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controlling a generator

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pyager1

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Hi, I have a propane generator with no governor on it. I would like to use the output frequency to control a pwm to an RC servo. I am really having a hard time figuring this out! Can anyone help me out?
 

MikeMl

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I built a speed-control for a small gas engine using an RC servo for the throttle butterfly. A PIC controlled the throttle and spark ignition module to vary advance. Speed was sensed off the magneto.
 

alec_t

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What range of output frequency do you expect, and at what voltage?
 

crutschow

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You could use a phase-locked-loop to do the frequency control.
That would consist of a 50/60 Hz crystal oscillator square-wave (whatever you main's frequency is) going to one input of a phase detector with the other input being the the squared-off output frequency (using a zero-crossing detector) from the generator.
The output of the phase detector would drive the PWM duty-cycle to control the throttle servo.
You will likely need some form of PID or Fuzzy Logic control loop compensation between the phase detector output and the PWM input to achieve stability and avoid hunting or oscillation of the generator speed under various loads.
A CD4046 PLL circuit has a digital type phase detector which should work well in your application.

This could all be done, including the PWM generation, with a micro of course, as Mike suggested.
 

dr pepper

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If the electronics has you confused, take a look at a briggs or similar mower engine, the governor is just a flap inlin with the engines cooling fan, a tensioned spring works against this flap, when the tension of the spring is overcome it operates a lever that shuts the throttle, its super simple. The spring tension sets the rpm of the engine.
If you want precision rpm control then the pll is required, but you'll need some tuned filtering to compensate for the acceleration of the motor.
 

alec_t

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I think the OP has got lost.
 

pyager1

New Member
Hi, sorry, I get busy doing things and forget where I'm at. !
I really would like to know how MikeMI had his set up? I wanot to keep the output at 60hz +/-2% . So my thought was to monitor the output frequency and have that be the determing factor for controlling the rpms of the engine. Knowing that the current draw will pull down the engine and in turn lower the frequency of the output. Voltage will remain correct if the frequency is at the rated output of 60hz. It's an 8Kw.
thank you for all of your help! !
Pete.
 

KeepItSimpleStupid

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Frequency and voltage regulation is totally separate issues. Frequency is RPM related and voltage regulation is done with field excitation.

RPM would need some sort of PID control.

Some sort of stability criteria is used before there is an auto transfer. You don't want to have the generator connected when your starting it, etc. Oil level usually a first order criteria for starting. Ideally, you don't want the generator trying to regulate or connected to a load until it starts and is at frequency and voltage.

Although not directly related, you have generator control panels: http://www.dieselserviceandsupply.com/Generator_Control_Panel.aspx

The kind of device your talking about is more like an ECM,You have been talking about a frequency regulator. Is the voltage regulator bad too? Is this a single or 3 phase generator? Are you planning some sort of autostart? Some sort of output OK? Is there a battery or is it self-excited?

Simplistically, a lawn mower engine, usually the throttle is full when starting and the mechanical governor prevents overspeeding. The generator would want to control RPM.

Car engines have a turn off the fuel mechanism when the throttle is to the floor and your cranking.

Then there is choke control like automobiles. You don't have vacuum and later engines had a choke unloader, of sorts. Early engines didn't, but you could "throttle" the choke. Then you had the fuel bulb that injected fuel into the carb.

so, there is probably an off, start and run throttle positions and there could be two off states. Fuel off an cranking and fuel off (flooded)

The choke is very much temperature dependent with an unload function. unloading the choke was to position it slightly open once the engine is running. That was like 1/8" in a lot of cars. Full throttle open the choke fully. Pressing the accelerator once set the choke position depending on temperature.
I've set up plenty of carbs. Later I used the propane enrichment method exclusively to set the mixture.

Your initial post was control generator RPM and not much else. Later, you sort of assumed thatRPM keeps the voltage in check. The AVR keeps the voltage in check.

An automobile has a few modes for RPM: off, start, open loop, idle, power, economy and cruise control.

It also has a charging system which is akin to the AVR of your generator.

Aside:
In some simple generators without a battery, residual magnetism is used to create voltage for the regulator usually via a separate winding until the unit gets up to speed.
The idle is a separate system because of temperature and the AC being on.
 
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pyager1

New Member
Well, it's like this. It's almost 30 years old, but works. It's a B&St 16hp single cylinder all cast iron. It has the governor shafts coming out of the crankcase, but doesn't work. It is a self excitation type generator. The only fuel shut off is a solenoid in line with the propane tank. I know RPM is what keeps the frequency correct, but being a self excited unit, it has no voltage regulation. So voltage has to take care of it self. This will not have any auto start and will not have a transfer switch because it will be in the back of the truck I have. It will just be used here and there on little job sites with no power. This is why my concern is with controlling the rpms. There isn't much else to do. Oh and there is no oil level switch. But everything you said is 100% in other cases. Just not mine. But please let anyone and everyone lend a hand.
Thank you
Pete
 

crutschow

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How about using a controller like one of these?
 

KeepItSimpleStupid

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Pete:

Thanks for all that info. I'm just the kind of guy that tends to look at everything and all scenarios, but that sure helps.

The governors, not speed controls, I'm familiar with are just a flap on the flywheel that essentially is an RPM limiter. My generator experience is extremely limited. Mikes is probably a LOT better.

My thoughts:

You have the car battery to use for a power source or at least one to use for starting. Take a look at some of the stuff at www.servocity.com and even here: https://www.pololu.com/category/102/maestro-usb-servo-controllers Their scripting controller MAY even be able t do the job. I don;t know.

You need two things A) to control the throttle and B) to measure the RPM.

An RC servo, like mike used might not be a bad idea. RPM could be optical or even a variable reluctance sensor on the flywheel magnets.
You may need to be able to select a few operational modes like: Off, Start, and Run. Start might just mean a fixed throttle position to start.

As suggested, you would need some sort of PID control. Software PID is relatively easy. I've only done it without auto-tuning.

PID means Proportional, Integral and Derivative. p is generally defined as the output from 0 to 100%. There are tuning constants that relate the output that's proportional to the error and added to the output is the constant that relates the integral of the error and the derivative of the error. When the constants are correct, the setpont and the measured value agree. With software, you have to prevent what's called reset windup which basically means don;t integrate the error term when it becomes >100%.

I just looked and found this: http://developer.mbed.org/cookbook/PID

As part of your control system, you could start the generator using the car's battery to run the electronics and then switch over to something powered by the generator output once the generator is running.
 

alec_t

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What voltage (approx) does the genny provide at ~60Hz ?
 

pyager1

New Member
About 118 on each leg. I am worried about any motors or transformers that won't be able to handle outside of 60 +/- 3hz.
I know I will never use 50% of the available output, so the engine will handle the torque.
Thank you
Pete
 
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