Power Electronics

Inertia Stalls Application of Brushless Motors in Cars

Brushless motor technology has been around for what seems like forever. Interestingly, the motor technology itself is simpler than the brush type, has better torque properties and is more efficient. Without all those pesky brushes and accompanying stuff, brushless motors should be cheaper and more reliable. If that is the case, then why has it taken so long for these motors to be accepted in the automotive world? Two issues come into play. The first is the cost and complexity of the motor-drive electronics. The second is not as easy to quantify — inertia.

Advances in semiconductor technology are addressing the first issue. The plummeting price of MOSFETs is helping to reduce the cost of drive electronics. Meanwhile, sophisticated but inexpensive sensorless-control ICs and methods are helping to simplify the design of the drive electronics.

I remember back in 1986 getting excited about the availability of a 40-mΩ MOSFET for around $1. I used three of these in an inverter for a three-phase permanent-magnet brushless-dc HVAC blower motor. Only three were required in that we drove the motor in a wye-wound configuration with the center pulled to B+ to save on MOSFETs. This configuration took a hit in efficiency and was tough to start, but at a dollar a MOSFET, it was the only choice.

Today, because of the relatively low MOSFET pricing, this configuration would not even be considered. That same MOSFET costs only about $0.15 to $0.20. And the reduction in pricing is even more dramatic when you compare the value of $1 back then versus $0.15 today.

With the cost of the inverter no longer a major stumbling block, we can focus on the control circuit. Yesterday, we had inflexible custom combinatorial-logic ICs that had trouble seeing back EMF at start up. Today, we have microcontrollers with brushless-control peripherals built into them.

These extra peripherals provide for sophisticated start-up algorithms, dead-time control for active rectification and excellent back-EMF detection. The end result is that the plethora of components needed to make a motor spin has been reduced to the bare essentials: a microcontroller, MOSFET driver, MOSFETs and just a few passive components.

This reduced bill of materials translates to a very cost-effective solution. The added expense for driving three phases versus one (in a brush-type motor) soon will be easily recovered in the cost savings accrued from using a brushless motor instead of a conventional brush type.

Tomorrow, the inverter (MOSFETs and drivers) could be a single-packaged part — or at least three sets of half H-bridges with integrated drivers — thus simplifying the assembly even more, reducing board size and complexity.

Despite these advances in the technology, we're still saddled with the massive amount of inertia inherent in the automotive industry. Brush-type motors have been in cars since before Kettering and Deeds made the first starter motor. As a result, they have all the cost squeezed out of them. And with the brush-type motors constantly improving brush-wear issues and reducing cost, the changeover to brushless keeps getting pushed back, because there is not enough delta in cost to justify the jump.

Recently, a major OEM was considering brushless for its flex-fuel fuel pumps, because E-85 fuel has a fairly corrosive effect on brushes (yes, the fuel flows through the motor). The OEM delayed the implementation of the brushless design because of improvements in the brush durability in E-85 fuel. How much of an improvement remains to be seen, but just the promise of the improvement made the difference. I think, for this application, the discomfort in making such a change was not overcome by the need to save money.

There are several applications that are moving in the direction of brushless technology — everything from fuel pumps to power steering. This includes HVAC blower motors, engine cooling fans and water pumps. I have even seen some movement in ABS pump motors to brushless. In hybrid vehicles, where every milliamp counts, ac-compressor drive motors and transmission-pump drive motors are going brushless, as well.

The opportunities are quite good for brushless motor technology to improve reliability and increase gas mileage while controlling cost. We've come a long way since the 1980s. We have a bit more to go before we can see brushless motors in large volume, mostly thanks to inertia. But even inertia can be overcome with patience and constant pressure.

David Swanson is a principal engineer in the Automotive Business Unit of STMicroelectronics. He has a BSEE from North Carolina State University and holds several automotive-related patents.

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