THREE-PHASE induction motors are the workhorses of industry. About 90% of electrical motors used in industry are of this type. Single-phase induction motors running from the 240V supply are among the commonest in domestic use. All induction motors are cheap, reliable, rugged, lightweight and reasonably efficient; in fact large (> 1 MW) induction motors have efficiencies of up to 98%. Their biggest drawback hitherto has been their small range of operating speeds, though this is changing with the introduction of solid-state, variable-frequency drives. Table 17.1 shows typical characteristics for a small and a large squirrel-cage motor.
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