Automotive Electronics Practice Test

The faster a starter motor rotates, what happens to the current drawn from the battery?

1. The more current it draws from the battery

2. The less current it draws from the battery

3. The greater the amount of torque produced

4. The less CEMF is generated

The correct answer is: The less current it draws from the battery

When a starter motor rotates faster, the current drawn from the battery generally decreases beyond a certain point due to the principles of electromagnetic induction. As the motor speed increases, the counter-electromotive force (CEMF) produced within the motor also increases. This CEMF opposes the applied voltage from the battery and effectively reduces the net voltage that drives the current through the motor windings. In an electric motor, when the rotor turns and generates CEMF, it acts as a kind of self-regulation mechanism. At lower speeds, the CEMF is minimal, requiring more current to produce the torque needed to start the engine. However, as the speed of the starter motor increases and CEMF becomes significant, the amount of current drawn will decrease. This is because the motor becomes more efficient and can maintain its operation with less energy input from the battery. This relationship between speed, CEMF, and current draw is key in understanding how starter motors function within automotive systems. Recognizing this dynamic helps explain why the correct answer is that the faster the starter motor spins, the less current it draws from the battery.