Motor control: the real source of energy efficiency

16 July 2012 Chris Edwards Better control over component-level electric motors could yield massive energy savings, and a two-pronged attack is yielding results across a range of products. Improvements to efficiency standards for small electric motors could easily outweigh the…

16 July 2012

Chris Edwards

Better control over component-level electric motors could yield massive energy savings, and a two-pronged attack is yielding results across a range of products.

Improvements to efficiency standards for small electric motors could easily outweigh the energy consumption reductions in computers and their displays from now until the end of 2035, according to a report published earlier this year by the American Council for an Energy-Efficient Economy (ACEEE). What’s more, this does not include the motors built into other equipment, such as air conditioners and washing machines.

Roland Gehrmann, manager industrial ICs and marketing at Toshiba Semiconductors in Europe, says: “The trend is also being driven by the European Community for higher efficiency and lower noise in motors.”

In addition to the designers of industrial and domestic machinery, the automotive industry has developed an interest in more efficient electric motors. These motors are required not only for electric and hybrid vehicles but also for the systems that go into engines powered purely by internal combustion. Klaus Meder, president of the automotive electronics division at Robert Bosch, says: “We think the internal combustion engine still has a strong future; we can reduce consumption by 30 to 40 per cent and this will compete with the pure electric vehicle. Electric vehicles will be mainly fleet vehicles. We think private demand for them will be quite small.”

But, he adds, “this 30 to 40 per cent reduction will go along with a lot of technology that goes into electronic vehicles, such as electric climate control. There is still a huge amount of room for improvement.” The efficiency trade-off, then, is in general terms one of hardware complexity versus running costs.

The energy used by a motor easily outweighs the hardware cost except, possibly, for the very small motors used in compact consumer appliances. Even in the latter, the pressure on battery life and the noise caused by cooling fans makes it worthwhile to invest in better motor efficiency.

The attack on wasted energy is proceeding on two electronic fronts. The first is to use digital signal processing to only pass electrical power to the motor when it needs it; the second is to optimise the electronic circuits that pass electricity into the windings, which get very hot as they do.

Reinhard Ploss, director of operations, R&D and labour at Infineon Technologies, explained at the ISS Europe conference earlier this year how changes to the materials used in power electronics result in dramatic differences to system-level performance. “The companies making motor drives are the heavy-metal guys of the industry,” he said. The power-electronics circuits need big heatsinks, take up a lot of space as a result and chew through a lot of power; “but when you change the system architecture, you can reduce by 90 per cent the total size of a motor drive and save a lot on material cost.”

Ploss went on to describe a new design based on silicon carbide, a material that works better at high temperatures than does conventional silicon. The silicon carbide devices run happily at a junction temperature of more than 150°C, which saves massively on heatsink surface area. The higher switching frequency makes it possible to run the drive without large DC capacitors, and with smaller filters and chokes. The peak power handling capacity increased tenfold to almost 20kW/l.

Diode juggling

Infineon Technology’s Reinhard Ploss says that the company is realising further space savings from system-in-package (SIP) techniques, using an approach the company calls Blade. This puts the power-handling and control devices into the same SIP.

Because wiring becomes a limiting factor in power electronics circuits, SIP provides better electrical and thermal performance and the manufacturing technique that the company employs makes it possible to process multiple products in parallel on large panels – conceptually similar to liquid crystal display (LCD) manufacture. The makers of power semiconductors are using a different form of scaling to their digitally oriented brethren: they are making more use of 3D techniques. Ploss explains: “The roadmap for power devices is not the linear shrink you find in CMOS technology.”

Diodes and transistors go not just on the top of chip, but on the bottom. In one design being used by Infineon, a bipolar power transistor goes on the top surface with the diode used to protect it going on the back.

The individual diode and transistor are made up of hundreds of cells to reduce the resistance through the semiconductors when they are switched on: if not, they will get very hot – and fail. But the connection between the diode and transistor also needs low resistance. This means using not just lots of parallel vias through the wafer but reducing the physical distance between them. The best way to do that is to shave the wafer so that it is less than 100’m tall.

As digital chipmakers have done, companies such as Infineon are moving to 300mm vinyl-LP sized for their power semiconductors to reduce manufacturing cost – as more chips can be made in parallel; but the thinned wafer feels more like a flexidisc. The wafers have to be moved around in a protective shield: removed from the case, the foil-like wafer buckles under the stresses imposed on the wafer by the various high-temperature manufacturing operations. “The stresses make the wafer bend by itself,” says Infineon Technology’s Ploss.

Further integration and size reductions are coming from putting the chips very close to each other in so-called SIP modules. With integration, because wiring becomes a limiting factor in power electronics circuits, SIP provides better electrical and thermal performance. The manufacturing technique that the company employs makes it possible to process multiple products in parallel on large panels – conceptually similar to LCD manufacture.

The space savings are not all good news, although it means more power can be squeezed into a small space. Meder says the automotive industry demands zero failures in the field from electronic devices: “Is it really zero? Practically yes. It is less than 1ppm for many parts – we now count failures in parts per billion… And for some it is easier to count mean time between failures because some users’ parts won’t fail at all.”

SoC heat concerns

Meder says shrinking dimensions can increase the chance of heat damaging parts or bending and delaminating from the circuit board due to heat and moisture. Some of the power circuitry is also moving onto more sensitive digital chips. At the VLSI Technology Symposium in June both Renesas Technology and Samsung talked about putting thin-film power transistors made from metal oxides into the metal stack that sits on top of regular silicon logic transistors.

Helmut Lang, who is responsible for testing devices at Freescale Semiconductor’s automotive and industrial group says high-power driver circuitry has moved onto system-on-chip (SoC) designs. For example, some of the company’s microcontrollers now sport the firing circuitry for airbags and, when they fire, they also get very hot. This is something that can affect other modules on the chip as, if they get too hot, they could fail, so the company wants to be able to model those effects to see how they can maintain reliable operation after the firing events.

Designers working on purely digital SoCs are also becoming more concerned about heat – so they can work out how far they can overclock the processors to support peak demands. Stephen Kosonocky, a design fellow at Advanced Micro Devices, explained at the VLSI Technology Symposium how the company is using a combination of thermal modelling and temperature sensors to work out how much headroom AMD’s processors have before they have to slow down the clock and, with it, reduce the heat that they generate.

As well as electrical power, the processors used to control motors are getting a lot of compute power in a bid to save energy in the motor windings themselves. Motors in large household goods used to be pretty primitive. They used wire brushes to maintain constant electrical contact between the moving part of the motor and the central stator.

These brushes incurred relatively large power losses from friction and resistance. Gradually, the brushed motors are dying to be replaced by permanent-magnet motors. Having the stator elements switch in sequence produces a constantly-changing magnetic field that forces the rotor to turn.

In principle, to make the motor turn faster you simply need to switch the stator elements at a higher frequency. There is a catch, however. In general, the faster the rotor turns the higher the voltage with which it is supplied should go. If not, the motor suffers a loss of torque and, after a point, it becomes unable to turn because the load it is trying to move is too large for the effective torque. This relationship is simple enough and does not need more than a simple microcontroller, if that, to decide on how much power to deliver to the motor.

Voltage and frequency balancing

The relationship between voltage and frequency breaks down at low speeds, and where you need precise control over the motor’s position. Voltage often needs to rise higher than expected to keeping the motor turning with sufficient torque; but if the voltage rises too far, then the motor windings can saturate, potentially, they will overheat. This simple-sounding problem is one that motor drive and electronic component designers have been wrestling with for almost 20 years: working out how high the voltage needs to be.

The answer, at least in theory, lies in a technique called flux-vector control. In effect, the motor drive uses a mathematical model of the motor to calculate how much flux is present in the windings at any given time. Early vector control techniques relied on sensors to work out where the rotor is at any point to help with the calculations; but the sensors have the drawbacks that they add cost and provide more components that can fail.

So, motor builders opted for computationally driven options. In general, the more processing power you throw at the problem the more accurate this estimated position will be, the more precise your flux prediction will be and the electrical supply tuned more finely to avoid wastage. More powerful digital signal processors (DSPs) and, in some cases,’dedicated silicon, are gradually improving the accuracy of the motor-control algorithms but designers are faced with an array of choices with no clear winner.

In a three-phase AC induction motor – stators with single-phase windings cannot be controlled as accurately – the stator has three windings that can be independently controlled. The stator moves the rotor as the result of the sum of the forces applied by the three phases.

The coils can be driven in such a way that they produce torque or simply apply force along the axis of the stator, so it does not generate a rotational force. In motor-control theory, these are quadrature and direct axes, respectively.

If you want to generate rotation, you want to maximise quadrature force and keep direct force to a minimum. How you do that typically relies on one of two basic techniques, as explained in the panel ‘Algorithmic contenders’.

In the automotive sector Tesla Motors has used flux-vector control in its electric sports car to control the AC induction motor at the heart of its propulsion system. The company claims its ability to maintain torque at lower speeds improves acceleration and overall efficiency compared to conventional cars powered by an internal combustion engine. Not only that, it does away with the need for gears.

To maintain high torque at speed, the power electronics in the Tesla needs to supply close to 1kA of current at peak output. The stator is packed with copper to reduce the resistance of the electrical interconnect. But heat can still be an issue.

The company has filed patents on algorithms that use models not only of the electric field inside the motor but its thermal profile. This lets the control algorithm work out how much power can be supplied to the motor safely, backing off the power if components are nearing their critical temperature.