Motor Control Efficiency: The Control Chip Factor

Today’s embedded system designers are faced with the challenges of delivering projects on schedule while hitting cost goals, responding to new customer and marketing requests and providing significant product differentiation. In addition, when the task involves electric motors, it’s hardly necessary to remind anyone of the importance of minimizing power consumption. If rising energy costs aren’t enough of an incentive, there is also the increasing government and institutional scrutiny of “green” scorecards. In short, making do with less electricity is a virtue. 

A key to achieving improved efficiency is the motor controller. Fortunately, available controller chip technology keeps improving. For example, now that the cost of 16-bit devices has dropped considerably, they are being adopted for more and more applications. So now, higher performance is possible even especially for three-phase motors.

Power factor correction (PFC) is becoming increasingly mandated for line-driven motor control applications.  It is now possible for a digital signal controller (DSC) to execute the PFC algorithm and motor control algorithms on one chip. 

Designers are also choosing to integrate motor controllers on the motor itself, though they often run into difficulties because of the physical size of the required chips -- traditionally a 9x9 mm footprint or larger. Fortunately, controller chips as small as 6x6 mm are now coming on the market, giving designers additional freedom and an opportunity to integrate controller and motor more easily.

But beyond the question of whether or not the controller is located directly on the motor, there is the question of what functions the controller can deliver.

Some vendors provide software for their controller chip at no cost and these software and chip combinations can provide an opportunity to develop capabilities including sensor-less controllers that can help with both performance and efficiency. Rather than having a separate device to sense shaft rotation, the motor can be built with a shunt resistor in the windings to make it possible to detect back-EMF and use that information to accurately estimate rotor position. This step can reduce cost and eliminate a potential failure point (the separate sensor).

But that’s not all software can provide.  To achieve better efficiency, for example with permanent magnet synchronous motors (PMSMs), designers want to apply the field oriented control (FOC) algorithm (or vector algorithm), which can help yield more precise torque control. FOC algorithms depend on “Clarke and Park” transforms. The Clarke transform, can identify real and imaginary currents while the Park transform can be used to accomplish transformation of the currents from the stationary to the moving reference frame relative to the stator vector current and rotor flux vector.

To visualize how this works, picture an AC motor operation from the perspective of the stator.  A sinusoidal input current is applied to the stator and this time variant signal generates a rotating magnetic flux. The speed of the rotor is a function of the rotating flux vector. Viewed from a stationary perspective, the stator currents and the rotating flux vector simply look like AC quantities. However, if you picture yourself inside the motor and traveling alongside the spinning rotor at the same speed as the rotating flux vector generated by the stator currents, during a steady state conditions, the stator currents look like constant values, and the rotating flux vector is stationary.

You must control the stator currents to achieve the necessary rotor currents. With the help of coordinate reference transformation, the stator currents can be controlled much like DC values using standard control loops. From there, control can be achieved with an ordinary PID (proportional, integral, derivative) control measures. 

To be sure, all of this takes significant computing power, but 16-bit digital signal controller chips are up to the task.  Of course, this is still a fairly complex undertaking. Those lacking experience might want to first work with brushless DC motors.

Still, according to the US Environmental Protection Agency, the efficiency of the one billion or so electric motors in use is a priority. Indeed, they estimate that even a one percent improvement in efficiency could save more than a billion dollars annually. So, given those incentives, adopting new and better control technology ought to be a priority.