Engineer Column

Motor Notes : Features and Selective Use of Sensored and Sensorless Driving of Brushless Motors


All around us there are a multitude of motors being used. Innumerable motors are active in places where we do not see or notice them–in air conditioners, washing machines, vacuum cleaners, and other household appliances; in the cooling fans, hard disk drives, DVD players, and other devices in our personal computers; and in automobiles, trains, buildings, factories, and many other places.

Among these, many motors operate under rotation rate or torque control. Motor startup and driving methods can be broadly divided into two types: ① driving with a position sensor, and ② driving without a position sensor.

  • ①Position sensored driving: A motor driving method requiring a sensor that detects the motor rotation speed and rotation position
  • ②Position sensorless driving: A motor driving method not requiring such a sensor

The advantages of ① sensored driving are that it ensures position and rotation detection during motor startup and driving, and smooth motor driving and rotation control are possible from lower to higher speeds using a motor driver that incorporates a hard logic controller. Disadvantages include the need to consider the precision of the sensor mounting position due to the need to mount a sensor such as Hall element/Hall IC in the motor, and the need for wiring to connect the sensor to the motor controller.

The advantages of ② sensorless driving are the ability to drive motors in which it is not physically possible to install a sensor, and the ability to drive motors subjected to harsh environments such as high temperatures and the presence of water, oil, and the like. Disadvantages include poorer performance at low speeds due to the use of the speed electromotive voltage, so that the rotation position is determined by computational estimation by an MCU, and overall responsiveness is worsened. Moreover, in place of a position and rotation detection sensor, the motor current, voltage, and motor parameters (R and L of the motor windings) are used in the computational estimation, so that the result is affected by differences between individual motors. In sensorless driving, there are also methods in which a hard logic dedicated controller other than an MCU causes startup by an external forced commutation signal, and thereafter the speed electromotive voltage is used as a position signal.

Thus each type of method has strong and weak points. Therefore, a motor driving method must be selected according to the environment in which the motor will be used, the degree of reliability sought, the type of motor load (constant torque, constant output, square of the rotation rate), and other requirements.

For Beginners

Hall element/Hall IC:
A magnetic sensor utilizing the Hall effect that outputs a voltage according to a magnetic flux density. The Hall effect is a phenomenon in which, when an electric current is passed through a solid and a magnetic field is applied perpendicularly to a surface of the solid, a voltage occurs in the direction perpendicular (orthogonal) to both the direction of current flow and the magnetic field direction. A Hall IC (also called a Hall sensor) combines an op-amp, a digitizer, and other elements.
Speed electromotive voltage:
The voltage corresponding to the speed electromotive force, which is the time derivative of a magnetic flux. One parameter used in position and speed estimation in sensorless control.
External forced commutation:
When a motor is in the stopped state, an induced voltage does not occur; hence in order to detect the motor position, at the time of motor startup the motor is forcibly rotated from outside in order to cause an induced voltage to occur. This is called forced commutation.

Information on Downloading Technical Documents

Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.

Download Technical Documents

Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.