Page 135 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 133

Figure 4.9 Examples of extreme distortion caused by thyristor loads on a local generator.

Rectifier harmonic currents occur in pairs such as the fifth, with neg-
ative phase sequence, and the seventh, with positive phase sequence.
Relative to the rotor the fifth will rotate in the opposite direction at six
times synchronous speed, and the seventh will rotate in the same direc-
tion as the rotor, also at six times synchronous speed. These two flux
systems with opposite rotation result in an alternating flux that is sta-
tionary with respect to the rotor. The alternating flux induces damper
winding currents that are stationary relative to the rotor, causing local-
ized extra heating. Within the rotor the effect is similar to that caused
by single-phase or unbalanced loads. Most machines are fairly tolerant
of unbalanced loads and overheating is unlikely unless the rectifier
loading (six-pulse) exceeds say 50 percent of the generator rating.
Machines with laminated pole faces are recommended for such duties.

Torque Pulsations
The alternating flux mentioned in the preceding paragraph and the
fundamental stator flux are both stationary relative to the rotor. The
reaction between these two fluxes results in a pulsating torque, the
magnitude being dependent on the spatial relationship between them.
The spatial relationship is dependent on the machine parameters and
the commutation delay angle.
The largest torque pulsations for a six pulse rectifier are at six times
the supply frequency, and for a 12-pulse rectifier at 12 times the sup-
ply frequency. They rarely attain a significant magnitude and are not
often troublesome in practice. The coupling between the flywheel and
the alternator will include some damping mechanism, and the effect
seen by the crankshaft will usually be small compared with the torque
irregularities it experiences in service. Where the rectifier loading is a
significant part of the rating of the set it would be wise to ensure that
the lowest natural frequency of the rotary system is well below the

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