Now while it is a piece of cake to control the power and speed of a combustion engine simply by throttling the fuel supply, which on the other side is a dreadful necessity while the electric motor more or less regulates itself, the “water tap” for electricity was finally invented in the seventies: now there are inverters available which convert AC to DC and the DC back to AC again with electronic components (and very low additional losses). The AC output can be controlled both in amplitude and frequency as to adapt it to the requirements of any motor at any desired point of operation. Speed and torque can now be controlled independently of each other. So the inverter overcomes virtually all disadvantages of the electric motor against any combustion engine, while the advantages remain as outstanding as they are, including the power feedback (inversion of energy flow) if a 4-quadrant inverter is used (2 rotational directions, 2 energy flow directions).
Broken down into very simple terms, such inverters build up a connection between the direct voltage in the DC link when the momentary alternating voltage in the line is higher than the DC voltage in the link, thus allowing energy intake, and disconnects both from each other when the voltage “out there” is lower. This is the principle of motor operation. For feeding back energy in generator mode the inverter, justifying its name, does the inverse thing: Connect when line voltage is low and disconnect when it is high. In this way the energy can go either way even though the line voltage is constant – and the DC voltage in the link circuit may also be kept at a constant level, depending on design.
The other end, the motor side of the power electronic inverter, is somewhat more sophisticated. Simplifying again, the principle is to switch the motor on and off very rapidly, much more rapidly than any mechanical switch could do. By varying the on / off time ratio the average motor current can be continuously varied, even if the DC voltage in the link circuit is kept at a constant amplitude. The principle is much more sophisticated and pretty much more expensive than controlling the water flow in the bath with a water tap, but the advantages are so salient that this principle is steadily making its way all through the realm of electric drives.
While old trams – and many of them are still around – could very well use their motors for braking, the electric power could not be fed back into the lines because the voltage the motor generates is, roughly speaking, a bit lower than the voltage on the line, so an inversion of the power flow was not possible. The electricity generated during braking was absorbed in resistors and went lost as heat. Nowadays inverters can chop the DC into AC, AC can be transformed (the transformer being the smaller, the higher the chopping frequency is chosen to be), rectified back to DC and fed back into the overhead line.
Now a combustion engine has a certain power output rating, and that’s it. If you try to get a little bit more torque out of it than what the rating plate offers you, you just stifle the engine.
What a difference to the behaviour of an electric motor! It also has a certain maximum power and maximum torque, but what does it do if you want more? It gives you more!
The speed of a DC or asynchronous motor drops a bit, while in a synchronous motor the angle between applied and induced voltage becomes a bit greater. Both leads to a higher current intake, which facilitates a higher output torque at approximately or exactly the same speed, respectively. The motor will offer you double the rated torque if you want it. Depending on type of design and size of the motor it may be more than 5 times as high as the rating. The only problem is that it allows for this only for a limited time because the excessive current generates excess heat in the motor, and in the long run the motor would burn out. Special motor protection switches which are adjustable to the current rating break the motor current if the rating is exceeded for too long. The better approach is monitoring the actual motor temperature. Or to use an inverter. Its electronic control offers unlimited programming options.
Since an electric motor starts up alone and many types even offer their greatest torque (breaking torque) in standstill mode, no clutch is needed in an electric vehicle.
Since an electric motor offers a much greater torque for a limited time than for continuous operation, no gear change is needed in an electric vehicle because vehicles always need their maximum traction force only for the limited periods of acceleration and uphill ride.
So, an electric motor is a much better and more sustainable option for vehicle operation than combustion engines of any kind. Together with a power electronic inverter they are close to ideal, while the combustion drive is more or less a makeshift manner to move a vehicle which only on account of more than 100 years of experience together with a huge and powerful market could be optimised by and large to the state we observe today. There is no further progress in sight.
All that is now still lacking is a usable battery. When it comes all land transportation will immediately go to electric drives. Wherever a catenary wire is available the electric drive is already demonstrating its superiority, and there are still some potentials left.