ASHRAE Journal - April 2010 - 61

Converter ac/dc

ac Supply

Induction Motor dc Bus Inverter M ac Source M

dc Rectifier

PWM Inverter

Figure 1: Block diagram of a variable frequency drive motor, M.

Figure 2: Diagram of a variable frequency drive.
100 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 % Speed

% HP and Torque

High Speed

HP Torque Flow

Low Speed

Figure 3: Pulse width modulation to generate a sinusoidal waveform.

Figure 4: Speed-cubed fan power relationship.

a variable torque application than for a constant torque application. There are numerous technical considerations and potential pitfalls in applying VFDs, including damage from reflected voltage waves, higher peak voltages within the motor windings, motor noise, increased motor heating, unwanted harmonics on the ac input line, and induced currents in the motor bearings. ASHRAE TC 1.11, Motors and Motor Control, maintains a chapter called “Motors, Motor Controls, and Variable-Speed Drives” in the HVAC Systems and Equipment volume of the ASHRAE Handbook (Chapter 44 in the 2008 edition). Technical considerations of VFDs are discussed in this chapter. In many instances in HVAC equipment, the variable frequency drive is applied in a packaged system (e.g., a variable air volume air handling unit, a variable air-volume packaged rooftop unitary air conditioning system, or a large chiller with a variable speed compressor), and the manufacturer of the system has engineered the drive-motor-load-control system so that these pitfalls are avoided.
Energy Saving Potential

According to the DOE Buildings Energy Data Book1 space cooling, ventilation, and refrigeration in residential and commercial buildings consumed 8.2 quadrillion Btus (quads) of primary electric energy in 2006, the bulk of which is consumed by refrigerant compressors, fans, blowers, and pumps, and therefore can be reduced by implementing VFD technology. Additionally, space heating in buildings consumed 1.7 quads of primary electric energy in 2006, a percentage of which (with
April	2010	

the notable exception of electric resistance heating) can be reduced by applying VFDs as well. In the next four articles, covering the major potential applications of VFDs in HVAC&R, we will break down the potential for energy savings in each of these areas. Here, we provide a brief overview of three of the major ways that efficient, variable speed operation of motors can save substantial amounts of energy in HVAC&R applications. • The speed cubed fan and pump power law dictates that for propeller fans, centrifugal blowers and centrifugal pumps in systems with fixed-flow resistance, the air or water flow rate will vary with the rotational speed (RPM) while the power will vary with the cube of the speed, as shown in Figure 4. For example at 50% of maximum speed and flow, the power input drops to only 1/8 of the power at maximum speed. HVAC systems operate at part load most of the time, so modulating the flow, rather operating cyclically at full flow, can save significant amounts of energy. • Continuous operation of cooling equipment at reduced capacity, instead of on-off operation at full capacity, results in less temperature lift, and hence increased compressor COP, as shown in Figure 5. • Continuous operation of cooling equipment at reduced capacity, instead of on-off operation at full capacity, eliminates on-off cycling losses.
Market Factors

Acceptance of variable frequency drives was slow initially (going back 20 or 30 years) because of three basic factors:
ASHRAE	Journal	 61



ASHRAE Journal - April 2010

Table of Contents for the Digital Edition of ASHRAE Journal - April 2010

ASHRAE Journal - April 2010
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
The Science of Evaporation is Key to Defense in Murder Trial
Selecting DOAS Equipment with Reserve Capacity
Technology Award Case Studies: Greening Hospitals
Technology Award Case Studies: Sustainable Remedy for Hospital
Building Sciences
Emerging Technologies
Technical Topics: Selecting Efficient Fans
Technical Topics: Dual-Capacity Heat Pumps
IAQ Applications
International Column
Classified Advertising
Advertisers Index
ASHRAE Journal - April 2010 - Intro
ASHRAE Journal - April 2010 - ASHRAE Journal - April 2010
ASHRAE Journal - April 2010 - Cover2
ASHRAE Journal - April 2010 - 1
ASHRAE Journal - April 2010 - 2
ASHRAE Journal - April 2010 - Contents
ASHRAE Journal - April 2010 - Commentary
ASHRAE Journal - April 2010 - 5
ASHRAE Journal - April 2010 - Industry News
ASHRAE Journal - April 2010 - 7
ASHRAE Journal - April 2010 - 8
ASHRAE Journal - April 2010 - 9
ASHRAE Journal - April 2010 - 10
ASHRAE Journal - April 2010 - 11
ASHRAE Journal - April 2010 - 12
ASHRAE Journal - April 2010 - 13
ASHRAE Journal - April 2010 - 14
ASHRAE Journal - April 2010 - 15
ASHRAE Journal - April 2010 - 16
ASHRAE Journal - April 2010 - 17
ASHRAE Journal - April 2010 - Letters
ASHRAE Journal - April 2010 - 19
ASHRAE Journal - April 2010 - Meetings and Shows
ASHRAE Journal - April 2010 - 21
ASHRAE Journal - April 2010 - The Science of Evaporation is Key to Defense in Murder Trial
ASHRAE Journal - April 2010 - 23
ASHRAE Journal - April 2010 - 24
ASHRAE Journal - April 2010 - 25
ASHRAE Journal - April 2010 - 26
ASHRAE Journal - April 2010 - 27
ASHRAE Journal - April 2010 - 28
ASHRAE Journal - April 2010 - 29
ASHRAE Journal - April 2010 - Selecting DOAS Equipment with Reserve Capacity
ASHRAE Journal - April 2010 - 31
ASHRAE Journal - April 2010 - 32
ASHRAE Journal - April 2010 - BRC1
ASHRAE Journal - April 2010 - BRC2
ASHRAE Journal - April 2010 - 33
ASHRAE Journal - April 2010 - 34
ASHRAE Journal - April 2010 - 35
ASHRAE Journal - April 2010 - 36
ASHRAE Journal - April 2010 - 37
ASHRAE Journal - April 2010 - 38
ASHRAE Journal - April 2010 - 39
ASHRAE Journal - April 2010 - 40
ASHRAE Journal - April 2010 - 41
ASHRAE Journal - April 2010 - Technology Award Case Studies: Greening Hospitals
ASHRAE Journal - April 2010 - 43
ASHRAE Journal - April 2010 - 44
ASHRAE Journal - April 2010 - 45
ASHRAE Journal - April 2010 - 46
ASHRAE Journal - April 2010 - 47
ASHRAE Journal - April 2010 - 48
ASHRAE Journal - April 2010 - 49
ASHRAE Journal - April 2010 - Technology Award Case Studies: Sustainable Remedy for Hospital
ASHRAE Journal - April 2010 - 51
ASHRAE Journal - April 2010 - 52
ASHRAE Journal - April 2010 - 53
ASHRAE Journal - April 2010 - Building Sciences
ASHRAE Journal - April 2010 - 55
ASHRAE Journal - April 2010 - 56
ASHRAE Journal - April 2010 - AP1
ASHRAE Journal - April 2010 - AP2
ASHRAE Journal - April 2010 - AP3
ASHRAE Journal - April 2010 - AP4
ASHRAE Journal - April 2010 - 57
ASHRAE Journal - April 2010 - 58
ASHRAE Journal - April 2010 - 59
ASHRAE Journal - April 2010 - Emerging Technologies
ASHRAE Journal - April 2010 - 61
ASHRAE Journal - April 2010 - 62
ASHRAE Journal - April 2010 - 63
ASHRAE Journal - April 2010 - Technical Topics: Selecting Efficient Fans
ASHRAE Journal - April 2010 - 65
ASHRAE Journal - April 2010 - Technical Topics: Dual-Capacity Heat Pumps
ASHRAE Journal - April 2010 - 67
ASHRAE Journal - April 2010 - 68
ASHRAE Journal - April 2010 - 69
ASHRAE Journal - April 2010 - IAQ Applications
ASHRAE Journal - April 2010 - 71
ASHRAE Journal - April 2010 - 72
ASHRAE Journal - April 2010 - 73
ASHRAE Journal - April 2010 - International Column
ASHRAE Journal - April 2010 - 75
ASHRAE Journal - April 2010 - 76
ASHRAE Journal - April 2010 - 77
ASHRAE Journal - April 2010 - 78
ASHRAE Journal - April 2010 - 79
ASHRAE Journal - April 2010 - 80
ASHRAE Journal - April 2010 - 81
ASHRAE Journal - April 2010 - 82
ASHRAE Journal - April 2010 - 83
ASHRAE Journal - April 2010 - 84
ASHRAE Journal - April 2010 - 85
ASHRAE Journal - April 2010 - Classified Advertising
ASHRAE Journal - April 2010 - 87
ASHRAE Journal - April 2010 - Advertisers Index
ASHRAE Journal - April 2010 - Cover3
ASHRAE Journal - April 2010 - Cover4
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