Energy efficiency is a continuing focus of the power electronics industry. Motors consume the most electricity in commercial and industrial applications (followed at a distant second by lighting). More than 85 percent of electric motors are single-phase motors that are installed in numerous applications - fans, pumps, compressors - that would benefit from variable speed control. Traditional approaches have been to complicate the motor design by tapping the motor windings and using electromechanical switches to change the number of windings, thus changing speed.
Electronic approaches have focused on using triac bridges that are phase-controlled. These systems generate significant power losses and conducted noise, making them limited in application. The drive industry has therefore been pushing for conversion of new systems to full three-phase motors with sophisticated three-phase drives. Such drives require not only a more expensive motor but also an electronic drive that is both complex and expensive. As these drives need to be set up by professionals, many users shy away from them.
Anacon Systems has taken the underlying technology of three-phase motor drives and applied it to single-phase motors. Using the latest IC technology - RISC-based processors - Anacon has developed a controller IC that meets the stringent performance requirements along with the lowered cost target. The ASIC uses an 8-bit RISC microcontroller with 128,000 SRAM, an 8-×-8-bit multiplier, analog-to-digital converter, four 10-bit pulse width modulators (PWMs), a 12-bit timer and a comparator to monitor faults and perform system shutdown. Design innovations also provide customization through programmability. The ASIC also includes an 8-kbyte EPROM and an SPI or PC serial port to allow for external programmability. The ASIC has digital outputs that can be used to signal operating or fault conditions using LEDs.
The resulting motor controller is created using this ASIC (see the figure). A power electronics design using all through-hole mount techniques results in a low-cost hardware platform that is easy to customize for unique market applications. The motor control uses an H-bridge power stage configuration containing power MOSFETs or IGBTs switching at 18 kHz to keep switching noise above the audible range. The driver stage interfaces with the ASIC and translates the PWM waveform into H-bridge on/off signals. Aside from the controller the only other function needed is a bias supply, providing the 12- and 5-volt power feeding off the input AC line, eliminating a low-frequency transformer or high-frequency power supply. With these few blocks - bias supply, controller, H-bridge, and driver stage - the motor controller can be realized.
The resulting product is:
- programmable through external software changes;
- rugged and flexible;
- energy efficient, because a heating-system blower working at 70 percent of full speed uses 50 percent less energy and also provides more comfort through continuous air flow and destratification.
The finished drive is not complicated to control. Firmware can be provided that sets the shape of the V/f tradeoffs. Trip points for idle speed and maximum speed can be set externally. Trip points for system protection can also be set externally. Thus, for a given application like a blower control, a simple DC voltage (0-5 V) can be used to control the motor speed while several potentiometers can provide the customer flexibility to set the critical trip points.