ASHRAE Journal - April 2010 - 60

Does Energy Savings Trump Costs?

Variable Frequency Drives, Part 1: The Technology
This five-part series will cover variable frequency drives and their applications in air conditioning and refrigeration in residential and commercial buildings. The first article will cover general variable frequency drive technology. The remaining articles will cover major applications.

By John Dieckmann, Member ASHRAE; Kurtis McKenney; and James Brodrick, Ph.D., Member ASHRAE

A

variable frequency drive (VFD) is a power electronic device that drives the common “squirrel cage” induction motor over a range of speeds by

converting standard frequency and voltage ac power from the electric utility to variable frequency, variable voltage power to energize the motor. Over the years, improvements have been made to VFDs’ effiency and reliabilty. They are now viable alternatives to other motor technologies. Induction motors operate at a speed that is proportional to the frequency of the input power (minus a small amount of slip that varies with the torque load on the motor). In HVAC&R, the common motor loads are refrigerant compressors, fans, blowers and pumps. By varying the speed of a motor and its driven load, the capacity can be varied to meet the realtime cooling, heating or ventilation load. As a result, significant energy savings can be realized, along with better comfort control. Other advantages include quieter operation and longer equipment life as a result of reduced average speed and soft starting of motors, which reduces the in-rush current and impact loading on equipment at startup. VFDs are alternately called variable speed drives, inverters, adjustable speed drives, or adjustable frequency drives. The focus of this article is on VFDs for induction motors. However, it is important to note that brushless dc motors, sometimes called permanent magnet rotor motors or
60	 ASHRAE	Journal	

electronically commutated motors, are another important class of electronically driven variable speed motors, used in applications such as blower motors and refrigerant compressors. Figure 1 is a simplified block diagram of a typical VFD. It has four basic subsystems: an ac/dc converter, a dc bus (also called the dc link), an inverter, and a control system. Alternating current from the electric utility is converted to dc in the ac/dc converter, generally a full wave rectifier bridge. The dc link maintains a steady dc voltage level using a capacitor upstream of the inverter. The inverter converts the dc back to ac at the frequency and voltage level needed to drive the motor at the desired speed. In most instances of VFD-driven motors, the ac output is three-phase to drive a three-phase motor. The control system manages the inverter so that it produces the desired voltage and frequency, and generally includes fault monitoring features.
ashrae.org	

Figure 2 shows the elements of the inverter in detail. The diode bridge shown is for three-phase ac input. The inverter section consists of six pairs of power transistors and freewheeling diodes. While there are various ways to synthesize a three-phase ac output from a dc source, the most commonly used method is pulse width modulation (PWM). As shown in Figure 3, the transistors are switched on and off rapidly (pulsed), at a carrier frequency that is much higher than the desired output frequency. The on-time of each pulse is varied to generate an approximation of a sinusoidal wave form. During the off-time of the pulse, the inductance of the motor winding draws current flow through the freewheeling diode such that the current flow is continuous and close to being sinusoidal. Applications for VFDs are sometimes classified as “variable torque” or “constant torque.” Variable torque does not mean randomly variable torque and constant torque does not mean that the torque is fixed under all conditions. Variable torque refers to applications where the maximum torque load on the motor decreases as the speed decreases from the maximum speed to lower speeds. Centrifugal pumps, axial fans and centrifugal blowers are examples of variable torque applications in HVAC. Constant torque refers to applications where the maximum required torque does not fall off appreciably as the speed decreases from the maximum. Positive displacement compressors and pumps are examples of constant torque applications in HVAC. As a general rule, a given VFD will have a higher motor power rating for
	 April	 2010



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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