In Part 1 of this series, we discussed basic automotive electricity. In Part 2, we went over automotive generators and alternators and their function, and in Part 3 we covered automotive voltage regulators and what they do.
So, by now you are becoming more familiar with how electricity is stored, generated and regulated in the automobile. It might have occurred to some of you that the electrical system only needs 80 to 100 amps of current for general running, even when all accessories are operating. Why, then, does our battery have a rating of 450 to 750 — and more — amps? Is all this storage necessary?
Yes it is! The main reason for the battery's storage capacity is to operate the starter, and a quick look at the numbers will demonstrate why this is so important:
Let's take a 500-amp-rated battery for example. At 12 volts, this 500 amp battery is capable of putting out 6000 watts (amps X volts = watts, remember?). We need all the wattage we can get to develop enough horsepower to turn the engine over for ignition and one horsepower (or the power necessary to lift 550 pounds one foot in one second) equals 746 watts. Our battery, therefore, puts out just over eight horsepower. That's just enough for a couple hundred revolutions of the engine before the charge is exhausted. If the engine doesn't start by then, well, you know...
Starters are incredibly strong motors that work in a hostile environment. They are the most important part of the starting system, or circuit, consisting of the following:
FLYWHEEL RING GEAR — This is a toothed ring that is fitted to the outside of the engine's flywheel. Matching teeth on the starter motor mesh with this gear in order to spin the crankshaft.
STARTER SOLENOID (Relay) — All cars are wired so that the battery's main cable connects to the starter motor windings (the thick cable is needed for large current flow, right?). This wire must be switched on and off, of course, and it would be costly and inefficient to route it through the ignition switch (not to mention the size of the switch's components required to carry such current!). Consequently, a relay is necessary...
Relays are devices that utilize a central iron core fitted closely to the inside of a coil of wire. When the wire is energized the iron core will be drawn down the length of the coil, the direction dependent upon the direction of current flow. If the relay's iron core is fitted with large, high current-carrying contacts it can be used as a high-current switch. Relays are used throughout cars (for horns, electric fans, air conditioning clutches, etc.) and the most important one is the starter solenoid.
The starter solenoid has very large contacts to carry the battery's full current. Its wire coil is actuated by a smaller current from the ignition switch, at which time the iron core slams down to make contact and turn on the starter motor. Most non-Ford starter motors employ a solenoid built into the motor itself. This type of solenoid not only provides the motor's electrical power but also mechanically engages the starter's drive gear onto the flywheel. It is commonly known as the BENDIX type of solenoid. Such solenoids operate in three stages, the disengaged, partially engaged and engaged. In the disengaged position the drive gear is released and no current is flowing. In the partially engaged stage, current from the starter switch flows through both the pull-in and the hold-in coils. Both coils draw the plunger inward, causing it to pull the shift lever and engage the pinion gear. When the plunger is pulled into the coil all the way, the pinion fully engages the ring gear. When the ring gear is fully engaged, engine cranking begins. When the engine starts the hold-in coil will cut out and the plunger will move out, retracting the pinion and opening the starter switch.
STARTER MOTOR — This is a powerful electric motor that engages the car's flywheel in order to spin the crankshaft. As in all electric motors, the starter is composed of windings of wire that form loops, ending at the commutator segments (remember these from the generator?). The armature coils are mounted on the motor's central shaft (supported with bearings) and the field coils are formed into four or more "shoes", placed inside the steel frame of the starter. Brushes are used to create electrical contact to the commutator segments and when current is fed into two of the four brushes, it flows through all the loops of the armature and shoe windings and out the other two brushes. This creates a magnetic field around each loop. As the armature turns, the loop will move to a position where the current flow reverses. This constant reversal of current flow allows the armature and field coils to repel each other and spin the motor. The greater the current flowing in the coils, the greater the magnetic forces, and the greater the power of the motor.
The copper loops and field windings are heavy enough to carry a large amount of current with minimum resistance. Since they draw heavy amounts of current, they must not be operated on a continuous basis for longer than 30 seconds. After cranking for 30 seconds it is wise to wait a couple of minutes to let the starter motor dissipate some of its heat. Starters heat quickly, so prolonged use can cause serious damage. A typical symptom of overheating starter motors is extremely slow, labored engine-cranking.
Various wiring designs are used in starter motors and one of the most popular is the four pole, three winding setup. Two of the windings are in series with themselves and the armature. One winding does not pass through the armature, but goes directly to the ground. This Shunt Winding aids with additional starting torque. However, as the starter speed increases, the shunt still draws a heavy current and tends to keep starter speed within acceptable limits.
Starter motors fail mostly due to overheating. They are placed in a hostile, hot environment and cannot be expected to last indefinitely. Another mode of failure is a shorted or open winding. This exhibits itself as a "dead spot" on the commutator. If a brush lands on a dead spot the motor won't turn at all.
A third failure-mode is a faulty pinion engagement. Sometimes the pinion assembly gets stiff or stuck due to lack of lubrication or wear. Starter motor rebuilding or replacement is required for all of these problems.
Before doing so, however, check to make sure the electrical connections on the starter (and the battery) are clean and tight. Most failures to start are due to loose or corroded battery cable connections or low-current solenoid connections, not to faulty starters.
Continuing with our Automotive Electrical Systems series, the next installment will cover automotive ignition systems and their function.
Classic Car Automotive Electrical Systems - Part 1: Basic Automotive Electrical Theory
Classic Car Automotive Electrical Systems - Part 2: How Generators and Alternators Work
Classic Car Automotive Electrical Systems - Part 3: How Voltage Regulators Work
Classic Car Automotive Electrical Systems - Part 5: Ignition Systems
Classic Car Automotive Electrical Systems - Part 6: How Automotive Relays and Fuses Work