Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

466 Centrifugal Pumps: Design and Application
Figure 18-27. Measured coupling torque during synchronous motor startup.
it starts to zero frequency when it is synchronized with the line at operating
speed.
Reliability Criteria. The overall system reliability depends upon the location
of the torsional natural frequencies with regard to the potential excitation
frequencies. An interference diagram generated for each system
helps to identify coincidences between expected excitation frequencies
and torsional natural frequencies within the operating speed range.
Whenever practical, the coupling torsional stiffness and inertia properties
should be selected to avoid any interferences within the desired speed
range. If resonances cannot be avoided, coupling selection can be optimized
based on torsional shaft stress calculations as well as location of
critical speeds.
When coupling changes are implemented that have different weights
than the vendor originally specified, the lateral critical speeds may be
affected. Generally, heavier couplings lower the lateral critical speeds,
while lighter couplings raise them. The lateral critical speeds should be
reviewed to evaluate the possibility of operating near a lateral response
that could result in high radial vibrations.
The acceptability of the torsional system is determined by comparison
to typical engineering criteria. Common criteria used for industrial machinery
(API Standards) recommend separation of the torsional critical