PRIOR ART

Ceiling fans are widely used in countries with tropical and semi-tropical climates. They are used extensively for most part of the year in tropical countries. Existing ceiling fans use almost exclusively single phase AC induction motors with external speed regulators. These motors are not very energy-efficient. Further there are losses in the speed regulators when the fan is run at less than the maximum speed. Hence there is considerable scope for saving energy by replacing the single phase ac induction motors with brushless DC motors. However for widespread adoption such a motor should have four important attributes: 1) the motor should have a very high efficiency  2) the motor should have a high power factor so that the mains supply is used efficiently with least harmonics  3) the motor should be capable of being supplied from the standard 2-wire [phase and neutral] supply available everywhere i.e. no change in existing wiring  and 4) the motor system should be cost effective.

An outline of the existing single-phase ac induction motor based ceiling fan is shown in Fig. 1. The single-phase ac induction motor is not energy efficient in low power applications such as ceiling fans. Another disadvantage with existing fans is that at a given regulator position the speed of the fan varies when the mains supply voltage or the mains supply frequency varies. In developing countries it is a fact that mains supply fluctuates in both voltage and frequency. This leads to undesirable speeding up or speeding down of the fan.

It is desirable to have a motor for ceiling fans which overcomes the above and other disadvantages in the prior art  and which has a low manufacturing cost. This goal is achievable with the present invention.

OUR INVENTION

Brief description of the invention

The present invention is suitable for driving ceiling fans having blades of various lengths.

Fig 2 shows an outline of the scheme of a motor drive system based on brushless DC motor for ceiling fans. The motor consists of an inner stator and an outer rotor. The electronic controller is a micro-controller based system consisting of a switched mode power supply with integrated active power factor correction [101 – 107]  an inverter [111] and a control system [110] to control the motor. The controller is an enhanced version of the controller described in the patent description of Ref [1]. Only the improvements from the controller of Ref [1] will be described here. The electronic controller is embedded within the motor body.

The motor in the present invention is a highly efficient brushless DC motor without any embedded rotor position sensors and uses ceramic magnets [204].

In the present invention the combination of stator slots and rotor poles and the magnet shape are designed to yield a back-emf waveform close to the ideal trapezoidal waveform. This design results in smoother torque and lower input current.

The usual method of starting a sensor-less brushless DC motor does not work in a ceiling fan application as ceiling fans are not as rigidly mounted as other machines for example a compressor or a blower. In the present invention the electronic controller detects the rotor position at rest accurately without the aid of any external position sensor. This is crucial to reliable starting of the fan as conventional methods of starting a sensor-less brushless DC motor result in oscillation of the fan and unreliable starting.

A novel aspect of the present invention is the design of the construction of the outer rotor back iron. In conventional designs the rotor back iron would be an annular iron ring of large diameter to the inside of which magnets are fixed. Such a ring is difficult to manufacture in a cost effective way. So a novel idea was thought about in which the rotor back iron is constructed from arc segments [205] that are formed steel strips of desired dimensions. When the back iron is made using the segments  a gap is necessary between adjacent segments for assembly. These gaps can result in reduction of magnetic flux which can impair motor performance. A novel idea in this invention overcomes this probable deficiency by taking advantage of a “null flux” zone around the center of a permanent magnet where there are no flux lines.

In this invention an infrared sensor is embedded in the motor and the wires are taken through the hollow shaft to connect to the electronic controller. This enables control of the fan using a hand-held remote control unit. This feature eliminates the standard fan speed regulator leading to further energy saving. This also makes fan control very convenient and thus enhances the user experience.

The present invention can be supplied power in the same manner as a conventional ceiling fan motor. Thus the existing wiring can be used.

Detailed description of the invention

The motor drive system consists of

1) a sensor-less three phase Brushless DC motor consisting of an outer rotor with ceramic magnets  an inner stator  and provisions to mount fan blades.
2) an electronic controller consisting of a switched mode power supply with integrated power factor correction  an inverter  a micro-controller based control system with firmware to control the brushless DC motor  and an infrared receiver. The controller and the infrared receiver are embedded within the motor body.

Motor

The motor in the present invention is a highly efficient brushless DC motor without any embedded rotor position sensors. The motor uses ceramic magnets [204] which are cost effective  can withstand high temperature and are not susceptible to demagnetization in operation. The absence of rotor position sensors and the use of ceramic magnets render the motor rugged and cost effective.

The wave form of the back emf voltage generated in a BLDC motor should be  ideally  trapezoidal in shape for best performance i.e. it should be constant between 300 electrical angle and 1500 electrical angle. In the present invention the combination of number of stator slots (S) and rotor poles (P)  and the magnet shape are chosen such that the resultant back-emf waveform is close to ideal. This design results in smoother torque and lower input current and thus lower power consumption.

The magnet shape and Slot/Pole combination is designed to cause a pronounced variation in stator inductance with respect to the rotor position. This is done in order that the electronic controller can generate distinct current pulses in the motor at rest and can detect the rotor position accurately using the pulse magnitude values. The rotor position detection is described more elaborately in the section on electronic controller.

The motor body is made of aluminium which does not have a high magnetic permeability. Hence a separate rotor back iron is required. In the prior art the rotor back iron is either an MS ring. Such a ring is difficult to manufacture in a cost effective way. These rings are generally made from the tubes of steel by cutting them to desired lengths and they are machined to get the required diameters on the inside and the outside. This process requires a cutting operation and a machining operation. These operations involve considerable costs and wastage of material. So a novel idea was thought about in which the rotor back iron is constructed from arc segments that are formed steel strips of desired dimensions. The segments are formed to the required shape using simple and cost effective tools. These steps result in a cost effective back iron.

The rotor back iron is made by fixing the segments [205] onto the rotor aluminium body as shown in the Fig 3. When the back iron is made using the segments a gap is necessary between adjacent segments for assembly. These gaps can result in reduction of magnetic flux which can impair motor performance. A novel idea in this invention overcomes this deficiency by taking advantage of a “null flux” zone around the center of a permanent magnet where there are no flux lines. The idea is to arrange the magnets such that the null flux zones coincide with the gaps between the segments as shown in Fig 4. This unique method ensures that there is no loss of flux and thus motor performance is undiminished. Fig 3 shows a section of the rotor with the segments. The magnets [204] are fixed to the inner sides of the segments. A section of the arrangement with the magnets and segments is shown in Fig 5 illustrating the coincidence of the center of the magnet (null flux zone) with the gap between segments.

The present invention embeds an infrared (IR) receiver in the bottom of the motor body. A plastic sensor holder is designed to hold the printed circuit board containing the IR receiver circuit. The sensor holder is as shown in Fig 6. It has a split stem which is flexible and has ribs around them for gripping the shaft. The printed circuit board is fixed onto the bottom of the sensor holder with the help of two tight fitting pins without any fasteners as shown in Fig 6. The sensor holder with the printed circuit board is then fixed firmly onto the motor by inserting its stem into the hollow bottom of the motor shaft. This method does not use any fasteners.

There is a translucent cover  shown in Fig 7  affixed to the bottom of the motor cover. This protects the IR receiver printed circuit board and allows the IR signal from the remote control to reach the IR receiver. This cover has snap fits  as shown in Fig 7  that mate into the slots provided in the motor body  as shown in Fig 8  to get fixed onto the motor without any fasteners.

In the present invention the electronic controller is mounted inside the motor body. A plastic PCB holder  shown in Fig 9  is designed to be mounted on the stationary motor shaft without any fasteners and holds the controller printed circuit board. The PCB holder has a hollow shaft. A draft angle is given to the inside of this shaft to make it slide over the motor shaft during assembly. The design provides a snap pin on the PCB holder and a hole on the motor shaft to retain the PCB holder in the desired position  as shown in Fig 9. The controller printed circuit board is slid into the PCB holder as shown in Fig 10  till the snap pins provided on the PCB holder engage with the corresponding holes on the controller printed circuit board. This ensures firm mounting of the controller printed circuit board.

Electronic controller

The electronic controller used in the present invention is an enhanced version of the controller described in the patent description of Ref [1]. The primary enhancement in this controller as compared to the one described in Ref [1] is the inclusion of a startup method specifically for ceiling fans. Another enhancement is the provision for interface to an IR receiver in both hardware and software.

Ceiling fans have a signification moment of inertia and multiple degrees of freedom.
The conventional method of starting a brushless DC motor involves exciting one of the phases such that a large current flows for an interval of time aligning the rotor to a known position. Then the controller excites the phases in a predetermined manner to get the rotor rotating. In a ceiling fan this method does not work due to the inertia and the degrees of freedom present. Such a method of starting leads to oscillations. This often causes a failure of the fan to start rotating. Thus a different method is required.

In the present invention the controller detects the rotor position without energizing the motor with a large current. This is done by energizing the motor phases [201 202 203] with short voltage pulses in various combinations using the inverter [111]. There are six combinations in which the phases can be energized. The resultant current pulses differ in magnitude depending on the rotor position. Fig 11(a) and Fig 11(b) show six voltage vector combinations and the resultant current pulses respectively. The current pulses vary in magnitude as the winding inductance varies with rotor position. The controller measures the magnitudes of the current pulses and identifies the rotor position with the voltage vector corresponding to the current pulse having the highest magnitude. This completes the detection phase of the startup. The controller then energizes the stator windings with the succeeding combination of voltages and proceeds with the manual commutation in the usual way. The magnet shape and the slot/pole combination are designed such that the current pulses are distinct in magnitude. Thus the controller and the motor are synergistically designed to start the ceiling fan reliably and smoothly.

The method of detecting rotor position based on inductance variation is a known method in sensor-less brushless synchronous permanent magnet motors (Ref [2]  Ref [3]). It has been used in other applications such as hard disk drives (Ref [4]). In this method a constant bus voltage is assumed for the inverter [111]. However as explained in Ref [1]  in an SMPS with single-stage power factor correction the bus voltage varies considerably. This can result in erroneous position detection because of unequal voltage vectors as the bus voltage is varying. In the present invention this problem is overcome by measuring the bus voltage concurrently when the voltage vector pulse is energized and adjusting the voltage pulse width to compensate for the bus voltage variation. This virtually eliminates the effect of bus voltage variation. To further enhance the reliability of position detection  taking advantage of the short time of the voltage vector application (a few milliseconds)  the detection method is executed a few times and the voltage vector corresponding to the maximum number of results in agreement is chosen.

The IR interface capability allows the use of an IR remote to control a ceiling fan driven by the motor of the present invention. Using an IR remote the user can set the speed of the ceiling fan. This obviates the necessity for a standard fan speed regulator and thus eliminates the losses in the regulator which occur when the fan is running at a speed lower than the full speed. As a consequence the efficiency of the fan increases.

The IR receiver printed circuit board described earlier has a light emitting device (LED) on board to indicate the fan status to the user. When the fan is powered on and at rest the LED is lit up continuously. If the fan is powered and is at rest there is still residual power consumption. The LED indicates this. The user can switch off the power to the fan to avoid the residual power consumption.

While the invention has been described in what is presently considered to be an embodiment  many modifications  variations and arrangements will become apparent to those skilled in the art. It is intended  therefore  that the invention not be limited to the disclosed embodiment but that it be given an interpretation commensurate with the appended claims.

References:

[1] Patent application number: No.2873/CHE/2009 A
Title: An electronic controller for a brushless DC motor
Publication date: 04/12/2009
Inventors: K. Durgasharan  M. Sundararajan

[2] Nobuyuki Matsui  “Sensorless PM Brushless DC Motor Drives ” IEEE
Trans. Industrial Electronics  vol.43  no.1  1996  pp.300-308

[3] S. Nakashima  Y. Inagaki and I. Miki  “Sensorless initial rotor position
estimation of surface permanent magnet synchronous motor ” IEEE
Trans. Industry Applications  vol. 28  No. 1  2000  pp. 1598-1603.

[4] Wook-Jin Lee  Seung-Ki Sul   “A new starting method of BLDC motors without
position sensor”  Industry Applications Conference  2004  39th IAS Annual Meeting.
Conference Record of the 2004 IEEE  pp. 2397 – 2402  vol.4

CLAIMS:
We claim
1. A cost-effective sensor-less three phase brushless DC permanent magnet motor optimized to run a ceiling fan in an energy efficient way.
2. A novel design of the outer rotor back iron in the motor of claim 1 which simplifies the manufacturing processes and thus reduces the cost of the motor.
3. A proper selection of combination of number of stator slots and rotor poles  and magnet shape such that a constant back EMF/phase is generated over a wide angle resulting in smoother torque and lower input current and thus lower power consumption.
4. An enhanced electronic controller based on the electronic controller of Ref [1]  supplied from a single-phase AC supply  driving the motor of claim 1; the enhancement consisting of a) an intelligent method to detect the rotor position accurately with bus voltage compensation  without the use of any external position sensor  and thus start the ceiling fan smoothly using the motor of claim 1 and b) incorporating an interface and the requisite software to connect to an infrared receiver.
5. A conscious design of the magnet shape and the slot-pole combination of the motor of claim 1 which aids the electronic controller of claim 4 in detecting rotor position accurately without external position sensor and starting the ceiling fan smoothly.
6. A constant speed operation of the ceiling fan regardless of the fluctuations in supply voltage and supply frequency due to the regulation of motor voltage by the electronic controller of claim 4.
7. An innovative method of locating and mounting the PCBs of the electronic circuit  firmly  using simple and cost effective plastic parts without the need for fasteners.