting in the Setup ⏐
System Ratings ⏐ Run Mode Delay menu.
Run Mode Delay is adjustable from five to 120 seconds.
Run-mode status is displayed in the Metering | System State
menu. One of three states are indicated: Motor:Start,
Motor:Run, or Motor:Off. The motor status line also
indicates one of two states for the Reduced Overcurrent
(ROC) feature: ROC:OFF or ROC:ON. See Section 5.5.
NOTE: See Appendix B for default set-point values.
Per-unit notation (pu) is used. 1 pu = 100%.
5.2 OVERLOAD
At a minimum, for customized thermal-overload
protection, motor data, must be entered for Full-Load
Current, Service Factor, Locked-Rotor Current, Cold
Locked-Rotor Time, and Hot Locked-Rotor Time.
5.2.1 THERMAL MODEL
OPI Menu: Setup | Protection | Overload
OPI Menu: Setup | System Ratings
A NEMA- or K-factor-based thermal-model algorithm
can be selected.
The NEMA-based algorithm uses the square of the
maximum RMS phase current as the thermal-model input:
2
2
rmsmax
II =
I in per unit of FLA
The K-factor-based algorithm uses a thermal-model
input based on true positive- and negative-sequence
component values:
A conservative value for k is:
k =
230
I
L
2
Where:
I
L
=
locked rotor current
full load current
The thermal time constant (τ) used by the thermal model
to provide starting and running I
2
t protection is:
unitperinfactorserv icesf
ondssecintimerot orlockedcoldT
:Where
I
sf
ln
T
C
2
L
2
C
=
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−−
=τ
1
When the motor is stopped, the thermal model uses a
time constant that is user selectable as a multiple (Cooling
Factor) of the thermal time constant (τ).