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How do over-current and i2t protections work? [GB/CM]

Author: AM Reference Number: AA-00280 Views: 89354 Created: 14-09-2011 15:55 Last Updated: 13-05-2014 16:32 100 Rating/ 2 Voters

The I2t protection works as a long time protection and the over-current as an immediate protection.

Let's take the over-current protection first. We assume that the over-current value is 4 A for 500 ms. With this setting, a current of 4 A or higher that is maintained through the motor for more than 500 ms will trigger the protection. Now, if we think a little about the reason for the over-current protection, we come to the conclusion that this is also a thermal protection — after all, a too big current in the motor can melt the windings to a complete disaster (worst case), or the motor will stop before the winding insulation is destroyed by the temperature, resulting (only) in a short circuit.

Coming back to the settings of the over-current protection, you must keep in mind that the protection will be triggered beyond 500 ms as well, for a current of 16.5 A (because it is higher than 4 A). Needless to say that as concerns the motor there is a very big difference between 4 A for 500 ms, and 16.5 A for 500 ms. To overcome this, you must also set the current limit of the drive, so if you want to be sure that the current through the motor won't exceed 4 A, you must set the current limit to 4 A - thus ensuring a maximum current through the motor of 4 A (due to the current limit parameter) and a maximum time that this current is allowed of 500 ms (due to the over-current protection). However, in some cases, you can let the current limit to be higher than 4 A, to allow for good accelerations/decelerations.

Now, if we think one step further and imagine the worst case scenario, where the motor is supplied with 4 A for 450 ms, and then 3.9 A for 10 ms; and then again, 4 A for 450 ms and then 3.9 A and so on - the result is that the motor will overheat and burn out without having the over-current protection triggered!

In this scenario, we can't use the current limit anymore to prevent it, but we can use the  I2t  protection.The  I2t  protection keeps track of the motor temperature by monitoring the motor current and computing the over-temperature. Any current above the nominal value will produce a bigger or a smaller overheat to the motor, and - on the same basis - a current below the nominal value will allow the motor to decrease its temperature (down to the ambient temperature, in the best case = current zero = motor stopped). And this is how the  I2t works: while the motor has a current higher than the nominal one, the drive will consider that the motor temperature is increasing; and when a certain threshold (based on the settings of the I2t protection from Drive Setup dialog) is reached, the protection will be triggered.



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