There are some drive systems which are so critical in process loops, stoppage for short duration (coast down to zero and then start up) is unacceptable. Does the requirement implies that machine/motor couplings are designed to withstand torque transients experienced when motors are re-accelerated following a power supply voltage dip. This is a matter of diligence.
Compliance with this requirement may lead to an unsafe situation by transmitting excessive forces from the motor to the machine which could result in significant equipment damage, loss of containment and/or injury.
High torque transients can be avoided by implementation of the correct electrical motor starter scheme. Over-design of the coupling to compensate for high torque transients is not an acceptable solution.
Re-acceleration is a method of automatically restarting process crucial drive motors after unexpected de-acceleration caused by system voltage events, such as, dips, outages, or bus transfers. Re-acceleration schemes are designed to minimize process disruptions by rapid detection of supply loss, recovery/monitoring of acceptable transient torque limits, and then automatic re-closure of the motor contractors. Depending on the connected load and minimum available fault current, the re-acceleration may be ‘instantaneous’ or a staged event.
Analysis of phenomena affecting the machine train
As the voltage dips down , the machine coasts down and due magnetic residual flux , the motor becomes a generator . As the voltage resumes within milliseconds , the inrush current is out of phase to “ Generator “ 180 degree and shall have torque impulse @5 times the starting torque .
This ripple may cause severe mechanical stress into mechanical train.
Analysis of problem (Mechanical)
If material of property and shaft FS is kept same , it shall lead to 58% increase in motor shaft diameter if evaluated on purpose of torque ripple due to out of phase currents , as σ= 16 T / π.d^3 ( using pure shear stress theory )
Next work may be check with standard NEMA frame dimensions, which specify larger shaft sizes than required .The shaft dimension can be back calculated to how much torque the motor shaft can handle and then compared with 1.58 times calculated shaft diameter.
Same is applicable for driven equipment as well.
Based on shaft diameter according to NEMA MG-1 frame size and actual driven shaft diameter, coupling hub diameter also shall found out . Couplings are rated for driver KW [P].
If we go back to coupling design calculation and take 1.75 as Service factor as mandated in API 671 , we can select a coupling meeting peak torque. T ≥ 9,550 x P/N x 1.75 Nm.
One of the machine components always should play a role of “ weak link “to meet fail safe design . Fail-safe designs are designs that incorporate various techniques to minimize losses due to system or component failures. The design assumption is that failure will eventually occur but system will fail in a safe manner.
In this case, it is imprudent to oversize the coupling; shifting the “weak link” role to pump / motor shaft.
It is important for the package vendor to undertake torsional sensitivity analysis of complete train on above aspects mentioned.
Hence, it is imperative to finalize the drive system with Intelligent Electronic Devices (IED) which can provide settings to ensure safe re-acceleration . Electrical department along with Process department shall create group of drives need re-acceleration and adequate relays are to be provided right at proposal stage . Electrical to update their motor data sheet template to include a check re-acceleration requirements.
Process to ensure details of permissive for equipment start/restart (if permissive conditions embedded in the start-up logic).
The responsibility of Package Engineer, right from RFQ at proposal stage, is to understand customer requirements and FEED specifications for statements relating to motors/package equipment such as:
‘Suitable for 100% overvoltage in order to re-accelerate after a short interruption of power supply’
‘able to withstand the mechanical stresses generated by 100% overvoltage caused by motor voltage 180 degree out of phase with incoming supply voltage.
A clear note for above shall be present in main RFQ document, motor and driven equipment data sheet and process data sheet. Same shall be applicable for PR
documents as well. During detail engineering torsional sensitivity analysis of complete train on above aspects to be reviewed.