Most engineers know that heating, ventilation, and air conditioning (HVAC) systems suck down an appreciable amount of electrical power. So they have been a focus for energy regulators and probably will remain so for the future.
Engineers who must design the next generation of more efficient HVAC equipment can take a few lessons from the Japanese. Japanese manufacturers lead the world in energy efficient air conditioning technology. One way they have gained efficiencies is through innovations in motor design and control technology. Specifically, they have employed variable-speed compressors to good effect in HVAC.
It is helpful in this discussion to review the ways in which an air conditioner dissipates energy. Air conditioners transfer heat by circulating a refrigerant fluid between indoor and outdoor heat exchanger units. Energy is consumed by the compressor circulating the refrigerant and by the fans that blow air over the heat exchanger coils. The seasonal energy efficiency rating (SEER) of the air conditioner is calculated as the ratio of the heat transfer capacity (btu/hr) divided by the average electrical power consumption in Watts. This average power consumption, calculated as a weighted average of input power for a range of cooling conditions, represents the average consumption during a typical cooling season.
The U.S. Dept. of Energy raised the minimum standard for SEER to 13 in January 2006 while the Energy Star standard is set at 14. It’s possible to improve efficiency incrementally through improved mechanical component design. However, studies show that efficiency gains of up to 40% are available by using variable-speed compressors.
In the U.S., compressors are typically driven by single-phase induction motors so they can run directly from the domestic ac supply. However, singlephase induction motors are far less efficient than three-phase motors. Conventional single-phase motors typically exhibit about 80% efficiency. In contrast, Premium three-phase units can hit 90.2%, and even ordinary three-phase motors come in at 87.5%
Three-phase motors also can deliver significant efficiency gains when driven at variable speed through a power inverter. Use of more efficient motors can partly offset the inverter cost because the motor itself can be smaller. Moreover, variable-speed control of the fan makes for quieter operation. Typically, permanent- magnet ac motors are used in variable-speed air conditioning systems because they have the highest efficiency of all three-phase motor types.
It is helpful to see why this is the case. There are a wide variety of ac motor types but two of the most common are the permanent-magnet synchronous motor and the asynchronous induction motor. All motors operate on common principles of physics governing forces acting on current-carrying conductors placed in a magnetic field. Motor types differ only in the arrangement of the windings and in the means of generating the magnetic field.
In a simple prototype ac motor, energizing one phase winding generates a torque that tends to align the rotor magnet with the winding’s axis. When the magnet aligns with phase A, the torque falls to zero and current is switched to phase B so torque production continues. Continuous rotation comes from switching the current between phase A and B windings as a function of rotor magnet position.
The rotor magnet rotation causes a changing flux in the windings that produces a voltage known as back EMF. Conservation of energy dictates that the electrical power input given by the product of the back EMF and winding current equals the mechanical power output given by the product of the rotor speed and shaft torque.
Traditional synchronous ac motors are driven with sinusoidal current waveforms. Their timing is such that the net magnetization effect from the phase windings rotates as winding currents change. To maximize motor output torque, the axis of the magnetization is kept 90° mechanically ahead of the rotor magnet axis.
A point to note is that some types of motors can be driven with modulated square waves rather than with sinusoidal waveforms. But sinusoidal currents produce a smoother torque output than switched currents because they avoid torque glitches at the current transitions. These motors are called synchronous motors because the electrical frequency of the winding currents and voltages is synchronized with the mechanical frequency of the rotor.
The second major motor type is the asynchronous induction motor. Its rotor mechanical frequency is typically less than the electrical frequency of the stator currents. The rotor does not use permanent magnets. The air-gap magnetic field between rotor and stator comes from rotor winding currents induced through transformer action by the stator winding voltages. The rotor current frequency is the difference between the stator electrical frequency and the rotor mechanical frequency.
When there is no shaft load, the rotor mechanical frequency almost matches the stator frequency and nearly no current is induced in the rotor windings. An advantage of the induction motor is that it starts directly from a fixed-frequency ac supply, so it is the most commonly used motor type in industry and appliances. However, the efficiency of an induction motor is inherently lower than that of permanent magnet synchronous motors. The difference arises because of the extra losses from the rotor winding currents and core magnetizing current.
There is a third ac motor type called the reluctance motor. It is less widely used than either of the above motors but has a very simple mechanical construction: The rotor has neither windings nor permanent magnets. The rotor iron has saliency (not truly round) which means that there is a rotor position that provides the minimum resistance (reluctance) to the flow of magnetic flux. When a stator winding energizes, it attracts the rotor to align with the axis of that winding.
As in the case of the permanent magnet synchronous motor, the reluctance motor generates continuous rotation by switching currents between two or more phase windings as a function of rotor position. A permanent magnet synchronous motor commonly has rotor saliency to simplify the mounting of the rotor magnets. Motor designers deliberately introduce rotor saliency to produce an extra reluctance torque component in addition to the magnetic torque. (Reluctance torque is the torque generated because the rotor is moving to a position where it sees declining reluctance.)
Variable speed drives (VSDs) use power converters to change the single-phase ac input voltage into three-phase variable frequency voltages to drive the motor. AC motor control schemes vary depending on the motor type but the power converter architecture is common. The usual control scheme for three-phase permanent magnet motors, is called six-step control. It is named for the six combinations of current flow possible in a wye-connected threephase motor. It connects two windings across the dc rail during each of the six connections and leaves the third winding unconnected.
Hall-Effect sensors typically detect the rotor magnet position to select the right windings during the appropriate part of the cycle. This control scheme is found in many air conditioning fan controllers.
But Hall-Effect sensors cannot be used inside hermetically sealed compressors. The alternative is to detect the zero-crossing of the back EMF on the unconnected winding in a so-called “sensorless” approach. The sensorless approach estimates the rotor magnet position based on winding current measurements. This allows application of field-oriented control (FOC) principles to align the stator current magnetic axis 90° ahead of the rotor magnet axis and so maximize torque production.
The FOC approach typically uses high-speed processors, such as digital signal processors, to manage stator current in real-time for maximum torque output. It is sometimes used in conjunction with interior permanent magnet (IPM) motors that have rotor saliency. An IPM motor has magnets in its rotor oriented so more torque gets produced than with a surface-type (SPM) motor, because it exhibits an additional reluctance-torque component. Air conditioning compressor manufacturers in Japan have adopted the IPM motor because it can deliver up to 15% more torque-peramp due to reluctance torque.
Example: Japanese HVAC
A new integrated design platform eases the adoption of variable- speed control of permanent magnet motors in air conditioning systems. The control IC includes a dedicated motion control engine (MCE) to implement sensorless Field-oriented control (FOC) for the compressor and fan motors. FOC enables use of highly efficient IPM motors for the compressor. It also incorporates special torque-compensation functions to minimize mechanical noise in the compressor when running at low speeds. Sensorless sinusoidal control of the fan motor minimizes acoustic noise and improves reliability by eliminating Hall-effect position sensors.
The MCE also has the bandwidth to control the input power factor thus eliminating the external control components that would otherwise be needed to handle PFC. The IC includes all the analog circuits needed to measure the motor winding currents and the input ac currents and voltages.
Finally, the IC integrates a separate microcontroller core to handle air conditioner system functions. Having a separate microcontroller core simplifies product development by decoupling the motor-control function from the application function. The power stage components are integrated into single modules that simplify the assembly and improve reliability. Highly efficient trench IGBT power switches along with optimized control algorithms maximize system efficiency.
These platforms are now in the field. A Japanese air conditioning company measured a system efficiency of over 95% on a 3.5kW HVAC inverter using this design platform. This is more than 5% higher than the typical system efficiency in Japanese HVAC systems. The results have prompted adoption of this design platform for the manufacturer’s next-generation outdoor unit system controller.
El Segundo, Calif.
Edited by Leland Teschler
Air conditioners are adopting variable-speed compressors to boost efficiency.
- New motor technology has been developed for variable-speed uses. Ditto for chip technology.
International Rectifier page on variable-speed control, www.irf.coom/product-info/imotion/
Update on develoments in control of IPM, PM and induction motors, tinyurl.com/477jqw
Wikipedia page VFDs, tinyurl.com/4cmhtb