Other

Harnischfeger Corporation

Yaskawa Electric Mfg. Co. Ltd.

David C Wycoff

Keith O Jurkowski

William A Hipp

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.

Method and apparatus for determining load holding torque

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. A motor speed deviation time period may also be provided and the hoist brake not applied to the motor until the time period is exceeded.

Growing community of inventors

Elm Grove, WI, United States of America

David C Wycoff