Power Electronics

Closed-Loop Control Benefits AC Motors

Feedback control methods implemented using low-cost microprocessors enable significant gains for single-phase ac motor speed controls.

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More 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 closed-loop, 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 single-phase 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 permanent-split-capacitor (PSC) motor is shown in Fig. 2. The TRIAC is driven to change the trigger phase angle for the ac voltage, reducing speed by reducing RMS voltage. A resistor-capacitor network is used to adjust the phase angle, approximating a linear phase change over the range. However, the speed of the motor does not change linearly with the phase angle, as depicted in Fig. 3a. The nonlinearity creates problems as small control adjustments can create very small effects or large speed changes, making accurate speed adjustments a challenge.

With a smart controller, the phase angle can be adjusted to provide a linear control feedback. With a closed-loop, smart controller, a true linear speed change can be provided. The speed sensor (i.e. a hall sensor) will provide an accurate measure of the RPM, which is then fed back into the speed control to provide the results seen in Fig. 3b. As the single-phase ac motor is inherently open loop, RPM measurements must be provided by an external sensor.

Startup Dynamics

Single-phase ac motors experience a surge in current during startup that is significantly higher than their run currents. We have measured startup currents of two to three times the rated run current listed on the motor. In many cases, where one motor (i.e. fan) is on the breaker line, the surge can be tolerated and is generally over-looked. However, we have seen many ac systems where there are grouping of motors on the same breaker line. At startup, the surge currents can exceed the breaker rating by two times or greater, and these are often overlooked until the breaker trips during 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 EMF” and reduce the current drain. Controllers can be designed so that users can customize the ramp-up period for the type of motor and system being driven. (We have found significant differences between the performance of an external rotor motor and a traditional internal rotor motor during startup). Fig. 4 provides an example of an uncontrolled and smart-controlled startup.

Fig. 4 shows that with soft-start, the peak current is reduced to just slightly more than the rated motor current. The time scales on the above graphs are different (soft-start extends the time to reach full speed from less than 7 sec to over 30 sec). The soft-start shows a double ridge that is the result of a novel three-wire control topology not covered in this paper. The ability to tailor the startup characteristics of a motor can extend the use of the power that is available and results in better-performing systems that can only be achieved through smart closed-loop control.

System Closed-Loop Control

Once you've added smarts to the basic TRIAC control topology, for negligible cost you can incorporate sophisticated closed-loop controls and accommodate more demanding applications. In the case of fan controls, controlling fan speed as a result of a measured temperature, pressure or airflow is very common. These applications have historically been addressed by more sophisticated and expensive motor/drive platforms (three-phase ac or dc).

The low-speed/low-torque requirements of a fan lend themselves well to TRIAC phase control. The closed-loop control can help overcome higher-torque startup issues, but will not inherently eliminate the drawbacks of the low-torque drive. High-torque applications need to move toward variable frequency drives (VFDs) to address low-speed/high-torque applications. Single-phase VFDs are not often a viable option, as they add cost and complexity and also can cause motor failure through winding insulation breakdown and pitted bearings. Only special single-phase ac motors can be reliably driven from a VFD.

With most low-cost microcontrollers, the key ingredients of a closed-loop system can be implemented. They are closed-loop feedback algorithms (i.e a PID control loop), a second feedback node to compare the external error signal to the desired setpoint and the availability of an external sensor that can provide the error signal in the appropriate form needed by the controller (i.e. 0 V to 5 V, 4 mA to 20 mA, etc.). Fig. 5 is a simple block diagram of the desired system.

An example of a closed-loop system is the use of a thermistor for temperature feedback, with results shown in Fig. 6. The smart control allows for setpoint control (desired temperature), slope of speed versus temperature (system sensitivity and reaction), and the PID setting 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 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.

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