Slide 1

Variable Frequency Drives and Irrigation PumpsStefanie AschmannBozeman, MT 11/30/1111Variable Frequency DrivesOverview of draft Variable Frequency Drives for Irrigation Pumping TechNote

ftp://ftp.wcc.nrcs.usda.gov/wntsc/Idaho%20pump%20Training/Variable Frequency DrivesIntroductionVSD vs. VFDHow VFDs workApplicationsOperation/Design ConsiderationsVFD Tool Demonstration

3Variable Frequency DrivesPumping water for irrigation can be a major expense for irrigated farms.In 2008 more than 570,000 pumps were used for irrigation$15.5 billion dollars spent on energyEfficiency is a big issueFor the most part, the enormous cost of inefficient pump operation and valve throttling goes unnoticedThere are several ways to vary the speed of a pump. Not all save energy.Electrical (variable speed motor using variable frequency drive, or VFD most common) Mechanical (diesel and gasoline engines)Hydraulic (hydraulic coupling)Variable speed pulley arrangementsChangeable gearboxMagnetic coupler

Variable Frequency Drives are a subset of Variable Speed Drives

5There are a number of ways to change the speed of a pump. The most common way for electrical pumps is to use a variable speed motor with a variable frequency drive. It is not the only way, however.

You can mechanically crank down on a diesel or gasoline engine to vary pump speed.Variable Frequency DrivesVariable Frequency Drives are used exclusively on devices that have electric motors.The frequency used in the term refers to the frequency of the electrical current.In the US, electrical grid power has a frequency of 60 Hz.Think about your rideTachometers (or rev counters) on automobiles, aircraft, and other vehicles show the rate of rotation of the engine's crankshaft.

8A motor has a set number of revolutions per minute based on the frequency of the current. Motors are designed to run at that one speed, regardless of the power needs of the pump.Variable Frequency DrivesPump speed (e.g. 1780 rpm, or 1175 rpm) refers to the rotational speed of the impeller.The motor shaft is connected to the impeller; the impeller adds energy to the water.Slowing the rotation of the impeller reduces the energy that is transferred to the water and thereby the power requirement of the pump. Slide 10

10What does changing the frequency do?The speed of an alternating current (AC) motor depends on three principal variables: The fixed number of winding sets (known as poles) built into the motor, which determines the motor's base speed. The frequency of the AC line voltage. The third variable is the amount of torque loading on the motor, which causes slip.

An electric motor consists of a set of poles with windings through which electrons travel. The number of poles and their associated windings affects the motors base speed. The frequency of Alternating Current voltage also affects the speed.Finally, the amount of torque, or the slippage will affect the motor speed.11What does changing the frequency do?It allows us to change the rotational speed of the pumps impeller.Changing the speed of the impeller has the same effect as changing the diameter of the impeller:It shifts the pump curve (usually down)

Why Variable Frequency Drives?Pumps often need to operate over a range of flow rates and pressurePump is designed to meet the greatest output for both flow and pressureAt other flows/pressure, the pump will be inefficient

Pump CurveGetting to Where We Want to Be

Often the pump operator will change the system curve to get to where they want to be.

Original Operating PointNew Operating PointHere, the operator throttles a valve, which increases the friction loss through the valve.

This changes the system curve and will reduce the flow rate, but it also wastes energy, because the new16

Original Operating PointNew Operating PointAnother option would be to shift the pump curve.

How? Reduce to rotational speed of the impeller by changing the frequency of the electrical supply

17Variable Frequency Drives

Variable Frequency DrivesConstant Pressure//Variable FlowConstant Flow//Variable PressureVariable flow//Variable PressureApplicationsVariable Frequency DrivesConstant Pressure//Variable Flow Example

3 center pivots running off one pump.Each pivot requires the same pressure.But the operator wants to be able to run 1, 2 or 3 pivots at the same time.

The VFD maintains the pressure by shifting the pump curve, thereby increasing or decreasing the flow.

Variable Frequency DrivesApplicationsConstant Pressure//Variable FlowConstant Flow//Variable PressureVariable flow//Variable Pressure

Variable Frequency DrivesConstant Flow//Variable Pressure Example

A well that experiences drawdown over the course of the irrigation seasonWant constant flow rate throughout the season.

Variable Frequency Drives

Variable Frequency DrivesVariable Pressure/Variable Flow Example

Multiple systems of wheel lines and pivots

Variable Frequency Drives

VFD Operation ConsiderationsInstalling a VFD for power savingsMotor load below 60%, electric motor becomes inefficientVFD itself is about 97% efficientSoft Start OptionSingle to 3 Phase ConversionAutomationOpen delta current balancing

What are VFDs potential problems? Additional losses heat, motor loading, etc.VFD systems may create electric power system harmonics (harmonic currents are caused by certain types of equipment that draw current intermittently while running ).Operation of pumps at critical shaft speeds, far from design (BEP), can result in deflection & shaft failurePump bearings and seals may wear faster at higher or lower shaft speeds

Slide 2727VFD potential problems (continued)Too much capacity that can burn out the motor. Explosion proof motors must be approved to operate over the entire operating range.At the lower rpms the cooling fan is not rotating fast enough. Mechanical seals must be designed to operate over the entire speed range. High shaft speeds lead to additional NPSH requirement to prevent cavitation. Slide 2828NPSH = Net Positive Suction HeadSizing

In the design process, the size of the VFD needs to account for all inherent drive inefficiencies the motor load including service factor. Drive size may also be increased to minimize, voltage distortion and interference with other electrical equipment. Almost double is needed when converting single to three phaseCare should also be taken to not select a VFD too large as the VFD output might exceed motor specifications and cause motor failure. Slide 29Motor loadSlide 30

Motor insulation class

The F class is standard for newer motors.31FiltersLine filters may be required for VFDs to regulate voltage. Imbalance in voltage generates more heat and loss of efficiency in the VFD and motor. The EMI and RFI generated by the installation should be measured. If the interference exceeds limits defined by the current IEEE 519, electric utility may require that filters be installed. EMI = Electromagnetic Induction wavesRFI = radio frequency induction.These create harmonics. This will affect systems both downstream and upstream of the pumping plant.32Environmental ControlAn increase in temperature will see a dramatic drop in VFD efficiency and may require installation of a cooling mechanism. External Heat Sinks Self-Contained Cooling Systems Environmental ControlVFDs cannot tolerate dust or dampness; Should be installed in enclosures that meet NEMA 4 standards (dust & water tight). Adequate sunshades or pump houses are required for all installations. Environmental ControlOther factors that may affect VFD efficiency are. Radio frequency or stray high frequency signals. Line voltage variation greater than 10%. Line frequency variation greater than 2Hz. Altitude greater than 3,300 feet (1000 meters) VFD Tool DemonstrationCase No. 3 (see handout)