Page 237 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SYNCHRONOUS MACHINES

11.12 CHAPTER ELEVEN

hydrogen-cooled generators have operated safely. Only a few rare incidents have been
reported. The hydrogen is normally maintained at 45 to 60 psi (300 to 400 kPa) inside the
generator. It is purged using CO to avoid explosion hazard.
2
Friction losses occur in the bearings and slip rings.

Core Losses
The core losses are the sum of hysteresis and eddy current losses. They can be reduced by
decreasing the thickness of the laminations and using low-loss magnetic materials such as
iron-nickel alloys, oriented silicon steel, or amorphous magnetic materials.

Stray-Load Loss

The stray-load loss is given by the increase in core losses with the current. It is caused
by the induced losses from the leakage fluxes of the armature and variations of the flux dis-
tribution in the air gap. This loss includes the eddy current losses that occur in large arma-
ture conductors. This component of the stray-load loss is minimized by laminating the
armature conductors using bundled conductors, strip conductors, etc. A typical value of this
loss is around 1 percent of the power output.

Armature Conductor Loss
The armature conductor loss is defined as the sum of ohmic (or dc) loss and the effective
(or ac) loss in the armature conductors. The effective loss is caused by the nonuniform flux
distribution over the cross section of the conductor. This is known as the skin effect. It depends
on the cross section of the conductor and the frequency of the armature current. The skin
effect can increase the armature copper loss significantly if the conductor is large. The
armature conductors are normally laminated or segmented to reduce this loss.

Excitation Loss
The excitation loss includes the loss of the field conductor and the automatic voltage regulator
which controls the voltage at the terminals of the machine. The heat losses from the field
conductor are normally included in the armature heat losses.

REFERENCE

1. A. Syed Nasar, Handbook of Electric Machines, McGraw-Hill, New York, 1987.

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