since 1940

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Identifying:

Alternator induced radio noise is a high pitched whine whose pitch and intensity increases and decreases with changes in engine speed. Turning the alternator master switch off also turns off the radio noise.

Solid state regulators that use a pulse-width-modulated field- control system can also create alternator whine. Whine caused by the regulator can be distinguished from the alternator in that regulator induced noise intensity changes in intensity and pitch with current load at a constant engine speed. Thus turning on the landing lights won't increase alternator whine but will increase regulator induced alternator whine. See Feedback below for a description of a regulator noise solution.

Morphology:

Current generated in the alternator stator windings is alternating and is converted to direct current before it leaves the alternator. Alternator diodes convert (rectify) the current from AC to DC. Six diodes are required to rectify the three stator phases. Three diodes comprise the positive cluster and three the negative cluster. As the voltage of each phase increases, a given pair of diodes becomes forward biased and allows alternator current to pass. Which stator winding and diode pair that is conducting at any moment depends upon rotor position. The combined result is a DC voltage with a slight amount of ripple voltage. Ripple voltage conducts into the electrical bus and then into the radio circuit.

Ripple voltage can be detected on the electrical bus with an oscilloscope; Another method is to use a volt meter set to AC volts. Connect a capacitor in series to the positive lead from the meter to block out the DC voltage so that only the ripple voltage gets to your meter. The capacitor is an open circuit to DC but passes AC. The volt reading is then the amount of ripple voltage on the bus. You will need to do comparison readings with other aircraft to determine what AC voltage level is normal.

Alternator cause:

Normally, there is not enough ripple voltage to cause radio noise. But, there are two conditions that can cause an increase in ripple voltage and radio noise: these are diode failure and increased circuit impedance.

If a diode fails the amount of ripple voltage increases. Alternator whine can be a symptom of a bad alternator diode. Two test methods are used to test the alternator without disassembly. This is a hand held unit with a probe that clamps over the alternator output wire. A bad diode will show up on the meter.

These meters were originally sold as the Ward Aero Alternator Tester model 647. They are currently sold by Support Systems Inc. as model 10-647-01. The second test method is to use an oscilloscope to check the alternator output for excessive voltage ripple or rectifier spikes caused by a bad diode.

Ward Aero 647 Alternator Diode Tester

With the alternator apart the diodes can be checked with a VOM meter. This test makes sure that each diode conducts in only one direction. Disassemble the stator leads from the rectifier. Calibrate the VOM on the R x 1 multiplier range scale so that there is zero reading with the VOM leads shorted together.

Positive diode test:

Connect one test probe to the large positive terminal stud and touch the other test probe to each of the three stator terminals. Note the three ohmmeter readings, they must be identical. Reverse the test probes and repeat the test. Note the three ohmmeter readings, again they should be identical to each other but not the same as in the previous step. Three of the ohmmeter readings should show a low resistance reading of approximately 6 to 20 ohms and three should show an infinite reading (no meter movement). Negative diode test: Repeat the test but connect one test probe to the small negative terminal stud.

Circuit causes:

Alternator whine can be caused by poor electrical connections, especially at the battery. Voltage ripple cannot occur in a zero impedance electrical circuit. Impedance is simply the amount of resistance to high frequency current. It is analogous to DC resistance and like DC resistance is measured in ohms. The low impedance of the battery holds the aircraft's electrical circuit at a DC potential. Any voltage ripple in the bus is absorbed by the battery. Thus, the aircraft battery acts as a large ripple voltage absorber. Alternator noise cannot occur if the electrical connections have zero impedance. Unfortunately will always be some impedance and ripple voltage in the electrical circuit but the better the electrical connections the less there will be.

Lets assume that the battery positive terminal is corroded. Although DC resistance as measured with an ohm meter may still be low, the high frequency resistance may be very high. The higher this resistance, the greater the amount of voltage ripple on the bus and the greater the radio whine.

Circuit impedance can be lowered by making sure the battery posts are clean and making good contact. Resistance should be less than .01 ohm. Also check the alternator ground connections. DC resistance between the alternator and the negative post of the battery terminal should be as low as possible.

The ideal low-noise circuit would have the alternator power output going directly to the battery's positive terminal. This dumps voltage ripple into the battery. The radio power lead would also go directly to a pure DC source, the battery.

If the alternator power lead and the radio power lead connects to a bus, then voltage ripple can go from the alternator to the radio power lead. The amount of voltage ripple at the bus depends upon the impedance between the bus and the battery. This impedance is higher than at the battery. Thus, in the ideal low-noise circuit, power termination occur at the battery. Power return (ground path) would be wired directly back to the negative post of the battery. This prevents conducting high frequency currents through the aircraft airframe.

With less than ideal circuits, the return path is from the alternator to the engine, engine mount, fireball, and through the fuselage to the battery. These connections should have low resistance. Flat braided ground straps are ideal for grounding the airframe to the engine mount. Flat braided straps are used because impedance is less with a braided, flat conductor than a round wire conductor.

Airframe Causes:

In 1976, Cessna started using audio panels that slide in and out of a rack just like a radio. Experience has shown that if the panel is not properly grounded, alternator whine can be heard, especially in the ear phones. All audio panel rack connector plugs will have a black ground wire that is part of the wire bundle. This ground wire should securely attach to the airframe, preferably with its own separate mounting screw.

Solutions:

There are two methods of filtering voltage ripple; diverting the ripple voltage back to the source, or blocking the voltage ripple so that it cannot pass. Capacitors divert noise currents whereas inductors block noise currents. The most effective approach depends primarily on the circuit impedance.

Capacitors divert noise currents back to the alternator return path (commonly referred to as ground). Capacitors must have a low impedance path back to the alternator to be effective. Install the filter capacitor as close to possible to the alternator. The capacitor is installed with one lead connected to the power output and the other lead to ground (placed in parallel to the circuit).

For DC voltages the capacitor forms an open circuit (high impedance) and doesn't allow any current to pass. At noise frequencies the capacitor forms a short circuit (low impedance) and passes noise currents to the alternator. In this manner we have formed a low-pass filter. The effectiveness of using a capacitor as a noise filter depends upon matching the capacitance rating of the capacitor to the frequency of the noise currents.

Alternator Filter Part Number CA63897-006 as used on Piper Aircraft. Cessna uses a similar filter part number S1915-1

Choosing the correct size capacitor

In order for the capacitor to be effective the impedance through the capacitor must be lower than the impedance of the original circuit. The capacitor represents an impedance of infinity (at DC voltage) to close to zero impedance at some higher frequency and then increasing impedance at even higher frequencies. One must select a capacitor whose impedance is the same or less than circuit impedance.

The effectiveness of the capacitor as a filter depends upon the capacitors capacitance and impedance, and the circuit's impedance. The higher the circuit impedance the better the capacitor filters. An ultimate high-impedance circuit is an open circuit. An example of an open circuit is the "P" lead from the magneto. With the magneto operating the "P" lead is open at the ignition switch. An example of a low-impedance circuit is a dead- short.

The frequency at which the capacitor's capacitance and inductance are equal is where it has the lowest impedance and the best filtering. This is the resonate frequency. The correct size capacitor is one where the frequency we wish to divert is the same or less than the resonate frequency.

Smaller size capacitors (Pico farad range) are effective at high frequencies and larger size capacitors (microfarad) range are effective at lower frequencies. As a rule-of-thumb if your filtering conducted interference, as you are in an alternator, then this is low frequency and your capacitor should be in the micro- farad range. If your filtering radiated interference where the conductor is acting as an antenna, this is a higher frequency and your capacitor should be in the Pico farad range.

Typically, the alternator uses a .5 to 50 microfarad capacitor. Cessna has a 5.72 microfarad capacitor filter available as part number S1915-1.

The best type of capacitor for filtering is a ceramic and then tantalum capacitor. Ceramic capacitors for the Pico farad range and tantalum for the microfarad range. The reason ceramic is best is because of the capacitor's low series resistance. Usually ceramic has the least series resistance and electrolytic the most.

Capacitor resonance can be approximated with the following formula:

Notice that total capacitor lead length has a significant affect on the capacitor's resonate frequency. For example, a 500 pf capacitor with 1/4 inch leads resonates at 100 MHz. But with 1 inch leads resonates at 50 MHZ. Lead length affects diminish as capacitance increases. As a practical rule-of-thumb capacitor lead lengths used in resonate circuits should be kept as short as possible.

 

Lonestar alternator filter part number LS03-01004 available here at Sky Ranch

Another solution is to block the noise before it enters the affected equipment. The Lonestar LS100003-06 (available at Sky Ranch) is effective at reducing power line noise from alternators, strobes, beacons, etc.

Blocking voltage ripple

The other solution to radio noise is to block the ripple with an inductor. The most common style of inductor is a ferrite core. These come in many different styles but typically the wire with the noise currents is wrapped around the core (placed in series with the circuit). DC current passes through the core but high frequency currents induce a magnetic field in the ferromagnetic material of the core. This magnetic field raises the impedance and effectively blocks noise currents. Ferrite's are effective on radio power input leads and strobe power input leads. In the first case they prevent noise currents from entering the radio and in the second case they prevent noise currents from exiting the strobe.

To be effective, ferrite impedance must be larger than circuit impedance. In a typical alternator circuit this would require a rather large ferrite. So alternator voltage ripple is usually diverted to ground by use of a capacitor. However, ferrites are simple to use and have an amazing filtering ability. Ferrites are best used in low impedance circuits whereas capacitors are best used in high impedance circuits. It is best to install ferrites on the radio power input lead rather than on the alternator power lead.

Feedback:  ADF noise and the alternator regulator

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