February 2014 Archives

Public Electric Car Chargers

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The power costs associated with public electric car chargers is fairly low; the infrastructure to support charging stands is not cheap, however.

The copper wire costs a bit, and the density is small. Additionally, due to the installation of the wiring in a public place, security must be provided to prevent damage. The stands themselves must incorporate safety features as well as a metering/authentication interface.

Were I looking to implement a public car charger system, I would be providing 20A 120V outlets and 208V or 240V 30A outlets.

Common datacenter PDUs provide the sort of metering interface necessary, and support single leg, dual leg, and three leg monitoring pickups. The same PDUs have internal circuit breakers, providing a manual or secondary means of protecting the circuit. It is assumed that a circuit breaker would be present at the service location for the end user to manually enable the power. Remotely operable circuit breakers may be used as well; these type of breakers are commonly used in remote controlled electric meters by public utilities.

One cannot assume that a breaker externally available to the end-user would not be held in the closed position in the event of a failure. A desperate customer may attempt to charge a car even if the current pulled through the circuit would exceed the protection ratings. Furthermore, one must also consider if the user would attempt to connect two 120V outlets from adjoining stands to get 208-240V for a higher power charger. While it is possible to do so, it would be a NEC violation.

I would implement the 120V connections as a grounded-wye configuration, and provide the 240V outlets as a grounded-wye from a 240V/416V wye transformer. This provides 240V, maximizing the available power through the circuit, while providing a common, single-phase interface at a compatible voltage to what may be delivered in the home.

A common denominator of a connector is needed as a standard charging interface. Tesla appears to have solved this.

Atmospheric Microwave Fading

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Everything you ever wanted to know about atmospheric diffraction of microwave signals.


This report, paid for by your tax dollars, details a survey done over open water to see where the RF goes during certain atmospheric conditions. The link used four receive dishes connected to a multiple-input diversity combiner system and the results cataloged. I highly recommend reading this paper if you do anything connected with radio communication. The golden nugget to understand here is my long title listed above. The refractive index of the atmosphere changes, and as it changes, the path available for RF changes from the transmit site to the receive site, necessitating multiple receive locations and associated wiring. Of course, all of this wraps together to increase the availability of the communications link or circuit and results in less expensive downtime.

Local copy: http://blog.catonic.us/kirby/Reference/90-256_ocr.pdf

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