The importance of reliable motor control in electric vehicles

7 Aug , 2017  

With the death knell sounding loudly for petrol and diesel powered cars, electric vehicles (EVs) and plug-in hybrids represent the future of both private and public transportation. The UK’s announcement that fossil fuel vehicles will be banned by 2040 follows a similar pledge from France. Indeed, countries across Europe are leading the way, with Norway spearheading a ban as soon as 2025. Add this to the announcement made by automotive giants such as Volvo, which says every car it produces from 2019 will be hybrid or electric, and the trend is clear to see. Get ready for the plug-in.

EVs are growing in profile. Marques such as the Chevrolet Volt, Nissan Leaf, Tesla Model S, Ford Energi, BMW i3, Fiat 500e and VW e-Golf are becoming more common on the world’s roads, pushing them into the public psyche. As demand grows, further improvements in technology will ensue, not least in motor control.

Just two components comprise the power system of an EV: the motor and the controller. Compare this to a traditional combustion engine, which needs a carburetor, oil pump, starter, exhaust system and so on, and the advantages are plain to see.

While early EVs used DC motors, predominantly due to their cost-effective motor/controller combination, the arrival of better and less expensive electronics has witnessed growing numbers of the latest EVs deploy AC motor/controller systems in order to leverage their enhanced efficiency and lower mass, not to mention reduced maintenance. It’s fair to say that an AC motor can last forever, or thereabouts. There are almost no wear parts except for the ball bearings, which are typically extremely durable. In addition, regenerative braking is achieved as welcome (and free) by-product.

In drive systems for battery voltages between 24 and 450V, technologies such as asynchronous (induction) motors with frequency inverters are used, or more recently, permanent magnet synchronous (PMS) motors. Permanent magnet technologies present new opportunities with regards to key design factors like efficiency, performance, size, weight, zero maintenance and low noise levels, making them extremely attractive to the EV industry’s main players.

Unlike induction motors, PMS motors use permanent magnets embedded in the steel rotor to create a constant magnetic field. The stator carries windings connected to an AC supply to produce a rotating magnetic field. At synchronous speed, the rotor poles lock to the rotating magnetic field, ensuring that the synchronous motor rotates in exact synchronism with the line frequency.

Induction motors still remain popular, however, this type of motor sees power supplied to the rotor by means of electromagnetic induction. Stator windings are configured around the rotor so that when energised, they create a rotating magnetic field that induces current in the rotor conductors and creates motion. Induction motors have advantages not dissimilar to PMS motors, including low cost, high efficiency, high reliability, zero maintenance and easy cooling.

Clearly, the effective control of motors, PMS or induction, is at the heart of every EV, and is the key for realising the optimum balance of top speed, acceleration and achievable distance per charge. In early EVs with DC motors, a simple variable-resistor-type controller looked after vehicle speed and acceleration. However, with systems of this ilk, a high percentage of battery energy is wasted as a loss within the resistor.

In contrast, many modern control systems adjust speed and acceleration by an electronic process called pulse width modulation (PWM). Here, simple switching devices such as silicone-controlled rectifiers are deployed to instantaneously turn on and off the electricity supply to the motor. Logic dictates that high power (speed and/or acceleration) is achieved when the intervals are short – namely when the current is off – with lower speeds and accelerations resulting from longer intervals.

Technologies such as PWM vector control are becoming increasingly commonplace. The principal advantage of vector control is to make an AC motor function like a conventional DC separately excited motor, with independent control of torque and flux. However, the main difference – and benefit – is that the brushes and commutator of a DC motor are not found on an induction motor, for example, presenting opportunities for EV manufacturers to specify a more compact, light, reliable and efficient drive unit.

It is also worth noting that the motor controllers found on most modern EVs have a system for regenerative braking. Regenerative braking sees the motor used as a generator to recharge the batteries as the vehicle slows. When this process occurs, a quantity of the kinetic energy normally absorbed by the brakes and turned into heat, is instead converted to electricity by the motor/controller and used to recharge the batteries. The upshot is greater vehicle range, typically by 5-10%, not to mention reduced brake wear and maintenance costs.

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Chris Corfield

Chris Corfield