Table of Contents
Advertisement
S3 PROTECTION
6–66
T
– the temperature in degrees Celsius of the hottest RTD that is neither
HottestStator
open nor short and is declared to be a stator RTD. If there is no such RTD, use a value
of zero.
T
– the value of the setpoint, RTD Bias – Minimum T.
Min
T
– the value of the setpoint, RTD Bias – Center T.
Center
T
– the value of the setpoint, RTD Bias – Maximum T.
Max
TCU
– the RTD Biased Thermal Capacity determined by the hottest stator RTD
RTDbias
temperature.
HCR – the setting HOT/COLD SAFE STALL RATIO
Note that the RTD bias feature alone cannot create a trip. If the RTD bias forces Thermal
Capacity Used to 100%, the motor current must be above the overload pickup before an
overload trip occurs.
COOLING RATE
The Thermal Capacity Used value decreases exponentially when the motor equivalent
current (I
) is less than the Thermal Overload Pickup setting. This reduction simulates
eq
motor cooling. As a stopped motor normally cools significantly slower than a running
motor, the relay has two cooling time constant setpoints, one is used when the motor is not
in service (stopped, tripped, locked out, etc.), the other is used when the motor is in service
(starting, running). The time constant is, in each case, the time in minutes for the motor's
temperature to cool by 63% of the difference between the initial temperature and ambient
temperature.
Motor cooling is calculated as:
where:
TCU
: the TCU caused by an overload condition
used_start
TCU
: the TCU dictated by the hot/cold safe stall ratio when the motor is
used_end
running (=0 when the motor is stopped)
t: time in minutes
τ: Cool Time Constant (running or stopped)
I
: equivalent motor heating current
eq
O/L
: overload pickup setting as a multiple of FLA
_PKP
HCR: hot/cold safe stall ratio
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS
Eq. 13
Advertisement
Table of Contents
