582 ROTATING MACHINES
TABLE 13.2.1Ranges of Standard Power Ratings for Single-Phase Induction Motors
Motor Power RangeCapacitor start 1 mhp to 10 hp
Resistance start 1 mhp to 10 hp
Two-value capacitor 1 mhp to 10 hp
Permanent-split capacitor 1 mhp to 1.5 hp
Shaded pole 1 mhp to 1.5 hp
Source:NEMA Standards Publication MG 1,Motors and Generators,New York, 1987.3002001000 20 40 60 80 100
Speed, % synchronous speedA or BCDTorque, % full-load torqueFigure 13.2.17Typical speed–torque curves for
squirrel-cage induction motors with NEMA design
classifications A, B, C, and D. (Adapted from NEMA
Standards Publication MG 10,Energy Management
Guide for Selection and Use of Polyphase Motors,
New York, 1988.)13.3 Synchronous Machines
Large ac power networks operating at a constant frequency of 60 Hz in the United States (50 Hz
in Europe) rely almost exclusively on synchronous generators to generate electric energy. They
can also have synchronous compensators or condensers at key points for reactive power control.
Generators are the largest single-unit electric machines in production, having power ratings in
the range of 1500 MVA, and we can expect machines of several thousand MVA to come into
use in future decades. Private, standby, and peak-load plants with diesel or gas-turbine prime
movers also have alternators. Non-land-based synchronous plants can be found on oil rigs, on
large aircraft with hydraulically driven alternators operating at 400 Hz, and on ships for variable
frequency supply to synchronous propeller motors. Synchronous motors provide constant speed
industrial drives with the possibility of power factor correction, although they are not often built
in small ratings, for which the induction motor is cheaper.
This section develops analytical methods of examining the steady-state performance of
synchronous machines. We first consider cylindrical-rotor machines and discuss the effects of
salient poles subsequently.