Sensorless Washer Motors Simplify Design
Motors in fans, pumps, air conditioners, refrigerators, washing machines, elevators, conveyors, and other rotating applications consume over half of the world’s electricity, according to the Electric Power Research Institute (EPRI). Most use wasteful electromechanical drives that only turn a motor on or off. Replacing these inefficient motors with variable-speed devices could cut wasted energy as much as 60% in appliance applications alone.
Most appliances use universal dc or single-phase-ac induction motors. Speed control methods are rather crude and use either on/off or phase-angle triac control. Typical system efficiency is around 50% at best. However, the advent of highly efficient power switches and advanced digital controllers has made motors and controllers more efficient.
Options in ac motors
Shaft torque output of any ac machine depends on the spatial angle between the stator and rotor fields. Torque is generated when the magnetizing forces of the stator winding currents interact with the air gap flux produced by the rotor. This torque tries to align the rotor flux with the stator magnetizing field and is at a maximum when the stator magnetizing current vector is 90 deg out of phase with the rotor flux vector. In a dc machine, the magnet is fixed but the switching action of the commutator and brushes ensures that armature [stator] magnetization is correctly aligned. In ac machines, the air gap field rotates, however, constant torque is still produced as long as the frequencies of the stator and rotor fields are synchronized.
Two main classes of ac machines exist; these are synchronous and induction or asynchronous.
In the synchronous ac machine, the rotor field is generated by dc currents in the rotor windings or by a permanent magnet. The stator currents must be synchronized with the rotor angle and frequency to produce a constant torque.
In the induction machine, the rotor field is generated by rotor currents induced through the stator’s transformer action. Frequency of the stator and rotor fields are automatically synchronized. When the induction machine stops, the flux coupled by the rotor windings is at the stator frequency and so rotor currents are also at the stator frequency. When the machine is rotating, frequency of the rotor flux coupled is the difference between the stator frequency and the rotational frequency of the rotor and is known as the slip frequency . When the machine rotates at the stator frequency, the rotor flux is constant, no rotor currents are induced, and the torque output is zero.
Less speed, more torque
Induction motors always run at some speed less than the stator frequency. If the load increases then the speed drops and the slip frequency increases, producing a larger rotor current, which generates more torque.
These motors are widely used in industrial and appliance applications especially when a fixed speed is required. A significant advantage of induction motors is ability to be started when connected directly to the ac line. In contrast, the open-loop output voltage of the synchronous machine must be closely matched in magnitude and frequency before it can be connected to the ac line. Large synchronous machines are commonly used in power generation, where multiple machines are connected to a common power grid.
In variable-speed applications the choice of motor for the drive system is less obvious. Open-loop (V/Hz) speed control methods are commonly used for inverter-driven induction motors. Closed-loop control of an induction motor is also possible using a speed sensor, with the motor slip frequency varied to control motor-generated torque. However, high dynamic control of an induction motor is difficult because the rotor currents cannot be measured and the rotor circuit has a large time constant. On the other hand, high dynamic torque control of a synchronous motor is quite simple once the rotor angular position is known.
Permanent magnet synchronous machines (PMSM) have been used for many years in industrial servo applications. The permanent magnet rotor makes this motor very efficient and so it can deliver a much higher continuous torque than a similar sized induction motor. However, shaft angle must be measured by a position sensor, such as a Hall-effect sensor or a resolver. This requirement for a rotor angular position sensor once limited applications to high-end industrial drives, but development of “sensorless” control algorithms in recent years has seen an increasing number of applications in home appliances.
Compressor speed control
One of the first applications of PMSM in appliances was in compressor speed control. The traditional compressor for air conditioning or refrigeration used an induction motor running at a fixed speed depending on the line frequency. The compressor had to be sized to meet maximum load conditions after power up, but in normal running the compressor was cycled on and off at a fairly low duty cycle to maintain the set temperature. However, when compressor speed control is applied, the most efficient operating speed can be selected in normal operation. Speed control increases efficiency over 30%; higher PM motor efficiency adds 15% more. Today, in energy cost-sensitive markets like Japan, compressor speed control is used in almost 90% of air conditioning applications and more than 50% of domestic refrigerator applications.
The first sensorless controllers used a six-step commutation sequence for the motor windings and estimated rotor position by monitoring back electromotive force (emf) of the open winding. This approach provides robust speed control but does not deliver smooth motor torque. First, to deliver a constant torque using six-step commutation, the motor should have a trapezoidal-shaped back-emf profile rather than the usual sine-wave shape. A second, even bigger, problem is the “torque glitch” introduced as current is switched to the next winding during commutation. The problem worsens at higher speed as motor back emf aids the decay of current in the outgoing winding but opposes current rise in the incoming phase.
Torque glitch generates audible noise in fans, washers, pumps, and air conditioners because high harmonic content of motor torque easily excites system mechanical resonance frequencies. The controller, however, is very simple to implement and is still used in applications that do not require smooth torque control.
An alternative, sensorless control method has become popular in recent years, as the introduction of cost-effective digital signal processor (DSP) and reduced instruction set computer (RISC)-based controllers has allowed implementation of more complex control algorithms.
The “current sensorless” controller allows the PMSM to be driven with sinusoidal voltage and current waveforms and estimates rotor position based on measured motor currents. This algorithm efficiently delivers constant torque without the audible noise issues associated with the six-step controllers described previously. In addition, this algorithm is implemented on a new control hardware architecture that allows efficient implementation of complex controllers without any software coding.
The current sensorless controller is at the heart of an application-specific integrated design platform. Compatible chips complete the platform approach by addressing additional integration challenges surrounding the control and power electronic components. These include three-phase inverter driver integrated circuits (ICs) and high-voltage current-sensing ICs to link the digital control IC and the power stage.
Without traditional software
The current sensorless algorithm is based on the simple PMSM model shown in the “Permanent magnet synchronous machines equivalent circuit” graphic. Motor winding back-emf is a sinusoidal function of the rotor angle, so it can measure the rotor angle. Back emf is calculated by measuring the current flowing into the stator windings for the applied stator voltages. To simplify the mathematics, the three-phase circuit is transformed to a two-phase equivalent model using the Clark transformation. Rotor back emf is represented by sine- and cosine-function equations shown in the “Rotor angle and the equivalent circuit” graphic.
To extract the rotor angle, back emf terms are integrated to calculate the rotor flux function, which is now independent of speed. Finally, the ratio of sine and cosine flux-terms is independent of flux magnitude and so can be used to accurately estimate rotor angle and speed.
The angle estimator is the key to the control algorithm but many other functions are required to complete the control system described by the block diagram in the “PMSM sensorless FOC system” graphic. The controller consists of an outer speed loop that generates a torque reference and an inner stator currentloop that controls the voltage applied to the windings. The stator current control loop is implemented in the rotating reference frame using field-oriented control (FOC) techniques. A vector rotation as a function of rotor angle transforms the stator currents into two quasi-dc components, ID and IQ.
IQ current is the component in quadrature with the rotor flux and is of the torque-producing current. IQ reference input is taken from the speed-loop output. ID current—aligned with the rotor flux—and can support or oppose the rotor flux. Over most of the speed range the ID setpoint is zero. However, if an extended constant power speed range is required, ID can be set to weaken the rotor flux. This can be very useful in applications such as washers, which require very high spin speeds.
The sensorless field-oriented control algorithm is implemented on new controller architecture. Each function in the control system in “PMSM sensorless FOC system” graphic is applied using hardware macro-blocks rather than software. Functions, such as PI controller, vector rotator and Clark transformation, are common to all ac motor control systems. The graphic, “Function block library,” of ac motor control and other general-purpose coding shows one way to eliminate traditional software programming from the development process, saving time and reducing errors.
The library is available on the motor control IC and also includes functions for analog inputs and space vector PWM control. The control system developer uses a graphical tool to pull components from the library into the control system design. A graphical compiler then translates the control design into sequencer instructions that connect hardware macro blocks in the correct sequence to implement the control system.
Accurate drum-speed control is important to control wash action in front-loading and top-loading washers. Front-loading machines, used for many years in Europe, are becoming more popular in North America. Top-loading machines require total immersion of clothes in water; but front-loading washers’ tumble action only requires water to fill the bottom of the drum. Significant water reduction saves substantial water-heating energy.
In front-loading washers, drum speed determines washing action. Depending on drum radius, rotation above a certain velocity causes clothes to press against the drum. At this speed the centrifugal force balances clothes’ weight. Below that speed, clothes stick to the side of the drum until the weight along the radius exceeds centrifugal force.
At that point the clothes fall back into the base of the drum. Drum speed determines how vigorously the clothes are washed, allowing a gentle wash cycle to be selected for delicate items. In top-loading machines, agitation action is produced mechanically, using a gearbox and clutch. Speed control systems simplify the mechanical system and control the wash cycle. Control of speed and angle-of-stroke allows better management of washing action and development of wash cycles using less water.
Several motor-speed control options described earlier are available for washer applications. European front-loading washers do not use ac motors, typically using a universal “brush type” motor. However, U.S. washers use a larger drum size, which requires a motor with a power range above one motor.
Three-phase induction motors are being used today, but recently the PMSM is becoming the preferred solution. The induction motor field is supplied by current and must be generated by stator magnetizing current. Total copper losses are more than double the PM motor losses; the torque-producing current flows in the rotor and stator windings. Since the PMSM is more efficient than an induction motor, it can use less steel and copper than an induction motor with the same rating.
In recent years, the price of copper and steel has almost doubled while the cost of magnetic material has dropped, allowing higher efficiency at lower cost. Several appliance manufacturers are introducing PMSM solutions into top- and front-loading machines; some use the software library and IC to develop a controller.
An integrated design platform helps simplify the design process and lowers the cost for advanced energy-saving appliance motor-drive applications. It uses a sensorless controller IC with all the control elements necessary to perform closed-loop sensorless sinusoidal control without tedious, and error-prone, software programming of other types of DSP or microcontroller.
|Aengus Murray, director of technical marketing, Digital Control IC Center, International Rectifier,|