What is Power Density and How Does It Affect my Choice of Motor?

Power density is the amount of power that a given motor can handle per unit volume. In really simple terms we might call it the power to mass ratio of a particular motor.

The reason this particular measure is important is also relatively straightforward – namely that, in applications where size (and also weight) are important factors, the power density of a motor is a critical performance measurement. This is because it indicates the relationship between the size of a motor and the power that motor can deliver. In lightweight (often battery powered) applications such as drones or automotive applications, high power density can be hugely important.

Power density is also a factor for controllers (although often considered less critical). The power density of a controller is the amount of power a controller can handle per unit mass. This was not considered a major factor several years ago but, as with motors, the measurement has become increasingly important for applications such as drones and any other mobile device where mass can have a significant bearing on functionality and/or energy use.


OK - so the higher the power density, the better for my application?

Not necessarily no.

There are other factors to be considered when thinking about the right motor for your application. For example, the relationship between speed and torque that you require, do you need reliable positioning or intelligent control in this way?

A typical (though not always the case) feature of many motors with high power density is higher speeds and less torque. This may well mean that you then require additional gearing to get the speed and torque combination required for your project. Adding gearing adds inefficiencies and weight and therefore goes against the power density of the whole package.

Likewise stepper motors offer very poor power density but for certain applications are the only type of motor which can really achieve what needs to be done.

Equally, it is very much the case that high power density motors and control electronics are using relatively new technology and are therefore quite expensive at the current time. It is therefore quite likely if you’re working on a project that doesn’t require high power density (for example where mass is not a critical factor) then high power density options will make the project a lot more expensive than it needs to be.

Equally, it is worth noting that high power density motors are not more energy efficient in and of themselves. The major energy efficiency gains only come from mass critical projects such as drones or mobile devices where reducing mass can reduce the power requirement to achieve a given task.


I need the best possible power density I can - what motors should I use?

Almost certainly you will want brushless DC motors. However, the exact type and design of brushless motor you require will depend on the specific application you have. To find out more about the different types of brushless motor and the pros and cons of using each type please click here.

Depending on your application you should also think carefully about whether you opt for sensored or sensorless brushless DC motors as this could also have an impact on the performance options you are able to achieve. There are a large range of potential brushless DC motor controllers available to drive all types of brushless motor but you will need to think about this when designing your product.

Equally, it is important to note that not all brushless DC motors are alike. Many standard BLDC motors will have an improved power density over a brushed DC or AC motor but there are certain manufacturers of brushless DC motors that are really pushing the envelope with new technology to increase power density.


An increasingly important factor for a large number of applications

The need for improved power density has been a hugely important factor for a large number of applications, especially as the electrification of transport has developed rapidly in the last few years. Any application where the total mass of the system will have a significant bearing on energy expenditure (for example drones, robotics and certain pumps) will have a pressing need for increasing power density to reduce the overall mass of the system. 

This trend has led to rapid developments in motor technology to try and reduce the overall mass by changing the manufacturing process for building motors. Where windings were now the main method for building motor coils – now there are huge amounts of R&D spending going into producing new methods for driving motors. Typically these systems run at much lower inductances and resistances than more ‘traditional’ windings. Some of these technologies are even claiming to have no theoretical power limit!

Final thoughts: an important consideration but don't lose sight of other factors at the same time

For certain applications – largely space and weight sensitive applications – power density is undoubtedly a very important factor. However, for other applications it may be less of an issue and other factors may trump the issue of power density as a factor.

If you have any questions or have a project or idea you would like to discuss with our team we would love to hear from you. Please feel free to contact us here.

Category: Brushless DC Motor FAQs
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