Variable Frequency Drives for Grain Drying Fans
Large fans are being installed on large bins, but it should first be determined if it is feasible to dry grain in the bin. Frequently the fan size required to natural air-dry grain in large bins exceeds what is economical or practical. For example, a 90 hp fan is required to provide an airflow rate of 1.0 cfm/bu to dry 33,000 bushels of corn in a 42-foot diameter bin. A fan smaller than might be expected may be adequate to cool grain. For example, a 7.5 hp fan will cool 58,000 bushels of corn in a 48-foot diameter bin in 75 hours.
Variable frequency drives (VFD) are being installed on farms to enable three phase motors to be operated on single-phase electrical lines. Single-phase motor size is generally limited to about 10 hp. The VFD converts single-phase alternating current to direct current and then electrical pulses create a simulated three-phase alternating current.
Three phase motors are normally smaller in physical size, but require the same amount of electrical power as a similar horsepower single phase motor. A 10 hp single-phase fan motor and a three-phase fan motor will use the same amount of kilowatt-hours of electricity.
The VFD does not reduce the amount of electrical power used if the fan is operated at the normal speed. Power used will only be reduced if the fan rotation speed is reduced. This will also reduce the quantity of air supplied by the fan and the pressure that the fan can develop. For example, reducing the rpm of the fan by 10% reduces the airflow, cfm, by 10%. A 20hp low speed centrifugal, LSC, fan operating at 1,750 rpm will deliver 18,500 cfm at 5 inches of static pressure. Reducing the rpm by 10% reduces the airflow delivered to about 16,650 cfm. It also reduces the operating static pressure to about 4-inches and the horsepower to about 15hp. In comparison, a 15 hp LSC fan operating at 1,750 rpm delivers about 15,200 cfm at 4-inches of static pressure. Since the quantity of airflow determines the speed of drying, it is not generally appropriate to reduce the quantity of airflow.
Operating a motor at a lower rpm than at which it was designed may result in overheating the motor, since the cooling fan for the motor will provide less airflow at a lower rpm.
VFDs consume some power during operation. The amount varies depending on several parameters, but about a 4% power consumption might be expected. For example if a 30 hp motor is supplied power through a VFD, the electrical consumption will be equal to that used by 31 hp motor.
The VFD allows the motor to be started slowly, which reduces the electrical current required for starting the motor. A normal motor start results in a current draw of 5 to 7 times that of the operating current. For a motor with a 50 amp running current, the starting current could be 250 to 350 amps for a short period. The large current draw may cause problems for the electrical system and requires that the electrical system be sized to accommodate the large current draw. The electric utility may assess demand charges based on this electrical demand. A motor starting slowly using a VFD will require a much smaller starting current. Starting the motor slowly is sometimes referred to as a soft start. This feature is available for three phase motors.
Several problems can occur with VFD sizing and installation, so it is extremely important that the electrical system is designed and installed by people experienced with VFDs. For example, electrical noise created by the VFD depends on how electrical cables are installed. It also can create radio frequency interference that affects AM radios, cordless phones and other devices involving a radio signal. Shielded motor cables can effectively suppress radiated interference if installed correctly. However, incorrect installation may cause additional problems. A VFD should be sized for a larger current than is listed on the motor nameplate. For example, a 25hp motor might require a 40hp inverter.
Starting a fan that is rotating opposite to its operating direction, backward, results in a severe torque surge that may cause overload currents and brownouts. The fan may be rotating backward due to the wind moving the fan or due to one fan starting before the next fan and air being pushed into the fan that is not operating. The fan should be slowed to a stop before starting rotation in the desired direction.
The utility electrical system may not be adequate to serve additional electric motor loads, so contact your power suppliers before adding additional fans and motors. Inadequate system capacity may cause problems for both you and your neighbor.