Last year, the switched reluctance motor gained a high profile when Maytag announced that it had used the technology in its Neptune line of front loading washing machines (see Design Engineering, December 1998). This year, US-based Ametek Lamb Electric has introduced the Infin-A-Tek motor-blower, the first switched reluctance (SR) electric motor-blowers for the floor care market.
The first models are designed for larger commercial floor care products and systems such as central vacuums, larger canister vacuum cleaners, and scrubbers. But long-range plans at Lamb Electric, however, will have Infin-A-Tek motors being made available to the company’s full line of floor care equipment – from industrial commercial to household vacuum cleaners.
Switched reluctance motors, though not commercially viable at the time, first became a motor design option in the 19th Century. Back then, the motors were being used – albeit without much success – in early locomotive design and development.
Many of the obstacles confronted then – excessive vibration, electromechanical interference (EMI), and high cost – were the same ones that had to be overcome before Ametek was able to offer its updated version of SR design.
Switched reluctance motors are viewed in some corners as a hybrid of series universal motors and brushless direct current (DC) motors – and they present to customers the best of the two technologies.
This generation of SR motors have neither the carbon brushes of series universal motors nor the positioning and drive magnets of brushless DC motors. These SR motors have a smart electronics package that helps drive an unwound rotor (or armature) through a wound stator (or field).
Switched reluctance motors have a life expectancy of three to five times that of a series universal motor. This long life in the Infin-A-Tek motor is made possible by a number of design features, some of which are unique to switched reluctance technology in general and others that are specific to Ametek Lamb Electric.
The Infin-A-Tek motor’s microprocessor-based controller requires no carbon brushes. These brushes, in addition to needing replacement, also produce carbon dust that can affect air quality.
In fact, the closest items to `consumables’ contained in these Ametek motors are the electrolytic capacitors that regulate electrical flow in the microprocessor motherboard.
The extended life of the Infin-A-Tek motor is also enhanced by the bearing lubrication and cooling features built into each model. A patented air seal bearing protection system keeps the sealed ball bearing and its lubricants free of moisture. In other universal motors, such contaminants as moisture and detergents can degrade the integrity of the bearings. The Ametek Lamb Electric patented cooling system, helps keep the entire INFIN-A-TEK motor running smoothly and efficiently.
With the first generation of these SR motors being designed as tangential discharge bypass motors, working air cannot be used to aid the cooling effort. This is because the air is drawn into the working-air fan system through the opening in the bottom of the motor and is collected and discharged tangentially.
Such an operating system means the motor must be cooled in another way. In the motor, this meant the use of a fan, mounted below the rotor core on the shaft, to move air through the upper part of the motor to cool the electronics package, the rotor and stator, and even the sealed ball bearings.
Effective cooling is made even more critical in switched reluctance motors due to the tight air gap tolerances that are key to basic SR design. The fact is that fields and armatures that are kept cool run more efficiently.
In the motor, precise alignment is enhanced by a series of construction concepts borrowed from Lamb Electric’s cup-style design of its Worldlamb motor-blower.
In a cup-design motor, the cup, bracket, stator, and cooling fan system all are linked together into a stabilising outer frame for the rotor and bearing. This differs greatly from many conventionally designed series universal motors in which all of the above parts are stacked, one on top of another, and are susceptible to the sliding and shifting that can compromise bearing alignment.
Precision alignment of the entire motor also helps the bearings run cooler, enhances bearing alignment, reduces bearing noise, and makes motor balance operations during final assembly much quicker and easier.
One other feature that not only contributes to cooling but also adds to the overall robust construction of the motor is the cast aluminum bracket and housing. This heavy-duty casting increases structural stability and acts as a thermal heat sink, drawing away some of the heat created by the stator windings and the electronic controller.
Another key design concept of SR motors that helps make them robust is the simplicity of the motor. The rotor is simply a stack of laminations mounted on a shaft. The simple, unwound rotor replaces the armature, a complex system of wound coils connected to a commutator, and eliminates many potential failure possibilities.
The geometry of both the rotor and stator are critical to the performance of an SR motor. To make the motor run, current is applied to two opposite coils of the stator, setting up a magnetic flux or inductance across the air gap with the rotor. The rotor wants to align itself to maximise this inductance, and thus, produces torque. As the rotor approaches this aligned position, the adjacent set of stator coils is energised, forcing the rotor to move yet again. This process repeats several times per revolution.
The switching of electrical currents in the stator is controlled in the motor by an onboard electronics package that sits on top of the rotor/stator combination. The microprocessor-based package is able to sense the position of the rotor and can begin the electronic switching of current to slowly start the motor and to control the speed and torque during the performance cycle.
Most slow starts in other brushless, electronically controlled motors require positioning magnets on the rotor to indicate the location of the initial excitement current. This slow start, or soft start, helps protect the motor from high inrush current.
Onboard electronics also offer the feature of a simple two-wire connection (including ground) for a single-unit device. Many other electronically controlled motors require a separate – and in most cases, bulky – control unit be mounted in the end-product and be separately linked to the motor and to an external power source.
Ametek designers have also shown that they can offer such possible add-on features as performance shaping for peak power and optimum performance in specific applications. The motor can sense external conditions (such as overloads or blockage) and boost the power or shutdown. There is a separate, on-demand, `boost power’ control or button, external speed control with a potentiometer, a speed/tachometer output and display, remote on/off switching and control, and a timer to monitor the amount of `on’ time of the motor.
Ametek Lamb Electric Tel: +1 330-673-3451