Littelfuse MPS - Page 48

Page 5-2
MPS Motor Protection System Rev. 6-F-022117
Protective Functions
The cold-curve time-to-trip (t) for current above
FLA × sf is:
τ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−−=
2
2
1
I
sf
lnt
The MPS provides indication of thermal trend and used
thermal capacity. Thermal trend is the value that used
thermal capacity is tending toward and it is a function of
the square of motor current. For currents greater than or
equal to FLA × sf, time-to-trip is displayed in Metering |
Thermal Capacity. The thermal trend value (Trend I
2
t) is:
%100
2
2
2
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
sf
I
tITrend
For currents less than FLA × sf, the thermal trend value
is:
ondssecintimerotorlockedhotT
Where
T
TT
sf
I
tITrend
H
C
HC
=
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
:
%100
2
2
2
The curve shown in Fig. 5.1 is a Class-20 thermal-
protection curve (20-s trip @ 600% FLA) with a service
factor of 1.15. FLA multiplied by service factor is the
current at which used thermal capacity begins to trend
towards a trip. Time-to-trip approaches infinity when
I = FLA × sf. Service factor has little influence on time-to-
trip when motor current is greater than 300% FLA.
MPS thermal-overload protection is dynamic. Time to
trip at any overload current depends on the value of Used
I
2
t ⎯ as Used I
2
t increases, time to trip decreases. This is
illustrated in Fig. 5.1 by the protection curves labelled 25%
Used I
2
t, 50% Used I
2
t, and 75% Used I
2
t. Programming
software SE-Comm-RIS has a plot function to display and
export MPS protection curves.
An overload alarm occurs when Used I
2
t reaches the I
2
t
Alarm Level set point.
An overload trip occurs when Used I
2
t reaches 100%.
When an overload trip occurs, reset is not allowed until
Used I
2
t falls below the I
2
t Inhibit Level set point. The
time-to-reset in minutes is:
t = - τ × Cooling Factor × ln(I
2
t Inhibit Level) / 60
Time-to-reset is displayed in the Metering ⏐ Thermal
Capacity menu. The thermal model has three reset modes;
Normal, Auto, and Multiple Motor Seq. The thermal-
overload reset mode is set using the Setup ⏐ Protection ⏐
Overload ⏐ I
2
t Reset Type menu.
In the Normal mode, a thermal-overload trip reset is not
allowed until Used I
2
t falls below the I
2
t Inhibit Level
setting. A reset input is required to reset the trip. Normal
is the default reset mode.
In the Auto mode, a thermal-overload trip is
automatically reset when Used I
2
t falls below the I
2
t Inhibit
Level setting.
NOTE: If the starter circuit is configured for 2-wire control,
the motor can start without warning when Auto mode is
selected. A warning label may be required.
In the Multiple Motor Seq. mode, Used I
2
t decreases
exponentially with a fixed two-second time constant when
there is no motor current. This mode is used in
applications where one overload relay is used to protect
several motors operating in sequence with only one motor
running at any one time. A two-second stop is required
between starts so that Used I
2
t decreases sufficiently to
allow the next motor to start. It is assumed that each motor
is allowed to cool between starts. Motor life may be
decreased if this feature is used in single-motor
applications. When an overload trip occurs, the trip is
latched but can be reset two seconds after the trip.
When I
2
t Start Inhibit is enabled, the I
2
t Inhibit Level set
point can be used to prevent a start with insufficient I
2
t
available. When Used I
2
t is above the I
2
t Inhibit Level set
point and motor current is not detected, Alarm1 is issued,
starter functions Start1 and Start2 are disabled, and the
relay assigned to Start Inhibit is energized. The time until
a start is permitted is displayed in Metering | Thermal
Capacity, and I
2
t Inhibit Alarm is displayed in the Trip and
Alarm message window. When Used I
2
t falls below the I
2
t
Inhibit Level set point, the relay assigned to Start Inhibit is
de-energized, the inhibit alarm is cancelled, and starter
functions Start1 and Start2 are enabled. Trips require a
manual reset. I
2
t Start Inhibit is removed when current is
detected. This applies in both Starter and Protection-only
modes. The Start-Inhibit relay is shared with the Starts-Per-
Hour function. See Section 5.23.
If the motor is equipped with RTD sensors, the thermal
model can compensate for high ambient temperature and
loss of ventilation. See Section 5.27.
I
2
t used during each start is recorded in Messages ⏐
Event Records. This information can be used to determine
the I
2
t Inhibit Level set point to ensure sufficient I
2
t is
available to complete a start, and to minimize thermal-
overload-reset time.
FLA Rating ........................... 1.00 to 5,000.00 A
Service Factor ....................... 1.00 to 1.25
Locked-Rotor Current ........... 1.50 to 10.00 x FLA
Hot Locked-Rotor Time ....... 0.10 to 100.00 s
Cold Locked-Rotor Time ...... 0.10 to 100.00 s
Cooling Factor ...................... 0.10 to 10.00
Model Type: ......................... NEMA, K-Factor
I
2
t Reset Type ........................ Normal, Auto,
Multiple Motor Sequence
K-Factor ................................ 1.00 to 10.00
I
2
t Alarm ............................... 0.50 to 1.00 pu
I
2
t Inhibit Level 0.10
to 0.90 pu