Electronics > Mechanical & Automation Engineering

I’m toying with the idea of a car engine + generator for power backup

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langwadt:

--- Quote from: AaronD on August 07, 2022, 11:56:28 am ---
--- Quote from: Berni on August 07, 2022, 09:30:24 am ---There is no need to adjust the throttle because the the cruise control does the job on its own. It will keep the wheels turning at exactly the same speed no matter the load put on it, so the more load you put on the generator the more the throttle opens to keep it going. You also don't need to hit exactly 50/60 Hz, going +/- 10Hz is going to work just fine.

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The cruise controls that I've used aren't exactly "precision".  In terms of PID control (not the only way to make a control system, but very common), I wouldn't be surprised if they were purely proportional (throttle = gain x error), with low enough gain to guarantee that it doesn't oscillate ever.  That goes for both vacuum and electric ones.  The speed does droop with increasing load (non-zero throttle requires non-zero error), though it might manage to keep +/-10% at highway speed.

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you also want it to be a bit sluggish so that if you hit resume with it set at 130km/h while doing 20km/h it doesn't just floor it

Ed.Kloonk:

--- Quote from: james_s on August 07, 2022, 07:11:58 pm ---I think a cruise control is going to be precise enough for this application, there is quite a lot of mechanical inertia in such an arrangement and the governors on conventional portable generators aren't exactly precision devices. It's normal for the frequency to be around 64 Hz at light load and 56 Hz under load. For most things this doesn't really matter.

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Yeah.
I think that if our hero has only a generator that requires a car to spin it, running old mechanical clocks to the precise second or perhaps listening to a vinyl record on an old turntable at the correct speed is prolly among the least of things to worry about.

A good reason might be when using a radio that still gets it sync from the line frequency. If the radio is out of tune, you might miss the broadcast announcement from the truth ministry instructing you to prepare to be put on the train bound fo...

Ahem.

Berni:
The cruise control in my 2013 Volvo keeps the speed nailed pretty much nailed on the set speed. The dash is digital and so you can see how the spedometer needle stays perfectly covered up over the green set speed marker.

They tend to use the old "fade in" control trick to get it to behave more comfortably. You don't need to instantly give all throttle control to the PID loop, but instead you can gradually ramp up how much it is allowed to affect the throttle. That way is slowly starts accelerating and it gives the PID loop some time to get settled in before it fully takes control. This trick can be used on most PID loops to smoothly switch them on without the whole thing going crazy in its first moments.

If you are to run a generator using cruise control and suddenly go from no load to turning on a 10kW flowtrough shower heater then yeah pretty sure it will not keep the the speed perfectly steady, likely dropping in RPM a lot before recovering back up, that would happen to all generators because the throttle response is not instant, so it has to rely on the inertia of the system to fill in the gap a bit, in order to get kinetic energy from inertia back you have to slow down the RPM. But as long as the engine doesn't stall and quickly recovers it is fine. The small generators indeed tend to have very simple mechanical governors and they work well enough for the job.

AaronD:
I may have mentioned before (didn't go back and look) that at one time I was thinking about making a temporary generator with my Jeep.  The idea was to drive with the front axle, using the 4x4 setting, and "somehow" spin the generator from where the rear driveshaft used to be.  5-speed manual transmission: 4th is 1:1, 5th is overdrive.

I also happen to have the remains of a failed aftermarket cruise control, so I was going to resurrect it with my own electronics and software.  Given that, I was wondering about the wisdom of adding a feedforward component based on either an ammeter or a torque sensor (engine- or generator-mounts on load cells?)...and that's about where I abandoned it.

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