Abstract: A motor control method and apparatus for a hoist is disclosed in which a drive motor is connected to an adjustable frequency power supply in which the frequency of the power supplied to the motor can be selectively varied. At the initiation of a hoist operation, when the hoist brake is holding the motor and an object constituting a load on the motor stationary, power is supplied to the motor at a predetermined low frequency which is sufficient to provide load holding torque upon release of the brake. The actual current level of the power supplied to the motor is sensed at the initiation of the hoist operation and a signal representative of that current level is compared with a reference signal representative of a current level which is sufficient to provide the load holding torque. If the signal representative of the actual current level of the power supplied to the motor exceeds the reference signal, an output signal is provided which will result in the release of the hoist brake from its holding condition.

Abstract: An adjustable frequency motor drive system for a hoist is disclosed in which the frequency of the power supply to the motor can be selectively varied and the speed of the motor will follow the varying frequency at a slip speed relative to the frequency. The slip speed of the motor may not exceed a value beyond which the motor can control the hoist load. Accordingly, a frequency value based on the frequency of the power supply to the motor is selected as the frequency which is not to be exceeded by the motor speed expressed as a frequency. Signals representative of the actual motor speed and the selected frequency value are compared and, if the signal representative of the motor speed exceeds the signal representative of the frequency value which is not to be exceeded, an output signal is produced which may be used to apply a brake which stops and holds the motor and hoist and thereby maintains the load suspended.

Abstract: The invention relates to control and braking systems for apparatus driven by single and polyphase alternating current induction motors. The system prevents destructive and dangerous overspeeding where the nature of the driven load is such that it can furnish sufficient torque to overdrive the apparatus in any one of several potential failure modes. A capacitor selected according to this invention is permanently connected across the motor winding such as to cause the drive motor to automatically develop retarding torque in the absence of power supplied thereto sufficient to restrict the overdrive speed to a value not appreciably exceeding the normal operating speed. The system also incorporates drive motors specifically designed with a predetermined value of starting torque appreciably less than that conventionally applied to similar apparatus for the specific purpose of limiting the load handling capability of the apparatus, thus preventing dangerous overloading.