Dear experts,

In an existing plant we have a big machine with high inertia that it is moved back and forth by 6 squirrel cage motors.

Each motor is 400kW / 690Volts and controlled by a Motor Module 6SL3320-1TG37-4AA0 (710kW / 735A).

All six inverters are connected to a common DC-BUS supplied by three Smart Line Modules  6SL3330-6TG41-7AA0 connected in parallel.

We've had a problem during blackout because the machine could not brake the kinetic energy back to the network and heavy damage was sustained.

To prevent such an event in the future we want to:

- install in each inverter three braking modules 6SL3300-1AH32-5BA0 (P15=250kW) and connect braking resistor 6SL3000-1BH32-5AA0 (P15=250kW). Total 18 choppers and 18 resistors (3 for each inverter/motor);

- the temperature switch of braking resistor wired to inhibit the braking module in case of overtemperature;

- provide UPS power supply for auxiliaries (CU320, SMC, 24V power supply, PLC, fieldbus, etc).

- the Vdc controller of each inverter is deactivated (p1240=0)

The network voltage is 690V ±10% 50Hz.

The torque overload during acceleration and deceleration is roughly 180% for 2 seconds.

The idea is: During normal operation the Smart Line Modules will do the braking because the machine brakes very often. The VDC usually stays within 1035Vdc (I think) and the braking modules should not activate. 

In the event of a blackout or Smart Line Module trip the upstream AC circuit breaker will open, the braking modules will start braking at 1158Vdc threshold. 

All sounds good, but I have the feeling that I am missing something.

So if you have any comments or suggestions, they will be greatly appreciated.

Manny thanks for your time,

Gigi.