Motor Overload Protection (i2t)

This feature limits the overcurrent injected into the motor to prevent damage from overheating.

Overview

If the Drive generates a current above the nominal current, the heat in the motor’s surroundings accumulates which the cooling systems will have to dissipate.

If the accumulated heat exceeds the cooling system’s capabilities, the system reaches its thermal limits and the motor or the surrounding elements can be permanently damaged.

This feature will evaluate the “overheating” status of the motor. If the motor is overheated more than the allowed limit, this feature will limit the RMS value of phase currents to protect the motor from being overheated beyond the permitted limits.

Note

This feature doesn’t directly measure the motor temperature. It is only a limiting feature which doesn’t allow the motor to overheat.

Requirements

To use this feature, the following parameters must be set properly according to the motor datasheet:

Motor rated current (rms value)

Motor maximum current (rms value)

Parameters to activate the i2t feature

Description of Subitems for 0x200A:

0x200A:1

Enable: Set this flag to enable the feature

0x200A:2

Peak Time: Maximum time period [ms] in which the motor is allowed to have its maximum current

0x200A:3

Percent max: This tracking variable counts the percent of the overheating status. At 100 %, the protection will be active.

0x200A:4

Protection active: When this flag is set, the output current is limited to equal or less the nominal current

Operating principle

The difference between copper losses under rated conditions and overload conditions is calculated, and is used to limit phase currents in case of motor overheating.

Once the motor phase currents are higher than their rated values, the overheat calculation starts by integrating the difference between losses under rated and overload conditions.

When a current is injected that is above the nominal current, the tracking variable will rise. If the overcurrent is less then the maximal value the peak time is extended accordingly.

Once the tracking variable reaches 100%, the calculated thermal energy storage is reached and the output current in all phases will be limited to their rated values.

When the output current has fallen below the continuous limit, the difference between the actual current and the continuous limit current will be negative and the i2t tracking variable will decrease.

When the i2t tracking variable falls to 0, current limiting is turned off and the motor phase currents can reach higher values again.