We have 10 DC motors are controlling from Siemens-DC Drives -6RA24. There is only one drive & these all motors are Armature controlled and connected on same DC bus controlled by DC Drives. . Fields of all 10 DC motors are seperately excited. 2 nos of Tacho connected for feedback on 2 Motors out of these 10.. One tacho is selected at a time and used for feedback to the drives..
Now the problem is that when we changeover the tacho from tacho-1 to tacho-2 all 10 motors armature fuse blown out.
But when we changeover the tacho from tacho-2 to tacho-1 Drives working fine and none of the fuse blown out.

Also Drives working fine on either tacho selection, problem comes only during tacho changeover..

Note: Both tacho feedback voltages are almost same.

Can somebody help me . Thanks in advance.

Hello micron_76

Welcome to the forum.

The simple answer is to not switch tachos while the drives are running.

It sounds to me as though there is a problem with the switching which is causing the drives to suddenly supply full output voltage to the armatures. Perhaps there is an opne circuit during switching one way, or there is a "negative" spike. Very hard to say from here.
Perhaps someone else has experienced this before.
I assume that this used to work OK??

Best regards,