The following diagram illustrates how the current and the Motor Overload Hot/Cold Ratio parameter determines 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© 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© 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© Ratio x 50% FLA = 15%).
At time T2, the OL H© Ratio is set to 80%. The motor overload content exponentially rises to a new steady state value of 40% (80%
H© 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© Ratio x 100% FLA= 80%).
7.1.7 Separate Starting and Running Motor Overload Settings
If desired, separate overload classes can be programmed for use during starting and during running. The motor overload protection may
also be disabled during starting or during normal running. In order to enable separate overload settings the Independent
Starting/Running Overload parameter needs to be set to "On" to allow independent overload operation. Once set to "On", the individual
Motor Starting Overload Class and Motor Running Overload Class parameters can be set to either "Off" or the desired overload class
settings.
The Motor Starting Overload Class parameter value is used for the motor overload calculations when the starter is starting the motor
(kick mode, acceleration, and running before up-to-speed has been declared). Once the motor has reached full speed and during
deceleration or braking, the Motor Running Overload Class is used for the motor overload calculations. As the motor protection curves
shift from the acceleration curve to the running curve, the accumulated overload content is retained to provide a seamless transition from
one mode of operation to the other.
Disabling the Starting OL function or using a higher OL class for the Starting OL can be useful on extremely high inertial loads such as
large centrifuges or high friction loads that require very long starting periods.
z NOTE: When the Independent Starting/Running Overload (P44 / PFN 13) parameter is set to "OFF", the running OL is used at all
times.
z NOTE: The Hot/Cold motor compensation is still active when either the starting or running overload is disabled. Therefore the
motor overload content may still slowly increase or decrease depending on the measured motor current. However if the motor overload
is disabled, the motor overload content is limited to a maximum of 99%. Therefore, a motor overload trip can not occur.
CAUTION: When both overloads are disabled, the accumulated overload content is set to zero (0%) and the starter does not provide
any motor overload protection. External motor overload protection must be provided to prevent motor damage and/or the risk of fire in
the case of a motor overload.
136
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 27: Motor Overload H© Ratio Example
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