Objective:
Describing how the Inverse Time Ixt (Overload) function works on 690, 890 drives to protect the motor. The AC30 Inverse time functions similarly as well.
Explanation:
This function block electronically emulates a motor thermal characteristic.
It is UL certified on a UL508a listed drive and allows users of the drive to omit the panel mounted motor overload block for each drive section.
The principle behind this function is that the drive should either fold back the current limit or trip when the motor current exceeds allowable limits for a period of time that would ordinarily overheat the motor.
The rated overload for a drive is 150% current for 60s in Constant Torque mode, and 110% current for 60s in Variable Torque mode; but the aiming point (threshold of overload detection) is set at 105%. No action is taken until the current exceeds 105%.
How can we calculate the time it would take for a drive to trip or fold back for a certain overload level then?
Example (considering CT mode):
- Rated current and time for action is 150% for 60s
- Actual overload current is 150-105 = 45%
- Ixt calculation yields 45 x 60 = 2700 A-s
- At 120% current, time taken for action = 2700/(120-105) = 180s
