Counter-Electromotive Force

In taking a look at the starter motor as it has been described so far, it should be noted that all the conditions for generating a voltage have been met. When a conductor cuts through magnetic lines of force, a voltage is induced in the conductor. In the starter motor, this induced voltage acts in opposition to battery current. The induced voltage is known as counterelectromotive force (CEMF). The effect of CEMF reduces the current supplied by the battery.

Because voltage induced by a conductor is proportional to the speed at which it cuts through the magnetic field, CEMF increases proportionally with armature speed. As the armature is wound out to its maximum speed, CEMF gets close to but cannot exceed battery voltage. The ability of an armature to produce torque is determined by the current flowed through it. The absence of CEMF is one factor that determines that peak torque is produced at zero rpm in a starter motor. As the cranking motor rpm rises, CEMF rises proportionally, reducing torque potential. In other words, CEMF acts a bit like a magnetic brake to help limit starter motor overspeed.

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