Starter generators are most often found in golf carts and lawn mowers, although they are also found in aircraft.

They are used to provide an initial burst of electrical power to start the engine and then, once the engine is running, they feed electrical power back to the battery to top it up and to provide electrical power for the lights and to run the engine.

To see if your starter generator is producing enough voltage to start the engine, conduct a field test. On 12V systems, the starter generator can output up to 15 or 16 volts at the battery.

However, there are also 48-volt starter generators that can produce far higher voltages when cranking the engine.

How Does a Starter Generator Work?

The starter generator is a combination of a starter motor and a generator.

By combining the two components, cost savings and efficiency are maximized.

Instead of engaging the starter by twisting the ignition key or pressing a start button, the starter generator is activated when the accelerator pedal is pushed.

Initial Voltage

A switch in the accelerator pedal allows current to flow from the battery to the starter circuit.

This is a separate circuit for the generator portion of the unit.

The initial voltage turns the starter which in turn rotates the engine, providing impetus for the first part of the ignition cycle.

At the same time that the starter is turning the engine, the electrical current is directed to the ignition coil so that a spark appears at the spark plugs.

This then ignites the fuel/air mixture and starts the engine. The same thing happens in a turbine engine in an airplane, so that the fuel ignites, turning the turbine.  

How the Starter Works

starter motor

The starter is composed of a stator and windings as well as an armature with brushes.

In a 12 Volt system, the armature has a field coil made of wires wound around it.

The current passes through the wires, brushes, and armature. A magnetic field is created from the flow of current.

It is this electromagnetic field that causes the stator to rotate, which in turn rotates the crankshaft and starts the engine.

Once the engine is running, the starter portion of the starter generator disengages and the generator takes over.

How the Generator Works

The generator part of the starter generator is the unit that provides electrical power to the engine and all the other parts of the vehicle that require power.

It also recharges the battery so that it is ready for the next time the starter is needed to get the engine going again.

Depending on what type of vehicle is being powered, the required voltage is supplied by the starter generator for the lights, power take-off (PTO), computerized components, electronic navigation systems in aircraft, as well as emergency shutoff components.

The engine provides the rotational power needed to turn the generator, creating electricity via electromagnetic fields in its coils.

This electricity is fed to the battery and to the ignition system to provide electrical power for the spark plugs.

Troubleshooting a Starter Generator


While the system is simple, there are a few common problems that develop over time due to wear and tear.

Heat is a major cause of failure.


A starter generator usually operates with a single coil. This coil heats up due to the amount of current generated.

While the coil is normally cooled by fans in normal automotive and agricultural equipment, the coil in the starter generator is inside the unit.

This means it is susceptible to overheating.

If the coil burns out, the unit will stop working and the engine will not run. The coil must be replaced if the engine is to run again.

Brushes Wear Out

The brushes that make contact with the commutator are the lifeblood of the generator.

If the brushes wear out, they will fail to make contact with the commutator properly and the generator will not produce electrical power.

The bearings inside the starter generator are also prone to wear.

This wear will cause the generator to rotate erratically, resulting in uneven wear on the brushes.

This causes them to bounce over the commutator, producing an intermittent current and eventually the system will fail.

This wear and tear also causes heat to build up, resulting in the starter generator burning out.

Why the Brushes Wear Out

brushes wear off in generator

The brushes are designed to wear out as they are in almost constant contact with the commutator.

The commutator has groves in it which create an oscillating electromagnetic field. It is this changing energy field that produces the alternating current to drive the various electronic systems in the vehicle.

What are the Brushes Made Of?

The brushes are manufactured from a carbon-based material. This material is formed into long rectangular bars with a stranded wire to attach to the vehicle's electrical system.

The brush is soft enough to make contact with the metal face of the commutator without damaging the surface. Instead, the brush slowly wears down.

Springs on the back of the brush keep constant pressure on it so that it pushes against the commutator.

The soft conductive material in the brush continues to wear down until it is too short to make contact and the generator stops working.

The brushes and springs are then replaced and the generator will work again as normal.

Spring Loaded Brushes

The brushes are spring-loaded so that they can push against the commutator. The engine transfers power to the starter generator via a toothed belt.

The belt transfers power to the starter generator so that it revolves at approximately 7,000 revolutions per minute (RPM).

The voltage that is produced is dependent on the number of windings there are on the armature.

When the starter generator is producing current, it should be supplying about 12-13 Volts to the battery. When it wears out, this figure will drop and the battery will not be charged properly.

The brushes should be inspected each year and replaced when they are worn down.

You can tell if the brushes are worn down when the top of the guides that hold them are level with the end of the brush. There will also be a lot of black dust from the brushes coating the inside of the starter generator.

Brush Guides

When the brushes wear out, the pressure from the springs is reduced.

Apart from the brushes becoming level with the end of the guides, the end of the brush has a pigtail that will make contact with the commutator bars. This results in arcing and damage to the bars and the guides.

To fix the problem, the starter generator is removed from the vehicle and then disassembled so that the damage can be polished away.

When reinstalling the start generator, the correct belt tension must be set so that the starter generator can transfer power properly and that there is little chance that the belt will slip or jump off the pulley.

What is the Advantage of a Starter Generator?

Starter Generator

Starter generators can boost the power and acceleration of vehicles through the additional torque they can supply.

They are also more efficient and can recover energy better than traditionally designed alternators.