MAIL: Induction vs. Synchronous

Confusion about the technical description of “induction generator” versus “synchronous generator”?

I have been bothered a bit by what appears to be confusion about the technical description of “induction generator” versus “synchronous generator.” Case in point: the HP96 article on the Crown Hill Farm hydro setup. An induction generator is the electrical analog of the common induction motor that is used in all power ranges from subfractional to mega-horsepower. It has a solid armature (squirrel cage) that is an electrical short circuit, and delivers electricity by slipping rpms against grid output (that is, 1,720 rpm vs. 1,800).

An induction generator would be the same device, except driven above the synchronous speed to, say, 1,850 rpm. It will not function except when connected directly to solid grid output. A synchronous generator on the other hand, has rotating poles, either DC excited or PM, and stands on its own at synchronous speed (1,800 rpm, say). It can be locked to the grid or other generators, but does not depend on them for excitation. It can be made to work from watts to gigawatts. I think the difference is worth noting; in the article in question, it is not clear which type is being employed. Sincerely,

W. Van Aller, PE and K3CZ • via email

Hello W., Your point is entirely correct, and there is an error in the sidebar on page 19 of HP96. It should have said “Induction or Synchronous?” not “Induction or Asynchronous?” Induction generators are asynchronous generators because, as you pointed out, they operate slightly above synchronous speed. For grid interconnection, induction generation is a lot easier because you don’t need a synchronizing device that ensures that frequency and phase are matched with the grid before bridging the two. An induction generator inherently has a lot of “slop” because the field current is magnetically induced in the rotor. In a synchronous generator, the field current is hard wired. Connect a synchronous generator to the grid out of phase (even if the frequency is right), and the generator will lose a fight with the grid. But an induction generator doesn’t care.

Chris Greacen

Hello W., The Crown Hill Farms turbines drive an induction generator. Canyon uses induction generators on smaller systems designed for connection to the grid. Interface is easy. In fact, if you want to waste the energy, you can first switch on an induction generator (motor), have it come up to speed, and then begin pushing it with the turbine. The water power pushes the induction motor to design speed, at which time it may begin its function as a generator.

We use synchronous generators for all our stand-alone projects. But there has been quite a bit of work using induction generators as stand-alone generators. They are made to work that way, but there are a few problems, mostly in voltage regulation. Regards,

Dan New • Canyon Industries

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