Current load of the motor, thermal motor load, (I²xt monitoring OC6, OC8)

Application:
Sensorless monitoring of the current load of the motor or thermal motor load as a protection against overload (OC6, OC8).
This I²xt motor monitoring is especially used if the motor is not provided with integrated thermal sensors (KTY, PTC).

Description:
The thermal motor load (I²xt value) is simulated without a thermal sensor, only via an arithmetic algorithm based on the detected motor currents. A simplified method is used.
Based on this I²xt value, the two adjustable monitoring functions operate for the motor load (OC6, OC8).
The current I²xt motor load is displayed in code C0066.
The implemented I²xt monitoring function for the motor load is no full motor protection! In case of unfavourable operating conditions, the simulated I²xt value can differ from the real motor load. Changed cooling conditions may have a great influence (e.g. interrupted cooling air flow or a too hot environment).

OC6-I²xt monitoring of the motor load (Trip):
The OC6 trigger threshold can be set in code C0120:   0%...120% (0%= deactive).
If the I²xt motor load exceeds the trigger threshold set in C0120, the 'Trip' fault response is triggered (setting of the controller inhibit).
The OC6 monitoring function can be deactivated via the trigger threshold C0120 = 0%.
In case of the 9300 Servo PLC the MCTRL_bI2xtMotorloadOc6_b system variable changes to TRUE.

OC8-I²xt monitoring of the motor load (warning):
The OC8 trigger threshold can be set in code C0127:  0%...120% (0%= deactive).
If the I²xt motor load exceeds the trigger threshold set in C0127, the 'Warning' fault response is triggered. This signalises an impending overload, whereby the drive can continue to run and the machine operation will not be interrupted.
In case of the 9300 Servo PLC the MCTRL_bI2xtMotorloadOc8_b system variable changes to TRUE.
The OC8 fault response (Trip, Warning, Off) can be set in C0606, default (Warning)
The OC8 monitoring function can be deactivated via the trigger threshold C0127 = 0%.

By default, both monitoring functions are deactivated (OH6, OH8) (C0120 = C0127 = 0%).

Behaviour of the calculated I²xt motor load if both I²xt monitoring functions are deactivated (C0120=0% and C0127=0%):

When the control board supply (mains + ext. 24V supply) is switched off, the current I²xt motor load is saved with mains failure protection (operating system record). When it is switched on again, the current motor load is initialised with this value.
If one of the two monitoring functions (OH6, OH8) has been triggered due to an exceedance of the set threshold, these fault messages can only be reset if the I²xt load (C0066) has fallen below the respective threshold by 5% (5% hysteresis).

Corresponding codes:

The 100% setting in C0120 and C0127 corresponds to the permanently permissible thermal motor load based on the entered rated motor data. The thermal properties of the motor used (for information please contact the motor manufacturer) are considered by the thermal motor time constant in C0128. The time constant for the Lenze motors can be obtained from the motor catalogue.

The update of the new 'MCTRL-LOAD-I²t' function block output is not carried out in the quick 1ms cycle but in the slower main program cycle. Since changes of the motor load are principally only made very slowly, no practical disadvantages will arise.

Additional analog signals:

Additional digital signals:

Additional 9300 Servo PLC system variables:

MCTRL_bI2xtMotorloadOc6_b
MCTRL_bI2xtMotorloadOc8_b

Additional fault messages: