Lehner Motoren Technik (LMT) Torqstar 1 (7025)

The Lehner Motoren Technik (LMT) Torqstar 1 (7025) is a high-quality German outrunner of the Torqstar series with a 70 mm diameter and a 12 mm shaft. Lehner Motoren Technik is a German maker of electric motors for model flying and industry. At 1,150 g the motor belongs to the size class in which large electric fixed-wing aircraft are powered.

The Torqstar is offered in windings from 5 to 40. The winding sets the speed the motor reaches at a given voltage: KV is a choice made at purchase here, not a fixed catalogue value. Added to that are the choice of cooling, air or water, and the choice between model and industrial versions. Water cooling carries the loss heat away through a circuit and allows higher continuous power at the same size, but it needs a pump, hoses and tight connections. A 12 mm shaft takes high torques.

The design follows the chain given in the frame of this part type: take-off weight, propeller, stator size and KV, cell count. Only once propeller and voltage are fixed can the matching winding be chosen; motor, speed controller (ESC) and battery must carry the same cell count. Power figures of other motors refer to time spans of 3 to 180 s and can only be compared on the same time basis. An aircraft with a drive of this size can reach the open-category limit of 25 kg; above that the specific category applies.

In large outrunners the bearings and the winding insulation age. With water cooling, seals and hoses join the list of wear parts, and with air cooling the motor needs airflow or propeller wash. The checks are shaft wobble, bearing play in axial and radial direction, the insulation resistance between winding and case and the equality of the phase resistances. If the motor cannot be turned evenly and quietly by hand, the bearings should be replaced.