Okuma reluctance motor used on CNC lathes

Okuma has announced that its reluctance motor is to be used as a drive for the main spindle and optional counter spindle on its LB lathes, replacing conventional brushless AC motors.

Previously the reluctance motor's high torque made it unsuitable for applications where accurate speed control is essential.

The torque ripple and speed control problems have been overcome using modern, computer-aided methods for designing the motors, and by embedding controllers that harness algorithms and real-time computing to tailor drive waveforms according to rotor position and current or voltage feedback.

Okuma said the cost of the drive electronics is offset by the simplicity of motor construction, as it uses no brushes, commutator, rotor windings or permanent magnets.

This also makes the motor reliable and maintenance-free.

Compared with brushless motors, the moment of inertia of the rotor is only half, wihch makes it high revving and easy to accelerate and decelerate.

As with an induction motor, the output can be precisely controlled over a range of speeds.

Comparative tests with conventional AC motors of the same class carried out at Okuma showed that PREX achieves 30 per cent higher power and a maximum efficiency of 90-95 per cent, with minimal heat generation even under peak load.

Okuma said the motor is powerful for its size, rigid, thermally stable, energy efficient, durable and can be controlled with high precision using modern electronic technology.

It is also easy to recycle as it only contains copper and iron.

The company offers its LB lathe spindles with bores of 80mm, 91mm or 100mm diameter.

On the Okuma LB3000 lathe, the PREX spindle motor provides an output of 30kW and develops a torque of up to 700Nm at 3,000 rev/m, accelerating up to 5,000 rev/min in 3.4s.

The larger motor on the LB4000 offers 37kW/1,200Nm/3,000 rev/m.

As gears and belts are dispensed with, the switchover between speed ranges takes less than 0.5s, reducing unproductive idle time.

When functioning as a C-axis, the spindle rotates at 200 rev/m and is indexed with a repeatability of 0.005 deg.

A PREX motor continues to be used in the 12-station turret, offering 7.1kW of servo power and generating 50Nm of rotary tool torque, allowing high metal removal during prismatic machining parts of a cycle.

Maximum tool rotational speed is 6,000 rev/m.

The smaller LB2000 lathe models are similarly fitted with PREX motors in the turret and to drive the spindle(s).

In a further development, Okuma has started installing two motors in the turret, one for indexing and another for tool drive, to increase the milling and drilling power.

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