Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
1999-06-23
2001-11-06
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S431000, C318S254100
Reexamination Certificate
active
06313597
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to vacuum and floor cleaners that are controlled by electronic controllers. In particular, it relates to such cleaners where the speed of the appliance is controlled during a starting period.
2. Description of Related Art
Vacuum cleaners largely fall into two broad categories. Firstly, those in which the whole cleaner is moved across the surface to be cleaned: such cleaners are normally referred to as “upright” cleaners. Secondly, those in which the main body of the cleaner is connected by a flexible hose to a cleaning nozzle that is moved across the surface to be cleaned: such cleaners are normally classed as “cylinder” cleaners. A variant of this second category has a centrally located, static body and a system of ducts and hoses to provide local cleaning. In each case, cleaning action is in part provided by suction that is produced by a fan unit driven by an electric motor.
Traditionally, the electric motor used in vacuum cleaners is a series-commutated motor with a wound armature and either an energizable winding or a permanent magnetic field. Such motors are well documented in the art, e.g. in “Electric Motors and Drives”, Hughes, Heinemann Newnes, 1980, which is incorporated herein by reference. A typical torque vs speed characteristic of this type of motor is shown in
FIG. 1
, from which it can be seen that the torque is relatively high when the motor is initially connected to the supply and falls off as the speed rises. A typical torque vs speed curve for a vacuum cleaner fan is also shown in
FIG. 1
, showing that the load presented by the fan is low at low speed but rises rapidly with speed. The difference between the two curves represents the accelerating torque that is available at any speed to accelerate the load. Hence, it follows that the fan will accelerate rapidly when the motor is initially connected to the supply but that the acceleration will fall as the curves come together and the motor will run stably at the speed where the curves cross.
Because the accelerating torque is high at standstill, and because there is a significant inertia associated with the rotor of the motor and the fan unit, there is considerable torque reaction at start-up. While this is not so much a problem with upright cleaners (because the mechanical layout is generally such as to cope reasonably well with the reaction torque), it is more troublesome in cylinder cleaners since the torque reaction causes the body of the cleaner to rock sharply as the motor starts. This can be at least a cause for user annoyance or even a source of danger to the user if the cleaner rolls over. With increasing suction requirements in cleaners, and hence more powerful motors, this is becoming a greater problem.
Similar problems occur in rotary floor cleaners where a rotating brush or mop is used to clean or polish a floor surface. The transient torque reaction produced can be annoying or even dangerous for the user because the machine can swing out of control when it is started.
Rudimentary forms of speed control for cleaning appliances have been available for many years and generally take the form of thyristors or triacs which are used to phase control the alternating supply voltage over a limited range. Typically these will allow the user to reduce the speed from 100% to some lower level, e.g. 70%. However, because of the inherent high starting torque of the motor, these forms of speed control are not particularly effective in controlling the starting transient.
One solution adopted for rotary floor cleaners has been to replace the series motor with a 3-phase induction motor driven by an inverter, e.g. as described by Kumaki in U.S. Pat. No. 4,992,718, which is incorporated herein by reference. In order to avoid a high starting current being drawn, Kumaki proposes a complex method to reduce the starting current for a given period of time using a microprocessor. While this proposal for reducing the starting current will have the incidental effect of reducing the starting torque, the system has no way to measure the speed and therefore no control over the speed at which the torque of the motor is varied.
There is therefore a need for a simple system capable of starting a floor cleaning appliance so that the transient torque is reduced to an acceptable level over the speed range from standstill to the working speed.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a floor cleaning apparatus comprising cleaning means, a switched reluctance drive having a rotor for driving the cleaning means, a controller for controlling energization of the drive and means for determining rotor speed of the drive and feeding a signal indicative of the rotor speed to the controller, wherein when the apparatus is switched on initially the controller is operable to vary the energization of the drive as a function of the measured rotor speed from a first value to a second higher value, thereby to control the torque produced by the motor during start up.
The controller may be operable to vary the energization smoothly from the first value to the second. For example, the controller may be operable to vary the energization level according to the following relationship:
E=a&ohgr;
n
+b
where E is the demanded energization level, &ohgr; is the rotor speed, a is an empirical constant, n is an index describing the shape of the torque curve of the fan, and b is a constant representing the required value of energization level at zero speed.
The controller may be operable to vary the energization in steps between the first value and the second value. The steps are preferably stored in a look-up table.
The energization level of the drive may vary from the first level to the operating level in a single step. In order to achieve this, the controller may comprise a first resistor connected to a reference voltage, a second resistor connected to the first resistor, a third resistor connected to the second resistor and another voltage, a microprocessor connected between the second and third resistors and an output between the first and second resistors for supplying a control signal to the drive, the microprocessor being operable to set its output impedance to substantially zero when the apparatus is switched on and subsequently set its output impedance to a high value when the rotor speed reaches a predetermined value. In this way, when the microprocessor has its impedance set to substantially zero, the controller provides a relatively low level of control signal at said output and so produces said first value of energization, and when microprocessor impedance is set to the high value, a control signal sufficient to produce the second value of energization of the drive is provided.
Preferably the controller is operable to vary the energization level by varying current demand of the drive.
Preferably, the apparatus is a vacuum cleaner. Typically, the cleaning means comprises a fan for creating suction and means operably connected to the fan for removing debris from a surface being cleaned. Alternatively, the apparatus may be a floor polisher. Typically, the cleaning means comprises a polishing pad or a brush head or a mop head.
According to another aspect of the invention, there is provided a method of operating a floor cleaning apparatus having cleaning means driven by a switched reluctance drive having a rotor operably connected with the cleaning means, the method comprising: supplying the drive with a first energization level to drive the apparatus at a first speed when the apparatus is switched on, measuring the speed of the rotor, and increasing the energization level supplied to the drive as a function of the measured rotor speed.
The energization level may be varied smoothly as a function of the measured rotor speed. The energization level may be increased according to the following relationship:
E=a&ohgr;
n
+b
where E is the demanded energization level, &ohgr; is the rotor speed, a is an empirical constant
Elliott Patricia M.
Slater Howard J.
Dicke, Billig & Czaja P.A.
Ro Bentsu
Switched Reluctance Drives Limited
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