540 ELECTROMECHANICS
Besides the stator and rotor iron-core losses,friction and windagelosses (which are generally
functions of machine speed, and are usually assumed to be practically constant for small speed
variations) are included inno-load rotational losses, which are effectively constant. Besides
copper losses (stator and rotor windingI^2 Rlosses),stray-load losses(which arise from various
causes that are not usually accounted for, and are usually taken to be about 0.5 to 5% of the
machine output) are included for the determination of efficiency (=output/input).
Much of the considerable progress made over the years in electric machinery is due to the
improvements in the quality and characteristics of steel and insulating materials, as well as to
innovative cooling methods. Modern large turbo alternators have direct water cooling (cooling
water circulated through hollow passages in their conductors, being in direct contact with the
copper conductors) in the stator (and the rotor in a few cases) and hydrogen cooling (with
hydrogen under 1 to 5 atmospheres of pressure) in the rotor. With hydrogen under pressure,
sealing the bearings appropriately needs particular attention for turbogenerators. For hydroelectric
generators, on the other hand, designing the thrust bearings for vertical mounting becomes a
prominent issue.
In general, every machine has a nameplate attached to the frame inscribed with relevant
information regarding voltage, current, power, power factor, speed, frequency, phases, and
allowable temperature rise. The nameplate rating is thecontinuous rating, unless otherwise
specified, such asshort-time rating.Motors are rated in hp (horsepower); dc generators in kW;
and alternators and transformers in terms of kVA rather than kW (because their losses and heating
are approximately determined by the voltage and current, regardless of the power factor). The
physical size and cost of ac power-system apparatus are roughly proportional to the kVA rating.
In order to fully utilize the magnetic properties of the iron and optimize the machine design,
the machine iron is worked at fairly saturated levels of flux density, such that the normal operating
point on the open circuit is near the knee of theopen-circuit characteristic(or theno-load
saturation curve,which is similar to the magnetizationB–Hcharacteristic). Magnetic saturation
does influence the machine performance to a considerable degree. Leakage and harmonic fluxes,
which exist in addition to the mutual flux (generally assumed to be sinusoidally distributed) and
which may developparasitic torquescausing vibration and noise, also have to be considered.
Accounting for, and including, these effects becomes too involved to be discussed here.
For motors the major consideration is thetorque–speed characteristics.The requirements
of motor loads generally vary from one application to another. Some may need constant speed
or horsepower, while some others may require adjustable varying speeds with different torque
capabilities. For any motor application, the starting torque, maximum torque, and running
characteristics (along with current requirements) should be looked into.
For generators it is the volt–ampere, or voltage–load, characteristics. For machines in general,
it is also vital to know the limits between which characteristics can be varied and how to obtain
such variations. Relevant economic features, such as efficiency, power factor, relative costs, and
the effect of losses on heating and machine rating, need to be investigated.
Finally, since a generator or a motor may only be one component of a complicated modern
power system, system-related dynamic applications and behavior (both steady-state and transient-
state) and proper models to study such behavior become very important when designing electric
machines.12.6 Learning Objectives
Thelearning objectivesof this chapter are summarized here, so that the student can check whether
he or she has accomplished each of the following.