Page 233 - Offshore Electrical Engineering Manual

220 CHAPTER 3 Motor Faults and Protection

ADDITIONAL PROTECTION FOR SYNCHRONOUS MOTORS

The following additional protection devices may be required for synchronous
motors:-

1. Pull-out protection
If the compression torque for a reciprocating gas reinjection compressor, for
example, exceeds the motor pull-out torque, the motor comes out of synchro-
nism and stalls. To prevent the motor continuing to draw a heavy stator current,
both the motor supply and the field supply are tripped.
2. Damper winding protection
The damper windings are often designed to allow the machine to be run up as
an induction motor. However, if an excessive number of starts are attempted
in succession, the windings will overheat. A thermal relay should be provided,
which will trip the motor supply before any damage is done.
3. Reverse power
If there is a disturbance in the supply system or if the supply fails, the machine
will generate using the inertia of the drive train for power. In this situation, the
motor should be disconnected if the stability of the system is not regained after
a short (timed) interval.

DETECTION OF MOTOR FAULTS ON LARGE MOTORS WITH THE
ROGOWSKI COILS

The basis of motor current signal analysis is that the stator current contains cur-
rent components directly linked to rotating flux components caused by electrical or
mechanical faults. These harmonic current components caused by faults can be used
for early failure detection. The Rogowski coil is a uniformly wound coil on a non-
magnetic former of constant cross-sectional area formed in a closed loop. The coil
surrounds the primary current conductor to produce a voltage that is proportional
to the time derivative of the primary current and the mutual inductance of the coil.
The Rogowski coils do not have ferromagnetic material in their cores, which means
that they will never be saturated. An integration module may be used for fast Fourier
transform analysis, where the large motor operates at a single speed. However, this is
a disadvantage with a VSD. A band-pass filter is utilised to concentrate analysis on
the harmonics which indicate the presence of faults, such as broken rotor bars. Refer
to Reference 7.