What is a Sensorless Brushless Motor and How Does It Work?

Quite simply, it's about the hall effect sensors

A sensorless brushless DC motor (sensorless BLDC motor) is quite simply a brushless DC motor without hall effect sensors. Hall effect sensors are sensors which are built into sensored brushless motors which are used to tell the brushless motor controller exactly where the rotor position is. This can be useful for maintaining set speeds and is particularly useful at start up as the rotor position dictates the start up sequence for the motor.

Operating a brushless motor without sensors - the importance of sensorless brushless motor controllers

Based on the fact that the motor has no in built sensors then this means that the brushless motor controller has to operate the motor without the use of sensors. One of the ways this can be achieved is through using the back-EMF of the motor to actively monitor the rotor position (and therefore speed). Put simply, back-EMF can be defined as the voltage created by the motor (as a generator).

A brushless DC motor is actually very similar to a generator such as one might find in a dynamo or turbine and so as it is driven round by the BLDC motor driver it will also start to generate electromagnetic force. This then works against the forward voltage produced by the driver but crucially the frequency of the force is directly related to the speed of the rotor and can be measured by an intelligent sensorless brushless motor controller such as the ZDBL10. By using this back-EMF frequency measurement it is therefore possible to measure the speed of the motor and compare it to the speed that the brushless motor controller is trying to achieve – any discrepancies can then be adjusted if required (a closed loop system) or left as they are (an open loop system)

Key advantages of a closed loop brushless system

With an intelligent controller such as the ZDBL15 brushless ESC closed loop sensorless brushless motor systems can achieve impressive performance both in terms of accurate speed maintenance but also dosing performance in small brushless gear pumps or diaphragm pumps. Key to this is the fact that accurate back-EMF measurement not only delivers accurate speed monitoring but it can also be used to count revolutions of the motor.

Of course, a further additional advantage is that sensorless brushless motors and BLDC motor controllers are also lower cost than Sensored Brushless Motors.

If you have any questions in relation to Sensorless Brushless Motors please feel free to contact us to discuss.

Closed loop vs open loop mode in sensorless brushless DC motor control

One of the most important distinctions in sensorless brushless DC motor control is between open loop and closed loop methods of control. Strictly speaking, using back-EMF as a speed reference is not a properly closed loop system. However, the feedback it enables is very close to this type of system in practice. 

A back-EMF controlled system clearly offers greater control over the motor and can also be used in applications where it is important to maintain a fixed speed under a variable load. However, it can be limiting the speed output of the motor you are using (depending on the speed you need) and often requires setup of the particular motor and controller to meet your application’s specific requirements. Optimisation of a sensorless brushless DC motor controller is something that we can do as a free service. Please click here for more information on this service if you are interested.

Is it right for you?

Ultimately the question as to whether a particular system is right for you needs to be answered on a case by case basis. However, there are certain key advantages of a sensorless brushless DC setup over other alternatives. Summarised, these are;

  1. Sensorless brushless DC motors are typically the fastest possible option available – especially in open loop.
  2. Sensorless brushless motors and controllers have less parts that can be damaged in the a sensored or brushed DC motor does and therefore tend to be the most reliable in the long term.
  3. If your application doesn’t require excellent low speed performance then it is quite possible that a sensorless system could match the performance of a sensored system but with the lower cost.
  4. Sensorless system can typically be used in harsher environments than sensored systems so if this is an important consideration for you then it would be worth thinking about sensorless options.
Category: Brushless Motor Controller FAQs
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