induction motor generator regulator?

[mab2]

Hi all,

I'm trying to build an (off grid) hydro power system, and I'm planning on using a 3ph 2HP induction motor as a generator (using the C-2C excitation method to get single phase 50ish Hz ac 230v (ish) out ).

I'd like to build a regulator to 'dump' power to a resistive load when there's a surplus. Ideally the reg would use PWM at a frequency >> than the generator output, so that the load maintains a power factor of 1

I've built a trial cct using a PWM controller and an IRF740 but it has the problem that it sees the 100Hz ripple from the rectifier and instead of maintaining a near constant mark space ratio, it turns on 100% for the peaks and then off again for the zero crossings, with a bit of PWMing in between.

first question: do I really need to worry about maintaining a PF of 1 or will a regulator that trims the peaks work OK? (I've seen simple ccts online that are basically a fet & a zenner diode, but I read somewhere that PWM would be better.

2nd question: if I do need to use PWM; any ideas how to get the PWM controller to ignore the 100Hz ripple without using huge amounts of filtering (I tried using more filtering and reducing the gain of the error amp, but if the filtering was sufficient to block the 100Hz, it just caused the controller to go into low freq oscillation (like, 1-2 Hz).

 

[alec_t]

how to get the PWM controller to ignore the 100Hz ripple

Perhaps use a peak detector with a time constant of several cycles to give a steady voltage for controlling PWM duty cycle?

 

[mab2]

Perhaps use a peak detector with a time constant of several cycles to give a steady voltage for controlling PWM duty cycle?

OK, I'll have to do a bit of searching on peak detector ccts... Thanks.

I do wonder, if it has a time constant of several cycles, will it respond fast enough to sudden load changes to stop the output voltage of the generator going too high? I guess I could have a 2nd simple peak trimming reg' to keep voltage within safe limits whilst the PWM reg' catches up.

m

 

[alec_t]

Something based on this perhaps?

The peak detector is just a diode and cap, with the cap voltage decaying via a voltage-setting pot. PWM frequency = 20kHz.
Isn't response time likely to be more than a few cycles anyway, because of the mechanical inertia of the 3ph motor?

 

[mab2]

Hmm...that's interesting - that cct looks simpler than the one I built - although It was basically a diode and cap so ought to work the same. I think I need to have a look at what I've done again - probably a stupid mistake (again)

thanks

m

 

[Changwalz]

Don't worry mab2, with these mistakes, you will learn it more nicely. Now at least you have learn how to built it.