Page 349 - Compression Machinery for Oil and Gas

334 SECTION II Types of Equipment

l Synchronous motors have larger air gap. In induction motors, the electro-
magnetic force induced in the rotor winding is mutually induced electro-
magnetic force. If the air gap is large, then the leakage flux will increase
and the mutual flux would reduce. As a consequence, the rotor electromag-
netic force and torque would be reduced. In a synchronous motor, the mag-
netic flux is derived separately from the field winding at the rotor. The
electromagnetic force induced in the stator armature winding is a dynami-
cally induced electromagnetic force due to relative motion between the field
and the conductors. The air gap can be larger and noise and vibration are
generally less than with the induction motors.
l When the synchronous motor is overexcited, it generates reactive power,
which improves overall consumption power factor of the plant. Power factor
will have a significant impact on the electric utility bill costs. Electric utility
companies have a minimum power factor threshold, typically 0.9, that
industrial customers must maintain in order to prevent additional power fac-
tor charges. Synchronous motors help improve overall power factor and may
eliminate the need of power factor correction equipment, for example,
capacitor banks.

Current Pulsation
In a synchronous motor, AC power is supplied to the stator to generate a rotating
magnetic field. DC power is supplied to the rotor which results in discrete North
(N) and South (S) poles. The poles in the rotor then lock onto (synchronize) and
follow the opposing rotating magnetic pole (N follows S). At zero load, they
follow exactly, but at load they follow slightly behind by a load angle which
varies between 0 electrical degrees at zero load to typically between 20 and
30 degrees at 100% load and approximately 70 degrees at stall. Fig. 7.21 shows
example where load angle at full load is 32 degrees. There are 180 electrical
degrees between each adjacent N and S pole. So take the previous example
where the torque variation was 40%, the torque would vary between 60%
and 140% and the magnetic lag would vary between 0.6 32¼19.2 degrees
and 1.4 32¼44.8 degrees. In a synchronous motor, however, the exciting
amps are varied to keep the power factor constant with load and so the amps
would also vary between 60% and 140% nameplate, the average amps would
be 100%, average power 100%. The current pulsation would be (140–60)/
100¼80%. So in this case, the NEMA limit of 66% current pulsation is ade-
quate to protect the motor because a synchronous motor is less affected by tor-
que pulsations. API 618 also recommends 66% as a current pulsation limit.

Natural Frequency
Note that the rotor lags the stator magnetic by an amount proportional to the
torque. The magnetic field acts as a spring and the rotor inertia and drive inertia
will have a natural frequency that is equal to: