RECALL how Apple’s reputation took a knock after the firm admitted it had surreptitiously hobbled the performance of peope’s iPhones when their batteries started deteriorating with age. Toyota now faces similar opprobrium for throttling back the batteries of some of its older Prius hybrid cars, to prevent them frying their electronics. The software tweak—performed during a voluntary recall of Priuses sold in America between 2010 and 2014—forces the vehicle to rely more on its petrol engine and less on its electric motor/generators. That causes fuel economy to fall and exhaust emissions to rise. Prius owners, who bought their hybrid vehicles precisely because they were lean and clean, have not been best pleased.



More galling still, reports continue to come in of overheating problems that leave Prius owners in limp-home mode or stranded in traffic unable to move. Online forums (see priuschat.com) have been abuzz with tales of woe from afflicted drivers. Apparently, owners of vehicles that are out of warranty have faced repair estimates of $3,500 or more. Toyota has since extended the warranty of the cars affected, and promised to reimburse those who have been left out-of-pocket.



Even so, the carmaker is now facing litigation over its recall remedy. A lawsuit brought by one of California’s largest Toyota dealers alleges that more than 100 of its customers with Priuses that had already received the software update have returned with the same overheating problem. Toyota disputes the allegations and has sought to have the suit dismissed. A judge has since ruled that the case can go forward, and has set a date for the trial early next year.



The source of the trouble is a box of tricks under the bonnet known as an inverter. This takes the 206-volt direct current (DC) from the Prius’s nickel metal-hydride battery and converts it into the three-phase 500-volt alternating current (AC) needed to drive the car’s two electric motor/generators. (One of these is used to propel the car when operating under battery power, and to assist in recharging the battery when braking; the other’s main job is to keep the battery charged, while supplying supplementary power to the drive assembly, as well as serving as an electric starter for the petrol engine.) Both motor/generators produce mechanical torque when driven by AC electricity—or, when spun backwards by an external source (ie, the hybrid’s petrol engine or the wheels during braking), generate AC electricity.



Managing this merry-go-round is the job of the inverter and its associated electronic-control units. Inside the inverter is a hefty circuit board with a bank of heavy-duty switching components called insulated-gate bipolar transistors. These flip the direction of the DC supply from the battery thousands of times a second, to produce the three-phase AC needed for powering the vehicle’s electric motors. Switching such a heavy current rapidly generates a serious amount of heat. To cope, the inverter has its own glycol-based cooling system.



This all works fine in normal conditions. Occasionally, however, a glitch in the inverter’s software lets matters get out of hand. Microscopic voids then build up in the solder beneath the transistors, causing parts to short-circuit or simply to overheat. Either way, the temperature inside the inverter can rise rapidly. If that temperature becomes excessive, the transistors get fried and the vehicle shuts down completely. More often, the overheating merely causes the current to the inverter to be reduced, the instrument panel to light up like a Christmas tree, and the vehicle to go into limp-home mode while things cool down.



The quick fix adopted by Toyota has been to reset the inverter’s software in order to reduce the likelihood of the transistors being stressed unduly. That has meant lowering the current from the battery and forcing the petrol engine to work harder to compensate—hence the higher fuel consumption and increased exhaust emissions.



The experience has baffled many owners. Online queries have focused on why so complex a contraption as an inverter/converter is needed anyway. Some have even wondered why the Prius uses AC electric motor/generators when perfectly good DC motors and alternators have been around for ages? The presumption is that using a DC motor would allow the vehicle to be powered directly from the battery, obviating the need for an inverter.



Well, not exactly. Electric motors, whether AC or DC, work by causing a magnetic field in the rotating part (rotor) to interact with a magnetic field in the stationary part (stator) of the machine. Magnetic fields can be produced by permanent magnets or by electric currents passing through bundles of wires wrapped around an iron core. Whatever the mechanism, all electric motors, whether their input current is AC or DC, require an alternating current in their stator windings to make their rotors spin.

In a conventional DC motor this AC current is generated by carbon brushes that rub against a split copper ring on the motor’s spindle. As it rotates, this arrangement, called a commutator, chops (ie, rapidly connects and disconnects) the direct current to create a crude form of alternating current. Unfortunately, this causes numerous problems. Friction and electrical arcing at the commutator means the brushes must be replaced regularly. Arcing at the commutator also limits the voltage that can be used, and thus the power produced. And because of the heavy current coursing through a DC motor’s inner windings, the rotor can get extremely hot and become difficult to cool. Conventional DC motors, moreover, require gear boxes because, while huge at start up, the torque they produce falls off rapidly with rotational speed. Finally, DC motors are not particularly good at being run backwards, in order to generate electricity for recharging purposes. At best, they are good for golf carts and fork-lift trucks.

Hence the “brushless” DC motors, with their transistorised switching in place of mechanical brushes and a commutator. These have no brush-related friction or arcing, and control of the current means torque can be controlled without the need for multiple gears. Like other hybrid vehicles, this is what the Prius uses, though it calls them synchronous AC permanent magnet motors. For all intents and purposes, brushless DC motors and synchronous AC motors are essentially the same. Both use permanent magnets embedded in their rotors and both need inverters.

That, though, raises an interesting question. Why not go the whole hog and use the output of the transistors to drive what is known as an AC induction motor? That is what Tesla does. An AC induction motor does not need permanent magnets, and can thus do without the expensive rare-earth metals that are an essential ingredient of modern, high-performance magnets. The answer is that while induction motors are ideal for electric vehicles built for speed (like Teslas), they are not all that efficient when used for pottering around town or cruising lazily down the highway. As cars and lorries tend to spend 90% of their time using little more than 10% of their maximum power, brushless DC motors are a better bet for typical hybrid vehicles like the Prius.