ECE major here that slept through all his power classes. Say I have a basic single-phase generator like the kind you get at Home Depot. If I have long string of irons and light bulbs (essentially purely resistive loads) and draw more than the rated wattage, what happens? My intuition says that the engine must work harder, but what is the specific mechanism that causes this.
In: Engineering
Current flowing through the coils of a generator makes an magnetic field. The more current, the stronger the magnetic field. The higher the load, the more current. This magnetic field is opposite to the one generated by the magnets of the generator.
The gasoline motor is trying to move the magnets past the magnetic field generated the the coil. The stronger this field is, the more work the gasoline motor needs to do.
Think of it through conservation of energy. If you want to output more power, then you have to work harder to produce the power.
If you’re running a generator and try to draw more then it is rated for, then it could get hot from the resistance and burn out / catch fire. Another possibility is that the voltage will drop to alow for the increase in current (same power output). Another possibility is that it cuts out because it has sensors that detect that the voltage is dropping.
The reson that the voltage would drop is a very simple proof thankfully. First, the setup:
* For your purely resistive load, V=IR
* By putting lots of lights in parralell, R is made very small, but has a set value independant of your power source
* P=VI, and P has a max (it’s rated wattage), so let’s assume that its running at max.
So we have 2 equations with 2 unknowns (V and I). Solving gives:
I=P/V=V/R => V=sqrt(PR)
V=P/I=IR => I=sqrt(P/R)
You can then see that as the resistance decreases, then the current increases and the voltage decreases.
Usually when overloading such small generators the voltage will drop and the generator will slow down and eventually stall. You can hear the difference in the generator engine as you load it up.
The generator always tries to maintain the fixed speed to output the right frequency of voltage. As you load it up it will slow down, and the engine governor will increase fuel to the engine to being the speed back to nominal.
Furthermore, increased electrical load results in increased fuel consumption.
If by “specific mechanism” you mean how does the engine compensate for increased load on the alternator, there is a simple throttle governed by engine vacuum. As speed drops, vacuum drops, this opens the throttle a little, engine speed increases to original. Therefore yes, more work is being done and more fuel is being consumed even though the difference in engine speed is barely perceptible.
From the practical side it is true.
I’m a former ECE major and also a live event audio engineer. I’ve stood next to a large deisle genny during a hardcore metal show and when the lighting director blasted the audience blinder lights you could easily hear the genny motor working harder.
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