Motors and Variable Speed Drives for Pumps and Fans. Reid Hart, PE

Transcription

1 Motors and Variable Speed Drives for Pumps and Fans Reid Hart, PE APEM Summer Forum June 2016 Wilsonville, Oregon PNNL-SA

2 Outline Energy Performance Code Packages Variable speed drives & motors Types of motors & drives VFD drive losses and motor interaction ECM motors efficiency & limitations Affinity laws and reality Affinity laws Reality in air and hydronic systems More accurate modeling of VSD fan and pump savings Application & codes Balancing and controls to maximize VSD savings When not to use VSD s Changes coming in variable flow code requirements 2

3 HVAC Motors & Variable Speed Drives

4 Motor Efficiency by Type Motor efficiency impacts Type Size Loading Poles/RPM Motor Master: RPM is pump or fan rotational speed in revolutions per minute 4

5 ECM Characteristics and Limitations Characteristics Constant airflow or torque means controlled to: Energy varies based on external conditions If ductwork bad, motor speeds up to approach desired airflow Larger, shorter ductwork means less energy use Various versions: Variable speed & constant torque Limitations Must be direct drive limits size of application Starting to see 10 hp or 15 hp; most are fractional hp 5

6 EC Motor Efficiency Electronically commutated motor (ECM) aka permanent magnet brushless dc motors (PMBLDC) Permanent magnet rotor; reversing (commutated) stator field Most controls: constant torque Replaces Shaded pole motors Permanent split capacitor (PSC) motors Efficiency Improvements Much better than PSC Tracks part load better Will increase power with static So less savings as filters clog 6

7 Types of Variable Speed Drives (VSD) Gear driven Belt driven with variable sheave Magna-drive Eddy current drive Variable frequency drive (VFD) or inverter-fed induction motors; pulse width modulation (PWM) Electronically commutated motor (ECM) a.k.a. permanent magnet brushless dc (PMBLDC) motor Switched reluctance motors (SRM) 7

8 Overall Fan System Efficiency Multiple Conversions = Multiple Losses Losses occur for each conversion of energy Wire Losses 2% Motor Efficiency: 80% = Losses of 20% Fan Belt Losses 6% Air Flow Fan Efficiency: 70% = Losses of 30% Efficiency Loss kw Meter 9.65 Meter kw Wire 2% 9.46 Motor Input kw Motor 80% 20% 7.57 Motor Shaft kw Belt 6% 7.14 Fan Shaft kw Fan 70% 30% 5.00 Air kw = Work System 52% 48% 4.65 Losses Overall system efficiency = [5.0 kw work] / [9.65 kw in] = 52% 8

9 Overall Fan System Efficiency with VSD Adding another Device Adds to Losses Losses occur for each conversion of energy Wire Losses 2% Motor Efficiency: 80% = Losses of 20% Fan Belt Losses 6% Air Flow Fan Efficiency: 70% = Losses of 30% VSD Efficiency Loss kw Meter 9.65 Meter kw Wire 2% 9.46 Motor Input kw Motor 80% 20% 7.57 Motor Shaft kw Belt 6% 7.14 Fan Shaft kw Fan 70% 30% 5.00 Air kw = Work System 52% 48% 4.65 Losses What is impact of VSD (the bad news)? The old rule of thumb is added constant losses of 5-10% of peak input Newer or larger units closer to 2%-3% of peak 9

10 VFD Drive Losses Compared to Throttling Ride the pump curve (Throttling) VSD speed control (No throttling) 10

11 VSD Drive Losses and Motor Interaction Drive losses Bernier, 1999 Doe-OIT, 2012 Sellers, 2010 (next slide) Motor efficiency impacts VFD Efficiency 100.0% 80.0% 60.0% 40.0% 20.0% 0.0% Compare VFD Efficiency Methods DOE-OIT Bernier The Bernier method uses an equation published in a December 1999 ASHRAE Journal article by Michel A. Bernier PhD. and Bernard Bourret PhD. Flow gpm The DOE-OIT method is based on a table from OIT "Energy Matters" Winter It was subsequently republished in the August 2004 Engineered Systems magazine, and again in "Energy Efficiency Of Variable Speed Drive Systems," a paper by James A. Rooks. P.E. and Alan K. Wallace, Ph.D., Relative motor efficiencies are from Electric Motor Efficiency under Variable Frequencies and Loads. April Burt, Piao, Gaudi, Busch, and Taufik. Journal of Irrigation and Drainage Engineering. 11

12 VSD Efficiency: Size Matters Sellers, David. A Field Perspective on Engineering. Dec. 18,

13 VSD Efficiency: Vintage Matters Sellers, David. A Field Perspective on Engineering. Dec. 18,

14 Affinity & Reality

15 The Beauty of Affinity For fans and pumps Flow is proportional to speed in RPM (at same pressure) Pressure in a static system reduces with the square of speed Power reduction is a cube function of speed (with no change in physical system configuration, natural pressure reduction) 15

16 System Reaction to VSD; Real vs. Ideal Ideal speed control (No throttling) Actual speed control (Some zones throttled) Valves partly closed Begin with 91 Energy Units 16

17 Static Pressure (SP) Reset Control Speed control for VSD is typically by pressure sensing Static pressure (SP) reset for fan airflow control Differential pressure (DP) reset for pump hydronic flow control Static or differential pressure setpoint can be reset Required by codes in some situations Can provide additional savings in addition to flow reductions; however, flow reduction provides much greater savings Condition Reduction Diff in Bhp % Savings VSD Flow & Speed (27%) [=SP] % Throttling (ride curve) Flow (27%) [increase SP] % Reduce SP Static Pressure (25%) % Here VSD adds about 11% savings over throttling control; lower flow can result in lower SP 17

18 Modeling realities of SP or DP reset Modeling realities (Major building simulation programs) Zone hydronic and air flow are calculated and modeled correctly DOE2 cannot model actual hydronic or airside static pressure requirements a change to the fan part load power ratio is made Energy Plus (or E+ with other system simulations) could model actual reset, but it would be extremely time consuming to set up, would significantly increase runtime and is not typically done Static or differential pressure reset thinking errors SP reset does not reduce flow (unless boxes/coils are starved) SP reset is driven by the critical zone, not average use See discussions by Bill Koran in his commissioning tips: The credit for pressure reset is a single power, not a cube! Minimum pressure limits VAV boxes have a minimum controllable operating pressure Hydronic valves may have minimum to report flow accurately Experimentation can show satisfactory operation below recommended minimums for both boxes and valves

19 Default FAN EIR-fPLR Curves FAN-EIR-FPLR Curves equest defaults EIR-fPLR: Energy Input Ratio as a function of Part Load (flow) Ratio Dimensionless Other formulas or inputs define 100% energy input Systems finds CFM for the hour Fan Energy Input 100% 80% 60% 40% 20% 0% 0% 25% 50% 75% 100% Fan Output eq_fc-dd eq_fc-iv eq_vav-vsd EIR-fPLR curve quickly translates hourly fan output (cfm) to energy input Does not require complexity of a ductwork model inside building simulation Allows any separately calculated relationship to be input Default curves in the equest DOE2 program for: Forward Curve or Air Foil; Discharge Dampers or Inlet Vanes; Any VSD; VaneAxial Shown: Forward Curve Discharge Dampers (FC-DD), Inlet Vanes (FC-IV), VSD 19

20 Improved VSD Fan Power Curves Source: Hydeman. Advanced Variable Air Volume System Design Guide

21 Actual Field Data is Limited Large HVAC Field and Baseline Data CEC A-21 21

22 More Accurate Modeling of VSD Fan and Pump Savings Temperature reset very easy Static Pressure reset requires a new FAN-EIR-fPLR curve The CCC FanSysCalc tool is a good source to generate realistic EIRfPLR curves Fan Input 100% 80% 60% 40% 20% 0% FAN-EIR-FPLR Curves CCC FanSysCalc vs. equest defaults 0% 25% 50% 75% 100% Fan Output ccc_base ccc_losp ccc_resetsp eq_fc-dd eq_vav-vsd Tools to analyze variable flow fan and pumping systems that include 1) the interaction of throttling and VSD control, 2) VSD drive losses, and 3) part load motor efficiency reductions are hosted by the California Commissioning Collaborative: 22

23 Reality in Air and Hydronic Systems 23

24 Application & Codes

25 Balancing/Controls to Maximize VSD Savings Standard balancing procedure says Operate pump at full load/speed Open all control valves Set main balancing valve for design gpm Optimum balancing procedure says (90.1 requires) Open all control valves Open main balancing valve (or better; do not have one) Adjust VSD speed to achieve design gpm Set maximum speed in DDC system Standard pumping controls Install pressure differential sensor at critical coil Maintain ~5 psi pressure difference by varying pump speed Advanced controls Measure pressure differential sensor for monitoring purposes Adjust VSD speed directly until one valve is 95% open 25

26 DDC Reset of Discharge Air Temp & Static Pressure Setpoints Reset DA temperature setpoint Increase if most zone dampers < 90% Decrease if zones have dampers > 90% Decrease to minimum (about 53 F) when OA > 70F to reduce fan energy, only if box minimums optimized Reset static pressure setpoint Opposite of DAT reset on damper position Coordinate by sequencing resets, or by making static pressure reset slow and DAT reset faster Similar resets on CHWS & HWS pumps Coordinate with water temperature reset 26

27 Sequenced DAT & SP Reset As cooling decreases: Max SP at Min DAT Reduce SP to box design SP Constant SP increasing DAT Max DAT reduce SP to min Go to min DAT: Discharge Air, F Sequenced VAV Primary DAT & SP Reset SP set in wg DAT set F Set Min DAT when > 70F OA SP, in. wg When economizer is locked out and Reheat needs are limited Cooling Demand Typically above 70 F outside air temperature DAT=Discharge Air Temp; SP=Static Pressure 27

28 When Not to Use VSD s When it s a constant volume application e.g., chiller flow; primary boiler; outside air preheat Avoid the VSD drive loss Adjust to match actual constant load Trim pump impeller rather than throttle flow Adjust fan speed with belt rather than use damper Exception high performance constant volume Example art museum needs flow to maintain even humidity High design airflow is conservative Can reduce with actual field measurement of actual exhibit Reducing speed provides great savings 28

29 Changes in VSD Energy Code Requirements ASHRAE (Similar proposed for 2018 IECC) Multi-speed or VSD on smaller units ( 65,000 Btu/hr) VSD on more fans ¼ hp on CHW Cooling towers from 7.5 hp to 5 hp Pump VSD requirements Heating pumps now covered for larger sizes C B (dry) A (moist) Cooling pumps down to 2 hp; adjusted by ASHRAE climate zone

30 Conclusions ECM motors provide an improvement over PSC VSDs reduce energy, but have losses and motor efficiency interaction The affinity curve is our friend Significant reduction in power with reduced flow and speed Need to account for losses: Drive efficiency losses Reduced motor efficiency at part load Damper or valve operation on non-critical zones Create a custom fan or pump power curve for accurate analysis (need more actual operating data sets) Balance and control to maximize VSD benefits New code requirements for fans and pumps 30

31 Questions? Reid Hart, PE