Identifying the problem
Alternator induced radio noise is a high pitched whine whose pitchand intensity increases and decreases with changes in engine speed.Turning the alternator master switch off also turns off the radionoise.
Solid state regulators that use a pulse-width-modulated fieldcontrol system can also create a whine in the radios. Regulator-causedwhine can be distinguished from the alternator-caused whine inthat the intensity and pitch of regulator-induced noise changeswith changing current load at a constant engine speed. Thus, turningon the landing lights won’t increase alternator whine but willincrease regulator whine.
How the alternator works
Current generated in the alternator stator windings is three-phasealternating current, but diodes convert it from AC to DC beforeit leaves the alternator. Six diodes are required to rectify thethree stator phases. Each of the three stator windings is connectedto a pair of diodes. Three diodes are connected to the positiveoutput terminal of the alternator, and the other three are connectedto the negative (ground) terminal.
As the voltage of each stator winding increases, the correspondingpair of diodes becomes forward biased and allows alternator currentto pass. Which stator winding and diode pair is conducting atany moment depends upon rotor position. After the diodes rectifythe three AC phases and sum them all together, the combined resultis a DC voltage with only a slight amount of AC ripple voltageremaining.
The best way to detect ripple voltage on the electrical bus iswith an oscilloscope. Another method is to use an ordinary volt-ohmmeter(VOM) set to measure AC volts. You may have to connect a capacitorin series with the positive meter lead to block out the DC voltageso that only the ripple voltage gets to your meter. (Some metersdo this automatically when you select AC volts.) The capacitoris an open circuit to DC but passes AC, so the voltmeter readingyou see is the amount of AC ripple voltage on the bus. You willneed to do comparison readings with other aircraft to determinewhat AC voltage level is normal.
What causes alternator whine?
Normally, there is not enough ripple voltage to cause radio noise.But, there are two conditions that can cause an increase in ripplevoltage sufficient to create radio noise. These are diode failureand increased circuit impedance.
If an alternator diode fails, the amount of ripple voltage increasesmarkedly. Alternator whine can be a symptom of a bad alternatordiode. Two test methods can be used to test the alternator withoutdisassembly.
There is a hand held unit with a probe that clamps over the alternatoroutput wire. A bad diode will show up on the meter. These meterswere originally sold as the Ward Aero Alternator Tester model647. They are currently sold by Support Systems Inc. as model10-647-01.
The second test method is to use an oscilloscope to check thealternator output for excessive voltage ripple or rectifier spikescaused by a bad diode.
Checking the diodes
With the alternator apart, the diodes can be checked with a VOMset to measure ohms. This test makes sure that each diode conductsin only one direction. You need to unsolder the stator leads fromthe each diode. Calibrate the VOM on the R x 1 multiplier rangescale so that there is zero reading with the VOM leads shortedtogether.
Connect one test probe to the alternator’s positive output terminaland touch the other test probe to each of the three solder terminalsof the diodes mounted to the positive rectifier plate. Note thethree ohmmeter readings: they should be identical. Now reversethe test probes and repeat the test. Note the three ohmmeter readings:again they should be identical to each other, but not the sameas in the previous step. Three of the ohmmeter readings shouldshow a low resistance reading of approximately 6 to 20 ohms andthree should show an infinite reading (no meter movement).
Repeat the same test procedure for the three diodes on the negativerectifier plate, connecting one test probe to the negative outputterminal and checking all three diodes with the other probe. Thenreverse the leads and check again. The diodes should show lowresistance in one direction, and infinite resistance in the oppositedirection.
Alternator whine can also be caused by poor electrical connections,especially at the battery. Normally, the low impedance of thebattery keeps the aircraft’s electrical circuits at a DC potential.(Impedence is simply resistance to an AC current.) Any AC ripplevoltage in the aircraft bus is absorbed by the battery. Thus,the aircraft battery acts as a big ripple absorber.
If the battery provided zero impedence (i.e., a short-circuitfor AC current), alternator noise could not occur. In the realworld, there will always be some impedance. But the lower it is,the less ripple voltage there will be.
Let’s assume that the battery positive terminal is corroded. AlthoughDC resistance as measured with an ohmmeter may still be low, thehigh-frequency resistance (i.e., impedence) may be very high.The higher this impedence, the greater the ripple voltage on thebus and the more whine you hear in your radios.
Circuit impedance can be lowered by making sure the battery postsare clean and making good contact. DC resistance should be lessthan 0.01 ohm…virtually zero. Also check the alternator groundconnections, including the engine grounding strap. DC resistancebetween the alternator and the negative post of the battery terminalshould be as low as possible.
The ideal low-noise circuit would have the alternator power outputgoing directly to the battery’s positive terminal. This dumpsripple voltage into the battery, where it is absorbed. The radiopower lead would also go directly to a pure DC source, the battery.
If the alternator power lead and the radio power lead connectsto a bus, then voltage ripple can go from the alternator to theradio power lead. The amount of voltage ripple at the bus dependsupon the impedance between the bus and the battery. This impedanceis higher than at the battery.
The return path is from the alternator to the engine, engine mount,firewall, and through the fuselage to the battery. These connectionsshould have low resistance. Flat braided ground straps are idealfor grounding the airframe to the engine mount. Flat braided strapsare used because impedance is less with a braided, flat conductorthan a round wire conductor.
There are two methods of filtering ripple voltage: bypassing theripple voltage back to the source, or blocking the voltage rippleso that it cannot pass. Capacitors are used to bypass ripple voltage,whereas inductors are used to block noise currents. The most effectiveapproach depends primarily on the circuit impedance.
Capacitors bypass noise currents back to the alternator returnpath (commonly referred to as ground). To be effective, a capacitormust have a low impedance path back to the alternator. Consequently,a filter capacitor must be mounted as close to possible to thealternator. The capacitor is installed with one lead connectedto the power output and the other lead to ground, so that it isin parallel with the circuit.
For DC voltages the capacitor forms an open circuit (high impedance)and doesn’t allow any current to pass. At noise frequencies thecapacitor forms a short circuit (low impedance) and bypasses noisecurrents back to the alternator. In this manner we have formeda low-pass filter. The effectiveness of using a capacitor as anoise filter depends upon matching the capacitance rating of thecapacitor to the frequency of the noise currents.
The frequency at which the capacitor’s capacitance and inductanceare equal is where it has the lowest impedance and the best filtering.This is the resonant frequency. The correct size capacitor isone where the frequency we wish to bypass is the same or lessthan the resonant frequency.
Smaller size capacitors (picofarad range) are effective at highfrequencies, while larger size capacitors (microfarad range) areeffective at lower frequencies. If you’re filtering conductedinterference (as you are in an alternator), then this is low-frequencyand the capacitor should be in the microfarad range. If you’refiltering radiated interference (where the conductor is actingas an antenna), this is high-frequency and the capacitor shouldbe in the picofarad range.
Typically, an alternator filter uses a .5 to 50 microfarad capacitor.Cessna has a 5.72 microfarad capacitor filter available as partnumber S1915-1.
The best types of capacitors for filtering are ceramic and tantalumcapacitors, ceramic for the picofarad range and tantalum for themicrofarad range. Electrolytic capacitors are relatively poornoise filters, and also have a short life.
Capacitor resonance can be approximated with the following formula:resonant frequency (in MHz) equals 1/2 pi times the square rootof lead length times capacitance. Notice that lead length hasa significant effect on the capacitor’s resonant frequency. Forexample, a 500 pf capacitor with 1/4 inch leads resonates at 100MHz. But with 1 inch leads, it resonates at 50 MHz. So capacitorlead lengths used in filter circuits should be kept as short aspossible.
The other way to filter radio noise is to block the ripple witha series inductor. The most common style of inductor for noisefiltering is a ferrite core. These come in many different stylesbut typically the wire with the noise currents is wrapped aroundthe core, creating an inductor in series with the circuit. DCcurrent passes through the core but high frequency currents inducea magnetic field in the ferromagnetic material of the core. Thismagnetic field raises the impedance and effectively blocks noisecurrents. Ferrites are effective on radio power input leads andstrobe power input leads. In the first case they prevent noisecurrents from entering the radio, and in the second case theyprevent noise currents from exiting the strobe.
To be effective, ferrite impedance must be larger than circuitimpedance. To filter the output of an alternator would requiredan impractically huge ferrite core. So alternator voltage rippleis usually bypassed 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 capacitorsare best used in high impedance circuits. It is best to installferrites on the radio power input leads, and to use a filter capacitorat the alternator output terminal.