Electronics Interview Question:

Is it possible to reverse the order of a Variable Frequency Drive? Ex: I want to be able to create 120v power from a variable speed engine, with at least 6kw of power.

Submitted by: Administrator

You say many alternators, but really, you need only one. The more I think about this, the more I think it is not so hard to do. If you have a 6KW alternator, it's possible that it has some kind of voltage regulation, maybe the rotor is not a permanent magnet, but an electromagnet with ring brushes, and there's a regulator circuit that drives this to keep the voltage constant regardless of frequency. If this is the case, what I would do (not many would try this) is rectify this to make 150 volts DC with a simple rectifier bridge and filter cap (total maybe $30). Then buy some 1KW inverters, and tap into them after the DC-DC converter (the part where it takes in 12VDC and steps it up to 150 VDC), drive the 150VDC in at that point, then let it switch this to AC for you. You could put 6 of these all running from the same alternator, so you could run 6 1KW loads.

Now taking this idea a step further, I would realize that the DC-DC converter is most of what you are paying for in the inverter, and it is the source of inefficiency in the inverter. So you are wasting a lot of money and watts, and so I would copy the circuit out of it that does the switching (4 power FETs and a 60 cycle driver) and have my inverter done much cheaper.
Submitted by: Administrator