Dual-Drive Permanent-Magnet Belt Conveyor with Load Sharing for a Coal Mine

Driving a long mine conveyor with two induction motors and two drives has a well-known weakness: the two motors rarely share the load evenly, because induction slip and small differences in drum diameter decide which one pulls harder. The result is one drive running hot while the other loafs, plus a transmission that wears unevenly over the years. The customer's existing conveyor had exactly that reputation — poor drive stability, uneven master/slave power split and high energy use.

Yolico replaced it with drum-integrated outer-rotor permanent-magnet synchronous motors. The motor is built into the conveyor drum, so the gearbox, the coupling and the belt-drive arrangement between them disappear along with their losses. Two YDS8160 T4 N drives feed the two motors, running at a very low electrical frequency — 14.67 Hz for 44 rpm at the drum — which is the natural operating point of a direct-drive conveyor and not a corner of the envelope the drive has to be pushed into.

Sharing the load is handled in the control scheme rather than by mechanical tolerance. One drive acts as master and runs in speed control; the second acts as slave and runs in torque control, following a torque command sent from the master over an analogue link alongside the speed reference. Because the slave is commanded in torque and not in speed, it cannot fight the master: it simply supplies its share. Accepted speed deviation between the two is 5 %.

Commissioning was about verifying the electrical and the mechanical behaviour together. At full load the belt and idlers ran smoothly and the drive current sat steady at 240 A without hunting, which is the clearest sign that the two motors had settled into a stable split. Starting from a loaded belt drew about 360 A momentarily, comfortably inside the drive's overload capability, and the belt accelerated and decelerated in a smooth fashion with no slip-back or idler vibration.

Two further points mattered to a mine that cannot afford an unplanned stop. The drives work with a mains-frequency bypass so the conveyor can keep running if a drive is taken out of service, and the changeover does not disturb the drive. And the interface to the plant controller behaved cleanly — communications and monitoring operated throughout with no interference or dropped connection.