GB2039159A - Electric motor - Google Patents

Abstract

An electric motor for controlling a fuel burner has a stator comprising a main winding (1) and an auxiliary winding (2) which is connected, e.g., in series with a temperature sensitive resistor (3) having a positive temperature coefficient. The resistor has a Curie point at which its resistance increases greatly to de-energise the auxiliary winding. The resistor allows a current to flow in the auxiliary winding for between 0.5 and 2 seconds. With the rotor stopped, the rate of increase of temperature in the main and auxiliary windings is not greater than 2.5 DEG C/second and 7 DEG C/second respectively. The P.T.C. resistor may be in parallel with a capacitor, as in Fig. 3 (not shown). <IMAGE>

Description

SPECIFICATION Electric motor The invention relates to electric motors and may find particular application in a motor which forms part of a fuel burner and a fuel-oil burner comprising such a motor.

Originally, fuel-oil burners were normally equipped with single-phase asynchronous motors including a resistance auxiliary winding for starting.

Special devices such as relays or a centrifugal coupler were used to cut out the auxiliary winding after the motor reached the operating speed.

This kind of conventional motor, which was very cheap, had electric characteristics which were quite appropriate for burner operation, i.e., a starting torque of 3 cm Kg or above and a small drift under load.

The motor was not thermally protected, but protected by a flame-monitoring device.

If the flame went out unexpectedly the monitoring device, which used a photo-electric cell, switched off the motor after a predetermined time.

The safety monitoring device was adapted to operate as soon as the motor was put into operation.

Thus, one or more successive failures of the motor to start may be sufficient for its fuel supply to be cut off, as a result of the absence of a flame for a maximum of 10 seconds in the case of domestic burners having a power less than or equal to 100 000 Kcl/h or 2 seconds for gas burners having a power greater than 60 th/h.

After the motor had been disconnected by the monitoring device, it couid not be restarted except by manual resetting.

After many years of using motors of the kind comprising a resistance auxiliary winding for starting, the burner manufacturers considered it was no longer possible to ignore the thermal state of this kind of motor.

As a result of its design, a motor comprising a resistance auxiliary starting winding has a highresistance auxiliary winding which has to be made of thin wire. Consequently the current density in the starting winding is very high and the rate of temperature increase varies between 8 and 1 5on per second. This means that if there is an accidental failure to start by the motor when hot, the temperature rise in the auxiliary winding will destroy the wire, since the response time of the monitoring photo-electric cell is too long.

Furthermore, as soon as the motor is abnormally loaded, the speed drops and the relay or centrifugal coupler is very often re-connected and supplies power to the auxiliary winding for a prolonger period without action by the monitoring device, since there is still a flame.

This inopportune event results in a decrease in the air flow rate, since the fuel flow rate is not sensitive to variations in the motor speed. The result is faulty combustion.

If the auxiliary winding is restarted for long periods, the wire will be destroyed.

Some manufacturers have thought of protecting the winding by mounting a thermal protection device on the motor.

Unfortunately this aim was quickly abandoned because of the increased cost of the device and particularly because of the difficultly of matching the protector-triggering time with the burner control device.

At present, it is very rare for the auxiliary winding to be destroyed as a result of accidental perturbations during starting. This marked improvement is because motors comprising a resistance auxiliary starting winding have been progressively replaced by more powerful motors designed with a much lower-resistance auxiliary stage, using larger diameter wire. The auxiliary stage is connected in series with a permanent capacitance. In this kind of motor, the auxiliary stage is coupled in parallel with the main stage and kept energized during operation.

The torque-speed curve of the motor shows that it reaches the operating speed without "hollows", since the acceleration is constant.

In spite of the elimination of the starting device, this motor is of course more expensive than the preceding motor. This is because a permanent capacitor has been added and there is a greater weight of copper on the auxiliary phase. Nevertheless, the price is justified relatively because of the increased reliability under abnormal operating conditions.

However, at the purely technical level, particularly with regard to operating states and efficiency under load, this method is not as suitable as the preceding method, i.e., using a motor comprising a resistive auxiliary starting winding. It is known that in the case of a fuel-oil burner, a large starting torque is essential for driving the fuel-oil pump and the pump-motor coupling. The starting torque is normally above or equal to 3 cm-Kg.

The starting torque can be expressed in the following simplified form: Cd = KI . K . Ip . Ia sin (Ip la) Rr, in which K is the effective turns ratio of the main phase to the auxiliary phase (PP/PA), Ki is a coefficient depending on the magnetic circuit, frequency, etc., when the rotor is disconnected, Ip is the current in the main phase, la is the current in the auxiliary phase, IpXa is the phase shift between these quantities, and Rr is the resistance of the rotor.

In order to obtain a starting torque of the order of 3 cm-Kg, using motors having a permanent capacitance, manufacturers have had to construct high-power engines including relatively highresistance rotors. The reason is that K is generally less than 1.5 since, above this value, the voltage at the terminal of the capacitance becomes greater than 450 V. This results in a limitation in supplies and an increase in cost of the capacitance.

The capacitance always has considerable impedance in order to limit the auxiliary current. As a result, the most commonl used capacitances are below 5 microfarads.

The phase-shift angle (Ip . aha) is always approximately 7r/2 radians due to the presence of the permanent capacitance.

The remaining two parameters are ip and Rr.

Ip depends mainly on the power of the motor. The motors thus have considerable maximum power relative to their loads, and considerable drift owing to the rotor resistance (see Fig. 1).

The increased power obtained with this kind of motor, though necessary for starting, has an adverse effect on stabilized rotation.

In Fig. 1, curve F shows that the motor has a maximum efficiency of 67% but the efficiency is only 52% when it operates on a fuel-oil burner, i.e., at the point of intersection N between curve F and the straight line representing the resistance torque.

For the previously-mentioned reasons, it is difficult to adapt the motor power for use. This kind of motor, therefore, is always used at a third of its maximum power and has low efficiency at the operating point.

The aim of the invention is to construct an economic, reliable electric motor having suitable electric characteristics for starting fuel burners and operating them under stable conditions. The invention also relates to a fuel-oil burner comprising an engine of the aforementioned kind.

An electric motor according to the invention, forming part of a burner using fuel, e.g., fuel oil, and having a stator comprising a main winding and an auxiliary winding comprising a resistor having a positive temperature coefficient, is characterised in that, in its auxiliary winding, it comprises a resistor having a positive temperature coefficient with a time of 0.5 to 2 seconds during which its ohmic resistance increases from a minimum value of a few ohms to a maximum value of several thousand ohms, i.e., a time during which a non-negligible electric current can flow through it.

The invention will be more clearly understood from the following description of a number of embodiments illustrated by the accompanying drawings in which: Figure 1 represents two groups of characteristic curves of electric motors. One group consists of rpm speed curves in dependence on the torques in cm-Kg-i.e., a torque-speed curve A of a motor comprising a resistive auxiliary winding, a torque-speed curve B of a motor comprising an auxiliary winding in series with a resistor having a positive temperature coefficient during the conduction time of the resistor according to a first embodiment of the invention, and a torquespeed curve C of a known motor having a permanent capacitance.The second group consists of efficiency curves, i.e., the resistance torque curve D of the burner in dependence on speed, the efficiency curve E of a motor according to a first embodiment of the aforementioned invention, the efficiency curve F of a known motor having a permanent capacitance and a straight line R representing the abscissa of the resistance torque of a burner using fuel, e.g., fuel oil.

Figure 2 is a diagram of the electric circuit of an electric motor according to a first embodiment of the invention; Figure 3 is a diagram of the electric circuit of an electric motor according to a second embodiment of the invention; and Figure 4 shows various operating curves of a fuel-oil burner, curve G showing the flow rate of air in cubic metres under normal temperature and pressure conditions, in dependence on the time in seconds, curve H showing the flow rate of fuel oil in kilograms in dependence on the time in hours, curve I showing the instantaneous average richness of the fuel mixture in cubic metres of air per kilogram of fuel oil and region J on the richness curve I being the place where ignition occurs.

In a first embodiment of the invention illustrated in Fig. 2, an electric motor of the kind comprising a resistance auxiliary starting winding comprises a main winding 1 permanently supplied with electric current during operation and an auxiliary of "starting" winding 2 energized only during the starting period. According to the invention, a resistor 3 having a positive temperature coefficient is connected in series with the auxiliary winding in order to deenergize it at the end of the starting-up process.

When the motor is energized, resistor 3, which is placed in series with the auxiliary winding, transmits the current required for feeding the winding.

When the current flows, resistor 3 is heated by the Joule effect and its resistance increases in dependence on its temperature.

As soon as resistor 3 reaches a temperature corresponding to its Curie point, there is an abrupt change in its ohmic value, i.e., from a few ohms to a few thousand ohms. Consequently, the starting winding 2, after this change, is in series with a very high impedance which limits the current travelling through the winding to a few milliamperes and eliminates the effect of this winding on the operation of the motor.

According to an important feature of the invention, when the motor is applied to a fuel-oil burner, the conduction time of resistor 3 must be defined in dependence on the starting conditions on the fuel-oil burner.

Figure 4 shows the instantaneous flow rates of fuel oil and air of a domestic burner equipped with a motor according to the invention. As shown in Fig. 4, ignition occurs when the richness of the fuel mixture is 0.7 or more, and the burner conditions became stable after 0.5 seconds.

The richness of the fuel mixture depends on the ratio F/O F/O St in which: F is the amount of fuel in kg, O is the amount of combustion-supporting agent in m3 and St is the stoichiometric ratio (kg/m3).

Consequently, the conduction time of resistor 3 must be greater than 0.5 seconds but need not be above 2 seconds.

Resistor 3 automatically protects the auxiliary winding and can be used according to the invention to design a motor in which the temperature of the auxiliary winding increases relatively rapidly, the rate of increase being greater than that allowed in conventional motors.

The most critical case usually occurs when the hot motor re-starts after a normal operating cycle. In such cases, the motor cannot re-start since resistor 3 is always at a temperature above its Curie point. The main winding is thus energized during the operation of the photo-electric cell monitoring device i.e., for a maximum of 10 seconds in the case of a fuel-oil burner having a power less than 100 000 Kcal/hour.

Conventional motors having class B and F insulation can withstand a peak winding temperature of about 200"C.

If the motor operates under stabilized conditions at 130"C (the maximum value accepted for B insulation) and if, for an unknown reason, the rotor is suddenly stopped (without energizing the auxiliary winding) during the 10 seconds before the cell monitoring device comes into action, the rate of temperature increase does not exceed 7"C per second in the main winding.

In a motor constructed according to the invention, a rate of temperature increase not exceeding 2.5"C per second is obtained in the main winding, the current density being 1 6 A/mm2.

The most critical case when protecting the auxiliary winding occurs during re-starting after a stoppage of about 2 minutes. This is equivalent to the time necessary for resistor 3 to change from a resistance of a few thousand ohms to a few ohms.

After two minutes, the motor changes from 1 30 C to about 90"C, thus giving a temperature difference of 110"C, relative to the maximum temperature 200"C for class B insulation, during a conduction time of up to two seconds. This corresponds to a rate of temperature increase equal to or below 55"C per second.

In a motor constructed according to the invention, the rate of temperature increase does not exceed 7"C per second in the auxiliary winding, the current density being 47 A/mm2.

The previously defined limiting values of the rate of temperature increase can be used for comparison with the values obtained with the motor according to the invention shown in Fig. 2.

As can be seen, the amount of protection of this motor is very considerable.

Rate of temperature Maximum Values increase permitted obtained values Main winding: 7"C/sec 2.5"C/sec Auxiliary winding: 55"C/sec 7"C/sec Curve E in Fig. 1 shows that the maximum efficiency of the motor according to the invention is 67%. The efficiency point P occurs exactly at the resistance torque corresponding to use of the motor on the burner.

The gain in power consumption relative to the earlier motors shown by curve F in Fig. 1 is watts or about 22%. There is also a 9% gain in the weight of sheet metal.

In a second embodiment of the invention, illustrated in Fig. 3, an electric motor of the kind comprising a permanent capacitance has a main winding 4, an auxiliary winding 5 having a capacitance in series 6, and a resistor 7 having a positive temperature coefficient and connected in parallel with the terminals of capacitor 6.

In the auxiliary winding 5, since the impedance of the resistor is very small compared with that of the permanent capacitor, the motor on starting behaves like a previously described motor having a resistance auxiliary stage.

The advantage of this embodiment, compared with earlier permanent capacitance motors used on fuel-oil burners, is that the power is just sufficient to drive the pump and turbine and the rotor resistance is low since the problems of overcoming the resistance torque on starting are solved by providing the aforementioned resistor 7.

At the operating point Q on the torque-speed curve D in Fig. 1, the motor has an efficiency of 72%, greater than that of the other solutions represented by efficiency curves at E and F.

In order to obtain a starting torque above 3 cm/kg, an efficiency greater than the efficiency of the first example represented by curve E, a very small amount of dissipated power in resistor F and a saving resulting from a reduced weight of copper and reduced capacitance, the motor according to this embodiment of the invention is constructed having the following main characteristics: An effective turns ratio of the auxiliary winding to the main winding of between 0.40 and 0.60.

A capacitance between 1 and 3 yF.

An ohmic value of resistor 7 near that of the auxiliary winding 5.

A conduction time of resistor 7 between 0.5 and 2 seconds, the rate of temperature increase in the main and auxiliary windings of the motor being below 7"C/sec and 55'C/sec respectively.

Another advantage of the effective turns ratio of the auxiliary to the main winding is that relatively low voltages are delivered at the terminals of capacitor 6. The reason is that when the motor is energized at 220 V, the operating voltage at the terminals of the capacitor is of the order of 250 V instead of 450 V in a conventional motor. This also results in reductions in the size and price of the capacitors.

To illustrate the aforementioned advantages, the following data are for (I) a known permanentcapacitance motor, (II) a motor according to the first embodiment of the invention, having the same diameter of sheet metal, and (III) a motor according to the second embodiment, having the same diameter of sheet metal: I.KNOWN MOTOR WITH PERMANENT CAPACITANCE Outer diameter of sheet metal 81 mm Length of iron 57 mm Resistance of rotor (relative to primary winding) 45 ohms Weight of copper (windings) 355 g Maximum efficiency 67% Efficiency at the operating point on a burner 52% (3 cm kg) Starting torque 3.15 cm kg Maximum torque 8.50 cm kg Effective turns ratio of auxiliary winding to main winding 1.45 Permanent capacitance 5 microfarads Voltage at terminals of capacitance 450 V Speed or rotation on burner 2860 rpm Current density when rotor is stopped:: Main winding 27 A/mm2 Auxiliary winding 5 A/mm2 II MOTOR ACCORDING TO THE FIRST EMBODIMENT External diameter of sheet metal 81 mm Length of iron 52 mm Rotor resistance (relative to primary winding) 28.7 ohms Weight of copper (windings) 360 g Maximum efficiency 67.5% Efficiency at point of operation on burner 67% (3 cm kg) Starting torque 3.35 cm kg Maximum torque (main winding) 6.60 cm kg Maximum torque (two windings) 8.65 cm kg Effective turns ratio of auxiliary winding to main winding 0.41 Speed of rotation 2860 rpm Resistance with positive temperature coefficient 50 ohms Instantaneous power dissipated in this resistance (on starting) 300 W Current density when motor is stopped: Main winding 1 7.4 A/mm2 Secondary winding 63 A/mm2 Gain in consumption relative to motor l: : 39 W Ill MOTOR ACCORDING TO SECOND EMBODIMENT Outer diameter of sheet metal 81 mm Length of iron 52 mm Resistance of rotor 27.8 ohms Weight of copper 300 g Maximum efficiency 72% Efficiency at point of operation 72% Starting torque 3.32 cm kg Maximum torque (without PTC resistance). 6.80 cm kg Maximum torque (with PTC resistance) 9 cm kg Effective turns ratio of auxiliary winding to main winding 0.46 Speed of rotation 2860 rpm Permanent capacitance 2 microfarads Voltage at terminals of capacitor 235 V Resistance with positive temperature coefficient (PTC) 40 ohms Power dissipated in PTC resistance on starting 260 W Current density when rotor is stopped: Main winding 22 A/mm2 Auxiliary winding 66 A/mm2 Gain in consumption relative to motor 1: 47 W

Claims (8)

1. A burner of fuel, e.g., fuel oil, comprising an electric motor having an auxiliary starting winding comprising a resistive impedance having a positive temperature coefficient, characterised in that the electric motor, which ensures that the burner operates under stable conditions after a time of the order of 0.5 seconds and gives efficient protection to its windings, comprises firstly a resistor having a positive temperature coefficient, i.e., a resistor taking 0.5 to 2 seconds for its ohmic resistance to increase from a minimum of a few ohms to a maximum of a few thousand to a few million ohms, and secondly, when the rotor is stopped, the rate of temperature increase in the main winding and the auxiliary winding is less than or equal to 2.5 C/seconds and 7 'C/seconds respectively.

2. An electric motor having a stator with a main winding and an auxiliary winding for starting, the auxiliary winding having associated therewith a resistive impedance with a positive temperature coefficient of resistance and with a characteristic such that at a particular temperature a large increase in the value of the resistive impedance occurs, the electrical and other characteristics of the motor being such that said particular temperature occurs when the motor reaches operating speed.

3. An electric motor as claimed in claim 2 wherein the resistive impedance is a resistor connected in series with the auxiliary winding.

4. An electric motor as claimed in claim 3 including a capacitive impedance element connected in parallel with said resistor.

5. An electric motor as claimed in claim 4 wherein the auxiliary winding has less effective turns than the main winding.

6. An electric motor as claimed in claim 5 wherein the effective turns ration of the auxiliary to the main winding is in the range 0.4 to 0.6.

7. A fuel burner having an electric motor for stabilising the operation of the burner as claimed in any one of claims 2 to 6.

8. An electric motor substantially as hereinbefore described with reference to, and as illustrated, in Fig. 2 or Fig. 3 and the associated operating curves of Fig. 1 and Fig. 4 of the accompanying drawings.