Electronics > Projects, Designs, and Technical Stuff
AC booster for startup surge (similar to grid-tie)
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jbb:
I’m assuming this is single phase?

Regarding the H Bridge:
- yes, the diodes will be critical because reverse recovery could be really high. Standard or superjunction MOSFETs will probably not be acceptable.
- the classic topology for this sort of system is a big DC bus capacitor and then the power switches (H Bridge for single phase) and then a filter inductor. This is referred to as a Voltage Source Converter (VSC).
- switching frequency will depend on the acceptable current ripple, the desired current ramp rate and the required closed loop bandwidth
- with care, the VSC can draw a little active power from the main AC circuit to top up its capacitor bank

Regarding generator paralleling without extra wires: this can be done using droop controls. Frequency droop control is used to share the active power and voltage droop control is used to share the reactive power.  If your generator has these droop controls built in it could be much easier to make it play nice with a STATCOM.
David Hess:
Thinking about it, I could design a power stage and control system which operates in parallel with an inverter to stiffen the output but that is hardly an economical solution.
jonroger:
No doubt that a one-off build isn't cost effective.   But cheap grid-tie inverters (a similar device) are < $300 on ebay.   That's much less than $1000 (plus size, weight, noise, gas) to add a second generator.
David Hess:
The problem comes down to a solution better than just using a larger inverter and I do not think there is one.

A synchronous motor is not quite the right solution for this but a doubly fed motor could do it.  More practical would be a motor-alternator and it should be possible to put one together from surplus parts for a reasonable price.  With a large rotating mass, the motor-alternator isolates much of the surge from the inverter.  Of course now the inverter has to be able to start the motor-alternator.

Easier would be to install a variable frequency drive on the motor configured to ramp it up as quickly as needed.  These have become very economical.
jonroger:
I checked and evidently it's hard to find VFDs that will take in 120VAC and output single phase 120VAC.   Do such things work well anyway (or do I need to switch to a 3 phase motor (difficult))?
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