# The Good, the Bad and the Noisy

##### Small power generators, Part 1: The big picture of the small end of power generation

Small generators are characterized generally in that they are most often powered by a single cylinder engine using an Otto/four-cycle configuration with gasoline as fuel, operating at 3,600 RPM. Further, in portable units (carry or roll about), their power capability is 10 kW or less, contrasted to permanently installed small units, which are in the 25 kW or less capability.

Generating electricity through a mechanical process was first recognized by Faraday in 1851, when he passed a magnet through a coil of wire. Sliding the magnet in caused his primitive galvanometer to move first one way, and pulling it out caused the meter to move to the opposite voltage polarity. Increasing the strength of the magnet or enlarging the coil by turns increased the fields, hence the power available — all important concepts.

“Modern” generators are really no different, and whether you move the coil or the magnet (or both in the unusual compound generator), that’s still how it is done.

In elegant simplicity, what we have, then, is a small engine directly connected to an alternator, although the devil, as usual, is truly in the details.

Keeping with a KISS philosophy — in the typical generator, the magnet spins (the rotor) and power is taken from a stationary coil (the stator). As our goal is to replicate as closely as possible the commercial power we use, the typical output is 120/240 volt at 60 Hz.

A two-pole alternator (north and south to make an alteration) turning at 3,600 RPM accomplishes this requirement. The math works out to be: 3,600 RPM divided by 60 seconds equals 60 Hz. If we have a deluxe long-run or continuous-duty unit, these are most often 4-pole configuration alternators running at 1,800 RPM.

Many major generator manufacturers supply small units in 25, 50 and 400 Hz frequencies, which use unique speeds of 1,500, 1,600, 2,400, 2,600 or 3,000 RPM, depending on the alternator configuration and the gearing. 3,600 RPM in small, one-cylinder engines is the sweet spot for fuel economy and power output, but not MTBF nor torque.

 Fig. 1: A Fundamental Alternator

Functionally, a pair of bar magnets is wrapped in a coil of wire to form the rotor. To minimize mass and enhance efficiency, this “field coil” is wrapped tightly around the magnets. The field induced by a DC voltage maximizes the magnetic field and thus the induced current in the stator. The DC voltage is coupled to the field coil by slip ring brushes tracking/riding on insulated shaft rings (see Fig. 1).

INERTIA

That addresses the voltage and frequency concerns, but what about varying load, which is recognized as varying current demand? Let us digress for a moment and consider two elements of related motion physics: inertia and momentum.

In the instant case, inertia is the resistance of the mass of the rotor to change velocity, and momentum is the resistance of the rotor’s mass to not change its velocity. These factors are important, as any change in velocity will affect frequency and the available power output from any generator. Simply put, small generators are challenged to stay on speed, and hence frequency, with varying loads.

Not only do we need to be concerned with just the rotor mass, but also the force fields presented against it.

 Fig. 2: A typical small generator, in this case a 7.5 kW two-cylinder 120/240-volt single-phase Coleman.

Let’s take the extreme case: no load to full load. With little load on the generator, the impedance of the load is very high — infinite in the case of an open circuit. So very little current flow is required, and in compensation, very little energy is needed to overcome the counter magnetism of the coil.

Full load then puts a low impedance on the coil, and hence more currents flow and more energy from the engine is needed to supply the energy needed for the power supplied/required.

So, the design engineer then must decide how much variation in frequency can be tolerated and how fast the correction has to be. Tradeoffs are inevitable and compromises are involved.

A critical number in this design is the available horsepower. The typical engine on a 5 kW generator is between about 9 and 11 horsepower. The calculation is somewhat complex, but for the first of many times we will encounter that issue with small generators — you get what you pay for.  The dynamic response to changing loads is helped by more horsepower, which you pay for!

Other pragmatic details are involved, such as selecting a notably overbore engine (stroke length is less than piston width) to close the torque window.

 WHAT’S THAT SMELL — A DISCUSSION OF FUEL As noted, most small generators use gasoline as fuel. Why? A plethora of pragmatic reasons: • Ubiquitous availability • Convenient storage and handling • High caloric output • Ease of engine starting • Good shelf life • Comparatively higher gravimetric energy density (GED), which is the reducing of the factors of weight, volume, energy calories per unit etc. to a single number. See the chart below. This last quality of GED is quite interesting, and a reflection of our new perception and evaluation of the total energy picture. Reviewing energy not in a microcosm but in the big systemic vision, we evaluate the total continuing cost of moving energy from source to use. A related example in this discussion is the difference between a small generator powered by gasoline or LP. Gasoline has a better value of GED than LP. Why? Gasoline has a higher quantity of calories per comparable unit with a lower volume and far less investment in storage and delivery handling. As a side note, home heating from heavy oil came into vogue from a similar analysis — highest calories in the most dense, smallest volume, which reduces transportation costs, historically lowest cost per calorie, and reduced safety concerns, as heating oil #2 has very low volatility compared to gasoline or kerosene, etc. Admiral Halsey converted the U.S. Navy from coal to oil using a similar GED analysis in the 1930s. More on fuels focusing on choice and storage in the next edition.

REGULATION

Concurrent with frequency regulation is voltage regulation. Think of this generator as a rotary transformer. If the output is on frequency, the voltage should be close to the desired 120/240 with that voltage only varying with the surge impedance of the windings. The size of the wire (copper is our friend) and the size of the coils lowers the surge impedance effect. Once again, you get what you pay for.

[Related Story: "The Mice vs. The Generator"]

That surge impedance factor can produce another undesirable circumstance, and that is sine wave distortion. Caught in between all these factors is the quality of the generated waveform.

As you can readily see, speed control is critical and is maintained by the engine’s speed governor. Governors on the small engines we’re considering are usually air-vane type or mechanical, reading the tachometer (in RPM) or crankcase pressure. Some professional-grade units even have electronic (smart) speed regulators that work in conjunction with the voltage applied to the rotor winding for improved voltage and frequency control.

The designer has to make choices for the scheme he chooses, the speed of adjustment response, the damping factors, etc. All of this plays into overall performance satisfaction.

In light of all of the above, you should understand that inexpensive (read, cheap) small generators do not do well with varying loads. Regulation aside, the flux density of the waveform is quite often distorted, which translates into a less than sinusoidal perfection.

In a past RW Certification Corner article, we discussed the various AC waveform distortions and how AC power transfer is measured essentially in equivalent power to a workable quantity of DC power. This equivalency is why a generator with a horrendously distorted sine wave can still run electric heaters and power lightbulbs without issue. However, that same ugly waveform, when it encounters transformer windings or a capacitor in the first filter of a switching supply, produces all sorts of undesirable results.

Charles “Buc” Fitch, P.E. is a registered professional consultant engineer, senior member of the SBE, lifetime CPBE with AMD, licensed electrical contractor, former station owner and former director of engineering.