The battery, and the rest of the electrical system, must be protected from excessive voltages. To prevent early battery and electrical system failure, regulation of the charging system voltage is very important. Also, the charging system must supply enough current to run the vehicle’s electrical accessories when the engine is running.
AC generators do not require current limiters; because of their design, they limit their own current output. Current limit is the result of the constantly changing magnetic field because of the induced AC current. As the magnetic field changes, an opposing current is induced in the stator windings. This inductive reactance in the AC generator limits the maximum current that the AC generator can produce. Even though current (amperage) is limited by its operation, voltage is not. The AC generator is capable of producing as high as 250 volts, if it were not controlled.
Regulation of voltage is done by varying the amount of field current flowing through the rotor. Hie higher the field current, the higher the output voltage. Control of field current can be done either by regulating the resistance in series with the field coil or by turning the field circuit on and off. By controlling the amount of current in the field coil, control of the field current and the AC generator output is obtained. To ensure a full battery charge, and operation of accessories, most voltage regulators are set for a system voltage between 13.5 and 14.5 volts.
FIGURE. The regulator can control the field current by (A) controlling the resistance in series with the coil, or (B) by switching the field on and off.
The regulator must have system voltage as an input in order to regulate the output voltage. The input voltage to the AC generator is called sensing voltage. If sensing voltage is below the regulator setting, an increase in charging voltage output results by increasing field current. Higher sensing voltage will result in a decrease in field current and voltage output. A vehicle being driven with no accessories on and a fully charged battery will have a high sensing voltage. The regulator will reduce the charging voltage until it is at a level to run the ignition system while trickle charging the battery. If a heavy load is turned on (such as the headlights), the additional draw will cause a drop in the battery voltage. The regulator will sense this low system voltage and will increase current to the rotor. This will allow more current to the field windings. With the increase of field current, the magnetic field is stronger and AC generator voltage output is increased. When the load is turned off, the regulator senses the rise in system voltage and cuts back the amount of field current and ultimately AC generator voltage output.
FIGURE. Chart indicating relationship between temperature and charge rate.
Another input that affects regulation is temperature. Because ambient temperatures influence the rate of charge that a battery can accept, regulators are temperature compensated. Temperature compensation is required because the battery is more reluctant to accept a charge at lower ambient temperatures. The regulator will increase the system voltage until it is at a higher level so the battery will accept it.
To properly test and service the charging system, it is important to identify the field circuit being used. Automobile manufacturers use three basic types of field circuits. The first type is called the A circuit. It has the regulator on the ground side of the field coil. The B+ for the field coil is picked up from inside the AC generator. By placing the regulator on the ground side of the field coil, the regulator will allow the control of field current by varying the current flow to ground.
FIGURE. Simplified diagram of an A circuit field.
The second type of field circuit is called the В circuit. In this case, the voltage regulator controls the power side of the field circuit. Also, the field coil is grounded from inside the AC generator.
Note: To remember these circuits: Think of “A” for “After” the field and “B” for “Before” the field.
The third type of field circuit is called the isolated field. The AC generator has two field wires attached to the outside of the case. The voltage regulator can be located on either the ground (A circuit) or on the B+ (B circuit) side.
FIGURE. In the isolated circuit field AC generator, the regulator can be installed on either side of the field.
Regardless of which type is used, the field circuit is designed to control the amount of voltage output by controlling the amount of current through the field windings. The relationship between the field current, rotor speed, and regulated voltage is illustrated in Figure. As rotor speed increases, field current must be decreased to maintain regulated voltage.
FIGURE. Graph showing the relationship between field current, rotor speed, and regulated voltage changes depending on electrical load.
The electronic regulator uses solid-state circuitry to perform the regulatory functions. Electronic regulators can be mounted either externally or internally of the AC generator. There are no moving parts, so it can cycle between 10 and 7,000 times per second. This quick cycling provides more accurate control of the field current through the rotor.
Pulse width modulation controls AC generator output by varying the amount of time the field coil is energized. For example, assume that a vehicle is equipped with a 100-ampere generator. If the electrical demand placed on the charging system requires 50 amperes of current, the regulator would energize the field coil for 50% of the time. If the electrical system’s demand was increased to 75 amperes, the regulator would energize the field coil 75% of the cycle time.
FIGURE. Pulse width modulation with 50% on time.
The electronic regulator uses a zener diode that blocks current flow until a specific voltage is obtained, at which point it allows the current to flow. An electronic regulator is shown.
FIGURE. A simplified circuit diagram of an electronic regulator utilizing a zener diode.
Battery voltage is applied to the anode side of the zener diode as well as to the base of transistor number 1. No current will flow through the zener diode, since battery voltage is too low to push through the zener. However, as the AC generator produces voltage, the voltage at the anode will increase until it reaches the upper limit (14.5 volts) and is able to push through the zener diode. Current will now flow from the battery, through the resistor (R1) to the anode, through the zener diode, through the resistor (R2) and thermistor in parallel, and to ground. Since current is flowing, each resistance in the circuit will drop voltage. As a result, voltage to the base of transistor number 1 will be less than the voltage applied to the emitter. Since transistor number 1 is a PNP transistor and the base voltage is less than the emitter voltage, transistor number 1 is turned on. The base of transistor number 2 will now have battery voltage applied to it. Since the voltage applied to the base of transistor number 2 is greater than that applied to its emitter, transistor number 2 is turned off. Transistor number 2 is in control of the field current and generator output.
The thermistor changes circuit resistance according to temperature. This provides for the temperature-related voltage change necessary to keep the battery charged in cold-weather conditions.
Many manufacturers are installing the voltage regulator inside the AC generator. This eliminates some of the wiring needed for external regulators. The diode trio rectifies AC current from the stator to DC current that is applied to the field windings.
FIGURE. AC generator circuit diagram with internal regulator. This system uses a diode trio to rectify stator current to be applied to the field coil. The resistor above the indicator lamp is used to ensure current will flow through the terminal 1 if the lamp burns out.
FIGURE. Current flow to the rotor with the ignition switch in the RUN position and the engine OFF.
Current flow with the engine off and the ignition switch in the RUN position is illustrated. Battery voltage is applied to the field through the common point above R1. TR1 conducts the field current coming from the field coil, producing a weak magnetic field. The indicator lamp lights because TR, directs current to ground and completes the lamp circuit.
FIGURE. Current flow with the engine running and AC generator producing voltage.
Current flow with the engine running is illustrated. When the AC generator starts to produce voltage, the diode trio will conduct and battery voltage is available for the field and terminal 1 at the common connection. Placing voltage on both sides of the lamp gives the same voltage potential at each side; therefore, current doesn’t flow and the lamp goes out.
Current flow as the voltage output is being regulated is illustrated. The sensing circuit from terminal 2 passes through a thermistor to the zener diode (D2). When the system voltage reaches the upper voltage limit of the zener diode, the zener diode conducts current to TR2. When TR, is biased, it opens the field coil circuit and current stops flowing through the field coil. Regulation of this switching on and off is based on the sensing voltage received through terminal 2. With the circuit to the field coil opened, the sensing voltage decreases and the zener diode stops conducting. TR, is turned off and the circuit for the field coil is closed.
FIGURE. When the system voltage is high enough to allow the zener diode to conduct, TR2 is turned on and TR1 is shut off, which opens the field circuit.
FIGURE. Computer-controlled voltage regulator circuit.
On many vehicles after the mid-1980s, the regulator function has been incorporated into the powertrain control module (PCM). The operation is similar to the internal electronic regulator. Regulation of the field circuit is through the ground (A circuit). However, in recent years there has been an increase in manufacturers that will control the field circuit by use of high-side drivers.
The PCM’s decisions, concerning voltage regulation, are based on battery voltage and battery temperature. When the desired output voltage is obtained (based on battery temperature), the PCM switches the transistor on or off as needed. This transistor grounds the AC generator’s field to control output voltage.
General Motors’ CS series generators may be connected directly to the PCM through terminals L and F at the generator. The voltage regulator portion of the PCM switches the field current on and off at a frequency of about 400 times per second. Varying the on and off time of the field current controls the voltage output of the generator.
The computer-controlled regulation system has the ability to precisely maintain and control the changing rate according to the electrical requirements, battery (or ambient) temperature, and several other inputs.
FIGURE. The PCM will use various inputs to regulate AC generator output.
FIGURE. A generator with internal microprocessor using a bus network for communications to the PCM.
Another generator control method is that like that used by Mercedes Benz. This system uses a generator that is actually a slave module on the LIN bus network. The PCM communicates power requirements over the LIN bus concerning voltage set point and a set point governing time to the generator. The LIN interface chip translates the request and output control is achieved by the integrated electronics with driver stages and an 8-bit microprocessor. This is referred to a power on-demand and reduces fuel consumption since the generator field is turned on only when needed. This also reduces underhood noises.