QUICK REFERENCE GUIDE
SYSTEM DATA
469 MOTOR MANAGEMENT RELAY – QUICK REFERENCE GUIDE
51
Bearing RTDs:
The Bearing RTD alarm and trip settings will be determined by evaluating the temperature
specification from the bearing manufacturer.
Unbalance Bias Of Thermal Capacity:
Enable the Unbalance Bias of Thermal Capacity so that the heating effect of unbalance
currents is added to the Thermal Capacity Used.
Unbalance Bias K Factor:
The
K
value is used to calculate the contribution of the negative-sequence current flowing
in the rotor due to unbalance. It is defined as:
(EQ 0.1)
where:
R
r
2
= rotor negative-sequence resistance and
R
r
1
= rotor positive-sequence
resistance.
A formula based on empirical data states that
K
is equal to 230 divided by the per-unit
locked rotor current squared.
From the data sheet, the locked rotor amps = 631% FLA or 6.31
×
FLA. Therefore,
(EQ 0.2)
Hot/Cold Curve Ratio:
The hot/cold curve ratio is calculated by simply dividing the hot safe stall time by the cold
safe stall time or use the motor thermal limits curve. For this example, both are available.
Using the data sheets the, safe stall time H/C or Hot/Cold Curve Ratio is given as 16/18 =
0.89
Enable RTD Biasing:
This will enable the temperature from the Stator RTD sensors, to be included in the
calculations of Thermal Capacity. This model determines the Thermal Capacity Used based
on the temperature of the Stator and is separate from the overload model for calculating
Thermal Capacity Used. RTD biasing is a back up protection element, which accounts for
such things as loss of cooling or unusually high ambient temperature. This measured
temperature is used to bias or modify the thermal capacity value stored in the relay.
RTD Bias Minimum:
Set to 40°C, which is the ambient temperature, obtained from the data sheets.
RTD Bias Mid Point:
The center point temperature is set to the motor's hot running temperature and is
calculated as follows:
Temperature Rise of Ambient Temperature.
The temperature rise of the stator is 80°C + 10% hot spot allowance, obtained from the
data sheets. Therefore, the RTD Center point temperature is set to 90°C + 40°C or 130°C.
K
R
r
2
R
r
1
--------
=
K
230
(per-unit locked rotor amps)
2
------------------------------------------------------------------------
230
6.31
----------
6
≈
=
=
