<P> In recent years, alternator regulators are linked to the vehicle's computer system and various factors including air temperature obtained from the intake air temperature sensor, battery temperature sensor and engine load are evaluated in adjusting the voltage supplied by the alternator . </P> <P> Older automobiles with minimal lighting may have had an alternator capable of producing only 30 Amps . Typical passenger car and light truck alternators are rated around 50--70 A, though higher ratings are becoming more common, especially as there is more load on the vehicle's electrical system with air conditioning, electric power steering and other electrical systems . Very large alternators used on buses, heavy equipment or emergency vehicles may produce 300 A. Semi-trucks usually have alternators which output 140 A. Very large alternators may be water - cooled or oil - cooled . </P> <P> Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges . Efficiency reduces dramatically at high speeds mainly due to fan resistance . At medium speeds efficiency of today's alternators is 70 - 80% . This betters very small high - performance permanent magnet alternators, such as those used for bicycle lighting systems, which achieve an efficiency around 60% . Larger permanent magnet electric machines (that can operate as motors or alternators) can achieve today much higher efficiencies . Pellegrino et al., for instance, propose not particularly expensive designs that show ample regions in which efficiency is above 96% . Large AC generators used in power stations run at carefully controlled speeds and have no constraints on size or weight . They have very high efficiencies as high as 98% . </P> <P> Hybrid automobiles replace the separate alternator and starter motor with one or more combined motor / generator (s) (M / Gs) that start the internal combustion engine, provide some or all of the mechanical power to the wheels, and charge a large storage battery . When more than one M / G is present, as in the Hybrid Synergy Drive used in the Toyota Prius and others, one may operate as a generator and feed the other as a motor, providing an electromechanical path for some of the engine power to flow to the wheels . These motor / generators have considerably more powerful electronic devices for their control than the automotive alternator described above . </P>

Is a car starter motor ac or dc