Power Systems World 2003
Phase Control Alternatives for Single Phase AC Motors
Phase-Control Alternatives for Single-Phase AC Motors
Offer Smart, Low-Cost, Solutions
by Howard Abramowitz, Ph.D EE , President, AirCare Automation Inc.
Abstract - Single Phase AC motors continue to be the primary solution for air-movement,
pumping and compressor applications. Their low cost and availability make them ideal for lowperformance systems. DC Brushless platforms attempt to address these applications but their
higher cost and complexity continue to form impenetrable barriers of entry. They are viewed as
overkill for most applications. Single-Phase Inverter Drives have come on the scene, making
headway, but remain complex and costly. Phase-Control solutions are being revisited and
upgraded to fill the void. This paper explores the performance limitations and trade-offs of the
Phase-Control solution and clarifies the boundaries under which the TRIAC Phase-control is a
preferred method of speed control and the improvements provided by smart phase-control.
INTRODUCTION
Variable speed motor controls have been available for many years. As technology
progresses, major innovations and improvements have been made in sophisticated
systems improving the motor drive performance and reducing cost and complexity of the
available solutions. Thus, improvements in the performance and availability of AC
inverter (Inverters) solutions as well as DC brushless drives (BLDC) and motors have
brought these solution options into the market in greater numbers.
There are many articles and technical papers that address the improvements and
innovations in brushless dc drives as well as improvements in AC drives. What we see
little of is the improvements in the existing low-cost control techniques focusing on phase
control or other low-cost AC control techniques that don’t require the AC power to be
converter to twice. This paper revisits the single-phase AC motor control and outlines
improvements and value added features now available that make phase-control drives a
viable alternative for speed control in most applications.
Of greatest interest is going back to revisit the Phase-control platform that has
been around for many years and look at the innovations and improvements that are
available. Comparisons will be made to the single-phase inverter designs but we will not
directly cover Inverter design innovations here. Most single-phase motors are not
designed to work well with high frequency inventers resulting in bearing pitting and
motor performance degradation.
The results demonstrate that with careful attention to the application requirements
and applying state-of-the-art technology to the basic TRIAC topology a good costperformance solution can be provided that will allow variable speed to be incorporated
into numerous applications that were not viable for the manual controlled TRIAC or the
expensive Inverter solution. Upgrading the TRIAC control results in smart, performance
competitive solutions at a lower cost.
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H. Abramowitz
Power Systems World 2003
Phase Control Alternatives for Single Phase AC Motors
SINGLE- PHASE AC MOTOR SPEED CONTROLS
The two primary ways to control the speed of a single-phase AC motor is to either change the
frequency of the line voltage the motor sees or by changing the voltage seen by the motor,
thereby changing the rotational speed of the motor. An Inverter will convert the AC
waveform into a DC Voltage and them create a PWM signal output that will filter into a
waveform with a predetermined voltage (controlling torque) and frequency (controlling
speed). Figure 1 has a block diagram of the Inverter design and the various speed waveforms
are shown in Figure 2.
Key Benefits: Voltage / Frequency control Æ Wide Speed range Æ low-speed / High efficiency
Key Drawback: Higher Cost Æ Hi-Voltage PulsesÆFull Speed lossesÆ Motor Reliability
Figure 1: Single-Phase AC Inverter Block Diagram
DC
Rectifier Bridge
AC
POWER
IN
HF PWM
OUTPUT
INVERTER STAGE
PWM BRIDGE
FILTER /
LOAD
AC
POWER
OUT
Figure 2: AC Waveforms for Inverter Drive
Consta nt V olts/He rtz
180
160
140
120
100
80
60
40
10Hz
V olts
20
20Hz
0
-20 0
0.02
0.04
0.06
0.08
0.1
40Hz
60Hz
-40
-60
-80
-100
-120
-140
-160
-180
s e co n d s
The TRIAC, Phase-Control design is simpler. There is a single switch that is in line with
the AC line. It chops the AC waveform causing the power to shut off during a portion of
the AC Cycle. Figure 3 shows the general schematic for a TRIAC controlled drive. The
motor shown is a permanent split capacitor motor having 2 windings and a capacitor for
phase shift.
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H. Abramowitz
Power Systems World 2003
Phase Control Alternatives for Single Phase AC Motors
Figure 3: AC Chopper / TRIAC control of 1-Phase AC motor
Chopped AC
Primary
AC Input
Auxiliar
y
1-Phase PSC Motor
The AC motor acts as a low pass filter causing the resulting current waveform to be
sinusoidal at the same operating frequency with a slight lag. The lower RMS voltage
created by the chopper increases the slip to the motor, thereby reducing motor speed.
Essentially starving the motor of its power, the motor slows down and eventually stops
because there is not enough energy to maintain rotational speed. Figure 4 shows typical
waveforms; the TRIAC driven output and the resulting AC waveform that the motor sees.
Figure 4: Phase Control AC Waveforms for Single-Phase Motors
Phase Control of AC Motor
Normalized AC V & I
1.2
Original AC Voltage
0.8
0.4
AC INPUT (V)
Motor Voltage
Motor Current
0
-0.4
-0.8
Chopped AC Voltage
At Motor
Resulting Motor Current
-1.2
Time
Key Benefits: Low Cost Æ Ease of Control Æ Robust & Reliable Æ Multi-loads
Key Drawback: Motor Speed range Æ Lower Efficiency Æ Power Factor
Page 3 of 8
H. Abramowitz
Power Systems World 2003
Phase Control Alternatives for Single Phase AC Motors
TRIAC CONTROLS REVISITED
A key problem with TRIAC controls is that the resulting voltage (and ultimately resulting
speed) is reduced by essentially starving the motor. However, where very inefficient
motors were the norm years ago, many fan applications are using more efficient and
better designed motors. These motors can be driven off a phase-control scheme down to
30% of motor speed before running out of torque. The motor speed is very sensitive to
small changes in the phase angle. Figure 5a depicts the RPM change by phase angle for
an efficient fan motor down to 20% speed range. The result is non-linear with minimal
speed change as the phase control begins and then enters a steep change mode as the
phase angle increases to the heart of the sinusoidal waveform. A well known
phenomenon, this characteristic is a frustrating element of existing manual controls as
speed changes too fast when using a speed potentiometer. In applications such as clean
rooms the constant adjustment and difficulty of setting the speed add much technician
time to the set-up and recalibration process.
Figure 5a: RPM of motor changes non-linearly with phase angle shift
Figure 5b: RPM tracks within 6% of RPM for fan motor over high speed range
230 V, 60Hz, 1550 RPM AC Fan Motor
Vrms & RPM at Phase Angle Control
1400
1200
1000
RPM
800
600
400
Steep RPM change
200
0
0
40 68 83 94 102 111 121
120%
100%
80%
Voltage
60%
40%
20%
RPM
Vrms and RPM track within 6%
from 100% to 60% speed range
0%
21
41
58
71
81
93
104
114
127
134
143
153
166
Slow RPM change
1600
N o rm a liz e d R P M a n d V rm s
F a n R o t a t io n S p e e d ( R P M )
1800
Phase Angle firing
Phase Angle Cut off (Degrees)
Figure 5b shows the relationship between the VRMS and RPM of a single phase motors.
Surprisingly, the speed of the fan (RPM) will track with the RMS voltage across the
motor. This is impacted by the loading (all the information presented is based upon fan
loading). Thus, if a control methodology could be provided that will linearize the RMS
Voltage as a function of controller change, the blower speed will likewise be linear.
Microprocessor control of the TRIAC drive provides this improvement. The digital
control can be designed to match the loading tables of the drive, making the response of
the drive to a linear input (i.e. the turning of a speed knob) or the digital input of 0-100%
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H. Abramowitz
Power Systems World 2003
Phase Control Alternatives for Single Phase AC Motors
controller signal (0-5 volt or 4-20 mA control signal). Thus, the microcontroller provides
a non-linear phase-angle control to keep the speed changes linear. This can be seen in
Figure 6 where significant phase angle needs to be transversed before the speed changes
even 10%.
Figure 6: Linear RPM Output Using Micro-control of TRIAC Phase Angle
100% Linear RPM change with input
80%
RPM adjusted
60%
40%
Phase Angle varies non-linearly
Phase Angle
20%
%
60
%
50
%
40
%
30
%
20
10
%
0%
0
Normalized RPM and
Phase Angle
Smart Control of Phase Angle
for Linear RPM Control
Normalized Speed Adjustment
EFFICIENCY REVIEW
No evaluation of the TRIAC Phase-control is complete without discussing the main
reason to NOT use TRIAC controls. Other than the limitation of speed range we already
covered, the biggest reason cited for not using TRIAC controls is the poor efficiency of
this approach. Although there is less energy consumed when the drive runs at lower
power, alternate approaches save more energy. In a semiconductor cleanroom facility
where you may find thousands of fans, the extra 20% or more energy savings add up and
can be used to offset the significantly higher investment in more energy efficient motors
and drives. Figure 7 shows a ¾ HP blower being driven by several technologies:
BLDC – Highest efficiency motor – uses >100 watts less at full power. Best power
consumption alternative at highest cost.
3 Phase AC – motor is comparably efficient to the single phase choice. Notice how the
power consumed is higher than TRIAC at 95-100% speed.
1 Phase AC – PSC motor – power consumption is slightly more efficient than 3-Phase
but that seems to be more attributed to the motor differences than the drive. At 95-100%
speed the double power conversion shows the inherent inefficiency of this at full speed
1 Phase TRIAC Phase Control – Energy saving at lower speeds do not compare to the
other options as power savings is less as the speed reduces.
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H. Abramowitz
Power Systems World 2003
Phase Control Alternatives for Single Phase AC Motors
Figure 7: Power Consumption Comparisons of Motor/Drive Combination
Power Consumed (Watts)
230 Volt Circulating Fan Comparisons
650
600
550
500
450
400
350
300
250
200
150
BLDC
3PHS-AC Inverter
1PHS-AC Inverter
1PHS Phase Control
900
800
700
600
Fan Speed (RPM)
One other note with regard to the TRIAC Drive consumption. The performance can be
enhanced by moving to a 3-Wire approach. Figure 8 shows a small fan being driven by a
3-wire circuit topology. The Auxiliary winding is connected directly to the AC line
maintaining full voltage as the RMS voltage across the primary is reduced.
Figure 8: 3-Wire Topology & Improved Power Consumption Data
AC Fan - Inverter Vs. TRIAC (50 Hz - 2/3-Wire)
160.0
140.0
120.0
100.0
80.0
TRIAC as efficient as
Inverter to <90% speed
60.0
Inverter
2-wire TR
3-wire TR
19
45
13
13
85
12
66
11
32
11
2
44
10
97
82
3
40.0
4
Chopped AC
Primary
AC Input
Full Power
efficiency
91
1-Phase PSC Motor
Auxiliary
Auxiliary at
full Voltage
P o w e r C o n s u m e d (W a tts )
3-Wire Topology
Fan Speed (RPM)
As can be see by the above, the efficiency of the TRIAC control is improved and is
competitive with Inverter solutions down to almost 90% of speed and continues to
provide efficiency improvements through the normal operating range of fan control.
Page 6 of 8
H. Abramowitz
Power Systems World 2003
Phase Control Alternatives for Single Phase AC Motors
SIMPLE GUIDELINES FOR PHASE-CONTROL
After the above overview it is useful to define the preferred boundary conditions to
expedite a decision if the TRIAC phase-control is a viable alternative and the criteria that
lend themselves to a smart-phase-control system solution.
To summarize the above evaluation there are a few well defined issues to consider:
1) Existing System – many existing systems use a single-phase motor for their needs. Adding
2)
3)
4)
5)
6)
7)
speed control can add to their performance. Taking a manual speed control and adding smarts or
network communications (needing smarts) are reasons to consider a TRIAC phase control
solution.
Motor Reliability- AC motors are not designed to stand up well under inverter high frequency,
high-voltage spikes. Motors today are specially designed “inverter ready” or having isolated
bearings to avoid pitting. If motor reliability is an important criteria the reduced efficiency may
be a worthwhile tradeoff against reduced motor life.
Speed Range – If the speed range needed is within the capabilities of the chosen AC motor, the
phase-control approach is viable.
Speed Accuracy – Phase-control is not traditionally an “accurate” speed controller. Slip,
loading and numerous factors contribute to the actual speed as the drive is usually an open-loop
controller. “Smart phase-control” will close the loop giving full, accurate speed control. The
added smarts will allow for added features to be added at low cost.. still maintaining a low cost
power platform.
Efficiency – As discussed, if power consumption and efficiency are critical elements of the
system design, there are better methods using more efficient motor and drive technologies. Here,
the bigger issue is the AC motor inefficiencies, more so than the drive inefficiency.
Cost – The phase control power and control platform are low cost. Adding smarts, feedback, and
network communications add cost at the same rate as the other topologies, however, the basic
power and control platform is a fraction of the cost of rectification and inverter stages. The AC
motor is also a less expensive product than alternative motors and will stay that way for
foreseeable future.
Robust & Reliable – most of the electronic drives have significant sensitivity to voltage
spikes, temperature swings, and current increases. The results in inverter drives is to do major derating to protect the product. Over-design can add 20-30% to the unit cost. The phase control
drive provides additional current and voltage buffers at a fraction of the cost.
CONCLUSION
Phase Control has been making great strides by adding smarts, improving performance
and adding microcontrollers and network communications to make them viable and
useful tools in today’s systems. Added to their robust design and low cost (power
platform) these systems will continue to grow as viable alternatives when requiring
variable speed control to enhance system performance.
BLDC, Inverter Drives and new system topologies will continue to make great strides in
reducing cost, improving performance and closing the higher cost gap with phase control
will continue. The inertia of significant volumes of single-phase motors will continue to
pose barriers of entry to these solutions beyond unit-to-unit cost comparisons. In noncritical applications where the higher performance is not necessary, “smart phasecontrol” will evolve to be the platform solution of choice.
Page 7 of 8
H. Abramowitz
Power Systems World 2003
Phase Control Alternatives for Single Phase AC Motors
makers of AirCare VariPhase™ speed controls
and
AirCare Consoles for System Integration
AirCare Console
www.aircareautomation.com
Customer Service: 512-233-6219
Page 8 of 8
H. Abramowitz