Classic AC motors: The asynchronous generator

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DC machines are reversible in the sense that when mechanical rotational power is supplied to the shaft, you have an electrical generator. Can this also be said of AC machines? The alternator in the charging system of an automobile could be used as an AC synchronous motor if three-phase AC were applied to its stator windings (with the rectifiers removed or disconnected). The speed of such a motor would be dependent on the frequency of the AC source. Interesting too, this automotive alternator is essentially a miniature version of the large alternators used in utilities power generating stations in which case the speed of the prime mover must be carefully regulated to provide the desired 60-Hz output from the alternator. Such alternators, incidentally, are synchronous alternators—the rotor has fixed magnetic poles usually ex cited from an auxiliary DC source.

Now focus on induction machines, that is AC machines with either squirrel-cage or wound rotors so that there are no fixed poles. For the sake of simplification, think about single-phase induction motors. Such motors must be specially designed to start inasmuch as there is no starting torque associated with a simple single-phase magnetic force. However, this is not our present problem. Rather the natural question arises as to whether such a machine is also reversible—if its shaft is driven faster than its ordinary speed as a motor, does generator action take place?

It’s not easy to find a common-sense solution. You might be prone to accept the possibility of generator action, but what about phase relationships? If you already had a source of 60-Hz power and you wanted to send some of it back into the utility 60-Hz line, it wouldn’t do to merely make the connection at some random time. It would have to be ascertained that both voltage and phase were correct. Such synchronization must be performed very carefully to prevent blown fuses, activated circuit breakers, or worse. How then can an induction motor be expected to smoothly and obligingly transform itself from a motor to a generator if its shaft speed is speeded up from an external source of mechanical power? Surely, it can be argued that either there would be no generator action, or that there would be fireworks if generator action did take place.

This is not merely an academic probe; the nature of such hypothetical operation has some very practical ramifications. If generator action takes place, regenerative braking should be possible on electric vehicles. Also, some utilities will pay for power injected into the AC line. It could be much simpler to use a speeded-up inductive motor as the generator than to attempt the use of a synchronous alternator by the environrnentalist interested in tapping alternative sources of energy, such as a fast-flowing stream, or the wind.

The actual behavior of an induction motor as a function of its shaft speed is shown in FIG. 2 6. It’s helpful to associate these curves with a physical system such as a train powered by an inductive motor. At standstill and while accelerating to its running speed, high mechanical power is required from the shaft and high current is consumed by the motor to meet this demand. The motor ultimately attains its running speed. This is somewhat below synchronous speed—the actual rate of rotation of the magnetic field supplied by the stator windings. The motor cannot attain synchronous speed even at no load, because there would then be no electromagnetic torque to cause the rotor to rotate.

FIG. 26 An induction motor becomes an induction generator above-synchronous speed. If appropriate mechanical power is imparted to the shaft of a line- connected induction motor, higher than synchronous speed will cause current to be injected into the line. The motor will have become a generator. Shown are: Motor action; Running speed; Generator action

Now, suppose the train encounters a down-hill grade. This can result in additional mechanical motion imparted to the shaft, causing the motor to run at higher- than-synchronous speed. The motor now behaves as a generator, sending current back into the AC line. This appears reasonable enough, but it’s only natural to ponder the effect of the frequency of the generated current. It fortunately turns out that the generated frequency will always be 60 Hz, or whatever the AC line frequency is. Moreover, this is true no matter how much the shaft speed of the machine exceeds synchronous speed. This wonderful situation exists because the magnetic field in the rotor is induced from the current in the stator windings, rather than being the result of a permanent magnet, or of DC from an external source. Such operation justifies the name of the machine as an asynchronous generator, it’s essentially a mechanically driven induction motor.

The curves in FIG. 26 show that at synchronous speed, the practical induction motor still consumes some current and still requires some mechanical power. This is because the magnetizing current and the electrical and mechanical losses are not zero as they might be postulated to be in an ideal machine. Also, as pointed out, no torque is developed at synchronous speed so an external source of mechanical power is needed to turn the shaft. Neither the practical nor the ideal induction motor can still behave as a motor at exact synchronous speed. And, unlike the synchronous generator, the phase of the current presented to the AC line by the asynchronous generator is always automatically correct.

Electrified railroads have, indeed, made good use of the induction motor in the manner alluded to. Because of its rugged construction that dispenses with the high current-carrying brushes of DC motors, it was long hoped that it could be used in electric automobiles. This, however, had to await the development of efficient and cost effective solid-state inverters. In the near future, it’s likely that there will be considerable competition between the use of AC and DC motors for electric vehicles.

A shortcoming of the inductive generator is that it does not like to deliver cur rent into a line or load displaying a lagging power factor. Static or synchronous condensers might be used to alleviate this problem, but such remedies negate the features of easy implementation and low cost.

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