Type-Specific Generator Maintenance
Portable generators come in three types: brushless, brush style and inverter. It’s important to understand the theory of each type so that problems can be better recognized if and when they occur.

Generator Types

A brushless generator is typically self-exciting, meaning that it produces needed DC excitation power internally. Power is produced through a magnetized rotating field, and the output power is taken off of stationary windings.

Brush-type generators usually are separately excited. In these models, the battery or power from the engine will put a DC charge on sliprings located in the back of the generator. The two sliprings, one positive and one negative, are specific to brush-style generators, as are brushes. A generator brush conducts current in and out of the generator. Often it consists of a block of carbon bearing against the commutator, which passes power into and out of the sliprings.

While also brushless, inverter generators differ from standard brushless and brush models through the ability to vary engine speed based upon the draw of connected equipment. This saves fuel and produces cleaner power at different engine speeds. An inverter module containing a microprocessor reads the load, or amperage, of the items plugged into the generator. The microprocessor then adjusts an electronic governor based upon the load size, with the engine rpm automatically increasing for larger loads and decreasing for smaller loads.

Contractors tend to prefer brushless generators, which offer fewer parts and typically cost less than brush styles. That also means less maintenance. Brush sets, however, usually have a cleaner sine wave, making them better suited for slightly more sensitive equipment.

A conventional generator produces a single sine wave of AC power per full engine rotation. In order to create the standard 60 hertz needed to power equipment, the engine must maintain a constant speed of 3,600 rpm. No matter how much load is needed, the generator runs at its full speed of 3,600 rpm.

Inverter generators, however, produce the cleanest sine wave — more than 300 overlapping AC sine waves per engine rotation. As a result, more electrical power is produced from each engine rotation. Before the inverter power can be used, it must first be transformed into the proper voltage and frequency required to power the user’s equipment. The inverter module “converts” three-phase, high-frequency AC power into DC power, which is then “inverted” to a clean and stable 120-volt, 60-hertz AC power signal. This single, sophisticated electronic module controls all functions of both generator and engine.

The “pure” sine wave of energy produced by an inverter generator has full width and amplitude and no voltage surge. It’s this clean, stable output that makes the inverter generator ideal for sensitive electronic devices, such as computers.

Type-Specific Repairs

Some differences also exist among generator types in the maintenance arena. One maintenance area specific to brush-type generators are, of course, brushes. The brushes can become worn from friction, dirt lodged between the brush and slipring, or improper seating of the brush, which curves around the slipring. Although brushes should last the life of a generator, be prepared to replace them if extreme wearing occurs. Some manufacturers recommend replacements after about 4,000 hours of use.

On brushless generators without permanently embedded magnets, the field coil can lose its residual magnetism, which will be indicated if there’s no voltage upon starting the generator. Consider a childhood science project as an example: Rubbing a magnet against a screwdriver will magnetize it so that both can function as magnets. Put both away in drawers and come back some time later, the magnet will still be magnetized but the screwdriver will have lost the effect.

Some generator manufacturers magnetize the steel laminations, also called the “stack,” rather than use a magnet to self-excite. The stack could lose its magnetism, however, after the generator is disassembled or the part is dropped or hit with a hammer. Lack of use also could lower its magnetism level, at which point a mechanic will need to remagnetize it, or “flash the field.” Most brushless generators will need at least 2-3 volts of residual magnetism to prompt the initial excitation. This level will always be available on generators with magnets embedded into the rotor, which will never need to be flashed since permanent magnets’ material will provide the residual magnetism needed to initially excite the generator.

Restoring a generator’s residual magnetism, or flashing the field, should always be done with the unit off and according to manufacturer specifications. Each manufacturer may require a different method — some, for example, may use a battery charger and cables while others may use a 12-volt battery — so it’s important to locate the service manual online or elsewhere.

Inverter generators also are brushless units, which equates to less maintenance. Like conventional generators, inverters have a rotating field and copper windings, along with special magnets and copper windings that produce the AC output, which require similar checks as described. Unlike conventional generators, however, a microprocessor in the inverter module controls the machine. It is an important part of the total generator and integral to producing AC output. Without the circuit board operating, the generator will not produce AC output.

There’s really no need to feel powerless over generator maintenance. Just make a point to understand the type of generator you have and what’s required to keep it in good working order.
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