3 phase Vrs 1 Phase Power

Rotary converters, fascinating machines, there is (or was) a website with pictures detailing the rotary converters used for the NY(?) subway system. If I remember correctly, a few of them were still in use at least into the late 1990s. Those were huge machines that ran from 25Hz AC and produced DC for the traction motors. Not only was the stator winding common, but the rotor windings as well. They had commutator and slip rings on the same shaft. The clever thing about these things is that they could be used either way, to convert AC to DC or DC to AC.

The electric lawnmowers I've seen all had series wound universal motors with brushes, and the cordless ones had permanent magnet DC motors. I've toyed with the idea of converting an ordinary lawnmower to an AC induction motor, friend of mine did it years ago with a surplus 2HP water pump motor. Those universal motors are almost as noisy as the gasoline engines and have a tendency to burn out rather spectacularly. If I found a deal on a nice beefy BLDC motor that would be nifty in that application too.

You're probably thinking of this site:

's amazing to realize that not just the last of these machines but also the supporting switchgear operated for nearly a century before the final conversion to solid state hardware. I've never seen any information on whether the New York subways ever installed mercury arc rectifiers, which was the technology between rotaries and solid state.

For a good explanation of these machines, go to Google Books and seach for "Theoretical Elements of Electrical Engineering," by Charles Proteus Steinmetz, 3rd edition, 1909.

--------- Essentially going back about 30-60 years when this was normal- advantages to both utility and customer arose due to the utility being able to predict future demand reasonably well I.e. deregulation in the power industry, a relatively recent phenomena has, in most situations, increased cost to the consumer because it encourages short term economic planning because of uncertainty of future loads with multiple suppliers in a region.

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Nothing has changed in this area. large customers will have enough load to take advantage of the actual simplicity of 3 phase and it is also to the customer's benefit to have balanced loads. In addition, large customers generally have larger motor loads and single phase motors simply don't cut the mustard compared to polyphase machines in terms of simplicity, performance and also with regard to harmonics when electronic control is used.

From its behaviour under load my lawnmower is not a series motor. It behaves much like a conventional DC motor with a PM field - this is close to that of a brushless machine and, since it is AC driven, It could well be a rectifier supplied DC motor with a PM field but I suspect that it is not (I haven't taken it apart and have misplaced any manual with information). A series motor will bog down under load, trading speed for torque- as a result, it chews heavy grass but is hard to stall. My lawnmower does exhibit some speed variation under load (have to check as due to medical conditions, I haven't been pushing it) but will then stall. Noise is not a problem and it is really difficult to take the cover off so brush replacement is something that has not been considered. I remember my brother converting a manual reel lawn mower by mounting a 1/4 HP(?) single phase induction motor to drive the reel- worked very well but was bloody heavy (2HP is overkill) Your description of the construction of a rotary converter is in line with what I remember of them The rotary converter that I met in a lab in 1954 was not what I would call a "sweet" machine. It combined the worst of a synchronous machine and a DC machine. Thankfully that was the only contact that I had with one of them. As I recall. the lab deteriorated into trying to adjust the breakers on each side so that they tripped simultaneously. I'm sure that the NY subway system had things worked out better. Note also that in 1954 the amplidyne was still "hot" in academic circles while industry had pretty well moved on to the magnetic amplifier (somewhat less interesting as a device but stationary). Have you seen either these days?

I take that back, I was thinking of something else, the electronic lawnmowers I've seen had permanent magnet DC motors with a rectifier in series, often just a single diode. I've never had to replace the brushes on anything like that, usually the rotor windings burn out before the brushes ever fail.

Another fun was is amplidynes. Used in some generating equipment for the exciter. A small change in field coil would cause a large change in a short-circuited armature's current. This armature current would create a large magnetic field that was displaced 90 electrical degrees from the original field coils, and would induce current in the output stage (somehow, I've forgotten the details).

daestrom