By Padmaraja Yedamale -- Design News, December 12, 2004
Every day engineers design products that employ single-phase induction motors. Speed control of single-phase induction motors is desirable in most motor control applications since it not only provides variable speed but also reduces energy consumption and audible noise.
Most single-phase induction motors are unidirectional, which means they are designed to rotate in one direction. Either by adding extra windings, external relays and switches, or by adding gear mechanisms, the direction of rotation can be changed. Using microcontroller-based control systems, one can add speed variation to the system. In addition to the option of speed variation, the direction of rotation can also be changed, depending upon the motor control algorithms used.
Permanent Split Capacitor (PSC) motors are the most popular type of single-phase induction motors. This article will discuss different techniques and drive topologies to control the speed of a PSC motor in one and two directions.
A microcontroller is the brain of the system. Often, the controllers used for motor control applications have specialized peripherals like motor-control PWMs, high-speed analog-to-digital converters (ADCs), and diagnostic pins. The PIC18F2431 and dsPIC30F2010 from Microchip both have these features built in.
Having access to the microcontroller's specialized, on-chip peripherals makes the implementation of control algorithms easier.
ADC channels are used to measure motor current, motor temperature and heat sink temperature (connected to the power switches). A third ADC channel is used to read potentiometer levels, which is then used to set the speed of the motor. Additional ADC channels can be used in the final application to read different sensors, such as the proximity switch, turbidity sensors, water level, freezer temperature, etc.
General-purpose inputs and outputs (I/Os) can be used for interfacing switches and displays in an application. For example, in a refrigerator application, these general-purpose I/Os can be used to control an LCD display, seven-segment LED display, push-button interface, etc. Communication channels like I2C™ or SPI™ are used to connect the motor control board with another board to exchange data.
Fault and diagnostics interfaces include input lines with special features like the ability to shut down the PWMs in case of catastrophic faults in the system. For example, in a dish-washer, if the drive is blocked due to accumulated waste, it could prevent the motor from rotating. This blockage can be detected in the form of over current in the motor control system. Using the diagnostics features, these types of faults can be logged and/or displayed, or transferred to the trouble-shooting PC of a service person. Often, this will prevent hard failures and reduce the downtime of the product, resulting in reduced service costs.
|The hardware interface for the PIC 18F2431 or dsPIC30F2010.|
PWMs are the main peripherals used to control the motor. Using the above inputs, the microcontroller's motor control algorithm determines the PWM duty cycle and pattern of output. The PWM's most valuable features include complementary channels with programmable dead time. PWMs can be edge-aligned or center-aligned. Center-aligned PWMs have the advantage of reduced electromagnetic noise (EMI) being emitted by the product.
Option #1: Unidirectional Control
VF control in one direction makes the drive topology and control algorithm relatively easy. The task is to generate a variable voltage and frequency power supply from a fixed voltage and frequency power supply (such as a wall-outlet power supply). The figure on page 85 shows the block diagram representation of this drive topology, with the three basic building sections discussed earlier. Motor windings are connected to the center of each half bridge on the output-inverter section. Many motors available off the shelf have both the main and start windings connected together with a capacitor connected in series with the start winding. With this configuration, the motor may have only two protruding wires (M1 and M2).
The MCU shown in the block diagram has a Power Control PWM (PCPWM) module, which is capable of outputting up to three pairs of PWMs with deadband in between the pairs. Deadband is essential in an induction motor control application to avoid cross conduction of the dc bus through the power switches when one turns OFF and the other turns ON. The diagnostic circuit may include motor current monitoring, dc-bus voltage monitoring, and temperature monitoring on the heat sink connected to the power switches and the motor.
|Block diagram representation of the drive topology with the three basic building sections. With this configuration, the motor may have only two protruding wires (M1 and M2). The MCU shown has a PWM module that is capable of outputting up to three pairs of PWMs with deadband between the pairs.|
|Bidirectional control using an H-bridge.|
Most PSC motors are designed to run in one direction. However, many applications call for bidirectional motor rotation. Historically, gear mechanisms or external relays and switches were used to achieve bidirectional rotation. When mechanical gears are used, the motor shaft runs in one direction, and the gears for forward and reverse engage and disengage according to the direction required. Using relays and switches, the polarity of the starting winding is electrically reversed based on the direction required.
Unfortunately, all of these components increase the cost of the system for basic ON and OFF control in two directions.
In this section, we will discuss two methods of bidirectional speed control for PSC motors using a microcontroller-based drive. The drive topologies discussed here produce effective voltages, which drive the main winding and start winding at 90-degree phase shifts to each other. This enables the system designer to remove the capacitor, which is in series with start winding, from the circuit permanently—thereby reducing the total system cost.
Option #2: H-Bridge Inverter
This method has a voltage doubler on the input side; on the output side an H-bridge or two-phase inverter is used (see figure above). One end of the main and start windings are connected to each half bridge; the other ends are connected together at the neutral point of the ac power supply, which also serves as the center point for the voltage doubler.
The control circuit requires four PWMs with two complementary pairs and sufficient deadband between the complementary outputs. PWM0-PWM1 and PWM2-PWM3 are the PWM pairs with dead band. Using PWMs, the dc bus is synthesized to provide two sine voltages at 90 degrees out of phase with varying amplitude and varying frequency, according to the VF profile. If the voltage applied to the main winding lags the start winding by 90 degrees, then the motor runs in the forward direction. To reverse the direction of rotation, the voltage supplied to the main winding should lead the voltage supplied to the start winding.
|Phase voltages when the motor is running in forward and reverse direction.|
This H-bridge inverter method of controlling a PSC type motor has following disadvantages.
The main and start windings have different electrical characteristics. Thus, the current flowing through each switch is unbalanced. This can lead to the premature breakdown of switching devices in the inverter.
The common point of the windings is directly connected to the neutral power supply. This may increase the switching signals creeping into the main power supply, and may increase the noise emitted onto the line. In turn, this may limit the EMI level of the product, violating certain design goals and regulations.
The effective dc voltage handled is relatively high due to the input-voltage doubler circuit.
Lastly, the cost of the voltage doubler circuit itself is high due to two large power capacitors.
A better solution to minimize these problems would be to use a three-phase inverter bridge, as discussed in the next section.
Option #3: Using a Three-Phase Inverter Bridge
The input section is replaced with a standard diode-bridge rectifier. The output section has a three-phase inverter bridge. The main difference from the previous scheme is the method used to connect the motor windings to the inverter. One end of the main and start windings are connected to one half bridge each. The other ends are tied together and connected to the third half bridge.
|Control using a three-phase inverter bridge.|
With this drive topology, control becomes more efficient. However, the control algorithm becomes more complex. The winding voltages, Va, Vb, and Vc, should be controlled to achieve the phase difference between the effective voltages across the main and starting windings, in order to have a 90-degree phase shift to each other.
In order to have equal voltage-stress levels on all devices, which improves the device utilization and provides the maximum possible output voltage for a given dc bus voltage, all three inverter-phase voltages are kept at the same amplitude, as given by:
| Va | = | Vb | = | Vc |
The effective voltage across the main and starting windings as given by:
Vmain = Va-Vc
Vstart = Vb-Vc
The direction of rotation can be easily controlled by the Vc phase angle with respect to Va and Vb .
Figures on page 87 show the phase voltages Va, Vb, and Vc, the effective voltages across the main winding (Vmain) and starting winding (Vstart) for forward direction and reverse directions respectively.
Using the three-phase inverter control method on a 300W compressor gave a power saving of 30 percent compared to the first two methods.
|Microcontroller Resources Required|
|Resource||Unidirectional||Bidirectional H-bridge||Bidirectional with three-phase bridge||Notes|
|Program memory||1.5 Kbytes||2.0 Kbytes||2.5 Kbytes||--|
|Data memory||~20 bytes||~25 Bytes||~25 bytes||--|
|PWM channels||2 channels||2 channels||3 channels||Complementary with dead time|
|Timer||1||1||1||8- or 16-bit|
|Analog-to-digital converter||3 to 4 channels||3 to 4 channels||3 to 4 channels||Motor current, temperature measurements, speed control potentiometer|
|Digital I/Os||3 to 4||3 to 4||3 to 4||For user interfaces like switches and displays|
|Fault inputs||1 or 2||1 or 2||1 or 2||For over current/ over voltage/ over temp, etc.|
|Complexity of control algorithm||Low||Medium||High||--|
|Unidirectional||Bidirectional with H-bridge||Bidirectional with three-phase bridge|
|Input converter section||Low - Single phase diode bridge rectifier||High - Due to voltage doubler circuit||Low - Single phase diode bridge rectifier|
|Output inverter section||Low - Two half bridges||Medium - Two half bridges. The power switches rated higher voltage||High - three-phase inverter. Using Integrated Power Modules (IPM) is better choice than discrete components|
|Motor||Medium - Starting capacitor required||Low - Starting capacitor is removed from the motor||Low - Starting capacitor is removed from the motor|
|Overall cost||Low||Medium||Medium - Efficient control for the given cost|
Another advantage of using the three-phase control method is that the same drive-hardware topology can be used to control a three-phase induction motor. In this scenario, the microcontroller should be reprogrammed to output sine voltages with 120-degree phase shift to each other, which drives a three-phase induction motor. This reduces the development time.
Single-phase induction motors are very popular in appliances, and industrial and consumer applications. PSCs are the most popular type of single-phase induction motors. Controlling the motor speed has many advantages, such as power efficiency, reduced audible noise and better control over the application. In this article, we discussed different methods of speed control that can be used with a PSC motor in unidirection and bidirection. Controlling a PSC motor using a three-phase inverter topology provides the best results.
|Phase voltage when the motor is running in forward and reverse directions.|
|An overview of motor types, motor classification, and applications using Microchip's PICs and dsPICs: |
|Datasheet programming specs and application notes for PIC 18FXX31 microcontroller: |
|Motor control development tools from Microchip: |
|Contributing writer Padmaraja Yedamale is a Senior Applications Engineer at Microchip Technology Inc. You can reach him at firstname.lastname@example.org.|