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CHAPTER 4: SETPOINTS
NOTE:
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
times to reset and operate, 40 points for reset (from 0 to 0.98 times the Pickup value) and
80 for operate (from 1.03 to 20 times the Pickup). However, when these curves are used
as an Overload Curve in Thermal Model protection, the 40 points for reset are not
required.
The EnerVista 8 Series Setup software allows for easy configuration and management of
FlexCurves and their associated data points. Prospective FlexCurves can be configured
from a selection of standard curves to provide the best approximate fit, then specific
data points can be edited afterwards. Alternately, curve data can be imported from a
specified file (.csv format) by selecting the Import Data from the EnerVista 8 Series
Setup setting.
If a thermal model curve is selected as "FlexCurve" then the TD Multiplier can be specified
between "0.00" and "600.00". The time dial multiplier (TDM) setting allows the selection of a
multiple of the base curve shape (where the time dial multiplier = 1) with the curve shape
setting. For example, all times for a TDM = 10 are ten times the multiplier 1 or base curve
values.
UNBALANCE BIAS K FACTOR
Range: 0 to 19 in steps of 1
Default: 0
Unbalanced phase currents cause rotor heating that is not shown in the motor thermal
damage curve. When the motor is running, the rotor rotates in the direction of the
positive sequence current at near synchronous speed. Negative sequence current,
which has a phase rotation that is opposite to the positive sequence current, and hence
opposite to the direction of rotor rotation, generates a rotor voltage that produces a
substantial current in the rotor. This current has a frequency that is approximately twice
the line frequency: 100 Hz for a 50 Hz system or 120 Hz for a 60 Hz system. Skin effect in
the rotor bars at this frequency causes a significant increase in rotor resistance and
therefore, a significant increase in rotor heating. This extra heating is not accounted for
in the thermal limit curves supplied by the motor manufacturer as these curves assume
positive sequence currents from a perfectly balanced supply voltage and motor design.
The thermal model may be biased to reflect the additional heating that is caused by
negative sequence current when the motor is running. This biasing is done by creating
an equivalent motor heating current rather than simply using the average current. This
equivalent current is calculated using the equation shown below.
where:
The motor derating as a function of voltage unbalance as recommended by NEMA
(National Electrical Manufacturers Association) is shown below. Assuming a typical
induction motor with an inrush of 6 x FLA and a negative sequence impedance of 0.167,
voltage unbalances of 1, 2, 3, 4, and 5% equals current unbalances of 6, 12, 18, 24, and
30% respectively. Based on this assumption, the amount of motor derating for different
values of K entered for setting Unbalance Bias K Factor is also shown in the following
figure.
PROTECTION
Eq. 14
4–119
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