Sensored or Sensorless Brushless Motors both have their place and Zikodrive can drive either...
This page discusses sensored and sensorless brushless 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 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 this 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 (such as the ZDBL Series) 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 between sensored or sensorless BLDC motors
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 to 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.
An understanding of this can quickly rule out several options, whilst quickly
pointing towards other solutions.
Got a question? Give us a call.
Our team of UK based engineers are always happy to discuss your requirements
and advise on the best possible solution for your application. We design and
manufacture both sensored and sensorless brushless motor controllers so we are
very familiar with the pros and cons of both and their suitability to a range of
applications.