Hitachi Drives PM Motor at Low Speed Without Position Sensor

Apr 18, 2013
Motohiko Hamada, Nikkei Automotive Technology

Hitachi Ltd developed a technology that enables to quickly start a permanent-magnet (brushless DC) motor and drive it with a high torque without using a position sensor.

With the new technology, it becomes possible to eliminate the need for a position sensor used for a permanent-magnet motor, enabling to reduce motor size and simplify installation and maintenance. Also, because the technology enables to quickly generate a high torque, it can be applied to conveyor belts, elevating machines, etc.

Permanent-magnet motors have higher efficiencies than induction motors, which are commonly used as alternating-current (AC) motors. To quickly start a permanent-magnet motor or quickly increase its rotation speed, a position sensor is normally used to measure the position angle of a rotor.

However, the location, installation accuracy and reliability of the sensor have been problems. And there has been a need for a control technology that does not require a sensor.

Therefore, motors that do not use a sensor are used for some applications. In this method, the position angle of a rotor is estimated from the voltage generated by a coil with a rotating permanent magnet. But, when the rotation speed lowers, so does the speed electromotive voltage, making it difficult to estimate the position angle of a rotor.

As a result, the use of such motors has been limited to air conditioners, refrigerators, industrial fans and pumps and other applications in which motors are not frequently started and just keep rotating. In addition, they use a large amount of current to increase torque at the time of starting, wasting a large amount of energy.

The new technology detects the position of a rotor by utilizing the fact that the inductance of a coil slightly changes due to the influence of the permanent magnet attached to the rotor. The inductance means the proportionality coefficient between the current applied to the coil and the generated magnetic flux.

The inductance is determined by the winding number and structure of the coil, etc, but it slightly changes due to the influence of the permanent magnet. The change of the inductance can be detected as the change of the motor's electromotive voltage, from which the position of the rotor can be calculated.

With this method, the position of a rotor can be detected even when the motor is not rotating or rotating at a low speed.