BENSHAW RediStart EXEXMVRMX3 Series, User Manual

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If no motor information is available, a Hot/Cold ratio value of 60% is usually a good starting point.
The MX
3
adjusts the actual motor overload content based on the programmed Hot/Cold Ratio set point and the present running current of
the motor so that the accumulated motor overload content accurately tracks the thermal condition of the motor. If the motor current is
constant, the overload content eventually reaches a steady state value. This value is derived as follows:
The running OL content is also adjusted based on the derating factor due to the presence of any current imbalances and or harmonics.
If the existing motor overload content is less than the calculated running OL content, the motor overload exponentially increases the
overload content until the appropriate running overload content level is achieved. If the existing motor overload content is greater than
the calculated running OL content level, the overload exponentially cools down or decreases to the appropriate running overload content
level. The rate of the running motor overload heating or cooling is controlled by the Motor Overload Cooling Time (PFN 32) parameter.
The following diagram illustrates how the current and the Motor Overload Hot/Cold Ratio (PFN 31) parameter determine the steady state
overload content. It assumes there is no current imbalance.
At time T0, the motor current is 100%FLA and the OL H/C Ratio is set at 30%. It is assumed that the motor has been running for some
time and the motor overload content has reached a steady state value of 30% (30% H/C Ratio x 100% FLA = 30%).
At time T1, the motor current drops to 50%FLA. The motor overload content exponentially cools to a new steady state value of 15%
(30% H/C Ratio x 50% FLA = 15%).
At time T2, the OL H/C Ratio is set to 80%. The motor overload content exponentially rises to a new steady state value of 40% (80%
H/C Ratio x 50% FLA = 40%).
At time T3 the motor current rises back up to 100%FLA. The motor overload content exponentially rises to a new steady state value of
80% (80% H/C Ratio x 100% FLA= 80%).
117
7 - THEORY OF OPERATION
Motor
Current
OL H/C
Ratio
Motor
Overload
Content
100%FLA
0%FLA
50%FLA
0 %
80 %
30 %
T0 T1 T2 T3
0 %
30 %
15 %
40 %
80 %
Figure 29: Motor Overload H/C Ratio Example
Factor Derate Imbalance Current
1
FLA
Current
Ratio H/C OL OL
ss
´ ´ =