Variable Frequency Drives and Irrigation Pumps Stefanie Aschmann Bozeman, MT 11/30/11 1

Variable Frequency Drives Overview of draft “Variable Frequency Drives for Irrigation Pumping” TechNote ftp://ftp.wcc.nrcs.usda.gov/wntsc/Idaho%20pump%20Training/

Variable Frequency Drives Introduction VSD vs. VFD How VFD’s work Applications Operation/Design Considerations VFD Tool Demonstration

Variable Frequency Drives Pumping 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 energy Efficiency is a big issue “For the most part, the enormous cost of inefficient pump operation and valve throttling goes unnoticed”

There 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 arrangements Changeable gearbox Magnetic coupler Variable Frequency Drives are a subset of Variable Speed Drives

Variable Frequency Drives Variable 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 ride Tachometers (or rev counters) on automobiles, aircraft, and other vehicles show the rate of rotation of the engine's crankshaft.

Variable Frequency Drives Pump speed (e.g 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

What 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.

What does changing the frequency do? It allows us to change the rotational speed of the pump’s 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 pressure Pump is designed to meet the greatest output for both flow and pressure At other flows/pressure, the pump will be inefficient

Pump Curve

Getting 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 Point New Operating Point

Original Operating Point New Operating Point

Variable Frequency Drives

Constant Pressure//Variable Flow Constant Flow//Variable Pressure Variable flow//Variable Pressure Applications

Variable Frequency Drives Constant Pressure//Variable Flow Example Constant 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 Drives Applications Constant Pressure//Variable Flow Constant Flow//Variable Pressure Variable flow//Variable Pressure

Variable Frequency Drives Constant Flow//Variable Pressure Example A well that experiences drawdown over the course of the irrigation season Want constant flow rate throughout the season.

Variable Frequency Drives

Variable Pressure/Variable Flow Example Multiple systems of wheel lines and pivots

Variable Frequency Drives

VFD Operation Considerations Installing a VFD for power savings Motor load below 60%, electric motor becomes inefficient VFD itself is about 97% efficient Soft Start Option Single to 3 Phase Conversion Automation Open delta current balancing

What are VFD’s 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 failure Pump bearings and seals may wear faster at higher or lower shaft speeds Slide 27

VFD 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 28

Sizing 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 phase Care should also be taken to not select a VFD too large as the VFD output might exceed motor specifications and cause motor failure. Slide 29

Motor load Slide 30

Motor insulation class

Filters Line filters may be required for VFD’s 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.

Environmental Control An 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 Control VFD’s 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 Control Other 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 Demonstration Case No. 3 (see handout)