Technical information

8 – THEORY OF OPERATION
Motor Overload
114
content level. The rate of the running motor overload heating or cooling is controlled by the Motor Overload Cooling Time
parameter.
The following diagram illustrates how the current and the Motor Overload Hot/Cold Ratio parameter determine the steady state
overload content. It assumes there is no current imbalance.
Figure 15 – Motor Overload H/C Ratio Example
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 %
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%).
8.1.7 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 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.