Perhaps no other technological development in the past 20 years has done more to revolutionise overhead crane design than the adjustable frequency drive (AFD). Today’s adjustable frequency controls and flux vector motor controls have become the industry standard. They are ideal for use in heavy-duty AC industrial cranes moving large loads, or where precise spotting, or high duty cycles are required.
We often overlook, however, the safety and performance benefits that AFDs provide for so called lighter duty applications, such as those in light manufacturing, machine shops, warehousing, etc. These applications usually involve small electric wire rope or electric chain hoists (referred to as monorail hoists or package hoists) with single-speed or two-speed controls, operating monorails or single girder crane bridges. These monorail hoists are more likely to be abused than their built-up counterparts. They are frequently subjected to overloads and higher duty cycles than their rating specifies. Finally, in many cases these hoists receive infrequent maintenance and are often operated by unskilled personnel.
Given these realities, the use of adjustable frequency drives provides many advantages for small hoist applications.
Increased versatility
No two applications are the same. Adjustable frequency drives provide the monorail hoist user with a variety of operating modes:
• Most single-speed squirrel cage motors can utilise an AFD, including conical-rotor motors used by many European hoist manufacturers.
• Multiple control modes that can be easily changed to suit the needs of specific applications or the desires of individual operators. Control modes include two, three and five discreet steps or infinitely variable speed selection.
• Wider range of selectable speed choices (up to 40:1, depending on the drive selected and the motor characteristics) are available to the user compared to the fixed speed ratios provided by typical two-speed motors (2:1, 3:1 or 4:1) or micro-drives (10:1). This also allows the OEM hoist builder to stock fewer motor/gearbox combinations.
Enhanced performance, prolonged life
The use of an AFD will enhance the performance and prolong the life of your hoist in a variety of ways:
• Reduces the characteristic high starting currents of AC induction motors (500% to 700% inrush) to less than 150% or 200% (depending on drive selected). This minimises the shock effect on both the load and the equipment, ensuring smooth movement of loads and prolonging equipment life.
• Provides controlled programmable acceleration and deceleration rates, permitting a softer stop and start. The result is reduced mechanical stress on the hoist, without a loss of productivity. With controlled deceleration, the motor mounted brakes perform the function of holding, not stopping the load, which greatly reduces brake wear and load slippage (when dynamic braking resistors are used), and results in longer brake life.
• Allows smooth transition between speed steps, eliminating the shock to mechanical components that occurs when two-speed motors change speed.
• Provides valuable monitoring and status information that can be used to evaluate hoist performance and establish preventive maintenance schedules.
• Features such as reverse plug simulation, stall prevention, inching control and micro-positioning control are built-in on some drives and are individually selectable, further enhancing the hoist’s performance.
Enhanced safety
Safety is a priority. That is why some manufacturers design their adjustable frequency drives with important safety features such as:
• Software designed to reduce the possibility of lifting an overload.
• Password protection to limit unauthorised modification of drive parameters.
• Software that reduces the possibility of programming unsafe parameters.
• Motor thermal overload protection which prevents overloading of the motor.
The advantages of AFDs are many. You can optimise the performance and safety, improve the versatility, and prolong the life of your small hoist with the use of an adjustable frequency drive.
