Can I use a Sensorless Brushless Motor Controller to Start a Sensorless Brushless Motor under load?

Starting a sensorless brushless motor under load with a sensorless motor controller - the short answer ...

Yes, although successfully (and reliably) accomplishing a sensorless brushless hard start is one of the hardest aspects of operation of sensorless brushless motors and does need some tailoring to your specific motor choice and application.

Click here to go straight to a short video outlining how this issue can be addressed.

The key problem with starting a sensorless brushless motor under load - How to apply start up power without knowing the rotor position ...

The key to successfully using a sensorless brushless motor controller to drive sensorless brushless motors in an energy efficient and practical way is the timing of the power input into the motor coils. If the timing is slightly out then this will cause major inefficiencies within the motor and can even act as a brake on the motor’s turning (depending on the rotor position).

This can also create excess heat and wasted energy (a common issue with lower cost brushless ESCs which often start to overheat and burn out relatively quickly).

When a motor is up and running it is relatively easy to know the rotor positions on a sensorless brushless DC motor as this can be detected using back EMF and then optimised at the testing and setup stage through testing and understanding the exact motor being used. 

However, at the startup stage it is impossible for the BLDC motor driver to know the inertia on the rotor or the rotor position on a sensorless BLDC motor because there is no movement from which to gain the back EMF information which the brushless motor controller can use to pinpoint the rotor location.

It is therefore quite common for sensorless motors to jump around a little at the start if they have not been properly optimised because the brushless motor controller being used is powering on coils on an assumed position with the aim of pulling the rotor into that pattern when it will then function normally.

Hence, the often stuttered startup followed by a sudden boost as the rotor aligns with the drive pattern (this is often compared to an engine struggling to start, coughing and spluttering, before suddenly roaring into life).

Optimising sensorless brushless motor controllers to start under load

The key to optimising a brushless motor controller to start under load is firstly to know the motor that you are working with and its application and then to optimise the brushless motor controller to the key specifications of this particular motor and the application. Key variables such as inertia, number of poles, inductance and more can all be used to optimise the controller to run well.

The Zikodrive brushless motor Vert Rotor case study provides a good example of this.

Once this is done the running of the motor will be as efficient as possible and this knowledge can then be used to inform the startup sequence for the motor. Knowing the number of poles and the motor inductance is a big part of this as this can be optimised specifically for the motor.

However, advanced programming in the ZDBL range of sensorless brushless motor controllers is also carefully optimised to read changes in back-EMF at the earliest possible stage of motor commutation therefore allowing it to quickly pick up the rotor position and apply power in the optimal way.


A tailored solution that delivers the best results

There is no doubt that starting a sensorless brushless DC motor under load is not the easiest aspect of motor control to master but with the ZDBL series of sensorless brushless ESCs we have developed a system which works well in a huge number of applications.

Sometimes this can require a significant amount of optimisation and hard work but in the long run it pays off, particularly where such performance is a key part of your application.

For more information you can browse the Vert Rotor Brushless Motor Controller Case Study or browse our range of brushless motor controllers (brushless ESCs) for more information.

If you have any questions at all please do not hesitate to contact us to discuss with one of our engineers.

Category: Brushless Motor Controller FAQs
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