Figure 1: Conventional, single-phase ACIM system diagram.

The use of electric motors is very common in home appliances. One of the most widely used motor types in the appliance market is the AC Induction Motor (ACIM). It can be found in refrigerators, microwaves, clothes washers, clothes dryers, and air conditioners. A typical single-phase ACIM design can be found in Fig. 1.

This particular example is a modified split-phase motor with a capacitor in series with the start winding to provide a start “boost.” It has a centrifugal switch, which disconnects the start winding and the capacitor when the motor reaches about 75 percent of the rated speed. Many times these motors have multiple windings for multiple speed operations. In this configuration, the motor is directly connected to the input power so the voltage and frequency remain constant; therefore, many times the motors are over-designed for peak performance. This results in an inefficient driving system, thus generating more motor torque than required by the load.

When the motor is operated at a light load, the Power Factor (PF) tends to be very low. This condition degrades the power quality of the supply line and may affect performances of other utility equipment connected on the same line. The PF is very important. Many distribution companies have started imposing penalties on the customer drawing power at a value less than the set limit of the PF.

This means the customer is forced to maintain the full-load condition for the entire operating time or else pay penalties for the light-load condition. Additionally, the supply line may experience a surge or sag due to the operation of other equipment on the same line. If the motor is not protected from such conditions, it will be subjected to higher stress than it is designed for, which ultimately may lead to its premature failure.

Figure 2: Three-phase ACIM variable-frequency drive using the PIC18Fxx31.

These problems, faced by both the consumer and the industry, strongly advocate the need for an intelligent motor control. With recent developments in today’s microcontroller (MCU) technology, motors can be controlled more efficiently, significantly reduce noise and provide cost reduction at the component level. Microchip Technology has recently introduced four new 8-bit PICmicro® MCUs known as the PIC18Fxx31 family.

The family touts several new specialized motor control peripherals including a power control Pulse Width Modulation (PWM) module, a 200 ksps Analog-to-Digital Converter that can be synchronized with the PWM and a motion feedback module. All of these features not only make intelligent-electronic motor control possible, but also simplify the design process for engineers. The following example will show a typical three-phase ACIM design using Microchip’s new PIC18Fxx31 MCU family.

The Voltage Frequency Drive (VFD) is the most popular electronic control for ACIM (see Fig. 2) and also enables users to drive a three-phase ACIM using a single-phase supply. The basic function of the VFD is to act as a variable frequency generator in order to vary the speed of the motor as per the user setting. The rectifier and the filter convert the AC input to DC with negligible ripple.

The inverter, under the control of the PICmicro MCU, synthesizes the DC into three-phase variable voltage; variable frequency AC. Additional features can be provided, like the DC-bus voltage sensing, Over Voltage (OV) and Under Voltage (UV) trip, overcurrent protection, accurate speed/position control, temperature control, easy control setting, display, PC connectivity for real-time monitoring, Power Factor Correction (PFC) and so on.

A single VFD has the capability to control multiple motors, is adaptable to almost any operating condition, and eliminates the need to refuel or warm up the motor. For the given power rating, the control and the drive provided by the VFD depend solely on the algorithm written into it. This means, that for a wide range of power ratings, the same VFD can be used. This control addresses all the issues mentioned with conventional motor control.

With global energy costs increasing, the PIC18Fxx31 MCU family becomes a logical choice for embedded intelligence in motor control. Not only does the family provide complete integration into a single chip solution; it can improve reliability, reduce noise and prolong the life of the motor. The ultimate payoff that is realized by using a VFD is in the efficiency improvement, which can be up to 30 percent in many home appliances. This improvement translates directly into cost savings for the end consumer.