Abstract: In a drive system for a grinding mill the load torque plus frictional torque may increase considerably as a loaded mill is inched from its rest position and will suddenly decrease when the load tumbles or cascades. The applied torque or driving torque must increase at a corresponding rate and should also decrease when the load cascades. If the applied torque exceeds by a significant amount the load torque plus frictional torque, a destructive hammering will occur in the gear train and this will be generally proportional to the excess of applied torque for a synchronous motor drive. The drive system has a DC source which is commutated by a commutating device to provide a low frequency stepped alternating voltage to the stator windings of the synchronous motors of the drive system for inching. The output of the DC source has an alternating current component which varies as the difference between the applied torque and the load torque plus frictional torque.

Abstract: There is provided a control apparatus for stopping a large grinding mill driven by two synchronous motors coupled to the mill through respective air clutches. The control includes a clutch actuator control which applies a predetermined amount of pressure of predetermined duration to the clutches sufficient to at least partially close the clutches but insufficient to fully close and lock-up the clutches. The control apparatus further includes an enable control which periodically enables the clutch actuator control to apply the predetermined amount of pressure. The enable control enables the clutch actuator control for a predetermined time duration sufficient to stop motion of the mill and synchronous motors. The present invention provides a system for bringing a mill to a stop relatively quickly without adding significant costs to the mill controls.

Abstract: There is provided an electrical inching control for inching a drive system comprising a grinding mill driven by one or more three phase synchronous motors. The electric inching control includes an inching power bus and contactors or the solid state equivalent thereof. The inching bus supplies energy for the stator windings of the synchronous motors. The contactors or switches connect the positive and negative sides of a direct current power supply to the different phases of the stator windings in a sequence which provides a stepped rotating magnetic field, and the rotor field windings are excited with direct current. The interacting stator and rotor fields causes a rotation of the rotor a predetermined amount with each step causing the mill also to rotate a respective predetermined amount.

Abstract: In the inching of a grinding mill or similar apparatus which is driven by a pair of synchronous motors through fluid actuated clutches, the motors must be angularly alined one with the other in order that the load be shared equally between the motors. The apparatus of the invention connects the field supply to both motor fields and then it connects a zero frequency supply (i.e. a DC supply) to the stator windings of both motors and ramps up or increases the amplitude of the supply. This brings the rotors into alinement without undue oscillation. When the amplitude of the low frequency supply reaches a predetermined level, the clutches are closed or locked and the motors are connected to the load with the rotors in angular coincidence or alinement.

Abstract: A control system, for a motor which may be subjected to stalling loads, provides a non-linear increase in motor current during stalling conditions to maintain rated horsepower. When the maximum transient overload current is reached, this current is maintained to keep the motor torque at a maximum until the motor is stalled. This provides maximum motor torque down to a stalled condition and resists stalling. A limit controller determines the maximum time a particular overload motor current can be maintained and when this time has passed the motor current is reduced to rated current. If the motor should stall completely a further reduction in motor current is made automatically.