Small Engine Problems? Ask an Engine Mechanic for Answers ASAP
Gain access to the Power Regulator Board (PRB).
There are 4 wires – 2 go to the brushes (numbers 1 and 4), and the other 2 come from the exciter winding (numbers 2 and 6).
Disconnect the brush wires. Pay attention to which wire goes to which brush.
Make up 2 jumper wires that will attach to the brushes, with a 5A fuse in one of the jumpers.
Connect the fused jumper to the positive (outer) brush and to the positive terminal of a 12V battery.
Connect the other jumper to the negative (inner) brush, but do not connect it to the battery at this time.
Gain access to the exciter wires.
Start the set, and connect the negative jumper to the negative terminal of the battery.
Set your meter to VAC and measure the AC output at the receptacle of the set. It should be a minimum of 60 volts.
Test the voltage across the 2 exciter wires. There should be a minimum of 60VAC.
How much voltage is there?
Set your meter to VDC and test the voltage at the brush wires that come from the PRB. Make sure you connect the positive lead to the positive wire. You should see at least 10VDC.
Disconnect the jumpers from the battery and shut the set down.
A genset is composed of 2 major components – an engine and a generator, commonly referred to as an alternator. hf
A generator is composed of 2 major components – a rotor, commonly referred to as an armature, and a stator. The stator typically has 2 or more windings – 1 or more power windings, and a smaller exciter winding.
There are 3 things required for the production of electricity – a magnetic field, a conductor (wire), and movement between the field and conductor. Since the field has no physical mass, it is much easier to move it than to move the conductor. We move the field by spinning it with the engine. This is why most gensets are known as revolving field gensets.hf
When you start the engine, the rotor starts spinning (it is connected directly to the crankshaft of the engine).
The rotor is a giant electromagnet.
When not running, the rotor retains a small bit of magnetism, known as residual magnetism.
As it spins, the magnetic field around the rotor is passed through the windings of the stator (conductor). The magnetic field aligns the electrons in the wire and makes them move. This flow of electrons is electricity. Residual magnetism will create a residual output voltage of about 3-6 VAC at the receptacles.
However, the amount of electricity flowing is not very much, so the PRB takes the AC power coming in from the exciter winding and converts it to a DC current, and feeds it back to the rotor through the brushes. This increases the strength of the magnetic field – which, in turn, increases the amount of electricity produced – until the PRB senses that there is 120 volts (give or take) available.
Remember, a magnet has 2 poles – north and south, or positive and negative. As the positive pole passes the windings, it moves the electrons in one direction (the positive pulse). As the negative pole passes by, it moves the electrons in the opposite direction (the negative pulse). One positive pulse and one negative pulse are one cycle, or Hertz. Since US power is 60 Hz, this process must happen 60 times per second (50 times per second (50 Hz) in much of the world).hf
We now have nominal voltage flowing (120 V in the US).
When a load is applied to the genset, this voltage drops correspondingly to the size of the load.
The PRB senses this power drop from the exciter winding and says ‘Hey, I’ve got a load on and the power is dropping. I’d better make more power’. It does this by increasing the DC voltage to the rotor, which creates a stronger magnetic field, which pushes even more electrons into movement.
The VR constantly monitors this voltage and adjusts the field accordingly.
We now have electricity.hf