DC braking with a soft-starter of a wood chipper

As to DC Injection Braking:
Siemens adds that contactor arrangement to attempt to even out the thermal stress on the motor windings and the SCRs in the soft starter, because they are using the same SCRs for the DCIB as they are for soft starting. Other soft starter designs add 2 more pairs of SCRs for handling the braking. Adding that contactor does nothing toward increasing the braking torque or anything.

All electronic braking methods in some way convert the kinetic energy of the rotating mass into some other form of energy, where it is dissipated or used elsewhere. DCIB has a drawback compared to Dynamic Braking in that the kinetic energy of that mass is converter to heat IN THE MOTOR, so that is a primary concern when using this. The quick and dirty rule of thumb is to count each braking cycle as if it is a motor starting cycle. So when you put that up against the Starts-per-Hour rating of the motor, it effectively cuts that in half. When you get into really large motors, i.e. wood chippers (typically), you have to get that S-p-H rating information on the motor to be safe. Sometimes it is as low as ONE Start-per-Hour!

That said, I have done a 900HP 480V whole-log chipper with a Soft Starter / Brake unit. They only braked it during production runs, because when the sawyers needed to change knives on the chipper head, they were waiting 25-35 minutes for it to coast to a stop. But because the motor would run for 3-4hours after starting and it took them an hour to change the knives, it didn't seem to have a big negative effect on the motor. We were able to stop it with DCIB in under 3 minutes, which increased their production by basically 1 hour every 2 shifts. So it was definitely worth doing despite the risks.

GaryS said:

That's DC Injection Braking
in this case the soft start has the ability to do the braking
the pulsing makes me think they are just turning on one SCR on phase 1 and phase 2
why the Q93 contactor it looks like they are trying to pass the DC current through all 3 motor phases
they must have a miss print ( direct current in phases L1 and L2)
L1 and L2 are the line side i think they mean U1 and V1 on the motor
but the 33% braking torque is standard and braking torque reduces as the motor speed slows down

Click to expand...

In a delta configured motor, two of the motor phases will be conducting.
In a star configured motor, three of the motor phases will be conducting, one with twice the current of the other two.


In both cases, all three motor terminals shown (U1,V1,W1) will be conducting.


Edit: I see I should read the entire thread before posting, as this point is already covered. Happy Friday.

Think about the electric flow\in Delta all 3 motor leads T1,T2,T3 would have th same current in each. Each motor winding would have the same current flow because each leg would have 2 paths for the current to travel
in Star you are correct the current flow would be unbalanced
dose anybody know if Siemens has 6 SCR's or 3 SCR's and 3 Diodes per leg on their soft starts it would make a big difference in the current available and the current flow
bot the breaking. years ago most soft starts used only 3 SCR's

I added motor diagram if it help figure out what happen when adding the contactor. For Y it seem obvious that it connect 1 more winding to the dc injection but for a Delta, it cancel 1 winding and give more current to the last one...

Siemens support told us that adding the contactor can permit to stop almost 2 time faster: (adjust time to about 3-4x the time it take to start for just DC injection but 2times if using the contactor)
and this with less current when using contactor. I did ask to confirm the last part because when i look at the drawing below, closing the contactor seem to be asking for more amps???