- It is forbidden to connected capacitors between Altistart and motor.

- In any case capacitors must be connected upstream Altistart (in the supply network). The further capacitors from Altistart are, the better.

- Some drawback of capacitor bank connected to network:
1) When capacitor bank is connected to network, overvoltage in the network (at the place where capacitors are connecting) or resonance can occur due to resonant circuit by inductance of the cables with these capacitors. Overvoltage in supply network is not good for thyristors. Thyristor voltage level is 1600V (ATS...Q)
2) Furthermore, synchronization for firing thyristor can be lost. Picture attached shows bad example when cable inductance and capacitor battery are close to resonance.

SOLUTIONS

If capacitors for PFC are required, there are two solutions (A,B):

A. Connect capacitors to network after bypassing Altistart. Starting process should consist of following sequence:
a) Disconnect all capacitors from network.
b) connect supply to Altistart
c) Start the motor via Altistart
d) After start finishes, bypass Altistart using bypass contactor
e) connect capacitors to the network.

B. Connect capacitors to the network before you connect voltage on Altistart. Thus, you will reduce overvoltage on Altistart or risk of losing synchronization during connecting capacitors. Be sure that PFC capacitors are not very close to Altistart to avoid excessive current peaks to power factor correction capacitors due to presence of harmonics during acceleration/deceleration.

In both cases you will need at least two contactors (one for Altistart supply and second for capacitor battery). If you decide for solution 1, you need one contactor more (for Altistart bypassing). As written at the beginning, the further capacitors from Altistart are, the lower risk of losing synchronization is.

Udgivet til: Schneider Electric Danmark