Variable Speed Pumping Jim Vukich Application Engineer ITT Corp. Malvern, PA
Why Do It?
Why Do It? Flow-Matching Minimize Starting & Stopping Flexibility Different Discharge Points, Flows Changing Conditions Present & Future
How To Do It? Adjustable-Speed Devices
How To Do It? 2-Speed Motor Two Windings in a Single Stator
How To Do It? Transmission Belts & Pulleys, Gears, etc.
How To Do It? VFD The Most Common Method Today
VFD Variable Frequency Drive
VFD Variable Frequency Drive We can change the speed of the motor by varying the frequency of the signal Frequency = Cycles per second = Hertz In the U.S. systems operate on 60Hz nominal
VFD Variable Frequency Drive
Types of VFDs Current Source Inverter Simple, but Noisy Flux Vector Control Precise, but Expensive Pulse Width Modulated Most Common for Pumps
Pulse Width Modulation Uses a Constant Voltage Pulsed with Integrated Bi-Polar Power Transistors Vary the Width of the Pulses to Generate a Sine Wave Switching Frequency Higher is Better (to a point)
Pulse Width Modulation
Effects on the Motor Harmonics Noise Torque Ripple Overheating Modern VFDs have eliminated most of these problems!
Reduced Torque
Effects on the Motor Steep Voltage Transients Short Rise Times Stress on the Insulation Material Reduced Motor Efficiency
Sizing the VFD What You Need to Know: Motor Full-Load kva Ratio of Maximum to Normal Torque
Sizing the VFD Do NOT size the VFD based on the horsepower!!! Size it on current and voltage Pumps are variable torque machines Pump VFDs often need to be oversized
Sizing the VFD Full-Load kva (Volts x Amps x 3) 1000 Size the VFD at least 10% higher than the motor
Sizing the VFD Older motors or cheap motors need to be de-rated 10% to 15% de-rating is typical Insist on a VFD-ready motor! Class H insulation High-fill impregnation
Sizing the VFD Make sure the VFD is rated for the same number of starts per hour as the motor
Why VFDs are Used in Pumping
Why VFDs are Used in Pumping A VFD allows you to change the flow and head performance of your pump
Why VFDs are Used in Pumping Flow Matching Match the inflow rate Maintain the outflow rate Maintain a constant liquid level in the sump Minimize the sump volume
Why VFDs are Used in Pumping Changing Conditions Over Time Increase flow without new pumps Maintain flow as new pump stations come online
Why VFDs are Used in Pumping VFDs can be used as soft start and soft stop VFDs can be used as phase converter
Why VFDs are Used in Pumping A VFD allows you to change the flow and head performance of your pump
Single Speed Curve
The Golden Rule for Pumps The pump can only operate at ONE point Where it intersects the system curve
The Golden Rule
Variable Speed Field
Many Possible Operating Points!
Efficiency vs. Speed
The System Curve
The Allowable Operating Range
Specific Energy kw-hrs per Million gallons
Different Kinds of System Curves
Risks & Limitations of VFD Pumping
Operation Outside of Allowable Range
The Allowable Operating Range
When you operate outside of the allowable range: Recirculation Clogging, Wear, Heat Rise Vibration Shaft Bending Stress on Seals and Bearings Cavitation
Recirculation Wear
Wear Heat Rise
Clogging Dirty little secret: There is no such thing as a non-clog impeller Any kind of impeller can clog if operated under the wrong conditions
Clogging Impeller energy drops off rapidly at reduced speeds If speed = 50%, impeller energy = 25% Turbulent zone moves further from leading edge Rags build up on the impeller Less available torque to pass the occasional mophead
The Turbulent Zone
Unstable Region Stall Point
Motor Cooling Motors with cooling jackets may require a minimum speed To provide heat transfer To prevent settling of solids Check with the equipment manufacturer
Natural Frequencies
Natural Frequencies All systems have a natural, or resonant, frequency. If that frequency is fed into the system from an external source, the system will begin to vibrate.
Natural Frequencies As the VFD ramps up and down, it may hit certain frequencies that excite the system The controls must be set to lock out those dangerous frequencies
Natural Frequencies Always install flex connectors to isolate the pump from the piping Always support the piping independent of the pump Pipe hangers Thrust blocks Base elbows
Running Above 60Hz Be very careful!!! 10% overspeed requires 33% more power NPSH goes up risk of cavitation recommends 63Hz maximum
Reliability A VFD is an in-line device Any in-line device will reduce the reliability and efficiency of the system
The Downside of Constant Liquid Level
Grease Ring Floating Debris
Operational Strategies or The Smart Way to VFD Pumping
Operational Strategies The #1 problem with VFD pumping is clogging Clogging usually results from running the pump too slow for too long
Prevent Clogging Know the limitations of your pump Set the minimum speed to stay within the A.O.R. Consider running in on/off mode at the minimum speed
Prevent Clogging Start at full speed and then ramp down This will clear clogs, and will open check valves fully Ramp up to full speed once in a while, to clear clogs and flush out cooling jackets
Grinder Pumps Don t run a grinder pump on a VFD Grinder pumps depend on torque to do their work Available torque is reduced when run on VFD
Keep the Sump Clean Vary the on/off level to prevent grease rings Run the pumps to snore to suck up debris
Keep the Sump Clean The controls may automate this for you!
Running Multiple Pumps on VFD Always ramp the 1st pump up to full speed before bringing the 2nd pump online Slow down the 1st pump to match the 2nd This is called synchronous operation
Use the Right Settings If using VFD as soft-start, ramp up to full speed as quickly as possible If using VFD as soft-stop, choose the correct setting! Otherwise, water hammer may occur
Use the Right Settings Make sure the VFD is set for a variable torque load This will maintain: Magnetization level Motor efficiency
Use the Right Settings Some VFDs have built-in presets for equipment: Pumps Blowers Compressors Fans Make sure you are using the pump settings!
Use the Right Settings Switching frequency Should be in the 3 to 4 khz range Any higher can damage the motor Any lower can cause audible noise
Installation Guidelines
Pump Sensors Some types of sensors may be sensitive to noise and harmonics generated by the VFD Filters may be required Check with the pump mfgr. Check with the VFD mfgr.
VFD Output Filter Reduces Noise Can eliminate need for motor de-rating Can reduce nuisance tripping (false alarms)
Pump Sensors Use shielded wire to carry the sensor signals, especially on long runs Run the power and sensor cables in separate conduits, spaced at least 12 inches apart
Power Factor Correction Capacitors PWM drives will keep the power factor at 95% or better Capacitors are not required Capacitors can actually be destroyed by the VFD!
VFD in same cabinet with other components VFD must be properly constructed to prevent interference
Bypass Contactor Insist on getting a bypass contactor installed Eliminates the VFD in emergency situations Helps with troubleshooting
Use Caution when Taking Readings You need a true RMS meter to take accurate readings of voltage, current, and power
Conclusions
Conclusions VFDs are NOT a cure-all!! They have their problems and you must use caution Be aware of the limitations of your equipment Pumps Pipe system VFDs
Conclusions VFDs are NOT appropriate for all systems High static head (flat system curve) may not work well with VFDs Don t use a VFD with grinder pumps
Conclusions VFDs can result in off-peak operation of the pumps This will lead to problems! Clogging Vibration Wear Lower efficiency Shorter life
Stay within the Allowable Operating Range
Restrict operation outside A.O.R. to short or intermittent duty ONLY
Conclusions Avoid VFDs if possible Consider the alternatives: Use different-sized pumps (jockey pumps) Make the wet well larger Choose pumps that can start more times per hour
Conclusions Avoid VFDs if possible If not correctly applied, VFDs may: Increase cost Increase complexity Decrease efficiency Decrease reliability
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