Field Coil Rotors provide the magnetism to drive charging system output in many bikes. These systems were used on many bikes in the 70's and 80's. Good examples include the Honda CB series, and Yamaha XS series motorcycles.

They function more similarly to car-type alternators than the permanent magnet flywheel systems described on the stator tech support page. They are more efficient systems in that they vary the magnetic field of the rotor to only generate as much charging current as needed by the battery at a given time.

Field Coil Rotors

Field coil rotors are an efficient way to generate the magnetic field for a motorcycle charging system. The rotor is made from two metal halves, pressed together. Inside the rotor is a plastic insulating core wound with magnet wire. The face of the rotor is a flat surface, with two isolated copper rings (called 'slip rings'). Each ring is internally connected to one end of the field coil. The field coil is powered by +12VDC provided by the voltage regulator. The current is passed to the rotor via brushes that ride against the slip rings as the rotor rotates. The rotor is mounted to the end of the engine's crankshaft, and rotates at engine RPM. The rotor is surrounded by an external stator with windings in close proximity to the edge of the rotor.

The field coil rotor is controlled by the voltage regulator in the system. The voltage regulator/rectifier in a field coil rotor system serves multiple functions. It measures battery voltage, and based on the level of battery charge allows varying amounts of current flow through the field coil. The current through the field coil then generates a magnetic field, which produces alternating current flow through the windings of the stator that surrounds the rotor. The rectifier side of the voltage regulator/rectifier then rectifies the alternating current from the stator to DC current to charge the battery.

General Rotor Failures

Rotors cannot be easily serviced or repaired, although if the field coil fails, it is possible to separate the rotor halves and rewind the field coil. There are only a few common failure modes of field coil type rotors. Most common is the field coil failing due to wire insulation wear. A bad spot on the field coil winding insulation will eventually develop, and short the field coil internally, either between windings, or to the body of rotor. The entire length of wire in the coil is necessary to generate a sufficient magnetic field, so any loss of winding due to short circuiting will cause a large drop or complete failure in charging output.

The other common failure mode of rotors is slip ring wear. As the brushes are constantly sliding on the slip rings and passing electrical current, the rings can eventually corrode, and the copper surface wears down and becomes uneven. This can cause a poor connection between the brushes and rings, resulting in high resistance in this connection, and poor current flow to the field coil. The surface wear can damage the brushes as well. The face of the rotor needs to be clean and even to ensure reliable operation.

General Rotor Troubleshooting Information

A Digital MultiMeter (DMM) is required to complete testing of the field coil.

Field Coil Resistance Test
Set DMM to resistance measurement mode (ohms) at lowest range.
Use meter leads to measure resistance between the two copper slip rings on top of the rotor.
Compare readings to the technical specifications of the part (available on the part description page for our parts, or OEM Shop Manual). If the reading does not match the target resistance, the field coil is suspect.
Much higher resistance can mean damaged windings.

Field Coil Short Circuit Test
Set DMM to resistance measurement mode (ohms) at lowest range.
Use meter leads to measure resistance between each ring on the face of the rotor and the metal body of the rotor.
This measurement should display infinite resistance ('OL', overload on some meters), indicating the field coil is completely isolated from chassis ground.
If there is any measurable resistance (especially low) or a short circuit in these measurements, this indicates the field coil is internally shorted to the rotor housing at some point, and will not operate correctly, if at all.