Temperature model for the external resistor

The surplus energy is converted into thermal energy via the external resistor. In the event of an error or in case of large motors, there is a possibility of the resistor overheating, which can damage the resistor itself as well as adjacent components. For this reason a temperature model has been integrated in the Bus Terminal that simulates the temperature of the resistor and, if a set temperature is exceeded, switches the resistor off until the temperature falls below the threshold value for the warning bit.
The switch-off can be deactivated via the CoE data (“Disable Chopper on Overtemperature”). In this case chopping also continues in the case of overheating.

The temperature model is adjusted via the CoE data for the resistor employed:

• The “Ambient temperature offset” parameter should be set to the ambient temperature of the external resistor. The temperature model calculates the difference to the ambient temperature due to the power dissipation. In order to be able to estimate the actual temperature, the ambient temperature is set by this parameter.
• The thermal resistance of the ballast resistor to the environment is set by the “Absolute thermal resistance” parameter. This value is required for the calculation of the temperature.
  When the ZB8110 is used, the value is 1300 m°C/W.
• Analogous to the previous parameter the thermal time constant of the resistor is set by the “Thermal time constant”. The thermal resistance and the thermal time constant can be taken from the data sheet for many resistors. If this is not the case, the data can also be determined manually by the user by means of a diagram or a measurement. (See the Section Measuring the resistance for this). When the ZB8110 is used, the value is 550 s.
• The “I2T warn level” parameter specifies the threshold value for the warning bit in the process data and the LED on the front panel of the Bus Terminal. In the case that the “Disable chopper on overtemperature” bit is set (default), the lower end of the switch-off hysteresis loop of the temperature model is defined by this parameter. The upper end is the “I2T error level” parameter.
• The switch-off threshold for the temperature model is defined by the “I2T-Model” setting value. If the “Disable chopper on overtemperature” bit is activated and this temperature is exceeded, the resistor is not switched on again until the calculated temperature in the temperature model has fallen below the “I2T warn level” threshold. If the temperature is allowed to rise still further due to the settings in the CoE data, the overtemperature is signaled via the “Error” LED on the front panel and the corresponding bit is output in the process data.