Improving the Efficiency of Residential HVAC
Systems Using Computer-Based Power-Electronic
Controls
Jason M. Anderson, Robert W. Cox,
and Prayag K. Parikh
Department of Electrical and
Computer Engineering
UNC Charlotte
Charlotte, NC 28223
Email: jmander1@uncc.edu, rcox3@uncc.edu,
pkparikh@uncc.edu
Abstract—Space heating and cooling are responsible for about
43% of energy consumption in the average American home [1].
That number can be reduced significantly through the use of a
computer-based power-electronic control system. This paper describes one proposed solution, which uses a PC to control the motors in the system. The central PC uses wireless communications
to coordinate fan and compressor speeds. The paper describes
some new control concepts designed to implement demand-side
management, and it shows how the system can be operated in
a diagnostic mode that regularly checks for increased energy
consumption resulting from degrading equipment conditions.
I. I NTRODUCTION
Buildings are responsible for approximately 39% of primary
annual energy consumption in the United States [2], and
heating, ventilation, and air conditioning (HVAC) systems
represent a significant portion of that consumption. Studies
released by U.S. Department of Energy (DOE), for instance,
indicate that space cooling and heating account for about
43% of the energy delivered to the residential sector [1]. In
large commercial buildings this percentage is lower because
power-electronic drives are used to better control cooling load
by varying equipment speeds. Given the projected growth in
energy prices, it will likely be necessary to bring the same
efficiency improvements to homes and small businesses. This
document describes a system that uses power electronics and
high-level, computer-based supervisory controls to generate
the necessary improvements at relatively low cost.
The system described in this paper makes use of a distributed network of motor drives and sensors, all under the
control of a single PC. In this arrangement, the computer coordinates the operation of motors in a residential or packaged
HVAC unit. At a high level, this system implements two types
of energy-saving features, namely variable-speed control and
routine motor diagnostics. Control commands are determined
by the central PC using set-point data, sensor information, and
other control inputs. These inputs include quantities such as
power-system frequency and voltage magnitude, and they can
Christopher R. Laughman
Laboratory for Electromagnetic and Electronic Systems
Massachusetts Institute of Technology
Cambridge, MA 02139
Email: crlaugh@mit.edu
be used to indicate when the power system is most able to
accept additional load. Use of these features would help to
support the SmartGrid initiative, which is a program aimed at
modernizing the electrical grid in the United States [3]. Each
motor drive can also be commanded to operate in a diagnostic
mode that allows the system to determine if equipment is
degrading. This capability is important because faults such
as refrigerant leaks can increase consumption by as much as
20% [4].
This paper describes the proposed system and presents
results from initial tests. Section II provides an overall description and explains the motor drives developed for this
application. Section III discusses diagnostic capabilities that
have been incorporated into the system, and it describes a
specific example that was tested in the field. Section IV
describes some new demand-side management features, and
finally Section V presents several conclusions.
II. S YSTEM OVERVIEW AND BASIC C ONTROL F EATURES
Figure 1 shows the layout of the system that is currently
under test in a single 2500ft2 home in Charlotte, North
Carolina. The system consists of three sets of components
- a central PC, several motors and drives, and a number of
sensors. These devices all communicate wirelessly over a mesh
network using the ZigBee protocol. This section describes the
overall layout of the system and it describes the custom motor
drives used throughout the network.
A. Test System Layout
Typically, HVAC plants feature three motor-driven devices a condenser fan, a compressor, and an evaporator fan. In most
homes, the condenser fan and compressor are located outdoors
as in Fig. 1. In commercial buildings, all three devices might
be packaged into a single roof-top unit (RTU).
The test system has been designed to demonstrate some of
the key features enabled through the use of distributed power-
electronic controls. Experimental motor drives have been deployed at the evaporator and condenser fans, and a simple
data-collection unit with current and voltage sensors has been
installed at the compressor. Data from this unit is used to
demonstrate some unique diagnostic possibilities. Temperature
sensors integrated onboard PICDEMZ demonstration boards
from Microchip have been placed both outdoors and indoors.
Other sensors could be used, but have not been considered
in this test application. A central PC located inside the home
can communicate with the motor drives, temperature-sensing
nodes, and the remote data-collection unit. A small voltagemeasuring device has also been installed at the central computer in order to demonstrate the demand-side management
features described in Section IV.
B. Custom Motor Drives and Basic Control Features
Speed coordination, which has long been used to realize energy savings in large commercial buildings, has been
shown to offer similar benefits in residential settings [5].
By controlling fan and compressor speeds, one can modulate
both compressor capacity and air flow so that overall cooling
capacity matches the heat gain in the conditioned space [6].
The goal of the currently proposed system is to achieve this
control at relatively low cost. The key components enabling
this savings are the low-cost wireless modules and the motor
drives. To keep costs down, the drives are designed to be easily
retrofit into existing HVAC equipment with split-capacitor AC
induction motors. Brushless DC machines were considered as
replacements, but it was decided to design a solution that
would keep nearly all mechanical connections intact, thus
minimizing both installation effort and cost.
1) Drives: Each motor drive is comprised of the subsystems
shown in Fig. 2. The power stage consists of a two-phase
rectifier and an inverter formed using the IRAMX16UP60A
integrated power module from International Rectifier [7]. At
present, both the condenser fan and the evaporator fan are
controlled by the drive circuits shown in Fig. 3. Both of
these motors are two-phase, 1/4 hp machines, and both were
previously operated with a capacitor permanently placed in
series with the B-phase winding. Note that this capacitor has
been removed and that the neutral point on the motor has been
connected between the two series capacitors that comprise the
DC bus.
240V AC Mains
Wireless
Tranceiver
Power
Stage
Microcontroller
Motor
Fig. 2. Block diagram of the motor drives used in the current system. Speed
commands are passed to the power stage from the wireless transceiver.
L
D1
+
Thermostat
Outdoor
Temperature
Sensor
vAC
Wireless
Transceiver
PC
VDC
Q1
-
C1
Q2
Q4
Q3
Q5
C2
iB
Fig. 3. Schematic of the power stage used for the condenser and evaporator
fans. The IGBTs are part of the IRAMX16UP60A integrated power module.
A boost rectifier was constructed using one of the three phase legs and a
separate single-phase rectifier module.
Outdoor Unit
Condenser
Fan
Evaporator
Fan
Compressor
Fig. 1.
A home with intelligent HVAC controls. Motor drives with
wireless transceivers are incorporated with the condenser fan, compressor,
and evaporator fan. Sensors can be placed as needed, and the locations shown
here do not indicate all relevant possibilities. Currently, the system uses
two temperature sensors - one placed at the thermostat and another placed
outdoors. The operation of the network is coordinated by the PC.
The power stage topology shown in Fig. 3 was selected
primarily on the basis of initial cost; specifically, it was desired
to integrate both rectification and inversion into a single, offthe-shelf three-phase inverter module. In initial studies, we
have also included a single-phase rectifier and an inductor
in order to form a boost rectifier. The decision to use the
boost topology was motivated by the desire to investigate
active power factor correction and harmonic cancellation [8].
Ultimately, the system will likely make use of a simple
doubling rectifier as shown in Fig. 4
Overall control of the rectifier and inverter is performed
using the dsPIC60F10A from Microchip. This microcontroller
monitors the inductor current, DC bus voltage, and rectifier
iA
+
Q2
VDC
vAC
The drives could be designed to always include the series
capacitor, but this leads to decreased efficiency, higher current,
and increased pulsating torque [9].
C1
Q4
Q6
Q3
Q5
iB
C2
-
Q1
iA
+
Fig. 4. Schematic of the power stage that is currently under consideration.
The switches Q1 and Q2 and their parallel diodes are used as part of voltagedoubling rectifier. This approach offers a cost savings over the boost rectifier
shown previously.
Q2
Q4
-
Q1
Q3
iA
iB
(a)
+
VDC
-
iA
Q1
Q6
Q4
Q2
iB
Q3
Q5
(b)
Fig. 6. Two other possible inverter topologies for a two-phase motor. (a)
Two-phase inverter with series capacitor. (b) Three-phase inverter with series
capacitor. Other topologies can also be considered.
500
0
−500
0
iA (A)
vAB (V)
output voltage, and it uses these signals to control the average current in the inductor [8]. Motor speed commands
are communicated using a wireless transceiver, and PWM
switching signals for the inverter switches are generated at
16kHz using a 256-entry sine table. Currently, each motor is
operated using an open-loop, constant volts-per-Hertz strategy.
When a speed change is needed, the PC sends the frequency
and modulation commands needed to set the desired operating
point. Figure 5 shows voltage and current waveforms recorded
during steady-state operation at the evaporator. Note that
the currents, which are separated in phase by 90 degrees,
have different magnitudes. This difference is the result of the
asymmetry in the two sets of stator windings.
VDC
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Time (sec)
0.5
0
−0.5
0
Time (sec)
iB (A)
0.5
0
−0.5
0
Time (sec)
Fig. 5. Waveforms recorded during steady-state operation of the evaporator
fan . Upper trace: Voltage between phases A and B of the two-phase machine.
Middle trace: Current flowing into the phase A of the motor. Bottom trace:
Current flowing into phase B of the motor.
Other inverter topologies were considered, and Fig. 6 shows
several possibilities. It is important to note that these other
approaches may be required in some cases. Many compressor
motors, for instance, contain an integrated capacitor that is
connected in series with the auxiliary winding. This capacitor
cannot be easily removed, so an inverter of the type shown
in Fig. 6(a) would be more appropriate. Commercial versions
of the final drives might include a switch that allows one to
indicate whether the capacitor will remain connected. Such an
input is needed because use of the series capacitor requires
the two output voltages to maintain different phase angles.
2) Basic Control Features: At present, the evaporator and
condenser motors can operate in a number of different modes.
During steady-state operation, these motors typically operate
using an open-loop, constant volts-per-Hertz strategy. Frequency and modulation commands are created at the central PC and transmitted wirelessly to the motor drives. If
communications are lost, the system resumes normal fullspeed operation. Each machine can also be operated in a
special diagnostic mode in which it is hard-started and brought
immediately to full speed. The purpose of this mode of
operation is related to the richness of transient signatures as
viewed from the perspective of on-line diagnostics. Further
details are provided in the next section.
Ongoing work aims to devise a control algorithm that
minimizes overall HVAC input power for a given cooling load,
Q. Total input power can be expressed using the objective
function
J = f (Q) = Wcomp + Wcond + Wevap
(1)
where Wcomp is the compressor input power, Wcond is the
condenser-fan input power, and Wevap is the evaporator-fan
input power. To minimize J for a given cooling load, further
information is needed and more complete component models
are required. Recent work indicates that the use of such
controls in combination with other techniques such as thermalenergy storage offer the potential for significant national
energy savings [10].
III. D IAGNOSTIC F EATURES
Faulty HVAC systems waste considerable amounts of energy. Researchers in the HVAC community have developed
diagnostic systems that use mechanical measurements to detect
key faults, including liquid slugging, fan imbalances, and
refrigerant leaks [4], [11]. The use of such systems is limited,
however, because the necessary sensors are both expensive
and potentially unreliable. An alternative approach is to rely
on electrical measurements. Using electrical current data, for
instance, one can often extract considerable information about
the status of the driven load [12], [13], [14]. This approach,
which is potentially cheaper than more traditional techniques,
motivates the use of power-electronic drives with integrated
diagnostic features.
To demonstrate the potential, current transducers (CTs)
have been embedded into each motor drive and an additional
data-collection unit with a CT has been installed on the
compressor. Two primary analytical approaches have been
considered. First, faults such as load imbalances have been
detected using spectral techniques; other failure mechanisms
have been analyzed using transient current signatures.
Research has demonstrated that current transients are particularly rich in information indicating the status of the driven
mechanical load [15], [12], [13], [16], [17]. For instance,
changes in current transients have been used to detect the
build-up of liquid refrigerant in compressors [17] and to detect
clogs in fluid systems [16]. The rich quality of transient
signatures motivates the use of a special diagnostic mode
in which motors are hard-started and data is recorded for
later analysis on the central PC. This approach minimizes
data-storage and processing concerns, as current data is only
collected for a short period before and after each start. Once
the machine has been started, variable-speed operation can
begin.
A. Example: Refrigerant Leak Detection
Field tests were conducted in order to demonstrate the
critical diagnostic information that can be extracted from
electrical transients using the proposed system. A key fault
condition was introduced into the HVAC plant by removing
a small amount of liquid refrigerant from the compressor.
Depending on both the severity of the leak and ambient
conditions, refrigerant loss can cause an HVAC system to
significantly increase its frequency of operation. This fault
was simulated by removing approximately 20% of the required
refrigerant, an amount considered to be relatively modest [18].
Compressor start-up data was collected for several weeks
before and after the removal of the liquid, and analysis was
conducted off-line on the central PC.
Transient analysis of the type considered here can be performed by computing time-varying estimates of the frequency
content of the measured line current. Formally, these timevarying estimates, or spectral envelopes, are defined as the
quantities [19]
2
am (t) =
T
Z
2
bm (t) =
T
Z
t
i (τ ) sin (mωτ ) dτ
(2)
i (τ ) cos (mωτ ) dτ
(3)
t−T
and
t
t−T
These relationships are Fourier-series analysis equations evaluated over a moving window T [20]. The coefficients am (t)
and bm (t) contain time-local information about the frequency
content of i(t). Provided that the basis terms sin (mωt) and
cos (mωt) are synchronized to the line voltage, the spectralenvelope coefficients have a useful physical interpretation as
real, reactive, and harmonic power [19]. In this specific case,
analysis is performed using the coefficient a1 (t). When properly scaled, this coefficient accurately represents real power.
Figure 7 shows experimental results obtained as the refrigerant level was varied. Figure 7(a) shows the power recorded
both during and after the motor in-rush. Note that both
the transient and steady-state power levels are lower when
the compressor is slightly undercharged, i.e. refrigerant has
been lost. The average power level during the transient was
recorded and trended throughout the course of the experiment.
Figure 7(b) presents a scatter plot that shows how the transient
power level varied as a function of the difference between
outdoor and indoor temperature. Note that for any given
temperature difference, more power was absorbed when the
compressor was fully charged. This result is consistent with
physical arguments [18], and it has been observed reliably in
the field. A fault-detection algorithm based on clustering analysis is currently under development. The resulting algorithm
could ultimately be executed automatically by control software
on the central PC.
IV. C ONTROLS
FOR
D EMAND -S IDE M ANAGEMENT
The use of a central PC makes it possible to introduce a
number of new control features. For instance, the PC can
coordinate the operation of the HVAC system so that it
responds to changes in power-system condition. This concept
was first introduced in [21], where the authors used frequency
deviations to detect imbalances between the supply and demand for real power on the grid. The central PC allows this
concept to be expanded, as its computational power can be
used to track power-system quantities, including frequency and
voltage magnitude. Additionally, a connection to the internet
makes it possible to receive commands from the local utility
provider. All of these possibilities make this system attractive
as part of the SmartGrid initative in the United States [3].
A new demand-side management (DSM) feature has been
tested using the proposed system. This technique uses voltage
measurements to obtain information about the power drawn
inside the home as well as information about overall utility
load. Whenever a device enters operation inside of a home,
for instance, the line voltage will decrease with respect to the
neutral and the neutral voltage will increase with respect to
14
Fully Charged
20% Undercharged
am (t) =
12
Real Power (kW)
bm (t) =
8
6
4
2
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Time (sec)
(a)
13.5
Fully Charged
20% Undercharged
13
Real Power (kW)
Z
2
T
Z
t
vN G (τ ) sin (mωτ ) dτ
(4)
vN G (τ ) cos (mωτ ) dτ,
(5)
t−T
and
10
0
0
2
T
12.5
12
t
t−T
where vN G (t) is the neutral voltage with respect to ground
and the basis terms sin (mωt) and cos (mωt) are synchronized to the measured line voltage. Complete details of the
computational process are presented [22] and [23] and will not
be discussed here. Reference [23] describes how these measurements can be used to identify the operation of individual
household loads, i.e. vacuum cleaners, lamps, etc.
Figure 8 demonstrates the operation of the proposed DSM
input. Figure 8(a) shows how both the line-voltage magnitude and the neutral-voltage magnitude were affected when
a vacuum cleaner was started at a nearby outlet. Figure 8(b)
shows how the same quantities were affected by a drop in
utility voltage. The latter of these two events, which has been
simulated extensively in the lab, was observed almost daily
during field tests conducted in July and August 2007. The
high frequency of these events hints at the strong potential of
this measurement approach.
V. C ONCLUSION
11.5
11
10.5
−10
−8
−6
−4
−2
0
2
4
6
Tout −Tin (◦ C)
(b)
Fig. 7. Results from the refrigerant-loss experiments (a) Real power absorbed
by the compressor during and after the motor in-rush. (b) Scatter plot showing
how average transient power varies as a function of teperature difference and
refrigerant charge. Transient power is measured during the period when then
the power is approximately constant. In (a), for instance, the transient power
level was measured between approximately 0.2s and 0.3s.
ground. By comparison, if the utility decreases its voltage or
if nearby customers operate large loads, both the measured
line voltage and the measured neutral voltage will decrease.
Voltage measurements at a single outlet, therefore, can be used
to determine both changes in utility load and changes in load
inside the home. This information can be used to sense periods
of high demand and can thus be used to curtail HVAC load
accordingly.
Since changes in voltage magnitude are often extremely
small, measurements are conducted using specialized hardware. Analysis is currently performed using analog computers
that estimate the time-varying frequency content of both the
measured line voltage and the measured neutral voltage. In the
case of the neutral voltage, for instance, these circuits esimate
the quantities
This paper demonstrates the basic control and diagnostic
features implemented by the proposed system. Eventually,
brushless DC machines may become common in HVAC
systems in new residential construction. In the meantime,
however, this system offers a simple approach that can be
easily retrofit into existing homes. Additionally, the diagnostic
features described here offer the potential for increased energy
savings, and many of the diagnostic procedures can also be
used for brushless machines.
ACKNOWLEDGMENT
The authors would like to thank Prof. Steven B. Leeb and
Prof. Les Norford for their valuable advice and support. This
work was supported in part by the Grainger Foundation and
the University of North Carolina at Charlotte through a faculty
start-up grant.
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170
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3
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