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How to Test an Alternator with an Oscilloscope

written by: Swagatam • edited by: Lamar Stonecypher • updated: 8/23/2010

Alternator troubleshooting with an oscilloscope is definitely simpler and much accurate than any other means. Here we diagnose the various typical waveform signatures produced by an oscilloscope and study what indications they reveal through a step-wise analysis.

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    What is an Alternator?

    Alternator troubleshooting with an oscilloscope, without a prior knowledge about an alternator's basic operating principle, can become quite confusing and puzzling. The following explanations will briefly enlighten you regarding alternators and their working concept, let's go through it.

    An alternator is an electromechanical device that is able to convert externally applied mechanical energy into an electrical energy through Alternator Image electromagnetic induction, generated by a rotating movement of its shaft.

    A modern alternator may be made up of the following essential elements:

    A central rotating field winding assembly called the rotor.

    A stationary induction winding assembly surrounding the rotor, called the stator.

    A diode network or the rectifier.

    A precision circuit unit called the voltage regulator is nowadays often included within the outer aluminum enclosure of the alternators.

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    How an Alternator Operates

    An alternator is probably the most essential part of every automobile and is most extensively used here. Its functioning can be better understood by studying the various stages involved inside a typical vehicle alternator.

    Let’s walk through its different stages of operation:

    Typical Alternator Wiring Layout 

    To initiate the alternator, some amount of power is derived from the battery which is under charge and supplied to the rotor field winding via the voltage regulator. The rotor armature instantly develops a strong magnetic field around it and interacts with the stator assembly. The action forces the rotor to rotate. The rotating rotor’s magnetic field around the stator induces a strong AC current in the stator windings. This voltage is applied to the ignition chamber where it is converted into a high voltage spark. The spark ignites the present fuel mixture to create an explosion and start the engine. Since the engine is connected to the rotor of the alternator through a pulley and belt mechanism, it takes hold of the rotor rotation. The rotor now relieves the battery and starts rotating from engine power. After this, the alternator’s stator voltage is solely used to charge the battery and power other electrical equipment.

    A voltage regulator is specifically included to strictly monitor the charging level of the battery by correcting the stator output current accordingly. The corrections are made by appropriately increasing or decreasing the rotor field current.

    For example, as long as the connected battery is not fully charged, the regulator maintains a higher current level to the rotor field winding; this increases its electromagnetic effect over the stator winding, which in turn delivers a higher amount of current to the battery. This process starts reversing as the battery gradually gets fully charged.

    Primarily, the stator may comprise of three sets of windings, each of these being positioned differently to each other.

    3 Phase Alternator Output Waveform, Image Normally the above windings exists in two types of configurations, namely in Wye and delta designs as shown in the diagrams. The induced current from these windings result in a three phase output. Three phase output helps in generating more voltage even at lower speeds, thus making the functions more efficient.

    When an alternator is intended to generate electricity from wind power, the pulley-belt assembly of the rotor may be simply integrated to the turbine mechanism and the above procedure replicated.

    However, since the main idea here is to charge a connected battery most efficiently, any malfunction in the above explained procedure may drastically hamper the results. Also, since the entire operation involves many critical electronic and mechanical stages, pin-pointing a particular snag may be pretty difficult and can be properly located only with an oscilloscope.

    The next section shows the various possible outcomes produced by an oscilloscope for different faults that may possibly occur inside an alternator.

    Image Courtesy : http://www.autoshop101.com/trainmodules/alternator/alt101.html

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    Using Oscilloscope for Troubleshooting Alternators

    Connecting an Osciloscope to an Alternator, Wiring Diagram, Image To test a running alternator over an oscilloscope, commence the procedure with the following simple instructions:

    Start the engine of your vehicle or the generator, now carefully unscrew the battery from its clamps.

    It is assumed that you know how to use an oscilloscope, otherwise go to - http://www.doctronics.co.uk/scope.htm,

    Broadly, you need to select the AC or the DC positions over the scope while testing the various waveforms as indicated in the testing tips and keep it in parade position preferably.

    As shown in the adjoining figure, connect the scope's negative croc clip to the negative or the ground. Connect a prod to the posive clip so that you may just need to touch this prod point to the terminal which was previously attached to the battery positive. You may get one of the following readings as per the performance of the alternator:

    A typical oscilloscope waveform indicating a normal operating condition of an alternator is shown in the figure below. The curve structure also shows that the alternator is running at the fast idle speed setting.Normal Waveform at Fast Idle Alternator Speed, Image 

    A normal alternator should also produce a perfectly straight line response when monitored over an oscilloscope in DC position. The figure below shows one such response. Here we can clearly witness the proportionate increase in the voltage and the current with an increase in speed of the alternator.Alternator DC Waveform, Current and Voltage at Different Speeds, Image 

    The next figure indicates the correct alternator waveform running at the slow idle mode. Here we see that the waveforms are more uniform and rounded off than the fast idle mode. The curves are less sharp than the previous waveform.Normal Waveform at Slow Idle Alternator Speed, Image 

    As already discussed, a modern alternator may consist of a built-in rectifier module for the required rectification process. If any one of the diodes involved in it becomes faulty (open), the efficiency of the alternator may be hampered to a great extent and may affect the charging of the connected battery. The graph below clearly portrays the resultant waveform for a diode failure. The curves also point towards an open stator winding or windings. The patterns are typically during a slow idle speed of the alternator.

    Alternator Diode Short or Open Waveform, Image 

    Now if the speed is increased up to fast idle, the waveforms become more aggressive and prominent, as can be seen in the bottom figure. During alternator troubleshooting with an oscilloscope, such waveforms may not at all be a welcome sight and will certainly depict an impending malfunction.Alternator Diode Short or Open Waveform, Image