How a Sensored vs Sensorless Brushless Motor Works
A brushless motor receives power from the electronic speed control. (ESC) The speed control must know the position of the rotor in order to accelerate the motor from zero RPM smoothly. When it does not know the position of the rotor, the ESC must determine it. The ESC is able to determine rotor position by sending power to the motor and receiving back EMF (Electromotive Force – this really just means a voltage “signal” is produced and sent back to the ESC) as the rotor is in motion. It is this feedback loop that allows the ESC to understand the position of the rotor.
If you believe the term for the synchronization process of a brushless motor is cogging, check out the article on cogging.
In a sensored brushless motor, as the name implies, the motor contains sensors within it. These sensors are responsible for sending information back to the ESC about the position of the rotor. When the ESC knows the position of the rotor, immediate synchronization is established right from zero RPM.
Advantages of a Sensored Brushless Motor
Sensored motors are able to accelerate smoothly and strongly right from zero RPM. They also are able to produce more torque at slow speeds in comparison to a sensored brushless motor. Sensored motors also are able to allow safe operation of the motor by monitoring its condition. In some ESC’s, the ESC will not operate if there is something wrong in the sensor signals. Another piece of information that is captured from sensored brushless motors is the motor temperature. This is great for data logging motor temperatures.
Advantages of a Sensorless Brushless motor
There seems to be a lot of advantages for a sensored motor, but sensorless motors have their place as well. Although sensorless brushless motors do not perform well at low speed, their performance at high speed is excellent. It is so good that most ESC manufactures rely on sensorless synchronization of a sensored motor at high speed. At high speed most ESC’s will alter the timing for the sequence of power pulses that are sent to the motors windings. In turn improving efficiency and performance.
Sensorless motors also are lighter in weight, less complicated and less prone to failure. It is quite easy to damage the thin wire loom that sends all the data from the sensored motor to the ESC.
Applications of Sensored and Sensorless Brushless Motors
The number one reason why you may require a sensored motor for your application is due to requiring torque at extremely low speeds. We are talking around zero RPM. Applications where there is a requirement for torque at very low speeds are RC Car based. They include Rock Crawling, or any type of very slow speed RC Car action.
Also, cars with high gearing that require immediate torque can be quite problematic at low speeds. They may consist of high speed on road cars or high speed RC drag racing cars. The low gearing in these RC vehicles requires the motor to develop a lot of torque from a start. A request for the motor to accelerate quickly from a stop can cause a great amount of power to be drawn. When the brushless motor is not in sync with the ESC, the reliability of quick, smooth acceleration is compromised. A sensored motor in these applications would help.
When it comes to other applications, the requirement for a sensored motor is low. For example, RC helicopter, boats and airplanes of all types work very well with a sensorless motor. Most of the operation of these motors in these applications are done at higher speed. One of the most important parts is even at 0 RPM, there is a very low torque requirement in getting the system going. A propeller at slow speed does not load the motor significantly enough to warrant a sensored motor. Dropping the requirement of added sensors and wire looms to a motor in these applications saves precious weight and complication. Ultimately, this may lead to increased reliability.
Sensored / Sensorless Brushless Hybrid ESC’s
A perfect example of this system is in my 1/8 scale brushless buggy. I am using a castle system that is capable of both sensored operation and sensorless operation. What ends up happening is during acceleration from a stop, the motor/ESC combination is in sync utilizing the sensors inside of the sensored brushless motor. Once a small amount of speed is gained, the ESC switches from using the sensors to using sensorless synchronization. Here you gain all of the advantages as described above. It is an excellent solution if you are looking for smooth operation from zero RPM and excellent high speed performance.
Can a Sensored Motor work with a Sensorless ESC
Taking a sensored motor and using it with a sensorless ESC will work with no problem. The motor will simply act as a sensorless motor since the ESC will figure out rotor synchronization based on back EMF generated. In other words, a sensorless ESC will operate a sensored motor as a sensorless motor.
For obvious reasons you can not take a sensored ESC and use it with a sensorless motor. If the sensored ESC has the ability to operate as a sensorless ESC, the motor will also be treated as above – sensorless synchronization.