When operating the unit on a 480V(AC) mains and the mains rated voltage is not correctly parameterised, an overload of the braking resistor can occur under certain conditions (see below).
The reason for this is that the switch-on threshold of the braking transistor is too low due to the incorrectly parameterised rated mains voltage for a 480V mains under parameter 0x2540:001.
In the factory setting, parameter 0x2540:001 Mains rated voltage = 400 Vrms [1] is set.
For operation on a 480V(AC) mains, 0x2540:001 = 480 Vrms [2] must be set.
The unit-internal DC voltage thresholds for switching the braking transistor on and off (braking chopper thresholds) are set to match the mains via this parameter.
Delimitation:
The effect of 'pumping up' the DC voltage, which can be observed with some unit sizes of the i950, does not lead to an overload of the braking resistor:
'Pumping up' of the DC voltage can occur with i950 style 3 units and locked controller because the DC link is only very lightly loaded in this state and thus almost no discharge takes place.
If the DC voltage reaches the brake chopper switch-on threshold due to the pumping up, the brake transistor is switched on, the DC voltage drops accordingly and the brake transistor is switched off again. The braking transistor is therefore not permanently switched on by this pump-up effect and the braking resistor is also not overloaded by the low energy to be dissipated.
Background to the 'pump-up effect':
Normally, the rectified DC voltage is: U_DC = U_network * square root(2).
However, if X-capacitors are present in the unit and the DC link is unloaded, the DC voltage can rise to the peak value of the mains voltage (peak peak of the mains phase voltage against PE). This is approx. 15% more voltage: U_DC_max = U_mains * root(2/3)*2.
The X-capacitors are charged to the peak value with each mains wave. The X-capacitor on the plus side is charged to the positive peak value and the capacitor on the minus side is charged to the negative peak value with a time delay. Gradually, the DC link capacitor is charged. The time constant depends on the ratio of the X capacitors to the DC link capacitors. The charge current is, as a good approximation, the AC differential voltage divided by the resistance (Xc at 300Hz); e.g. Ic ~ 10mA at 480V mains voltage.
