TELECOM Digest OnLine - Sorted: Re: 25 Hz Power

Re: 25 Hz Power

William Warren (
Fri, 10 Mar 2006 22:11:26 -0500 wrote:

> I wonder how the telephone company rectified AC power for its DC
> batteries. For large installations, a motor-generator set known as a
> rotary converter was required. Early machines would need 25Hz as
> well. Later on mercury arc rectifiers came out.

The only motor-generator sets in use when I joined the phone company
in the 1970's were used for creating 90v, 20Hz ringing voltage and +/-
110 volt coin collect/return battery. All the MG sets ran off the -48V
CO battery plant, which was floated across the -48 rectifier outputs
powered from commercial mains. These MG sets were replaced with
solid-state units in the 1980's, although I think the MG sets were
kept as standby units well into the 90's.

Some CO's also had independent alternators in case of power failure:
many of them powered by jet engines, and so powerful that they were
actually used to generate power for the electric grid during summer

> Over the years the technology of power supplies dramatically improved.
> The size and cost of power converters had marked improvement.
> [snip]

> I don't know if true, but someone told me that in Europe that didn't
> bother with three phase for house supply, but just give everyone 220
> service. That does seem to be more efficient for house supplies.

I don't know why, but Europe uses 50 Hz AC power, instead of the 60Hz we
favor in the US and Canada.


Despite the different frequency, both US/Canadian and European homes
have single-phase power; nobody bothers with three phase for homes,
because there isn't enough demand in homes to justify the added
expense of installing three-phase power.

The difference is that European homes receive a 220-volt, single-phase
feed at their electric outlets, which is, of course, twice the "110"
volt standard used on this side of the Atlantic. The higher voltage
means lower current for the same wattage, thus allowing smaller wire
sizes and concomitant savings in home construction costs.


William Warren
(Filter noise from my address for direct replies)

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