Sensored versus sensorless brushless DC (BLDC) motors and controllers

This page discusses sensorless and sensored brushless DC motors (and controllers), the differences between them and the key issues you need to be aware of when using them. We give you some basic pointers about the mechanical and performance differences between the two as well as understanding their suitability to a range of applications. As always, if you are working on a project at the moment and are still deciding on the best solution for your project, please feel free to contact us to discuss your requirements.

We’ll start with a quick overview of the two types of motors and how they work…

What are sensored brushless motors?

Sensored brushless motors are brushless motors with positional sensors in them. These positional sensors feedback positional information to the controller which can then be used to ensure the drive pattern delivered to the controller syncs perfectly with the rotor position.
Different sensored motors may have sensors set up in different ways so it is important to always be aware of this when setting up a controller but typically these are arranged at either 60 degree or 120 degree intervals.
The main advantage of sensored motors is that they create a closed loop system which can enable the controller to know the rotor position and hence sync the drive pattern up very accurately. The downsides of these motors it that they are completely reliant on the sensors in order to function correctly so in applications where there may be a lot of dust, vibration or moisture they can (depending on the build quality of the motor) fail as a result of sensor performance.

What are sensorless brushless motors

Sensorless brushless motors are quite simply, brushless motors without built in sensors. Sensorless brushless motors require a sensorless brushless motor controller (sometimes referred to as a brushless electronic speed controller or ESC) in order to work. A sensored brushless DC (BLDC) motor controller will not work with a sensorless brushless motor.
On the face of it it would therefore seem logical that sensorless brushless motors are only able to operate in open loop because there are no in built sensors. However, this is not entirely the case. The reason this is not the case is down to an electrical concept known as back-electromotive force (or back-EMF).
A typical sensorless brushless motor has permanent magnets fixed on the rotor with the electro-magnets mounted around the edge of the motor. As the motor picks up speed it starts to generate electro-motive force in the stator coils. As an example, with the unit switched off and the rotor being spun (for example by a bicycle dynamo) this can be used to generate electricity.
The critical aspect of this for sensorless brushless motor control is that the frequency generated by this back-EMF is directly proportional to the speed of the motor. Therefore, if a sensorless brushless motor controller can read this frequency, it can then determine the speed of the motor and adjust the drive pattern accordingly.
The critical weakness with this method comes at low speeds where the back-EMF is very weak and therefore hard to read reliably. This is why starting up a sensorless brushless motor can be such an issue.

Things to think about when making the choice

As you will no doubt have worked out, the choice between sensored or sensorless brushless motors is a decision based on performance, application, environment, lifespan and cost.
In a situation in which you need a quick setup and a short duty cycle then sensored will probably be the simplest to use.
However, if you are looking at a long duty cycle in a harsh environment then there are considerable benefits to be had from using sensorless.
It is also important to remember that, whatever motor you choose, you will need a controller that can deliver the performance that you need.

What speeds are you typically looking to achieve?

If your application requires low speed for a lot of the time then it is likely that a sensored brushless motor will be the best solution for your application as the closed loop enables the driver to commutate the motor much more effectively at lower speeds.
However, if your project requires higher speeds then sensorless brushless motors will be the best solution as they are very stable at higher speeds and (depending on the controller that you choose) they can often go much faster than sensored motors. This is in part because open loop operation can run at faster speeds than closed loop but also because the controller does not need to process the signal inputs from the hall sensors.

Never underestimate the importance of the controller in your project

As a specialist motor control company, we are bound to say this aren’t we? However, that doesn’t mean it’s not true!
The type of controller that you use can dramatically influence the performance of the motor that you have chosen. Key variables such as power handling, the type of drive pattern that you have (sinusoidal, trapezoidal or FOC) or intelligence and programmability can all have an impact.
It is not an exaggeration to say that a sensorless brushless motor with an advanced controller can easily outperform a sensored brushless motor with a low quality controller. Again, the most important thing to understand in making this decision is undoubtedly the application that the motor is going into.

The importance of application specific solutions

One thing which has probably become abundantly clear (if you’ve read this far) is that making the decision about what is the best package to go with is heavily informed by the application the motor is to be used in. There are several reasons for this and we are always happy to help advise on the best possible option for your requirement.
An understanding of this can quickly rule out several options, whilst quickly pointing towards other solutions. Typical examples include the speed and torque range you are looking for. Low speed, high torque would be much better suited to a sensored brushless DC motor than a sensorless system which may struggle to retain a reliable speed. Equally, for high speed applications in harsh environments a sensored system would quite likely experience potential problems around reliability with a sensorless option being much more appropriate.