EPOS4 / IDX: What is the meaning and action of the I2t protection?

Question:

• What is the so-called I2t algorithm?
• What is the controller's reaction if the drive is overloaded?

Solution:

The so-called "I2t algorithm" is a permanently updated internal counter which is in use to estimate the motor winding's temperature depending on the motor's current draw over time. The I2t algorithm protects the motor winding of overload and damage without the need of a thermal sensor.

Cross reference:

• Chapter "3.11.2 Output Current Limitation according to I2t Method" of the "EPOS4 Firmware Specification".
  
• Support Center document:
  maxon controllers: I2t Limitation & Motor winding protection

How is the winding temperature estimated?

The winding temperature estimation by the "I2t algorithm" is based on the measured "Current actual value" over time within each current control cycle and the motor's "Thermal time constant winding" and its "Nominal current" (which are configured by EPOS Studio's "Startup wizard"). 

What happens when the estimated temperature gets too high? 

Hitting the I2t limit does not(!) switch off the power stage (like probably expected by you), it just limits the motor current to the configured motor's specified "Nominal current".

• The "Nominal current" is the current which still can be applied to the motor without increasing the winding's temperature any further.
• It is possible to record the "I2t level motor" (0x3200/01) and "Current actual value averaged" (0x40D3/01) by EPOS Studio's "Data recorder". It can be nicely observed then that as soon as the I2t level's maximum of 100% is hit, the "Current actual value averaged" is reduced to the configured motor's "Nominal current".
• If the motor current is lower than the "Nominal current" later on again (due to lower torque requirements of the load), the I2t level decreases and it is possible to demand higher currents for a limited period of time later on again.

Indirect possible consequences:

• If the output current (= output torque) is limited due to hitting the maximum I2t value (of 100%), the motor will not be able follow the commanded motion profile (= acceleration, speed, deceleration) anymore, i.e. the speed will drop.
• In case "PPM - Profile Position Mode" or "CSP  -Cyclic Synchronous Position" and a reasonable configuration of the "Following error window" (0x6065) is present, hitting the I2t limit triggers a "Following error" (0x8611) in consequence.

Additional remarks:

• The I2t levels can be even checked and recorded by the object "I2t level motor" (0x3200/01) and "I2t level power stage" (0x3200/02). 
• Bit 11 "Internal limit active" of the Statusword (0x6041) is set in case an internal limit (e.g. I2t's max. level) is hit. 
• The controller's stage temperature can be checked by the object "Temperature power stage" (0x3201/01).