Electronic sensors give AC drives a lift

Despite the simplicity and lower cost of AC induction motors, DC motors have dominated the fork-lift truck market mainly due to the cost of the control units. The situation has now changed as manufacturers of fork-lift trucks have found that using a bearing with a built-in electronic speed sensor they are able to match the cost of DC drives while giving better performance.

The bearing, produced by SKF, incorporates Hall-effect sensors to measure the rotational speed of the motor shaft. The bearing has been installed in an AC Superdrive motor control system developed for the latest range of fork-lift trucks from BT Industries of Sweden for both the main traction motor and the truck’s hydraulic pump motor.

An accurate feedback signal of the bearing’s rotational speed enabled the adoption of AC motors for these applications. Fitting the bearing is straightforward, no extra space is required, and it is protected inside the motor casing.

The bearing is fitted with a magnetic impulse ring and two Hall-effect sensors. The impulse ring is magnetised with 64 north and 64 south poles. As they rotate, the poles develop a magnetic field which creates an electromagnetic field in the sensors. By using two sensors, the bearing provides feedback on both speed and direction of rotation.

SKF Industrial Sales Tel: 01582 490049