Motor Overload Protection (i2t)
Overview
The motor overload protection (i2t) feature evaluates the “overheating” status of the motor. If the motor is overheated more than the allowed limit, this feature limits the RMS value of phase currents to protect the motor from being overheated beyond the permitted limits.
Note: This feature does not directly measure the motor temperature. Instead it estimates the accumulated copper losses inside motor windings.
Requirements
To use this feature, the following parameters must be set properly according to the motor data sheet:
| Object ID | Name | Description |
|---|---|---|
| 0x6075 | Motor rated current | Motor rated current (rms value) |
| 0x6073 | Motor maximum current | Motor maximum current (rms value) |
| 0x200A | i2t | Parameters to activate the i2t feature |
Description of Subitems for 0x200A:
| Object ID | Name | Description |
|---|---|---|
| 0x200A:1 | Enable | Enables and selects different modes of motor I2t protection |
| 0x200A:2 | Peak Time | Maximum time period [ms] in which the motor is allowed to have its maximum current |
| 0x200A:3 | Percent max | The percentage of the maximum transient energy allowable in the motor |
| 0x200A:4 | Protection active | When this flag is set, the output current is limited to equal, or less than, the nominal current |
| 0x200A:5 | Motor winding type | This object determines the winding type of the motor, which can be either concentrated or distributed. By default, concentrated winding is selected . |
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 than the maximal value, the peak time is extended accordingly.
Depending on the selected I2t mode, the following behavior is expected:
-
Motor I2t Protection: Once the calculated value of the motor thermal capacity index reaches 100%, the drive reports an error and stops.
-
Motor I2t Disabled: The I2T algorithm is disabled. In case of motor overload, no warning or error will be reported and the motor will not be protected.
-
Motor I2t Limitation: Once the calculated value of the motor thermal capacity index reaches 100%, the output current in all phases is restricted to their half of 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 50%, current limiting is turned off and the motor phase currents can reach higher values again.
To calculate losses, the RMS value of the phase current is used for electrical frequencies above 10 Hz. This applies to both types of motors (concentrated and distributed). However, for concentrated winding, the instantaneous current value is used for frequencies below 10 Hz.
Attention: The tracking variable in subitem 0x200A:3 has a maximum value of 200%. Values that exceed this limit are displayed as 200%. If the drive reaches values above 200%, the thermal energy in the motor cannot be tracked anymore!
Inverter I2t Overview
The inverter I2t protection protects the inverter in high temperature operating conditions.
Detailed Description
Internally, the servo drive measures each phase current individually. The RMS value of the motor phase currents is used to calculate the inverter thermal load.
Each phase's output has a permissible maximum accumulated heat energy. The inverter I2T algorithm roughly estimates the accumulated heat energy and is dependent on parameters such as drive phase currents, electrical frequency, and drive-measured temperature. When the inverter protection is triggered, an error is reported. This error message can be cleared before the inverter accumulated heat energy is decreased below the allowed limits.