Brushless Motors: Sensor-Based vs Sensorless


Sensor-Based brushless motors are often used in applications where starting torque varies greatly or where a high initial torque is required, such as in remote control cars. If one were to look at industrial or aerospace applications or brushless motors, it would be hard to find sensorless motors being used due to their lack of starting torque and unreliable operation under noisy electrical conditions. Sensor-based brushless motor systems always know the position of the rotor, which is especially critical at low speed as well as during the start condition when there is no rotor movement. Only with the proper rotor position information can the speed control apply power to the correct rotor phase combination.

Novak speed controls can apply 100% power to the correct phase set immediately after decoding the throttle signal from the transmitter when the rotor is stationary. This allows the sensor-based brushless motor to act more like a brush-type motor and deliver maximum torque at zero speed without the losses associated with brushes and the commutator. Another benefit of the sensor-based design is that the motor and speed control are always in sync with each other at all speeds.

The time required to attain the rotor position from the Hall Effect sensor is on the order of a few micro seconds. If a motor's rotor is turning at 80,000 RPM, that would be 1333 revolutions per second, or 1.33mS per revolution. Compared to a microprocessor running at 20MHz (or 26,600 clock cycles @ 50 nanoseconds per cycle), that is a very long time, which means that the microprocessor is capable of executing many instructions during that time frame.

In contrast, a sensorless speed control does not know the position of the rotor until it is spinning at a certain speed and generating enough back-EMF to calculate its approximate position. One cannot simply apply power to the coils and expect the motor to start up properly. A start-up phase sequence is required to get the motor running.

Most sensorless speed controls use a ramp-up method in which power is applied to two phase coils to get the rotor into a known position within the created stator flux (like a stepper motor). The motor phases are then energized according to a certain coil energizing pattern and the motor speed is gradually increased by slowly decreasing the commutation period. During the ramp up period the phase that is not being fired is monitored for back-EMF. Once the speed is high enough to produce detectable back-EMF, the algorithm switches over from the ramp-up method to a back-EMF or zero crossing feedback mode.

So, the myth that the sensor-based motor system is slower than the sensorless system couldn't be farther from the truth. As a matter of fact, the sensorless speed controls have to spend a fair amount of time filtering and making decisions on back-EMF detection while the sensor-based feedback system is much faster, more reliable, and overall far superior.

As far as the power output or top RPM of a given size motor is concerned, it has nothing to do with what type of sensing is being used. Motor performance is based on factors such as size of the rotor's magnet, stator design, copper content, air gap between the stator and rotor, etc.-- Not the type of sensing being used.

We design and build our own motors in-house at Novak, using computer-based motor design software. Our motors are purpose designed and built from the ground up specifically for remote control car applications. Through extensive testing and feedback from our design and testing staff and from numerous world class drivers, we verify design parameters and newly developed materials and continually improve the performance and quality of our brushless motors. Many other brushless motor suppliers simply import low cost, low quality sensorless (or sensor-based) motors becasue they do not have the in-house capabilities to design or build brushless motors themselves.

The cost of the additional sensors and timing hardware to produce a sensor-based brushless motor is minimal compared to the tremendous benefits they provide for the performance and efficiency of the system.

 

Article Viewed 3465 Times - Last Updated 2013-04-01

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