Tecktalk - Generators And Alternators

May 2001 Telltale page 3 - by Tom Winlow and Marcel Laroche.


Before about 1970, most cars and marine engines were equipped with a generator, which produced direct current (DC.) A generator uses magnetism to produce electricity. When current flows through a wire, a magnetic field is created around it. The strength of the magnetic field is directly proportional to the amount of current flowing through the wire. When another wire is moved across this magnetic field, a current is induced in this moving wire, even though it is not physically connected to the first wire. In a generator, electromagnets powered by the battery create a strong magnetic field. This magnetic field is stationary (STATOR). A center cylindrical piece (ROTOR), containing a multitude of wires, is rotated inside this magnetic field, cutting through the "lines of force" of the magnetic field (STATOR). This induces a voltage; thereby a current in the rotor wires. A generator can be compared to an electrical pump. The faster it turns, the greater is its output in terms of voltage (electrical pressure) and consequently current. At idle speed, it produces barely enough voltage to keep the generator red light out, as you older types well remember.

The red light indicated that the generator output voltage was less than the battery voltage. Therefore, the battery, having a higher voltage (electrical pressure), was powering the whole electrical system. As the rotational speed of the generator increased, the red light went out, indicating that its voltage was greater than the battery voltage (about 12.5 volts). As speed increased further, the voltage would also increase until it reached about 14 volts, at which point, the voltage regulator would step in and regulate the voltage at about 14 volts. The voltage regulator was made up of springs, coils, relays, heavy contacts, etc. What a mess! The voltage regulator disconnected the battery from the generator (red light "ON") at low speed to prevent the generator from becoming an electrical motor.

If you connect a generator directly to a battery, it would be a motor just like the starter motor. Electric cars use that principle. When the accelerator pedal is depressed, the battery powers the DC motor; when the pedal is released, the motor becomes a generator and recharges the batteries. A bit off the subject, but do you get the idea?

The wind speed transducer is a tiny generator putting out millivolts and milliamps. The stator is a permanent magnet instead of an electromagnet. The rotor is essentially the same as the big generator. Its construction is the same as a $1.50 toy motor, yet it costs $150.00 + PST + GST, of course! The knot meter is a generator only. The permanent magnet is contained inside one of the paddles of the paddle wheel, and in this case, the magnetic field moves and induces a voltage in the transducer coil. The higher the boat speed, the faster the paddle wheel turns and consequently, the higher is the induced voltage in the transducer coil. Since the bulkhead-mounted knotmeter has a fixed resistance, the higher voltage will increase the current flow in the instrument and consequently, a higher speed indication. Stone age electrical engineering that costs more than a colour TV?


In the 70s, with the advent of the semiconductor and solid state electronics, the alternator displaced the generator because it could produce more power at low rotational speed. It is also smaller, lighter and more powerful than the generator. An alternator initially produces alternating current (AC), yes AC. Before this AC current can be used by the boat or car electrical system, it has to go through a rectifier and a voltage regulator. Unlike DC, which has a constant polarity, AC's polarity continually reverses itself (ALTERNATES). The center part of the alternator (ROTOR) contains electromagnets with north and south poles on each magnet (positive + negative). The stator surrounding the rotor contains coils of wire. As the rotor turns inside the stator, both the north and south poles of the electromagnets induce alternatively, a positive then a negative voltage, hence an alternating current in the stator coils. Because AC can't be used by the boat's electrical system, it has to go through a rectifier to change it into DC. Not pure DC, like battery power, but a slightly rough DC form acceptable to your electrical system. The rectifier contains diodes, which are electrical one way valves, which allow current to flow only if it's of the proper polarity. This converts the negative phase of the AC into a positive phase. The end result is a rapidly pulsating (ROUGH) DC, rather than pure battery DC power. After the output has been rectified, it goes through the solid state (no spring or contacts) voltage regulator, which performs the same functions as the generator voltage regulator.

Which way does the current flow?

When Ben Franklin discovered electric current, he had no idea that electrons existed. Having a positive attitude, he arbitrarily said that current flowed from positive to negative. Much later, scientists discovered that current was a flow of electrons and that they travelled in the opposite direction; that is, from negative to positive. In the late 1960's some textbooks and school curriculums taught negative to positive current flow. The world is still using Franklin's "conventional" current flow theory, that is, positive to negative, to explain and dissect electrical circuits. No matter which way the current flows, it is still able to produce the same amount of work.

When connecting electrical devices, don't try to "second guess" the manufacturer. Connect positive to positive and negative to negative, because if you don't, there will be smoke and when the gismo stops smoking, it will not work anymore. That is proof that electrical gismos work with smoke!