Variable Frequency Drive (VFD’s)

Three phase commercial power is not common in rural areas. This creates an obstacle to today’s modern farmers who are progressively building larger grain handling systems. The farmer in most instances has a 240 volt single phase source of power. This is a problem in that motors over 10 Hp. are very expensive when compared to their three phase counterparts. To extend three phase power to these systems has been historically very expensive so one of the only options was to live with that or install clumsy and inefficient rotary converters. On the other hand American industry has utilized VFDs for many years because of the control they provide. These VFDs powered three phase motors from a three phase source. Much more recently the need for large motor control in rural areas has prompted the utilization of VFDs for grain handling and irrigation. The VFD is able to do this because of the way it is designed wherein it turns AC to DC then back to AC. The output AC has the three phases needed for these motors.

 

 VFD capabilities include:

■VFDs can be use single phase power to drive three phase motors
■VFDs replace conventional motor starters
■Settings on the VFD provide a “soft start” that can be programmed to virtually eliminate inrush current that is common in conventional motor starters.  The soft start also greatly reduces the wear on the mechanical system which occurs on start-up.
■VFDs are more efficient electrically than other forms of phase conversions
■VFDs provide a balanced flow of current through all three phases thus eliminating the need to derate motors
■VFDs when combined with a PLC can be programmed to automatically change speeds
■VFDs in addition to the “soft start” can be programmed to decelerate in a controlled manner thus saving additional energy
■VFDs have adjustable overload settings allowing specific machinery protection
■Initial cost may seem high, but with VFD ease of installation and inherent advantages these cost are quickly offset

 

 

Variable frequency drives advantages over rotary phase converters are many. A rotary converter is designed for a specific load. If the load is four motors totaling 100 Hp. any time that all four motors are not running the current in the three phases (legs) is unbalanced. This is inefficient and reduces motor life. In many applications one of these motors is larger than the rest so if the operator was running three motors totaling 60 Hp. then needed to start the remaining 40 Hp. It would not be uncommon for him to be required to shutdown the three running motors so that the inrush current to the largest motor did not overload his system.

Variable frequency drives allow the modern farm operator the level of control of his grain system that he has become accustomed to in the truck that he drives and the tractors that he operates. It gives him the ability to maximize the hours that he has available and to protect the large investment he has made in his grain handling equipment.

 

Typical Indoor Installation

 Typical Indoor Installation

 

 

Installation in Outdoor Enclosure

Outdoor Enclosure

 

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