Moped charging systems written by Peter Heid
Moped Charging Systems:
On almost every moped you will find an alternator that is built as part of the ignition system and it uses the flywheel magnets for generating both ignition power and light power. The moped charging system is typically very simple with no battery and no regulation. This is possible because the low output of the charging system is self regulating. A typical stock moped will provide around 30 watts at approximately 6 or 12 volts, which requires little regulation. Newer models may provide 65 or more watts of power, but with regulation. As a comparison, the Honda GL1800 has an alternator capable of producing 1100 watts. The non-regulated system components use an amount of power close to the alternator output. Because the maximum wattage is so low, if the power generated is cut off from the load, the charge coil and wires will not overheat. When you shut off the light switch the power generated in the light coil will not flow and must be dissipated as heat. Thirty watts of heat is not enough to overheat the wires and destroy the insulation.
Federal DOT regulations do not allow the headlight to be effected by other lights or devices, so often times the manufacturer would operate the brake light from a separate circuit that uses the ground side of the ignition coil for power. When turn signals became part of the moped electrical system, the manufacturers often installed a battery and a separate charge coil for the battery. The battery was typically used only for the turn signals, but sometimes other functions such as the horn may be used in this circuit also. If the moped is equipped with electric start, a battery is required. To charge the battery from the AC output of the moped charging system, a rectifier/regulator is required. This device converts the AC voltage to DC and limits the peak voltage to protect the moped electrics.
Every standard alternator is made up of a rotor that produces a magnetic field which rotates about a stator. The windings of the stator generate power as the magnetic field alternates between it's north and south poles. Alternators can be a stand alone device or they can be discrete components that work together. This is most commonly the case with mopeds and small motorcycles, with the flywheel being the rotor, and the coils on the magneto plate are the stator. The alternator, as the name suggests, produces AC power. Alternating current is often used to power light bulbs and some ignition systems on mopeds and small motorcycles. Any vehicle that has a starter motor must also have a battery to power it. Batteries cannot store AC current, therefore current must be converted in to DC current. A component known as the rectifier performs this conversion. The the regulator keeps the voltage from rising above the required voltage. Of course the alternator must provide more power than needed by the vehicle load so the battery will stay charged.
The Field Controlled Alternator:
Probably not used on any mopeds, but worth mention is the field controlled, or field wound alternator. This system just exchanges the permanent magnet for an electromagnet, and is often recognized by the use of brushes to provide DC power to the electromagnet. This is commonly called the "field". Most cars use this type of alternator. Because the field strength can be varied, this type of alternator can produce more power at low RPMs, and less if needed, at high RPMs.
Most field controlled systems rotate the field rotor inside the stator windings. To provide power the brushes contact two slip rings that are wired to each end of the field coil. The field rotor rotates with the engine and the stator winding surrounds it. The field current can be varied by the regulator to control the output. Because this type of alternator varies the output in conjunction with the load, it is more efficient then the permanent magnet alternator. It is not unusual to feed 20 watts in to the field coil to power it.
Older motorcycles used a field controlled alternator that appears somewhat different. It looks like a cage made up of fingers around the rotor, with the windings inside. The fingers alternate from one side of the rotor to the other and they are magnetized. The fingers on one side are north and the others are south for a varying field.
Permanent Magnet Alternator:
The power generator on a moped is producing AC current with a permanent magnet, or fixed field alternator. The alternator is composed of the stator and the flywheel. The stator is an assembly of copper wire coils on steel or iron cores located inside the flywheel that is subject to a varying magnetic field. The magnetic field goes through a complete magnetic phase as the magnet passes the the steel core of the winding. It does this by impressing the coil with a magnetic field from one pole, and as it rotates, it shifts to the opposite pole, reversing the magnetic field. The magnets themselves have north and south poles of opposite character. This magnetic field variance from pole to pole is what is needed to produce AC current. The greater the magnetic field change, the greater the power output. Stronger magnets and smaller gaps between magnets and coils, as well as greater RPM, will produce more voltage, and more magnets will produce more current. Most flywheels have 4 or more magnets, most generally in multiples of 2.
The coils that make up the stator usually include the magneto for the ignition, or in the case of CDI it will have a charge coil and usually a trigger coil also. These coils have no bearing on the charging system and are not included in this discussion.
The stator coils are arranged in 2 general formats, individual coils or a star shaped assembly with all the coils wound on it. Typically individual coils are older, and the more compact star assembly has a higher power density for the larger power requirements of more modern mopeds. The stator coils can be wired in many configurations, including multiple coils on one core. On stators with separate coils it is common to find one or more coils that are wound a certain number of turns and then the wire is split. At the split the wire exits the coil for a connection and it also continues to be wound around the core. Basically this is a center tapped coil which makes 2 separate coils with a common connection. The reason for multiple coils is for the operation of independent circuits. The headlight may have a separate coil from the horn and tail light. When the more modern star stator configuration came about, because it can provide much greater power output, the coils are typically run in series so there are only 2 wires for the lighting coils. The power is not usually split up in to independent circuits until after it is run through a rectifier/regulator and the battery. In some more modern charging systems the coils are grouped together in sets of three and wired for a 3 phase output. This is not normally found on small motorcycles or mopeds. The 3 phase system is easily identified by 3 charging wires, usually colored the same, going from the stator to the regulator.
Permanent magnet alternators are not very efficient, but they are very simple and quite reliable. Simplicity means low cost, and in combination with reliability, this explains why it is the most common charging system on mopeds and motorcycles. What makes this alternator inefficient is the short circuiting of the excess power in the system. This is accomplished by the resistance of the charging circuit and the regulator, if one is used. The permanent magnet alternator has a big advantage over a field controlled alternator because the alternator has no brushes to wear out. Also, a standard field-wound alternator requires 40 watts to energize the field, whereas the permanent magnet alternator does not require any power to start generating.
Permanent magnet alternators have an inherently poor load regulation curve. With no load the output voltage can vary from 20V at idle to well over 100V at high RPMs. When the load current is increased a magnetic field is created in the stator which is in opposition to the field of the permanent magnets. This tends to limit the short circuit current, and when the regulator does short the windings, the current is near maximum in the field which cancels out much of the permanent magnet field. This reduces the load on the engine, making the permanent magnet alternator appear more in efficient than it actually is.
The number of turns of wire that can be fit on a charge coil will determine it's power output, assuming the magnetic strength and RPMs remain constant. The power is the amount of wattage provided by the coil, which is the voltage multiplied by the amperage. With the magnetic strength and RPMs constant, the wire diameter is what will determine the voltage and current. The wire diameter is often an educated guess the first time a coil is made, and calculations can be derived from it for the required modifications. When modifying a coil by rewinding it, the educated guess has been done already. The first consideration is the maximum number of amps in a short circuit condition. Basically if the charging system is at full output the system amperage can not exceed the maximum amperage of the coil wire. When multiple coils are wired in series, the direction the coils are wound must be the same or they will generate out of phase of each other and cancel out the power pulses. When the power generation is in phase in a number of coils series wired together, the voltage will increased as the peaks of the phases overlap. The peak voltage will be the sum of all the coils in the same phase. If they are out of phase then the peak voltage will be that of a single coil, but the current will be multiplied by the number of coils.
To be able to produce DC voltage high enough to run the vehicles electrical system, the AC voltage must be high enough. Because AC voltage varies in the form of a sine wave, the average voltage is much less than the peak voltage. Since AC voltage is both negative and positive, it has an average value of zero volts. Both the negative and positive voltages do work, so they must be averaged together somehow. The averaging method used is called root mean square or RMS. To get the RMS value, first square the voltage to make all positive values, then find the average and take the square root. This means for a 6 volt system with an actual output voltage of 7.2 volts, a minimum of 10.2 volts AC must be produced (7.2V / 0.707 = 10.2V). A 12 volt system operating at 14.4 volts would need twice the AC voltage or 20.4 volts to work. This is the approximate voltage at which it will start charging.
6 to 12 Volt Conversion:
One of the most common modifications for mopeds is to convert them from 6 to 12 volts. There is one basic rule of thumb that should be followed to make it an easy transition. When the AWG number is decreased by 3 the area of the wire is doubled. Number 22 AWG is about twice the area of number 25 AWG. When the area of the wire is doubled, the current carrying capacity is doubled also, and the resistance is halved.
What this means is in a system of 30 watts at 6 volts, reducing the size of the wire by 3 AWG numbers will cut your amperage in half and it will double the voltage. Six volts at 30 watts would give 5 amps, while 12 volts at 30 watts would only provide half that or 2.5 amps. The 6 volt system might have 22 AWG wire which handles 7.0 amps, and for 12 volts, 3 sizes smaller, number 25 AWG handles 2.7 amps. The more copper that can be put on to a coil, the higher the output in watts. The maximum voltage the wire can handle is based on the insulation while the maximum current it can handle is based on the wire diameter.
When the output of the system exceeds the load by more than the charging circuit can dissipate through the load and it's internal resistance, a regulator must be used. The controlled field regulator works by varying the field voltage, while the permanent magnet alternator uses a shunt or series regulator. The shunt regulator works by sensing the battery voltage and short circuiting the excess power. This allows the excess power to be converted to heat and dissipated.
The shunt regulator has to dissipate the heat and often employs cooling fins on it's aluminum case. In higher power systems it can get get very hot and a good thermal connection to the chassis is required. They are also called a “parallel” regulator because the load shunt or resistance is in parallel with the load. The shunt resistance is constant and the load is switch on and off to try to keep a constant voltage. The shunt resistance tends run opposite of the load, an increase in the load gives in increase in shunt resistance. This is the most common type.
The series regulator, like it sounds, has the shunt in series with the load, and it varies the resistance for a constant output. The operating characteristics of this regulator are opposite of the shunt regulator, with the shunt resistance increasing as the load increases.
The battery is a storage device for the charging system, and provides the power to run the electrics when the engine is off or at low charging output like idling. Most batteries are lead acid types with a nominal voltage of 6 or 12 volts. Modern systems sometimes employ lithium ion batteries for a much greater power to weight ratio. The worst thing you can do to a lead acid battery is discharge it completely. Batteries can have the required voltage but may not provide sufficient current, and for this reason it is necessary to load test a battery. Either type of battery can last 10 years or more, depending on the care it is given, but lithium batteries require an occasional use of a dedicated charger for full life.
The first thing that should be done when testing any charging system is to inspect all the electrical connections. Loose or corroded connections will cause problems and must be remedied before accurate test results can be had. A poor connection can burn out a charging system because it is increasing the load. Heat is generated at poor connections that may melt the insulation or cause arcing that can destroy electrical connectors. A condition called “thermal runaway” is possible, where more heat causes an increase in current draw, which causes more heat, to the point of failure. All electrical connections should be subject to dielectric grease to prevent corrosion.
When testing a charging system it is best to put a load that is close to equivalent to the output of the system. Any restive load that doesn't drop the voltage below the system voltage will be adequate. A group of light bulbs is an excellent load for the charging system. On a 30 watt system three 15 watt bulbs would make a good load for testing. The output voltage and current should be measured with successive tests using 1 bulb first, then 2, then all 3, in series. The test should be done at the engine RPM for the vehicles rated output, or at a reasonable operating RPM. This testing is done without a battery in the system so it doesn't effect the charging system output.
If equipped with one, measure the voltage across the battery terminals, first shut off, then with the bike running, at the rated charging RPM. If you don't get an increase in voltage from the battery voltage to 7.2 volts, or 14.4 volts, then the charging system isn't working. If the battery voltage is low, then the charging system may not give correct readings. Use a fully charged battery when testing.
The engine is used to provide the mechanical power that is converted to electrical power, and it is not available for propulsion. The theoretical amount of power is the total wattage burned, which includes the load, and any loss through regulation. Multiplying the output voltage and current with a full load will give the maximum output in watts. There 746 watts per horsepower and most alternators are around 90% efficient at generating power, so it is more like 821 watts actual. A moped with 50 watts of electrical power would use 50w/821w/hp = 0.06 horsepower. It is a small amount of engine power used, and even doubling the wattage would still consume less than 1/8th of a horsepower.
Because the alternator produces power proportionately to the engine RPM, at low engine speeds, the engine power is only a fraction of full output. With the permanent magnet alternator, the output is always at it maximum possible output, so that amount of power will be used for generation, regardless of the load. If the vehicle uses 30 watts and the charging system produces 50 watts, the engine will be required to provide enough power to generate 50 watts.
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