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How Generators WorkPosted 4/14/2005By Vince Fischelli
And they're easy to test ... when you know how.
It is customary to describe a voltage-generating source by the type of voltage it provides; alternators produce alternating current (AC) and generators produce direct current (DC). To be technically correct, it should be called a "generator" since the output is DC. A generator has two functions in a vehicle. First, it provides the electrical energy to operate the vehicle when the engine is running; and second, it charges the battery at the same time. This makes the generator's performance during engine run important so the computer-controlled engine gets the power it needs to function properly to maximize fuel economy and minimize emissions. The battery is actually "off-line" (not providing) when the generator is generating. If the generator fails and cannot produce electricity, the battery takes over and runs the vehicle until the battery runs down. Let's take a brief tour through a generator to see how the internal components work together to electrically power the vehicle and charge the battery. A generator contains several major components. Figure 1 shows the electronic voltage regulator, a field/rotor winding and a stator coil. The electronic voltage regulator controls the generator output by controlling the electron current flowing through the rotor. The rotor is a large coil spinning inside the stator winding. Transistor Q1 is a power transistor located in the voltage regulator to control rotor winding current. Current flows up through ground, into emitter, out Q1 collector through the rotor winding back to B+.
As the rotor spins inside the stator winding, electrical energy is induced in each of the stator windings. Each of the outputs from the three stator windings is called a phase and is an AC voltage wave form called a sine wave. The sine wave has a positive alternation (+ peak) and a negative alternation (- peak). Each of the phases reaches its positive peak 120¯ after the previous sine wave as the rotor continues to spin inside the stator. The AC sine waves are presented to a diode bridge inside the alternator to convert the AC sine waves to DC (B+), as shown in Figure 2. To understand how the diode bridge converts AC to DC, we need to understand how diodes work. Generators use solid-state diodes to do their job in converting AC to DC by a process called rectification (converting AC to DC).
Looking back at Figure 2, at the connections where a stator winding connects to the diode bridge, (P#1, P#2 and P#3), there are two diodes in series from each connection point. There is a diode connected to B+, and therefore it is referred to as the positive diode. There is a diode connected to ground, or B-, and therefore it is referred to as the negative diode. There is a positive and negative diode in a series network for each stator phase winding. Here is where diode knowledge comes in handy. The positive diode applies the positive alternation (+ peak) into positive voltage to draw electrons out of the positive post of the battery. When the associated sine wave is in its positive swing, the positive diode is forward biased and draws electrons out of the battery positive post. Electrons flow against the arrow in the positive diode. The negative diode applies the negative alternation (- peak) into negative voltage to force electrons into the negative post of the battery. When the associated sine wave is in its negative swing, the negative diode is forward biased and sends electrons into the battery negative post. Electrons flow against the arrow in the negative diode. This establishes a DC current through the battery to charge it. DC current means the electrons always flow in one direction, from negative to positive, through the battery and vehicle circuits. Any electrical circuit on the vehicle is connected in parallel to the battery between B+ and B-. Generator current flows through each circuit as it does through the battery. At no time does any diode network stop contributing its share of electrical energy to charge the battery or provide electron flow to vehicle circuits. Each phase of the generator simply "peaks" in numerical order as long as the rotor is energized and turning. The end result is a constant DC voltage and current source at the generator terminals. All DC current that runs the vehicle passes through the diode bridge. The diode bridge generates a lot of heat from all the hard work it must perform. If the heat is not dissipated adequately, the diode bridge will burn up and the generator fails - resulting in no charging voltage and no charging current.
Figure 4 shows the DC charging voltage and the ripple voltage riding on the output voltage as seen on a lab scope. The ripple is created by each of the three phases going through their positive peak one after the other. The smaller the ripple pattern the better. A generator is most efficient when the ripple is less than 0.5 V AC. Testing a Generator Let's get to the bottom line about testing generators. Forget the industry hype about the proper way to test generators. Old wives tales abound with misinformation. Forget dedicated and usually expensive generator testers. They are designed so that a tech with no training can test a generator if he can read instructions. And never put a carbon pile load on the generator to see if it can get close to 90 percent of its rated output in amps. That's a good way to smoke a perfectly good generator. Instead, do it this way with your digital multimeter (DMM):
Next, turn off all electrical loads and continue to monitor the DMM reading. Increase engine rpm to 2000 and watch for a rise in charging voltage not to exceed 15.10 volts. Above 15.10 indicates overcharging. This could be caused by a defective electronic voltage regulator or a bad connection between the battery and the generator. The charging voltage should never exceed 15.20 volts. If the charging voltage is 14.80-15.10, it should be cold weather. But even below 60 degrees Fahrenheit the charging voltage should never exceed 15.20 volts. In milder weather expect the charging voltage to be in the range of 14.10-14.50 volts. If the charging voltage is near 13.80 volts it should be warm to hot weather. After checking a few vehicles following these simple test procedures, technicians can learn what the charging voltage range should be for each make and model. Knowing what to expect for charging voltage readings allows a technician to test a generator charging system on the car.
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Copyright (c) 1996-2011. Automotive Service Association®. All rights reserved.
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The term "alternator," to describe what we have been calling an alternator since the late '50s, is now obsolete. Ever since the '96 model year, we are supposed to call them "generators." The name change more accurately describes the function of what we used to call an "alternator."
The stator winding shown in Figure 1 is a delta connection because the three windings are connected to electrically form a triangle like the Greek letter delta. Since a stator has three windings the generator is said to be a "three-phase system."
A diode lets electron current flow in one direction but not the other. But how does a diode respond to the voltage of the sine wave? A diode lets electrons pass through the opposite direction the diode's arrow is pointing, as shown in Figure 3. Or, we could say electrons only flow against the arrow. For electron current to flow against the arrow, the diode must be forward biased. That means the diode anode is more positive than the diode cathode. This is forward bias and allows electrons to flow through the diode in the direction against the arrow. Heavy-duty diodes form a diode bridge inside a generator to rectify (convert) the AC produced inside the generator to DC at the generator output.
Some technicians have tried to take a weak or dead battery and place it in a car with the engine running to recharge the battery. First of all, never disconnect a battery cable while the engine is running if the vehicle has any onboard electronics at all (engine control module, power control module, vehicle control module, body control module, transmission control module, digital sound system, etc.) The battery acts as a voltage stabilizer to help the generator keep its output below 15 volts. By disconnecting one of the battery cables, the generator loses the stabilizing action. The generator then may go crazy and produce a major energy dump (high voltage surge) into the electrical system, possibly destroying the generator itself but most assuredly a lot of electronics in the car.
It's so easy when you know how. Notice Figure 5. The DMM is connected to the battery terminals with the engine running and the generator producing a charging voltage. The DMM shows 14.20 volts of charging voltage; we conclude the generator is producing. Now, with the engine running at about 1500 rpm, turn on all electrical circuits, high beams, high blower, AC, wipers and radio. This creates an electrical current load on the generator. Watch the charging voltage. It should not drop below 13.5 volts under load. The higher the charging voltage is above 13.5 volts, the stronger the generator. Lower than 13.5 volts indicates a weak charging system, so look for a loose or worn generator belt. Check connection voltage drops for corrosion.
