Geraint Thomas, sales engineer at Control Techniques has wrote this interesting article on the importance of reliable motor control in electric vehicles.
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. An announcement was made by automotive giant Volvo, which said every car it produces from 2019 will be hybrid or electric, a trend is starting. Get ready for the plug-in.
EVs are growing in profile. 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.
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.
About Control Techniques – Nidec Control Techniques has been at the front of customer-focused drive technology for over 40 years. We’re dedicated to the advancement of automation. From product development at our headquarters to our 45 Automation Centers around the world, we provide solutions relevant to the industries in your region. We ensure high performance, reliability and energy efficiency across every application. We’ve always been a company who dreams big. Founded in 1972 as KTK in Newtown, Wales, we took our name from our three founders. Their vision was to provide a new drive that met the needs of the market. The following year they did just that. The KTK 415V DC thyristor drive improved performance, reliability and energy efficiency for motor control. As we grew, so did our network. We developed close relationships with our customers. This led to a wave of product innovations including the Commander AC and the Mentor, the world’s first digital DC drive.