GE Multilin MM300, Instruction Manual

Page: 145 / 198

CHAPTER 5: SETPOINTS PROTECTION ELEMENTS
MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL 5–35
• I
2
/ I
1
represents the negative-sequence to positive-sequence current ratio.
• k represents the value of the
Unbalance K Factor
setpoint, used to adjust the degree of
unbalance biasing.
The value for k may be estimated as follows.
Eq. 2
In the above equation, I
LR
represents the locked rotor current in per-unit values on an FLA
base.
If a
k value of 0 is entered, the unbalance biasing is defeated and the overload curve will
time out against the average per-unit motor current.
The following figure shows recommended motor derating as a function of voltage
unbalance recommended by NEMA (the National Electrical Manufacturers Association). To
illustrate the MM300 unbalance biasing, assume a typical induction motor with an inrush
of 6 × FLA and a negative-sequence impedance of 0.167. With this impedance, voltage
unbalances of 1, 2, 3, 4, and 5% on the motor terminals will result in current unbalances of
6, 12, 18, 24, and 30%, respectively. Based on these assumptions, the derating resulting
from the MM300 unbalance biasing for different values of
k is as illustrated in the GE
Multilin curve below. Note that the curve for k = 8 is almost identical to the NEMA derating
curve.
Figure 5-27: Motor derating factor due to unbalanced voltage
Hot/cold biasing
When the motor is running with a constant load below the overload level, the motor will
eventually reach a steady state temperature, which corresponds to a particular steady-
state thermal capacity used. As some thermal capacity is used, there is less thermal
capacity left in the motor to cover transient overloads than is available when the motor is
cold. Typically, the extent of this effect is calculated by taking the ratio of the motor's rated
hot safe stall time to its rated cold safe stall time. The safe stall time (also known as locked
rotor time) is the time taken with the rotor not turning for the motor to heat to a
temperature beyond which motor damage occurs at an unacceptable rate. The term 'cold'
refers to starting off with the motor at ambient temperature, while 'hot' refers to starting
off with the motor at the temperature reached when running at rated load. The method
the thermal model uses to account for the pre-overload state is thus known as hot/cold
biasing.
The MM300 calculates the steady-state thermal capacity used according to the following
equation.
Eq. 3
In the above equation:
estimate) ive (conservat
230
estimate); (typical
175
2 2
LR LR
I
k
I
k = =
853729A1.CDR
0.7
0.8
0.9
1.0
0 1 2 3 4 5
Voltage unbalance (%)
NE M A
k=2
k=4
k=6
k=8
k=10
0.7
0.8
0.9
1.0
0 1 2 3 4 5
Voltage unbalance (%)
GE M ultilin
Derating
factor
Derating
factor