3 hours ago
05 January 2010
Buck Boost transformers
A few posts ago, I speculated about a way to lower the AC power line voltage for those half dozen of us that are having trouble with the fast charging in cold weather. It is clear that the problem is related to a supply voltage that is slightly above nominal. The car should accept this voltage and indeed it does but only in warm weather. So OK, great, we found a small problem that AC Propulsion and Mini can fix before the production car comes out. But the problem remains for six of us in the frozen north.
The cost of a transformer that can sustain 12,000 watts for my 50 amp charger is prohibitive, as is the size. It occurred to me that all that is needed is a small transformer with a 12 volt 60 amp output, wired to oppose the 240 volts from the power company feed. This is only a 720 watt transformer, much cheaper and smaller than 12,000 watts.
Well, none of my engineering colleagues had heard of this but everyone thought it should work, at least theoretically. (None of us specialize in power transmission, so what do we know? Nada.) When drawing up my idea shown above, I looked around the net and behold, there is an entire class of such devices called Buck Boost transformers. Technically they should be called "autoformers" since the primary and secondary windings are connected.
I got the call today, Mini is coming by next week to either fix my charging problem or at least test a fix. I suspect they already figured out the Buck Boost Autoformer option. We'll see.
And yes, the National Electrical Code has a whole section on how to wire the things. So it is all kosher. Well, maybe not the switch I show in my fantasy schematic. That would let you easily go back to the higher voltage in summer and charge a few minutes faster. But I have not read the code in detail to see if it is allowed. I bet not. But theoretically it should work without burning down the house.