here. - Power Electronics

Closed-Loop Control
Benefits AC Motors
By Larry Grover
Grover, Senior Design Engineer
Engineer, and Jonathan Guy
Guy,
Vice President Engineering, AirCare Automation, Austin, Texas
Feedback control methods implemented using
low-cost microprocessors enable significant
gains for single-phase ac motor speed controls.
M
ore than 80% of the motors bought each
year continue to be low-cost single-phase
ac motors. Because these motors are not
very efficient when controlled using
conventional means, the industry has
focused much of its energies on displacing the single-phase
motors with higher-efficiency varieties. However, when
higher efficiency is desired, there is an alternative to changing
motors. Combining low-cost ac motors with low-cost smart
controls can improve performance, efficiency and system
implementation. It is estimated that 40% to 60% of the ac
motor applications would benefit from some form of closedloop, smart speed control.
Closed-loop control allows single-phase ac motors to
close the gap in performance and efficiency that exists
between these motors and other higher-performance,
higher-cost motors. These control techniques enable
design solutions that are competitive for a wide range of
applications while maintaining their low-cost appeal. This
paper explores several key aspects of these speed controls
utilizing a traditional low-cost phase-control (TRIAC-drive)
technique.
Speed-Control Linearity
The staple of speed control for a single-phase ac motor
remains a nonintelligent, TRIAC-driven phase control. It
should be mentioned from the outset that this discussion
relates to permanent split-capacitor and shaded-pole singlephase ac motors as a product family.
The cap-start motor (Fig. 1) uses a centrifugal switch to
allow the auxiliary winding to be taken out of the power
loop after the motor comes up to speed. The difficulty is that
the motor needs to maintain a fairly high speed to keep the
centrifugal switch out.
The traditional TRIAC control for a single-phase ac
Fig. 2. In a single-phase ac PSC motor, a TRIAC is driven to change the
trigger phase angle for the ac voltage, reducing speed by reducing
RMS voltage. An RC network adjusts the phase angle, approximating a
linear phase change over the range of speeds.
Fig. 1. In a single-phase ac cap-start motor, a centrifugal switch allows
the auxiliary winding to be taken out of the power loop after the
motor comes up to speed.
Power Electronics Technology June 2005
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CLOSED-LOOP CONTROL
a challenge.
With a smart
co n t ro l l e r, t h e
phase angle can be
adjusted to provide
a linear control
feedback. With a
Phase angle varies
closed-loop, smart
Nonlinearly
controller, a true
0
10
20
30
40
50
60
linear speed change
(%)
RPM Adjusted
0 40 68 83 94 102 111 121
can be provided.
Phase Angle
Phase-Angle Cutoff (°)
The speed sensor
(i.e. a hall sensor)
will provide an
Fig. 3. The nonlinear relationship between phase angle and RPM is shown for a 230-V, 60-Hz 1660-RPM ac fan motor
accurate measure
controlled by a conventional TRIAC driven, phase control (a). Smart control of the phase angle produces linear RPM
of the RPM, which
control (b).
is then fed back
into the speed control to provide the results seen in Fig. 3b.
permanent-split-capacitor (PSC) motor is shown in Fig. 2.
As the single-phase ac motor is inherently open loop, RPM
The TRIAC is driven to change the trigger phase angle for
measurements must be provided by an external sensor.
the ac voltage, reducing speed by reducing RMS voltage. A
resistor-capacitor network is used to adjust the phase angle,
Startup Dynamics
approximating a linear phase change over the range. However,
Single-phase ac motors experience a surge in current
the speed of the motor does not change linearly with the phase
during startup that is significantly higher than their run
angle, as depicted in Fig. 3a. The nonlinearity creates problems
currents. We have measured startup currents of two to three
as small control adjustments can create very small effects or
times the rated run current listed on the motor. In many cases,
large speed changes, making accurate speed adjustments
1800
1600
1400
1200
1000
800
600
400
200
0
(%)
Linear RPM Change With Input
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CLOSED-LOOP CONTROL
where one motor (i.e.
fan) is on the breaker
line, the surge can
be tolerated and
is generally overlooked. However,
we have seen many
ac systems where
there are grouping
of motors on the
same breaker line.
At startup, the surge
Soft-start - 1.25 A pk, <7 sec.
currents can exceed
Soft-start - 0.7 A pk, <40 sec.
the breaker rating by
-65 mV
250 mV
250 mV
-65 mV
two times or greater,
and these are often
overlooked until the
Fig. 4. A 277-V, 0.65-A rated fan motor experiences much lower surge currents in a smart-controlled startup (a) than in
breaker trips during an uncontrolled startup (b).
installation.
With a smart controller, users can
create a controlled delay to allow the
phase to go to full throttle more slowly,
giving the motor time to gain “back
PID
EMF” and reduce the current drain.
Loop
Controllers can be designed so that users
Control
can customize the ramp-up period for the
Temperature
type of motor and system being driven.
Pressure
(We have found significant differences
Humidity
Sensor
Airflow
between the performance of an external
RPM
rotor motor and a traditional internal
rotor motor during startup). Fig. 4
provides an example of an uncontrolled
Fig. 5. Closed-loop control of single-phase ac motors requires the use of external sensors.
and smart-controlled startup.
Fig. 4 shows that with soft-start, the
but will not inherently eliminate the drawbacks of the lowpeak current is reduced to just slightly more than the rated
torque drive. High-torque applications need to move toward
motor current. The time scales on the above graphs are
variable frequency drives (VFDs) to address low-speed/highdifferent (soft-start extends the time to reach full speed from
torque applications. Single-phase VFDs are not often a viable
less than 7 sec to over 30 sec). The soft-start shows a double
option, as they add cost and complexity and also can cause
ridge that is the result of a novel three-wire control topology
motor failure through winding insulation breakdown and
not covered in this paper. The ability to tailor the startup
pitted bearings. Only special single-phase ac motors can be
characteristics of a motor can extend the use of the power
reliably driven from a VFD.
that is available and results in better-performing systems that
With most low-cost microcontrollers, the key ingredients
can only be achieved through smart closed-loop control.
of a closed-loop system can be implemented. They are
closed-loop feedback algorithms (i.e a PID control loop), a
System Closed-Loop Control
second feedback node to compare the external error signal
Once you’ve added smarts to the basic TRIAC control
to the desired setpoint and the availability of an external
topology, for negligible cost you can incorporate sophisticated
sensor that can provide the error signal in the appropriate
closed-loop controls and accommodate more demanding
form needed by the controller ( i.e. 0 V to 5 V, 4 mA to
applications. In the case of fan controls, controlling fan speed
20 mA, etc.). Fig. 5 is a simple block diagram of the desired
as a result of a measured temperature, pressure or airflow
system.
is very common. These applications have historically been
An example of a closed-loop system is the use of a
addressed by more sophisticated and expensive motor/drive
thermistor for temperature feedback, with results shown
platforms (three-phase ac or dc).
in Fig. 6. The smart control allows for setpoint control
The low-speed/low-torque requirements of a fan lend
(desired temperature), slope of speed versus temperature
themselves well to TRIAC phase control. The closed-loop
(system sensitivity and reaction), and the PID setting
control can help overcome higher-torque startup issues,
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Power Electronics Technology June 2005
Drive Output (V)
CLOSED-LOOP CONTROL
Temperature (°C)
Temperature (°F)
Fig. 6. In a closed-loop system using temperature feedback, smart control of a fan allows setpoint control at a desired temperatue and
adjustment of the slope of fan speed versus temperature (a). The selected settings lead to the control loop response shown (b).
Key: Ch1: Setpoint Signal
Ch1 2.00 V
Ch3 2.00 V
Ch2: Sensor Feedback Signal
00 s Ch1
Ch3: Fan Speed
840.mV
Ch1 2.00 V
Ch3 2.00 V
Fig. 7. A low-cost microcontroller incorporates a PID control loop that is applied to a fan system maintaining a set differential air pressure.
Measurements reveal the responses to a stepup request going from 0-in. to 0.5-in. pressure (a), a stepdown request from 0.8-in. to 0.4-in.
pressure (b), and a significant drop in chamber pressure (c).
controls the response time and overshoot (not shown) when
temperatures shift at the targeted load. The resulting system
is low cost and easy to implement.
A final example is shown in Fig. 7. The same low-cost
microcontroller incorporates a PID control loop that is
applied to a fan system maintaining a set differential pressure.
The system is a small chamber being driven by a fan filter unit
driven by an ac fan and controlled with an AirCare VariPhase
speed controller. This controller has two analog inputs and
contains a full PID control feature. The differential pressure
is measured by a SETRA 265-type sensor with a 0-in. to 1-in.
range. In setting a strong “P” term and very weak “I” and “D”
terms, the following responses were observed. These settings
reflected customer requests for slow system response so that
turbulence and major airflow changes would not be felt by
the chamber occupants.
Fig. 7a shows a step request (Ch1 waveform) going from
0-in. to 0.5-in. pressure. The chamber pressure comes up
with no noticeable overshoot, and fan speed (Ch3 waveform)
comes up uniformly to adjust pressure. Next, Fig. 7b shows
that a stepdown request from 0.8-in. to 0.4-in. pressure also
provides a stable response. Finally, Fig. 7c depicts a system
Power Electronics Technology June 2005
where the setpoint is unchanged but the chamber sees a
significant drop in chamber pressure (i.e. a door opening
causing a large drop in pressure) and the resulting system
response.
In general, the airflow system had a slow response, so
loop stability was not hard to achieve. However, a bigger
challenge was dealing with small-chamber air turbulence that
caused measurement error. Proper attention to the sensor
placement and adjusting PID helps stabilize the flow against
the chamber air turbulence.
High-End Results
In conclusion, the above examples show how low-cost
solutions can be applied to single-phase ac motors to provide
high-end results. The above evaluation demonstrates a smart
system that enhances the performance of single-phase (PSC)
ac fans. Based on the traditional low-cost power control
topology (TRIAC phase control) and adding a low-cost
microcontroller, designers can create closed-loop control
and feedback to achieve efficiency and performance that
closes the gap with other, more expensive motor/drive
topologies.
PETech
22
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