The starter motor in your automobile is a DC motor. If you were to accidentally reverse the battery polarity, the DC motor would still rotate in the same direction. Reversing polarity of the battery will not cause the motor to rotate in the opposite direction.
All engines require a toyota starter motor to turn them over before firing. In conventional vehicles, this is a straightforward, but powerful, direct-current electric motor. When the starter switch is activated by the driver, current flows to a solenoid attached to the starter motor. This current moves a lever into the solenoid that then causes a cogwheel of the motor to mesh with the teeth on the circumference of the flywheel. At the same time, an electrical contact is closed to allow a large current to flow and rotate the starter motor as well as the engaged flywheel. Typically, currents of hundreds of amperes are required to start the engine and are provided by the battery, which is generally a 12-V lead–acid module. The battery is recharged by the alternator–rectifier combination when the engine is running. Automotive batteries have improved enormously over the years and have far longer lives than formerly, even though they may be called upon to power many more functions. Although guarantees may be for two or three years, in practice batteries often operate for eight years or longer before failing. Moreover, modern car batteries no longer require periodic ‘topping-up’ with de-ionized water. Further information on the evolution of the lead–acid battery is given in Section 7.4, Chapter 7.
A starter motor is required to run the internal combustion engine up to a speed sufficient to produce satisfactory carburation.
The starter motor is mounted on the engine casing and a pinion on the end of the BMW starter motor shaft engages with the flywheel teeth. The gear ratio between pinion and flywheel is about 10:1. A machine capable of developing its maximum torque at zero speed is required. The series wound motor has speed and torque characteristics ideal for this purpose.
To obtain maximum lock torque (i.e. turning effort at zero speed), the flux and armature current must be at a maximum, so resistance in the starter circuit (windings, cables, switch and all connections) must be a minimum; any additional resistance will reduce the starting torque. Generally, the inertia engaged mercedes starter motor is energised via a solenoid switch, permitting the use of a shorter starter cable and assuring firm closing of the main starter-switch contacts, with consequent reduction in voltage drop. The use of graphite brushes with a high metallic content also assists in minimising loss of voltage.
