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Motor Control Platform Cleans Up
Start with electricity, add water and save on both
Randy Frank, Contributing Editor -- Design News, October 23, 2006
The washing machine and dryer combination are among the larger consumers of energy in a typical home. In addition to energy, the washing machine also consumes large amounts of water. Conserving both energy and water has made more efficient washers and dryers a target of government agencies, such as the U.S. Department of Energy (DOE), and energy-conscious consumers. As a result, designs are transitioning in several ways.
The most obvious external change results by switching from top-loading to front-loading washers. The typical top loading machine in North America has an induction motor and gearbox. Older machines use an electromechanical control with relays to reverse the direction of the induction motor to provide agitation.
Current front loading machines use induction motors, too. The induction motor is quite large with a tachometer on the end for speed feedback. Using this machine as the reference point, an estimated 15 percent cost reduction can occur when washing machine manufacturers switch to permanent magnet (PM) synchronous motors and direct drives.
Earlier washing machine designs use gears and belt drives. However, manufacturers are moving to direct drive permanent magnet motors for both front-loading and top-loading machines. While the idea has been around for a long time, it has started to gain popularity. For example, LG Electronics makes the PM motor used in its machines. “Now that LG has started to make quite of a headway in Europe with their direct drive front loading machine, European companies are saying we should be looking at this,” says Aengus Murray, director of iMOTION product management, International Rectifier (IR). IR developed a motion control platform to specifically control permanent magnet motors in washing machines.
For some manufacturers, the transition to direct drive is occurring in two stages. In the first phase, a permanent magnet motor replaces the existing induction motor. Even though the magnets add cost, the PM motor is more efficient, uses less material and now is less expensive because of the reduced amount of copper and steel. Phase two implements direct drive and the design eliminates the gear and belt system entirely. The complete cost reduction comes after implementing the second stage.
Electrical Gears
Washing machine manufacturers can use a direct drive because of a large number of poles in the PM machine. “By having 36 to up to 50 poles, you build the gearing ratio into the motor,” says Murray. With two poles, the speed range of the electrical frequency is only cut in half. The pole pair number divides down the electrical frequency to the mechanical speed range.
Multi-pole motors gear down the speed and gear up the torque. It turns out the washing torque requirement is quite high. “You need something like 20 Nm of torque but you don’t need it at a very high speed,” says Murray. “Essentially the larger number of poles in the direct drive motor is the equivalent of replacing the gearbox by building it into the motor.” An advantage of a motor directly coupled to the drum is better dynamics, since the gearbox and belt drive are eliminated. “What we are seeing now is that washing machine companies want tighter control on their acceleration and the target speed because one of the ways they are saving water is by paying a lot more attention to the washing action,” explains Murray.
Manufacturers have found they do not need to turn the clothes as much but by agitating the water, they force the water to push up through the clothes. “The laundry action happens when you force the water through the cloth,” says Murray. By controlling the speed, the acceleration and the deceleration rate of the agitator, the way the water goes up through the clothes changes, requiring less water in the drum.
Sensorless Operation
Driving any PM motor requires knowing the position of the rotor. Designers need to know where the rotor magnet is so they can input the right amount of current when the magnet is underneath the coil. With the sensorless solution provided by IR’s platform, in addition to the cost savings of eliminating the Hall Effect sensor, improved reliability results by avoiding misalignment issues and failures.
With the PM motor, the field is reduced by the magnets in the rotor. In a very simple control, eventually the back electromotive force (EMF) generated because of the magnets starts to increase with speed and eventually the back EMF approaches the bus load. Once the back EMF equals the bus voltage, no further current can be supplied and the speed cannot increase. This is known as the base speed. “To go beyond this speed you have to be able to manage the phase of the current,” says Murray.
One of the aspects of the control algorithm in the IR platform is the field oriented control. Measuring the motor current and knowing the rotor position, allows the determination of the component of current providing the torque. Phase advancing the current determines the component of current acting against the field magnet. This component is used to reduce the back EMF and increase the motor’s speed, a process known as field weakening. The trade-off is a lot less torque because the magnet strength is reduced. Fortunately, in the spinning operation, torque requirements are minimal, since the machine is just overcoming inertia and there is not a lot of friction loss. The approach is particularly effective in the washing machine for increasing the spin cycle speed.
| Induction Motor | Direct Drive | |
| Mechanical parts | (Pulley and Gears)55% | (Permanent Magnet) 50% |
| Electrical parts | 45% | 50% |
| Cost Basis = 100% | Cost Reduction = 15% |
The design challenge was demonstrating that field weakening worked reliably during high-speed operation and also provided very reliable operation at the low speed where a high level of torque is required. Operation at medium speed is not a problem. “Most big washing machine companies said, 'We don’t believe you can really do this. Here is a washing machine. Show it to us, please.’” says Murray. By optimizing the start up process in the algorithm, cycle reversing can occur within a few hundred milliseconds instead of the time that washing machine manufacturers expected. The algorithm can reliably start and run at about 5 percent of the base speed, which brings the operation well within the washing machine speed range.
Since there is no back EMF to measure at start up, the operation is open loop. The transition to closed loop operation occurs at a fixed frequency. There is some work in characterizing and tuning the load that manufacturers must perform in the development phase for the start up mode. In the tuning process, the manufacturer must make sure that there is enough torque for the maximum load — then the operation is acceptable at the lower loads. Murray assures, “We have been talking to customers for the last few years and we have tailored the solution to make sure we can solve those problems.”
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| The IRMCF341 appliance motor control IC permits sensorless operation of a permanent magnet synchronous motor. A signle shunt resistor allows motor phase current reconstruction. The unit contains a microcontroller, a motion control engine (MCE) and an embedded analog signal engine (ASE). |
| Useful Links | ||
| //Check out the links below for more info// | ||
| Get more information on International Rectifier’s integrated washing machine technology . | Get more information on Energy Star clothes washers . | |
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Motor Control Platform Cleans Up
Start with electricity, add water and save on both
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