Configuration

Characteristic curves

Overview of function

The rated DC current (= maximum permissible continuous DC current) specified on the converter rating plate may be exceeded in operation. The amount by which the rated value may be exceeded and for what period are explained in more detail below.

The absolute upper limit for the absolute overload current corresponds to 1.8 times the DC current rating.

The maximum overload period depends both on the time characteristic of the overload current and on the load history of the converter and is specific to individual units.

Every overload must be preceded by an “underload” (load phase with load current < rated DC current). After the maximum permissible overload period has expired, the load current must be reduced to a value at least ? rated DC current. The dynamic overload period is made possible by a thermal monitoring function (I2t monitor) in the power section. This I²t monitor uses the time characteristic of the actual load current to calculate the time characteristic of an equivalent value for the thyristor junction temperature over ambient temperature. Converter-specific characteristics (e.g. thermal resistance and time constants) are included in the calculation. When the converter is switched on, the calculation commences with the initial values that were calculated before the converter power supply was last switched off or last failed. Allowance for ambient conditions (ambient temperature and installation altitude) can be made via a parameter setting.

The I2t monitor responds if the calculated equivalent junction temperature exceeds the permitted vale. Two alternative reactions can be parameterized:

• Alarm with reduction of armature current setpoint to rated DC current or
• Fault with shutdown of converter.

The I²t monitor can be switched off. In this case, the armature current is limited to the rated DC current.