Abstract:
A method drives a synchronous electric motor, particularly to start a fan/exhaust fan. The method includes measuring the supply voltage value of the electric motor windings being proportional to the mass flow rate. The method also includes measuring a real load angle, by a signal coming from a Hall sensor associated with the motor rotor; performing a comparison with an optimal load angle corresponding to a reference mass flow rate of fluid being exhausted; and changing the voltage value and the operating frequency applied to the electric motor up to reach a value of the real load angle corresponding to said optimal load angle value.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates, in its more general aspect, to a synchronous electric motor particularly, but not exclusively, used in smoke suction fans mounted in wall boilers.  
         [0003]     In particular, the invention relates to a driving method of a synchronous electric motor comprising a rotor equipped with a permanent magnet, which is rotation-operated by the electromagnetic field generated by a stator being equipped with pole pieces and the relevant windings.  
         [0004]     2. Description of the Related Art  
         [0005]     As it is well known to the skilled in the art of this specific field, wall boilers are equipped with smoke extraction fans, commonly called exhaust fans, generally operated by asynchronous electric motors.  
         [0006]     Fan models operated by synchronous electric motors have been only recently introduced.  
         [0007]     Wall boilers generally comprise, besides a burner, a driving electronic box allowing the burner to be turned on and off, eventually by means of a timer, as well as the increase or decrease of the plant circulation water temperature to be regulated.  
         [0008]     Smokes produced by the burner have different physical features according to the boiler operation speed and thus, in order to optimize the boiler operation, the fan should adjust the smoke suction to this operation speed.  
         [0009]     In particular, when turning on, an initial thermal-transient step occurs, after which a steady-state step begins, which can be defined as a fixed-displacement state. During the initial step, the smokes being produced have a higher density and the fan should thus change the motor operating conditions by sucking more in order to extract smokes and allow the boiler to operate in the best way.  
         [0010]     Obviously, the same thermal transient occurs if the plant temperature is changed.  
         [0011]     It is thus typical to position in the plant, downstream the fan, a smoke flow rate regulation lock which, according to the boiler position and more particularly to the flue lodgement and flow resistance level, allows a constant-in- time smoke mass flow rate to be kept.  
         [0012]     This lock undergoes, for a good operation, a constant and accurate maintenance without which a burner shutdown and/or an extremely dangerous toxic smoke saturation in the boiler room and eventually in adjacent rooms is threatened.  
         [0013]     Too sharp changes of the load flow rate can also cause a temporary motor shutdown, which would require a sudden manual or automatic intervention for a new starting.  
         [0014]     It is also convenient that the electric motor always operates at a speed approaching the highest efficiency speed, defined by an operation point corresponding to the lowest power absorbed by the line, also resulting in energy saving for the user.  
         [0015]     In order to obtain a speed approaching the highest efficiency speed, it would be very useful to have a measure of the load, i.e., of the smoke flow rate, available any moment.  
         [0016]     Different devices for measuring the flow rate, even through continuous surveys, are known in the prior art.  
         [0017]     Generally, these flow meters are devices being positioned in correspondence with the fan deliveries and, by exploiting different principles, determining the gas volume flowing through a given section in a predetermined time, in particular a differential manostat can be for example provided, positioned between the fan suction and delivery.  
         [0018]     All these meters, although satisfying the aim of providing a flow rate measure, thus have some drawbacks.  
         [0019]     First of all, a certain area, wherein they must be housed, is to be provided on the delivery.  
         [0020]     Moreover, it must be underlined that, in order to check the good operation thereof, these devices must undergo periodical precautionary maintenance activities.  
         [0021]     Other solutions can provide the use of current sensors in order to indirectly determine the fan flow rate by sensing a higher or lower current absorption by the stator windings.  
         [0022]     Nevertheless, this solution does not provide precise and reliable measures.  
         [0023]     A further prior art solution is described in the European patent application no. 0 403 806 relating to a centrifugal pump or fan for letting a controlled-temperature fluid flow, particularly in heating systems. Sensors for determining the fluid flow rate and temperature sensors for determining the fluid temperature are provided. A control device being associated to the electric motor processes the values received by the sensors for the motor driving in order to obtain an almost constant fluid temperature.  
         [0024]     Also this solution requires expensive sensors to be managed, complicating the structure of the fan and of the device for driving the relevant electric motor.  
       BRIEF SUMMARY OF THE INVENTION  
       [0025]     One embodiment of the present invention provides a driving method of a synchronous electric motor, particularly for the fan operation, allowing all the drawbacks mentioned with reference to the prior art to be overcome in a simple and cheap way.  
         [0026]     In particular, this driving method minimizes the power being absorbed at all motor operation speeds, solving possible critical load change situations which could stall the synchronous electric motor.  
         [0027]     The driving method allows the mass flow rate of a synchronous electric motor to be kept constant when the load changes.  
         [0028]     The features and advantages of the method according to the invention will be apparent from the following description of an embodiment thereof given by way of non limiting example with reference to the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]      FIG. 1  is a block diagram of the principle of an algorithm for driving the method realized according to the present invention;  
         [0030]      FIG. 2  is a general flow chart showing the steps of a method for driving a synchronous electric motor according to the present invention;  
         [0031]      FIG. 3  shows an embodiment of a fan for a wall boiler using the method of the present invention;  
         [0032]      FIG. 4  schematically shows a synchronous electric motor for operating the fan of  FIG. 3 ;  
         [0033]      FIG. 5  shows a power regulation circuit being incorporated, according to the invention, in a synchronous electric motor driving circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     With particular reference to the example of  FIG. 1 , a block diagram of an algorithm processing principle is shown, whereon a driving method according to one embodiment of the present invention is based. This diagram is globally indicated with  1  and it is suitable to drive a synchronous electric motor  12 , as shown in  FIGS. 3 and 4 , in particular to operate a fan  8 .  
         [0035]     The fan  8  has a body  9  for housing an impeller  10  coupled to an end of a rotor  14  of the motor  12 , allowing the impeller  10  to rotate and thus smokes to be sucked by a suction duct in order to push them towards a delivery duct  11 .  
         [0036]     The synchronous motor  12  comprises the rotor  14 , equipped with a permanent magnet which is rotated by the electromagnetic field generated by a stator  16 , equipped with pole pieces  18  and the relevant windings.  
         [0037]     Advantageously, the motor  12  comprises a magnetic flux sensor  20 , for example a Hall sensor, particularly an analog sensor, positioned on the stator  16  close to the rotor  14 . The sensor  20  outputs a signal defining the momentary position of the rotor  14 .  
         [0038]     As it is known, the signal of the sensor  20  is a signal phased with the counter electromotive force E in the absence of a constant. The phase angle between the counter electromotive force E and the network voltage V provides the load angle θ of the motor  12 .  
         [0039]     In the case of the fan  8 , like in all aerial and/or hydraulic applications, the load angle θ is related to the mass flow rate of the flowing gas or fluid whose calculation formula is: 
 
 P mass=δ* A*v  
 
 Where δ is the gas density, A is the delivery duct  11  area and v is the speed of the gas flowing through this delivery duct  11 . 
 
         [0040]     Therefore, the mass flow rate depends on the gas density, on the dynamic pressure thereof in convection and/or forced-circulation in the body  9  of the fan  8  and on the gas temperature.  
         [0041]     A change of the features of the gas flowing through the body  9  of the fan  8  involves a change of the mass flow rate and thus of the load angle θ of the motor  12 .  
         [0042]     The synchronous motor  12  has for each load, and thus for each load angle θ, an ideal operation point which will be called “optimum point”, wherein the motor  12  absorbs the lowest quantity of current and it conveys the highest power to the load.  
         [0043]     This situation can be represented by cusp curves having a well defined minimum being already identified by the prior art.  
         [0044]     Letting the motor operate near the “optimum point” allows the highest efficiency to be obtained with the lowest losses.  
         [0045]     Concerning the synchronous motor  12  power supply, as shown in  FIG. 5 , a possible solution provides to rectify an alternate voltage provided by the network by means of a diode bridge followed by a capacitor stage, allowing the rectified voltage to be leveled. The so-obtained voltage is conveniently modulated by means of some switches (IGBT or power MOS . . . ), conveniently controlled according to a PWM modulation technique, which allows a voltage to be provided to the windings of the pole pieces  18 , whose frequency and fundamental sinusoid amplitude can be varied.  
         [0046]     The voltage of half the bridge would be V BUS/2  if not being connected to the load. In fact the voltage at the bridge center varies around the value V BUS/2  according to a sinusoid law with a ripple whose oscillation frequency is equal to the PWM frequency.  
         [0047]     According to the method provided by the present solution, the operating conditions of the synchronous motor  12  are kept close to the “optimum point” by means of a first and second regulation loop  30 ,  40 , schematically indicated in  FIG. 1 , both feedback operating.  
         [0048]     Advantageously, the first loop  30 , also defined as inner or faster loop, regulates the supply voltage provided to the windings of the pole pieces  18  and indicated with Vwindings, according to the load angle θ car  deviation from the optimum angle θ optimum .  
         [0049]     The optimum angle θ optimum  is drawn from experimental measures, known in the prior art, and it is indicated in convenient tables. By operating according to the “optimum angle”, the motor absorbs the lowest current according to the different loads or load powers.  
         [0050]     The load angle θ car  of the synchronous motor  12  is steadily provided, based on the signal processing by a processing unit  42  of the signal provided by the Hall sensor  20 .  
         [0051]     In particular, the 90° complement of the load angle θ car  is measured and if it is higher than the optimum angle θ optimum , the voltage Vwindings is increased on windings L 1  and L 2 , and vice versa by a load angle/voltage converter  44 . Obviously, in order to regulate the voltage Vwindings, a frequency converter can be used, based on a boost converter microcontroller.  FIG. 5  schematically shows a power circuit  24  with an active element bridge  26  which can be used to this purpose.  
         [0052]     Advantageously, the second regulation loop  40 , also defined as external slower loop, regulates the frequency f of the synchronous motor  12  according to the voltage Vwindings provided to the windings L 1 .L 2  of the pole pieces  18 , based on the first loop  30 .  
         [0053]     In particular, this second loop  40  allows the synchronous motor  12  to operate in steady conditions. As it is known in the prior art, the synchronous motor has for each load power a lower stability limit given by a lowest voltage provided to the motor according to a predetermined operating frequency.  
         [0054]     Obviously, as above-underlined, the voltage Vwindings provided to the synchronous motor  12  is related to the fan mass flow rate P mass  and proportional to this mass flow rate. A voltage/flow rate converter  46  produces a measured mass flow rate P mis  from the voltage Vwindings.  
         [0055]     Therefore, if the mass flow rate being measured (P mis ) according to the second loop  40  by the voltage Vwindings is lower than the optimum mass flow rate, indicated in  FIG. 1  with P target , then the operating frequency f of the synchronous motor  12  is increased and vice versa by a flow rate/frequency converter  48 .  
         [0056]     In particular, the reference mass flow rate P target  is, in the case of the fan  8 , the one ensuring an effective and safe combustion, i.e., obtained through a convenient concentration of burnt gases carbon monoxide, carbon dioxide and nitric oxides.  
         [0057]     Concerning the method provided by the present solution on a fan  8  with synchronous motor  12 , after an initial transient when the burner reaches a steady state, an optimum load angle θ optimum  having a mass flow rate P target  prevails. Therefore a voltage value Vwindings to be provided to the windings L 1 , L 2  of the pole pieces  18  of the motor  12  and an operating frequency value f of the motor  12  are defined by means of a frequency converter.  
         [0058]     The voltage Vwindings is regulated according to the first loop  30  so that the load angle θ car  follows the optimum load angle θ optimum .  
         [0059]     The second loop  40  will thus allow the frequency f of the synchronous motor  12  to be changed in order to keep the P target  and avoid any instability of the motor  12 .  
         [0060]     The driving method according to the present solution will be now analyzed in detail with reference to the flow chart indicated in  FIG. 2 , this embodiment is shown by way of non-limiting example.  
         [0061]     First and second timers are set for the first or inner loop  30  and the second loop or outer loop  40 , respectively (step  50 ).  
         [0062]     The first timer is decremented during operation of the first loop (step  52 ).  
         [0063]     The conveniently rectified and leveled network voltage is measured, for example by means of a diode bridge associated with convenient capacitances; V Bus  and the center-bridge voltage V cp  are then measured; and the offset voltage, i.e., the deviation of the center-bridge voltage V cp  from V BUS/2 , is calculated (step  54 ).  
         [0064]     The voltage to be applied to the pole pieces  18  and the operating frequency f are calculated for the motor  12  (step  56 ).  
         [0065]     Advantageously, the method provides to set the optimum load angle θ optimum  and thus the reference mass flow rate P target .  
         [0066]     The rotor  14  is then analyzed and, by means of the signal of the Hall sensor  20 , the real load angle θ car  is drawn.  
         [0067]     It is checked if the rotor is locked (step  58 ) and, in such a case, the power supply to the windings is interrupted (step  60 ), otherwise a check is made to determine if first timer equals zero (step  62 ).  
         [0068]     If so, then the method decrements the timer of the second loop  40  and to reset the timer of the first inner loop  30  (step  64 ).  
         [0069]     Next, a check is performed to see if it is the first time executing the second loop (step  66 ).  
         [0070]     If so, then the method increases the voltage and the frequency up to reach the network synchronism (step  68 ).  
         [0071]     Next, the method calculates the mass flow rate Pmass starting from Vwindings (step  70 ).  
         [0072]     The method further compares the effective load angle with the optimum load angle (step  72 ) and adjusts variable that affect Vwindings if the optimum load angle is not achieved (step  74 ).  
         [0073]     A check is made to see if the second timer equals zero (step  76 ), and if so, the method:  
         [0074]     resets the second timer (step  78 ); and.  
         [0075]     adjusts the operating frequency f if the present mass flow rate differs from the set flow rate P target  (step  80 ).  
         [0076]     Advantageously, the method thus provides to compare the real angle θ car  with the optimum load angle θ ott  by changing Vwindings and the operating frequency f up to reach the optimum load value θ ott .  
         [0077]     As it is evident from the above, the control method provides to keep the mass flow rate constant by changing the motor operating conditions by means of two feedback cycles; a first inner faster cycle  30  allowing the winding voltage to be changed and a second outer cycle  40  allowing the frequency f to be changed.  
         [0078]     Advantageously, it can be observed how the frequency f change takes more time than the voltage Vwindings change and particularly when the winding voltage Vwindings is steady. Therefore the synchronous motor is avoided to operate under almost unsteady conditions wherein the motor could not succeed in following the supply voltage, risking a sudden interruption.  
         [0079]     Advantageously, the above-described method allows to avoid, in the case of the fan, hood, smokes, during the initial burner transient, the load angle change from being considered as a change of the aeraulic load. In fact, when the burner is turned on, the temperature increases according to a given transient, by applying a control system as indicated in the European patent application no. 03425571.1 of the same Applicant this temperature change can be conveniently determined.  
         [0080]     Obviously, the present method can have different alternative embodiments among which the case of the digital Hall sensor can be considered.  
         [0081]     Moreover, the mass flow rate P mass  being calculated could be provided as a signal for a convenient boiler control electronic box.  
         [0082]     Advantageously, the control method being shown allows any device for measuring the flow rate to be eliminated, as well as the lock positioned in the fan delivery and the differential manostat positioned between the fan suction and delivery. In fact this method allows the smoke mass flow rate to be calculated and kept constant in time and an output signal being proportional to the mass flow rate to be provided, or simply an alarm signal when the mass flow rate is not sufficient. The boiler control electronic box will provide the extinction of the pilot flame in case of alarm or it will process a flow rate signal by modulating the gas pressure at the burner nozzles.  
         [0083]     A further advantage of the suggested control method is the complete removal of the smoke flow rate regulation lock positioned downstream the fan, thus improving the fan operating conditions with respect to the emitted smokes and considerably reducing maintenance operations.  
         [0084]     A further advantage of the suggested control method according to the present solution is the constant control of the smoke mass flow rate, allowing the combustion efficiency to be improved in the fan operation areas outside the target point, as for example during the boiler starting transient.  
         [0085]     Another advantage of the suggested control method is to allow the smoke mass flow rate to be kept constant in time by changing the motor operating conditions.  
         [0086]     Experimental tests carried out by the Applicant allowed to prove that the method according to the invention is particularly competitive from the economic viewpoint with respect to all the other techniques presently used in this field.  
         [0087]     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0088]     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.