As greater and greater efficiency is sought in motor technology this has led to increasingly low inductance motors being developed, with the aim being to enable higher power handling in motors, thereby reducing the overall mass of the motor for a given power rating. This has led to major gains in power density of motors, something especially important in applications such as drones and other robotic technologies where batteries are the power source and where mass is hugely important. The benefits of this move are significant, and the technology is set to begin trickling down into other applications as it becomes more cost effective, however for controllers there are a number of challenges.
The two major benefits to low inductance motors are that they improve the power density available in a motor and they also increase the potential speeds that a motor can achieve. Increasing numbers of applications are looking at this type of technology for either or both of these benefits. Some industrial machinery benefits from increased speeds. Many applications do not require these high speeds but benefit from improved power density. Depending on the nature of your application there are a number of ways that the problem of low inductance can be addressed.
The simple route around the issue of low inductance in motors has been to add an inductor into the controller. This essentially replicates the inductance from the motor but is part of the controller. This can be a useful shortcut to get this type of motor turning but, where mass and power density are critical factors in the overall design of a system, it is essentially taking all of the mass savings that have been gained in the motor and adding it into the controller.
Inductors are not small components! That said, there are a number of advantages to this option if the overall mass of the motor and controller is less important. That is why this type of solution is currently the most widely used in motor controllers where low inductance is required.
Perhaps the most obvious issue that arises in low inductance motors is the disappearance of back-EMF from the motor. This means that a controller based on a back-EMF sensing circuit is unable to detect the rotor position or speed of the motor and this essentially renders them redundant. To a significant extent these circuits can be redesigned and respecified to be more sensitive to back-EMF (there is still some there) but there are limits with what they can do. One option is to use sensors until a back-EMF can be picked up (as power increases in the motor so too will the back-EMF signals). This can be an effective solution in this type of application. Another option is to use advanced motor drive algorithms that can use field strengthening technology to artificially increase the power in a coil and enable a back-EMF frequency to be picked up.
The lower inductance and resistance of the motor can also give rise to major power surges in such motors that would not have been possible with more ‘traditional’ motors. This means that current sensing circuits have to respond faster and safety features in controllers must be specified accordingly.
Another option in this type of situation is to use controllers based on a higher PWM frequency. These solutions often require the use of new components in the design, but these are especially relevant where inductors cannot be used and where mass is hugely important.
Low inductance motors have challenged the way in which many of the traditional motor drive technologies work but there are still a range of solutions available depending on the nature of the application. Many of the solutions above are quite new and are still being developed but there are a range of options available that can assist with getting these motors performing well. Ultimately, as with many things in motor control, the best option for the job will depend on the application specific details of your project.