Overload Monitoring

Overload monitoring models are used to ensure Infeed Modules, Inverters and Motors do not overheat when in an overload condition (application value > continuous rated value) for a prolonged duration. The continuous power and rms current and torque values cover most conditions. However, if the cycle time is very long, the rms and average values can be minimized. The Overload Monitoring accounts for this situation, by applying an exponentially weighted time factor to the overload value (utilization factor, U).

The Pxt model is applied to Infeed Modules
- Utilization, U = P / Pcont x 100%
- Pcont is the infeed rated continuous power

The Ixt model is applied to Inverters
- Utilization, U = I / Icont x 100%
- Icont is the inverter rated continuous current

The I2t model is applied to Motors
- Utilization, U = (I / In)² x 100%
- In is the motor rated nominal current

The Pxt and Ixt overload models are based on the Overload time (tOL) and Overload cycle time (tOLC) database variables, and is represented by the following chart:

Sample Scenario: Inverter Overload Using Ixt

Inverter Rated Values
- Continuous current, Icont = 7.5A
- Peak current, Ipeak = 20A for 1s
Application
- Cycle time = 20s
- 10A peak for 1.5s during accel
- 10A peak for 1.5s during decel

Therefore, the inverter utilization, U = 10A / 7.5A x 100% = 133%. Since U > 100%, the inverter is in an overload condition. Worth noting is that 10A is only 50% of the inverter's maximum output current. As shown in the chart below, in this particular case, the Ixt value peaks at 84%, so the inverter will operate below its maximum rated internal temperature.

Next, consider the same scenario as above, except that the accel and decel durations increase from 1.5s to 3s. In this new scenario, the inverter Ixt value exceeds 100% reaching a maximum value of 107%. This implies that the inverter will overheat. Either the inverter must be upsized or the application requirements must be reduced. The main point here is that the inverter is still operating at 133% of its continuous current rating (utilization, U = 133%). The difference is the duration of the overload condition.

The actual Ixt model for the above scenario is shown below. With U = 133%, the Ixt value reaches 100% at 2.9s, which is consistent with the results above.