Peak Torque

[electrolitic]

Rotary blade saw, 125kgm2,running at 1200rpm, driven by AC induction motor 610kW, 1780rpm. Motor starting torque 1.8 times Tn, motor max torque 2,7 times Tn.
VFD for no-load starting up to nominal speed. During open transition (applied VFD cannot run in parallel with the grid) a high peak torque appears caused by phase difference voltage angle (grid voltage x motor cemf), I suppose.
How to calculate this peak torque?
Thanks

 

[btrueblood]

Probably a question better asked in the Elect. Engr./Electric Motors forum.

 

[electrolitic]

Thanks. I agree but it is, in my oppinion, the mech effect of a
non synchronized electric motor braked by the grid against a rotating high inertia load that frequently breaks the machine fused coupling.
It is like the torque peak that appears when the motor connection changes from star to delta during a star/delta starting.
Anyway I will ask them.

 

[btrueblood]

As a WAG, I would use 2x the max. motor torque.

 

[electrolitic]

Motor nominal torque is 3200 Nm. Motor max torque is 2,7 Tn.
2 x 2,7 x 3200 = 17400 Nm
The coupling can be adjusted hidraulically up to 15000Nm but I
would like to keep it below 12000Nm to be protected in case of blade stoppage.
Belt driven blade as in the past is not used here. Rigid shaft do it.
I would like to know the sum of torque that appears:Motor torque{starting torque, I suppose) in one direction + rotary load torque in an opposite direction).
Is this assumption correct?

Thanks.

 

[btrueblood]

That was my WAG - complete step change reversal of motion. Dunno if it is right, valid, or nothing, just a gut feel.

 

[electricpete]

The worst-case transient torques can approach 20 times full load torque. Google motor

NEMA MG-1 suggests that the duration of interruption should be at least 1.5 times the "open circuit time constant" of the motor, in which case the residual voltage should decay to below 33% of original value so that worst case incoming plus residual voltage is 133%. This is what NEMA would recommend to prevent damage to the motor itself. To what value torque is limited in this case I'm not sure.

Open circuit time constant could probably be conservatively estimated at 1 sec for small motors.

If you deenergized for 1.5 sec you should probably eliminated a lot of problems. I think in common practice people are not as concerned about abusing small motors (such as by reclosing out of phase) as for large motors.

610kW or 610W?

=====================================
(2B)+(2B)' ?