Electric motors and generators combined with battery storage systems tend to be extremely useful for transmitting and storing mechanical power. With peak efficiencies of up to about 95% for both motors and generators and compact sizes possible with very high rotational speeds these electromechanical conversion devices would seem to offer endless possibilities for efficient transmission, storage and transformation of mechanical power. Getting those supper high efficiencies though requires either good matching of the design of the motor or generator to the application or very sophisticated microprocessor based controls. In the interest of more flexibility in application with simple controls a surprisingly large number of motors and generators from the past 100 years have used electro magnets instead of permanent magnets. While these electro magnets do accomplish the goal of allowing motors and generators to be tuned to work in challenging applications without sophisticated controls they also cut the maximum attainable efficiency of the device exactly in half.
Magnet Efficiency
Alternating Current Motors and Generators
Direct Current Motors and Generators
Brushless DC Motors
The Motor Generator
Limitations of Motors and Generators
Hybrid Electric Drive
Series Capable Two Motor Parallel Drive
Dual Transmission Parallel Drive
Three Clutch Electric Boat Drive
Series Electric Drive
Using electromagnets to provide the field for a motor or generator wastes all of the electrical power required to maintain that field. This is very significant because it means that the maximum attainable efficiency of the motor or generator is no longer 95%, but rather 48%. An analogy of what is going on would be to compare how much motive power is required to support a cow one foot off the ground by a helicopter versus how much motive power is required to leave a cow hanging from a derrick. Obviously once the cow is hoisted up to hang from the derrick no power is required to leave it hang there. Leaving a cow hanging one foot off the ground from a helicopter on the other hand requires that the helicopter engine be run at rather high output for the entire time that the cow is left hanging. In this analogy the derrick is a motor or generator with permanent magnets and the helicopter is the electromagnets. The analogy can be taken further to describe the added flexibility of using electromagnets. If the cow needs to be moved over ten feet to one side this can be accomplished by simply steering the helicopter in that direction. To move the derrick suspended cow over ten feet it must be lowered to the ground so that the derrick can be disassembled and then reassembled at the new location. Is hanging a cow from a helicopter worth that added flexibility of being able to move it over when desired? No of course not, and similarly maintaining the field for a motor or generator with electromagnets is generally not worth the radical reduction in efficiency.
The simplest electric generator is just a set of windings around a spinning magnet. As the magnet spins the magnetic field it generates also spins past or through the windings. As the magnetic field experienced by the windings changes an electric current is induced. This most basic generator creates a roughly sinusoidal waveform alternating current. If just one magnet and one set of windings is used then the frequency of the AC current is equal to the number of revolutions per second of the magnet. At this point it should also be pointed out that real motors or generators use armatures to take advantage of the fact that iron conducts the magnetic field and can therefore concentrate the changing magnetic field at the windings for high efficiency. The basic arraignment would be a magnet or a set of magnets embedded in a rotor made of steel or iron so that there is a single N pole and a single S pole to the rotor. This would be called a single pole rotor. The set of windings would be on a stationary armature that loops around from one side of the rotor to the other side. The set of windings could be located anywhere on the stationary armature, and are in fact equivalent to just a single winding around the stationary armature even if that single set of windings is divided into two individual windings on opposite sides of the armature.
A two pole generator can be built with either a two pole stator and a single pole rotor, a two pole rotor and a single pole stator or with a two pole rotor and a two pole stator. Normally a two pole generator would have a two pole rotor as well as a two pole stator. The frequency of the AC current would be twice the number of rotations of the rotor per second. Similarly a four pole generator would normally have a four pole rotor as well as a four pole stator and the frequency of the AC current would be four times the number of rotations of the rotor per second.
The simplest electric motor would be a synchronous AC motor with permanent magnets. This simplest synchronous AC motor would have the same layout as the simplest AC generator with a single pole rotor and a single pole stator. In the case of the synchronous AC motor a roughly sinusoidal AC current is connected to the set of windings. The changing current forced through the windings sets up a changing magnetic field, and when the motor is spinning at the synchronous speed this changing magnetic field opposes the magnetic field of the permanent magnet rotor to maintain the synchronous speed even when a load is applied to the rotor. The simplest AC synchronous motor is not necessarily self starting, and must somehow be set in motion before a current is applied to it.
The single pole AC motor would rotate at a speed in revolutions per second equal to the frequency of the applied AC signal. A two pole AC motor could have a one pole stator with a two pole rotor or a one pole rotor with a two pole stator. Like with the AC generator a two pole AC motor would normally have both a two pole rotor and a two pole stator. The two pole AC motor would rotate at a speed in revolutions per second of half of the frequency of the applied AC signal. A single pole AC motor running on 60Hz electricity would spin at 3600RPM and a two pole motor would spin at 1800RPM. There is no way to spin a basic type AC generator at more than 3600RPM to produce 60Hz current and similarly there is no way to get a simple type AC motor to spin faster than 3600RPM on 60Hz electricity.
Starting a synchronous AC motor can be accomplished with a starting motor or the design of the motor can be modified so that it does not get stuck between phases while starting. The self starting motors typically use large capacitors to create a lag in the applied current which prevents the motor from getting stuck and allows some torque to be generated even before the motor has come up to synchronous speed. If the capacitor is left electrically connected to the windings while the motor runs it causes the motor to run somewhat less efficiently, which is why many capacitor start motors use centrifical switches to disengage the capacitor once the motor has come up close to synchronous speed. The capacitor can also be disengaged using electronic control circuitry, but this technique has seen surprisingly limited use since the advent of cheap electronic controls in the 1960's.
Because three phase AC power can allow synchronous AC motors to self start with no additional equipment it has been widely used in industry for most of the history of electrical power distribution. Three phase power is three sinusoidal signals traveling on three wires instead of just a single sinusoidal signal traveling on two wires. The simplest three phase motor is a single pole rotor with a single N pole and a single S pole which rotates inside a single pole three phase stator that has three sets of windings. These three windings are connected to the three legs of the three phase power with either a delta or a wye configuration. Each winding experiences a roughly sinusoidal signal that is in phase with the rotation of the rotor. One rotation of the rotor for each cycle of the AC current. A two pole three phase motor would have a two pole rotor and six sets of windings connected to the three legs of the three phase power. This two pole three phase motor would make one revolution for every two cycles of the AC current.
A direct current (DC) motor can run off of a battery, and a DC generator can be used to charge a battery. The simplest type of direct current motor is the brush type permanent magnet motor. In a brush type motor the magnets are in the stator and the windings are in the rotor. The windings are connected to the supply current through the brushes which switch the polarity of the DC current so that the motor can run roughly as an AC motor. DC motors usually have a large number of poles for good starting torque and smooth operation. Each set of windings in the rotor has a set of contacts, and all the contacts are lined up on the race that the brushes ride on. As the contacts rotate under the brushes an electrical circuit is set up in one set of windings after another. A larger number of poles on a brush type motor is analogous to a higher frequency AC current in a synchronous AC motor.
Because brush type motors can produce very large starting torque and can run over a range of speeds they are also often used on AC power. A brush type motor used on AC power is referred to as a "universal motor", or as an induction motor. An induction motor is a motor that uses electromagnets. Since the polarity of the electromagnets changes with the polarity of the current applied to the windings through the brushes an induction motor can run directly off of AC current. For a permanent magnet brush type motor to run on AC current a bridge of four rectifying diodes (or six for three phase power) must be installed before the brushes.
Producing direct current always requires some sort of rectifying device, and the simplest way to rectify an AC signal into DC current is with a bridge of diodes. The diode bridge consists of four diodes wired so that each of the two AC leads is connected to both the positive and negative DC outputs. Prior to silicone based diodes the copper/copper oxide diodes used for rectifying electric current were very inefficient and had to be large and well cooled to last. Because of the lack of efficient diodes most DC current was produced with brush type generators, which are identical in layout to the brush type motor.
The efficiency of silicone diode rectification is proportional to the voltage of the signal being rectified since each silicon diode has a fixed 0.7 to 0.8V drop across it. This means that diode rectification for a 12V system is always less than 90% efficient. For a 120V system though diode rectification is extremely efficient, with just one percent of the power lost to the voltage drop across the diodes. To get lower voltage systems to work more efficiently SCR transistors (which have no fixed voltage drop across them) can be used instead of diodes, but this requires sophisticate control circuitry to monitor the phase of the AC signal and turn the SCR transistors on and off at just the right time. This is known as a synchronous rectifier.
Both brush type DC generators and brush type DC motors have the potential to run very efficiently, and they also can be made to work over wide ranges of speeds, loads and voltages. Generally though brush type motors and generators only attain very high efficiency over narrow ranges of speeds, loads and voltages. The speed at which motors and generators attain peak efficiency is proportional to the operational voltage. Double the voltage and the speed where peak efficiency occurs also doubles.
Direct current motors can be designed to run with no control devices of any kind, but such motors draw large amounts of current at low motor speed and generally cannot attain very high peak efficiencies up at the speed corresponding to the voltage they are run at. In order to design more efficient brush type motors some sort of control device must be used, and these devices have traditionally been referred to as "speed control" systems. Speed control is a somewhat confusing term since it is actually the output power that speed control devices modulate. The simplest type of motor speed control is resistive speed control, and this type of speed control was widely used from the late 19th century through the mid to late 20th century. Resistive speed control is simply one or more additional switches that allow one or more large power resistors to be placed in series with the motor windings. These large power resistors increase the total resistance of the winding circuit to reduce the current that flows through the windings. With less current flowing through the windings the power output of the motor is reduced. The problem with resistive speed control is that it is very inefficient, with the power resistors dissipating a large amount of power. Still though being able to limit the amount of current that a motor consumes during starting allows much more efficient motor designs that use significantly less power up within the operating speed range.
An improved method of motor speed control was introduced in the later part of the 20th century using electronics circuitry to rapidly switch the power supply on and off to limit motor power output. This is referred to as pulse width modulated motor speed control. Mostly what the rapid switching does is prevent the motor from overheating during starting so that more efficient motor designs can be used. Motor speed control that times the pulses of current to coincide with poles on the stator and poles on the rotor coming close to lining up can do an even better job of delivering good efficiency at reduced speeds and loads, but this type of motor speed control is so sophisticated that the brushes can be done away with entirely.
Timed motor control can improve the low speed efficiency of any brush type motor, but there are huge advantages to eliminating the brushes altogether. A brushless type motor has the same layout as an AC motor, that is the magnets are in the rotor and the windings are in the stator. The main advantage of brushless motors is that there are no brushes to wear out or corrode. If brush type motors are used a lot the brushes will eventually wear out and need to be replaced, and if water gets on them the motor probably will simply stop working until the brushes are cleaned. Eventually the contacts that the brushes ride on may also wear to the point that the entire rotor needs to be replaced, which normally means that the entire motor will be replaced. Brushless motors remove this maintenance problem while also allowing the motors to be smaller, lighter, cheaper and more efficient. The efficiency advantages of brushless motors are due to removing the frictional drag of the brushes sliding on the contacts, but this does not account for the large observed operational efficiency difference between brushless and brush type motors. The other big difference is that brushless motors normally have more capable electronic control devices.
All brushless motors have motor controllers that time pulses of current to coincide with the passage of the magnets, but some controllers do a better job of selecting the ideal part of the magnet passage to use for reduced load operation than others do. Some brushless motor controllers also use large capacitors to perform a type of capacitive voltage conversion to dramatically improve reduced speed performance of DC motors. In order to avoid confusion over just what this capacitive voltage conversion circuitry is capable of doing these systems are normally referred to as an "amperage multiplying" capability of the motor controller. There is huge potential for capacitive voltage conversion to widen the range of speeds over which very high efficiency can be obtained, but the complexity of capacitor switching and control circuitry required is substantial.
The basic concept of capacitive voltage conversion is that converting to a lower voltage involves charging two capacitors in series and then discharging them in parallel. Because capacitors have a voltage proportional to the amount of charge in them this capacitive voltage conversion requires many stages of conversion with many capacitors if it is to be efficient and produce a smooth output current. For running a DC motor there is a situation where a dropping voltage actually works well for attaining good efficiency in the part of the magnet passage before the poles line up. For this reason high power capacitive voltage conversion has been much more widely applied to motor controllers than other applications.
Just about any motor could also be used as some type of generator, and most types of generators could potentially be used as a motor. This is just another way of saying that motors and generators are devices that are laid out in the same ways. Motors however do not always make good generators and generators do not always make good motors. Brushless DC motors though generally make excellent generators, and devices used both as motors and generators are always of the brushless type and are used with very sophisticated control circuitry. The main application of the motor/generator is in providing regenerative braking for vehicles. Instead of wasting the kinetic energy of a heavy vehicle being stopped by friction brakes regenerative braking can store much of that energy in a battery storage system. The main challenge with regenerative braking is that the voltage of the current from the motor/generator is much less than the voltage required for the device to run as a motor. This is where capacitive voltage conversion comes in, and all of the power delivered to the batteries by regenerative braking must come through some sort of voltage conversion system.
The biggest problem with permanent magnet motors and generators is that the permanently magnetized rotor bumping past the iron in the stator creates eddy currents that suck down considerable power. For applications where the motor or generator needs to spin for long periods of time while not being used electro magnets tend to work better. When the field magnets are not energized the power required to spin the electro magnet type motor or generator is only the power required to overcome the friction of the bearings and the air resistance of the spinning rotor. Electromagnet type motors and generators also are more efficient for running under extremely light loads as the electromagnets need only be energized enough to supply that small load. Probably the best example of this is an automotive alternator which needs to deliver high current to rapidly recharge the starting battery immediately after the engine is started, but then is required to deliver only a very small amount of electrical power the rest of the time the engine is running. Since the automotive alternator runs under little or no load most of the time yet still needs to provide high maximum current capability electromagnets yield much higher overall efficiency.
Because motors and generators that use electromagnets to provide the field are half as efficient under load a much better way to provide for disengagement is to use a permanent magnet motor or generator on a clutch. Just how this would work would depend on the application, but some generalities can be made. The ideal electrical generator for most applications would be a three stage device with a small always spinning permanent magnet generator, an always spinning medium output field winding (electromagnet) type generator and a large permanent magnet generator on a clutch. The way this three stage generator would work would be that medium to heavy loads would be supported by the large permanent magnet generator, but when a large constant load was not demanded the large permanent magnet generator could be disengaged. Very small constant loads would be supplied by the always spinning permanent magnet generator. When the load was a bit more than the small permanent magnet generator could supply but not enough to justify clutching in the big permanent magnet generator the field winding type generator would handle the difference. Short duration medium to medium large loads would also be supported by the field winding type generator so that the clutch would not have to be cycled in and out so frequently. For intermittent drive applications a motor/generator on a clutch would work much better than either a permanent magnet or an electro magnet motor spinning all the time.
Electric motors and generators have traditionally been bulky and heavy both because they operate at slow rotational speeds and because large thermal dissipating capability is required for inefficient motors and generators. When a motor is running at 95% efficiency the amount of heat that needs to be dissipated is only one fifth of what needs to be dissipated if the motor runs at 75% efficiency. Efficiency and rotational speed go hand in hand with motors and generators. A fast spinning motor or generator can be much smaller and lighter, but needs to be more efficient since it cannot dissipate as much heat. Getting a motor or generator to run at 90 or 95% efficiency over a narrow range of speeds and loads is fairly straight forward, but for most applications it is a wide range of speeds and loads that need to be efficiently provided for. Limiting maximum current at low motor speeds is the main requirement for minimizing high thermal dissipation requirements, but for a motor or generator to work well and be very small the efficiency needs to remain high under heavy loads at low rotational speeds. This requires some sort of voltage conversion capabilities so that reduced speed operation of motors is possible with the correct voltage and likewise that increased speed operation of generators is possible at the higher voltage required for efficient operation at that speed. If the voltage is always fairly well matched to the speed of the motor or generator then the motors or generators can be smaller, lighter and more efficient over a wider range of loads.
A permanent magnet motor/generator on a clutch would allow for much higher overall efficiency in a hybrid drive system, while also providing more flexibility. For most applications each motor/generator would have it's own reduction gear before it's clutch. For hybrid drive the motor/generator might be clutched parallel to either the transmission input or output.
Series Capable Two Motor Parallel Drive-
Obviously a motor/generator clutched to the transmission input and another motor/generator clutched to the transmission output would allow for the highest level of flexibility. This would also allow part of the full load power output from the combustion engine to be routed through the motor/generators to take some of the load off of the gear box. With a large portion of the maximum power output of the engine routed through the motor/generators the gear box could be lighter and smaller and light load efficiency with a low cylinder count engine would be considerably higher.
There would be six distinct modes of operation of the hybrid drive system with four different clutch use combinations. These modes would be called normal, series, parallel acceleration, parallel charging, all electric and breaking. The clutch modes would be: Both motor/generators disconnected, both motor/generators connected, just the output side motor/generator connected or just the input side motor/generator connected. For normal sustained cruising at any speed above some very low maneuvering speed the vehicle would be driven on just the combustion engine with both of the motor/generators disconnected. In this mode the vehicle would essentially just be a conventional gasoline or diesel powered vehicle and the hybrid system would not interfere with efficient operation in any way other than the added weight of the motor/generators and the battery system. When heavier acceleration or rapid hill climbing was required series mode would be used to bypass some of the power around the gear box with both motor/generators engaged. When even heavier acceleration was required the input side motor/generator would be disconnected and the output side motor would run off of battery power. For maximum power output at very high vehicle speed both of the motor/generators would be run off of battery power. This maximum output mode would tend to stress the gear box more than any other mode of operation, but one mitigating factor would be that it would usually be used when the engine speed was so high that maximum torque could not be generated. Another way to reduce stress on the gear box in this highest output mode would be for the motor controller on the input side motor to back off on torque delivery during each of the torque spikes from the combustion engine. This would be particularly useful with low cylinder count combustion engines that tend to be hard on small efficient gear boxes.
For full electric mode one or both of the motor/generators would be used with the engine disconnected via it's own clutch. The most efficient all electric mode operation would be with just the output side motor clutched in so that the input of the gear box would not need to spin over. Because the output side motor would be operating at a fixed reduction ratio lower vehicle speed operation in this mode would rely on some form of voltage conversion. Depending on how efficiently the voltage conversion could be accomplished electric only operation at low vehicle speed might work better on just the input motor working through the selectable ratio gear box. Certainly for steep hill climbing at low vehicle speed the input side motor would be clutched in so that more reduction could be provided by the gear box. Maximum electric only acceleration and electric only top speed would be provided by both the motors running off of battery power.
The final mode of operation would be regenerative braking, which again would be done by one or both of the motor/generators. Any time that the brake pedal was touched the output side motor/generator would clutch in and the combustion engine would disconnect. For heavier sustained breaking the input side motor/generator would also clutch in to provide substantial additional braking force. In situations where some heavy acceleration using battery power was required but no breaking was used then the battery would have to be charged directly from the combustion engine. This would be accomplished by clutching the input side motor/generator in during coasting or sustained cruising.
In hilly terrain or in situations where substantial breaking was used the energy recovered by regenerative breaking would have to be used up in acceleration to make room for more regenerative braking energy recovery. The most efficient way to use this stored electrical energy would be to augment the combustion engine during acceleration. In this way the combustion engine could be kept at lower engine speeds where it could most efficiently provide relatively small amounts of power. If the combustion engine were a port injected gasoline engine though it might make sense to use substantial amounts of the energy recovered from regenerative breaking to provide for slow cruising and light acceleration with the engine completely shut down. The gasoline engine would then only be started for higher speed cruising and acceleration. An example of this would be a situation in mountainous terrain where substantial time was spent at lower speeds negotiating sharp turns. In climbing a hill the gasoline engine would be used to provide all of the power. In descending a steep grade regenerative braking would charge the battery, and then when a series of sharp turns kept the vehicle speed low the recovered energy would be used through one or both of the electric motors without the gasoline engine needing to run. When acceleration back up to high speed was required the combustion engine would once again start up to provide all or most of the driving power.
A racing mode might also be provided for where the combustion engine would run during coasting and light breaking to provide more battery charging. For this racing mode heavy acceleration with both the combustion engine and both of the electric motors would quickly drain the battery, so any opportunity to use the combustion engine to get some additional charging done would make the vehicle considerably faster. Running the battery much harder like this would of course require skinny individual cells and a high capacity battery cooling system.
Controlling all of these different modes and clutch combinations does present some significant challenges. Obviously this would require a great deal of sophisticated control circuitry, but even with computerized management of the entire drive system there are some general concepts that tend to always be applicable. Fully automated operation would be considered indispensable by most people, and adaptive programs that would learn both the driving habits of one or more drivers as well as the patterns of hills, turns and city traffic often encountered could probably be made to work quite well. For a more hands on approach some manual overrides would also be considered indispensable. Being able to manually select any one of the five drive modes with five separate buttons would be the best way to approach this. These manual overrides would also have to be divided into two classes, full manual override and temporary manual override. Normally full manual override would not be considered desirable, but rather a temporary manual override would be selected. A temporary manual override would involve manually selecting one particular mode of operation, but only until the automated system determined that some other mode of operation was more appropriate.
One way that a full manual override might be used would be to force the vehicle to stay in electric only mode for short trips around town where later plugging in to charge the battery was desired. A full manual override into conventional drive mode with both motor/generators always disconnected might also be used for long trips in wide open flat terrain.
Some indicators of system performance would also be considered indispensable. The most important of these would of course be battery state of charge, but some other ones could be very useful as well. An indicator of relative engine load would be very useful on any vehicle with or without a hybrid drive system. This relative load indicator would simply provide information about whether the engine was running at reduced efficiency due to underloading or running at risk of premature wear due to overloading. Another indicator that would be very useful for a hybrid drive vehicle would be a gear box overload indicator. If the drive system was manually forced into maximum output mode with both motors running off of the battery the gear box overload indicator would provide a warning that too much torque was being applied at a low engine speed.
It might be expected that all of the clutch engagement and disengagement would lead to excessive wear of difficult to replace friction disks. It would however be a fairly simple matter for the motor/generators to be electrically brought up to speed before being engaged. This would both eliminate nearly all clutch wear and also allow the clutches to be somewhat smaller, lighter and cheaper. The clutches would be engaged with a cam or eccentric so that no power was required to hold the clutches either engaged or disengaged. And of course a clutch that does not slip would be a dry type clutch.
Dual Transmission Parallel Drive-
Another totally different way that much of the same flexibility and efficiency in parallel hybrid electric drive could be attained would be with a single motor/generator on it's own separate selectable ratio transmission. The main reason to put a motor/generator on it's own selectable ratio transmission would be as an alternative to using voltage conversion, but there would be some significant advantages for high torque delivery from a small and lightweight drive system as well. A motor/generator on it's own separate selectable ratio transmission would be particularly good for delivering high maximum acceleration and high power hill climbing capability at low vehicle speeds. During sustained cruising on the combustion engine the entire electric drive system would disconnect via a clutch on the output side of it's gear box. So again the vehicle would function fully as a conventional gasoline or diesel vehicle without the hybrid drive system interfering with efficient operation other than it's added weight.
Three Clutch Electric Boat Drive-
For hybrid drive in a boat the ideal setup would be a single motor/generator on a three clutch system that would allow one of four different configurations. The combustion engine could drive the propeller shaft while the electric motor was disconnected, the electric motor could drive the propeller shaft while the engine was disconnected, the engine could drive the motor as a generator without driving the propeller shaft or both the electric motor and the engine could drive the propeller shaft. The advantages of this setup would be numerous, and well worth the complexity of the three clutches. The obvious advantage would be that the boat could be driven by either the electric motor or the diesel engine on the same propeller without the other device dragging. Being able to connect the engine to the motor/generator for charging batteries would also be extremely useful. Running both the engine and the electric motor at the same time could provide for very high peak output to deal with the most difficult maneuvering situations on what would otherwise be an under powered vessel. Spinning the propeller fast enough to deliver the power of the engine and electric motor together would mean that a voltage converting motor controller would be necessary to allow the motor to run efficiently over a wide enough range of speeds. Likewise the diesel engine would need to have an injection system capable of delivering an increased injection flow rate for the higher speed operation. The voltage conversion would probably best be done by converting the battery voltage up to a higher voltage for the maximum output in parallel mode. In this way the electric motor would drive the propeller at maximum efficiency with no voltage conversion durring maximum speed electric only motoring. Reduced speed motoring on the electric motor would then be supported by the battery voltage being converted down to a lower voltage. Another way that this voltage conversion could be provided would be by switching series/paralell combinations of battery cells. The last mode of operation for this hybrid drive system would be for battery charging from the spinning propeller while the vessel is under sail. The amount of power available from the spinning propeller is not large, but if the boat is a fast sailer this small charging source can add up to large amounts of battery charging during a long sail.
Series Electric Drive-
These parallel type hybrid drives are generally the most useful, but series type hybrid drives have also been used. In a series hybrid an electric generator and an electric motor take the place of a mechanical transmission. In practice the main advantage of this electric drive is that there is no gear box to wear out, but there can also be efficiency advantages in some applications. The main application of this type of drive is in large ships powered by huge single cylinder diesel engines. The electric drive can provide reduction to spin the propeller slower than the engine, and reversing can also easily be provided for all without the use of engine driven gear sets. Since transmission efficiency is normally quite low for single cylinder engines the electric drive can be made to work quite well. For more on why ships have so often used large single cylinder diesel engines see
Diesel Engine Basics.
In electric drive locomotives different series/parallel connections between the windings of the generator and the windings of the motor can allow for multiple selectable reduction ratios. Again the main reason to do this is to eliminate an expensive gear set type transmission that would eventually wear out and need to be replaced. To get an electric drive to work efficiently the generator and motor have to be able to attain very high efficiencies. The 95% peak efficiencies that can be obtained seem to allow an electric drive with an overall efficiency of 90%, which is pretty descent. The problem arises when the generator and motor are required to run at reduced speeds and loads where they are not as efficient. If the efficiency of each device drops off to 80%, which is typical for many motors and generators, then the overall electric drive efficiency will be only 64% which is not very good compared to most gear set based transmissions. Attaining even that 80% efficiency over a wide range of speeds and loads requires sophisticated control circuitry. It is not that electric drive systems cannot be made to work well, because they most certainly can. The level of sophistication required for really good electric drive efficiency is substantial though. A good old fashioned gear box is a whole lot easier to understand even if it might be more expensive to produce and will just wear out in a few thousand hours anyway.
Traditional electric drive locomotives used DC motors and generators to eliminate problems with frequency matching. In order to match the voltage of the generator to the voltage required for the speed of the locomotive different series and parallel combinations of windings were selected with large full current switches. The arrays of switches themselves are large and complex, and selectable ratios also require more brushes in the motors and/or generators. More selectable ratios require larger and more complex switch arrays as well as larger numbers of brushes. Newer locomotives which are called AC electric drive locomotives in fact are rather similar to the old DC locomotives except that they use solid state switching to take the place of the brushes and the arrays of large switches. Frequency matching is still done through rectification and switching.
An electric drive for a ship which does not require multiple selectable ratios can be accomplished without the use of any brushes or fancy electronic controls. The generator is a three phase AC generator, and the motor is a three phase synchronous AC motor. To reduce the speed of the propeller the speed of the diesel engine is reduced. The advantages of this type of synchronous AC series electric drive for single cylinder engines could potentially also be applied to single cylinder motorcycles. Motorcycles most certainly do require more reduction for starting out and climbing hills than they do for high speed cruising, but this could be provided for with a voltage converting motor controller. When cruising on flat ground at anywhere from about half speed to maximum speed the single cylinder engine would be "locked in" to one fixed amount of reduction with the generator running synchronously with the electric motor driving the rear wheel. In this locked in mode both the electric generator and the electric motor would operate without any control circuitry, and the overall drive efficiency could be close to 90%. When more reduction was required for very slow speeds, heavy acceleration or hill climbing the motor controller would rectify the AC power from the generator. The DC current would then be switched on and off in sync with the speed of the electric motor. Capacitive voltage conversion would not be absolutely essential, but it of course could dramatically increase the efficiency of low speed operation. When the drive was operating with increased reduction through the motor controller the efficiency would of course drop off somewhat, but when locked into what would be equivalent to high gear the efficiency of the drive system would remain very high since the conversion circuitry would simply be disconnected. For more on why single cylinder engine powered motorcycles with gear boxes work so poorly see
Fuel Hogs.