GB2296112A - Speed controller for a single phase AC motor in a water heating system - Google Patents

Abstract

A water heating system comprising a pump driven by an ac motor includes a coil switching device which automatically switches between high, low and intermediate speeds, according to the output of a temperature sensor attached to the return feed to the boiler. Changes of temperature are detected using a "staircase" window comparator. When the voltage across the sensor enters each window in turn a transistor operates a relay and simulates the operation of the original manual switch, altering stator coil connections so that different speeds can be produced. The invention includes a manual override switch so that the speeds can be controlled manually as well as when switched to "Auto".

Description

SPEED CONTROLLER FOR A SINGLE PHASE AC MOTOR SECTION 1 INTRODUCTION AND OBJECTIVE 1.1 Introduction This feasibility study addresses specific problems imposed by the inherent limitations of small and medium sized, pumped central heating systems. The distribution of heat by water circulation is poorly regulated, and consequently inefficient. The boiler, is unable to regulate its power output except by repeatedly switching ON and OFF.

The "engine" runs either at full power or tickover. Temperature controls on the boiler and system are imposed by ON/OFF thermostats. Consequently repeated cycling on and off eventually results in a system running at the required temperature.

Boiler design is very advanced and no attempt is made here to review their design or operation. If the water feed rate were itself regulated, in order to maintain preferential conditions for maximum heat transfer across the boiler heat exchanger, then improved system operation should be expected. In particular this would apply to any average sized household making use of thermostatic radiator valves (TRV's), or similarly equipped larger office premises. In these systems the capacity of the system effectively changes as room temperatures achieve their intended levels. A cold system may be supplying typically between ten and twenty radiators, but as TRV's regulate and then isolate, the overall capacity will reduce until only two three radiators may be in use.A single speed, off the shelf circulating pump may be well matched for a nominal sized system, but may have a speed inappropriate for the changing demands of the system.

1.2 Objective The objective of this project is to devise a means of optimising the circulating efficiency of a hot water system by automatically varying the pump speed, according to the changing thermal load on the heat exchanger.

The final design will result in reduced boiler cycling (extending boiler life and reducing maintenance), increasing economy (lower running costs) and reduced hydrodynamic noise. The prototype materials will cost approximately 50 to build, subsequent models will cost progressively less, until a production build cost of less than 25 is achieved, thereby allowing the manufacturer's mark up to be confined within the limit of a reasonable sales value. This device will then be available as an accessory that can be fitted with DIY levels of expertise, to virtually all small to medium sized pumped central heating systems.

1.3 The completed device will be known as the " P-TEC ", an abbreviation of Pdmp speed Thermo Electronic Controller. During development the prototype build standards will be defined by a suffix as follows.

eg: 1st prototype (design only) P-TEC.01(d) 2nd prototype (design only) P-TEC.02(d) 3rd prototype (build) P-TEC.03 The production model will be launched with the name WIZARD.

SECTION 2 JUSTIFICATION 2.1 The majority of domestic water heating systems all follow the general principles illustrated in Figure 1, page 5. Configurations do vary, but in almost every case the installer selects a coarse range circulating pump according to the approximate capacity of the final system. The wide range of possibilities is reduced by having a standard range of circulators, each of which can be preset to one of two, three or even four speeds. Generally the choice is very simple. Installers of domestic two or three bedroom house circuits favour a particular model or size, and usually preset the fastest pump speed, with little regard to the merits of slower speeds. By using the fastest speed the assumption is made that the system will waste no time in raising room temperatures.

2.2 The motors themselves are small ac single phase induction motors, based on a squirrel cage design. Speed adjustment is effected by manually selecting the number of stator pole pairs driving the rotor, or by changing the interconnection circuit between windings. Starting is ensured by the use of a starting capacitor in series with a seperate stator coil, together wired across the supply and in parallel with the main coil(s).

The motor/pump manufacturer enjoys a virtual monopoly and can boast that they are a "household name". An automatic speed control system has just recently become commercially available, but by employing a variable frequency inverter is expensive and only available for large capacity (office and industry), applications.

2.3 Measurements were made of temperatures around an eleven radiator domestic heating system, using twenty thermocouples. Some of these results are tabulated in Tables 1 to 4. The survey exposed shortcomings in the way the system was designed and installed. The tables compare measurements of heating applied to the hot water circuit and to the central heating circuit, both at two pump speeds. The results illustrated are not in themselves conclusive, other than to show a marginal reduction in the time the central heating water is raised 37 deg C (55 minutes) using pump speed 1, as compared to the 35 deg C rise (59 minutes), using pump speed 3. Added to this observation is the fact that pump speed 1 consumed less electrical power, 170 mA 40 Watts, than pump speed 3, 420 mA 105 Watt. The heat exchanger differential temperature is more advantageous.Boiler flame outs, if monitored, would be expected to be more frequent on the higher pump speed.

Further studies requiring controlled load conditions are considered essential.

After ten minutes into the water heating cycle, water leaving the heat exchanger has only increased 2 deg C in temperature. Eventually as the recirculating water increases in temperature so this differential increases to approximately 5 to 7 deg C, and useful work is now being done. The water should now circulate at the fastest speed consistent with the pump performance curve. All radiators will come up to heat rapidly. The faster speed will also generate a certain amount of noise, both from the pump and from the pipe system itself.

The temperature trial was repeated for different pump speeds and system configurations. It was found that system performance improved when the slowest speeds were used for cold starts, and progressively higher speeds were used once the entire circulation had achieved a certain level of heating. Eventually the action of TRV's will reduce the capacity of the system and a lower pump speed should be employed.

WATER CIRCUIT ONLY PUMP SPEED #1

ELAPSED COLD HOT DIFF.

TIME RETURN SUPPLY TEMP. Notes (MINUTES) oo 16 19 3 low start temp.

39 42 3 10 51 58 7 12 65 70 5 15 71 78 7 16 74 81 7 18 76 81 5 flame out Temperature change of 25 deg C in Average diff. 5.3 10 minutes WATER CIRCUIT ONLY PUMP SPEED #3

ELAPSED COLD HOT DIFF.

TIME RETURN SUPPLY TEMP. Notes (MINUTES) oo . 33 33 0 start temp.

4 52 52 0 8 70 73 3 10 79 81 2 12 | 84 86 2 flame out Temperature change Average diff. 1.4 of 32 deg C in 8 minutes TABLE 1 (top) Survey, Pump Speed #1 (1150 rpm) TABLE 2 (bottom) Survey, Pump Speed #3 (2250 rpm) CENTRAL HEATING ONLY PUMP SPEED #1

ELAPSED COLD HOT DIFF.

TIME RETURN SUPPLY TEMP. Notes (MINUTES) 5 25 33 8 start temp.

20 42 51 9 45 59 68 9 60 62 72 10 Temperature change Average diff. 9 of 37 deg C in 55 minutes CENTRAL HEATING ONLY PUMP SPEED #3

ELAPSED COLD HOT DIFF.

TIME RETURN SUPPLY TEMP. Notes (MINUTES) 6 26 31 5 start temp.

20 41 45 4 50 57 62 5 65 61 63 2 Temperature change Average diff. 4 of 35 deg C in 59 minutes TABLE 3 (top) Survey, Pump Speed #1 (1150 rpm) TABLE 4 (bottom) Survey, Pump Speed (2250 rpm) SECTION 3 APPLICATION AND DESIGN OPTIONS 3.1 Application The requirement is to adjust motor speed to suit the system loading, determined by the effectiveness of the boiler heat exchanger and the changing capacity of the system. The speed will be selected according to the temperature of recirculated water returning to the boiler. Cold water will be passed through the heat exchanger slower than hot water, in order to pick up more heat.

Substitution of the standard ac induction motor with a dc or universal (commutator) motor would be uneconomic, impractical and not at all likely to gain acceptance by the industry or consumer. The existing single phase induction motor remains in use, consequently a number of options are available as a means of changing speed. Such a motor can only change speed as follows: a) by varying the supply frequency b) by selecting a different configuration of pole pairs Method a) requires the use of a variable frequency inverter. Method b) requires that the motor is equipped with multiple poles. Standard pump motors change speed by changing the number of pole pairs, so adaption of this design should be possible.

An alternative method in common use, often incorrectly described as speed control, is voltage regulation.

Design options are available in terms of speed control and temperature sensing, these are considered briefly in the remainder of this section.

3.2 DESIGN OPTIONS - SPEED CONTROL 3.2.1 Frequency Adjustment (Variable Frequency Inverter) This method would allow smooth adjustment of motor speed between the designed maximum and minimum speed limits.

The speed of rotation of an induction motor possessing a fixed number of poles, is determined by the angular speed of the rotating magnetic flux generated by the stator windings. There is an element of slip between the stator flux and rotor, so the rotor does not quite achieve synchronous speed. If the supply frequency were increased so would the speed of rotation of the magnetic flux, according to the following equation....

n = f /p where: n is in revolutions per second f is supply frequency in Hertz p is the number of pole pairs By adjusting the supply frequency to the motor, so its speed will alter accordingly.

Inverters are generally circuits which have a dc source, and by switching of rectifying devices, enable an alternating voltage to be synthesised for supply to an ac load. Various circuits are in common use , such as the single phase centre tapped inverter, see Figure 2, and the dc chopper regulator in Figure 3. Analysis of the circuits is complex and the difficulties associated with efficient commutation of the SCR's render such systems impractical and uneconomic for small scale systems, and are primarily of use in large 3-phase loads.

To maintain optimum magnetic flux conditions in the induction motor, the ratio of voltage to frequency must be kept constant. This is because in any magnetic circuit the induced voltage is proportional to the flux level and the frequency, (v=d/dt).

If the ratio is not kept constant then the torque characteristic becomes less efficient.

Again, this demands a complex circuit, see Figure 4, and cannot be justified for such a simple application as P-TEC.

3.2.2 Voltage Regulation It is possible to obtain some measure of speed adjustment by simply reducing the supply voltage, with consequential increase in slip.

Thyristors, diacs and triacs are usually used and the simplest circuit is identical to that employed for lamp dimmers. A typical circuit for a motor speed controller is illustrated in Figure 5.

Apart from the obvious handicap of reduced efficiency with incresed slip there are other disadvantages associated with this method. The increased rotor losses with falling speed means that the load torque has to be restricted so as to avoid excessive rotor heating. Consequently this form of speed control is only practicable for lightly loaded systems, such as ventilation fans. At particular firing angles large harmonic components can be set up, producing reverse torque effects. Additionally problems of interference and a need for suppression, due to the repetitive action of the SCR's would be a further disadvantage.

3.2.3 Pole Pair Configuration This method represents the simplest solution to the problem and is the cheapest and easiest to implement. Two or three speeds can be obtained by having the stator poles interconnected in two or three different ways.

Because the motors are already designed to change speed manually then the motor requires no modification. P-TEC will be transferable between any multi-pole single phase motor, with only minor modifications or adjustment required for different speed ranges, or higher rated motors.

Torque is produced when the rotating magnetic flux links with the rotor conductor bars, and induces an emf in the bars. This emf drives a current in the rotor and creates a magnetic field around the rotor. The rotor and stator magnetic fields interfere to produce a torque reaction. In a two pole motor the rotor and flux rotate with with the same cyclic period as the supply frequency, and rotational speed can be shown to depend on the number of poles in the stator windings and the ac frequency.

As stated above in 3.2 n = f/p Due to the relationship between the torque reaction, induced emf, rotor current, friction and windage losses, an element of "slip" is introduced and the rotor runs at slightly less than synchronous speed. For a 4-pole winding at 50 Hz the rotor turns at lower than 25 rev/sec, typically 1200-1400 rpm.

Two and three phase motors are self starting, from switch on they immediately generate a cyclic rising and falling magnetic flux that rotates around the stator. A single phase motor will only start by an external influence or by artificially simulating the magnetic effect of a 2-phase motor. This is achieved by providing a second set of windings at 90 degrees to the main windings, and supplied through a capacitor. The currents in the two windings differ in phase by 90 degrees and set up a rotating magnetic field for the rotor to follow.

A suitable control circuit for switching between windings is a window comparator.

A cascade or "staircase" of comparators, provides a series of windows, each one capable of activating an output device specific to that stage. Each window can be adjustable and referenced to a different temperature range, so that changing inputs will operate relays to engage different winding circuits. Monolithic IC chips are available that could simplify the construction process, but would build in some redundancy and would be prohibitively expensive. A relatively simple circuit, using comparator IC's or Op Amps, resistors and diodes etc, provides the cheapest development and production solution. See Figure 6 for typical single window circuit.

3.3 DESIGN OPTIONS - TEMPERATURE SENSOR 3.3.1 The three temperature sensors considered were the thermocouple, platinum resistance temperature detector (PRT) and thermistor.

Each method is well proven and documented for practical applications. Final selection can easily be made based on criteria such as cost, desired accuracy, reliability and simplicity of circuit.

The principle advantages and disadvantages of each type, are listed below.

TEMPERATURE ELECTRICAL 110 TYPE CHARACTERISTICS COMMENTS Thermocouples Low source impedance, typ- Low voltage output requires ically 10#. Voltage-output low-drift signal conditioning.

devices. Output shift is 10's Small size and wide tempers of microvolts/ C. Outputs ture range are advantages. Re typically in the millivolts at quires reference to a known room temperature. temperature. Nonlinear response.

Platinum and Resistance changes with Highly repeatable. Good lin other RTD's temperature. Positive tem- earity over wide ranges.

perature coefficient. Typical Requires bridge or other impedance (0 C) 20# to 2k#. network for typical interface.

Typical sensitivities 0.1%/ C to 0.66%/ C, depending on material.

Thermistors Resistance changes with tem- Highest sensitivity among perature. Negative tempera- common temperature trans ture coefficient. Typical im- ducers. Inherently nonlinear pedances (25 C) of 50# to (exponential function) but 1MQ available. Sensitivity at accurate linearized networks 250C is about 4%PC. Lin. available.

earized networks available with with 0.4%/ C sensitivity.

Any of the devices above is suitable and can be inter-changeable within P-TEC, with only minor modification or adjustment required for temperature calibration.

Typical sensor circuits are illustrated in Figures 7, 8 and 9 (next page).

SECnON 4 DESIGN SOLUTION 4.1 The design solution was approached by considering practical circuits to satisfy the components of the block diagram, illustrated as Figure 10 below. A detailed circuit diagram is illustrated in Figure 11, pagel8.

4.2 Temperature Sensor After consideration of the three methods described in 3.4.3, the thermistor was selected as the preferred sensor, on the basis of its low cost, reliability and simple circuit. The sensor shall be sited as close to the boiler feed water inlet as possible, but subject to individual heating system assessment, may also be sited further upstream provided the system temperature remains closely aligned to that entering the boiler. Consequently it should be possible to locate the sensor circuit within the same enclosure as the control circuit.

It is intended the P-TEC will be installed as a single unit, mounted in close proximity to the motor and with easy access to the existing power supply point. The thermistor is supplied with a constant current from the + ve voltage supply rail and forms one part of a potential divider. Voltage sensed across the thermistor is fed to the inverting input of an Op Amp employing negative feedback. The feedback is adjustable so that an appropriate gain can be provided.

4.3 Control Circuit The control circuit is required to be a staircase window comparator. The temperature sensor will provide an output which will be identified within the threshold limits determined by adjustable voltage references.

The single window circuit of Figure 6, has been replicated to provide the required number of stages (windows). See Figure 11 page 18 for a detailed circuit diagram.

The output of IC1 is applied to the non-inverting input of IC2 and the inverting input of 1C3. The amplifiers have no feedback and hence operate with full gain.

For all practical considerations the outputs will be at either the + ve or -ve supplies, depending on the comparison of IC 1 output with the upper and lower limits of each window. When the output of IC1 is between the limits of one window both outputs will be + ve, and that window transistor will be switched ON. The associated relay will be energised and the contacts will close. When each window threshold is crossed by a rising sensor voltage, the Upper Limit comparator output (ULl), changes from "high" to "low", ie + ve to -ve, providing a current sink to the redundant transistor circuit. Consequently the transistor switches OFF and its associated relay is de energised. Meanwhile UL1 is also the Low Limit (LL2) for the next incremental window.Its low limit comparator output changes from "low" to "high", ie -ve to +ve. The upper limit (UL2), has not been exceeded and consequently its output is also high. The window transistor switches ON and its associated relay is energised.

As the sensor registers higher temperatures, so the output from IC1 is detected in progressively higher windows. Three windows correspond to the motor speeds. The final 4th window, is only activated when boiler return (feed) water is near to maximum working temperature. This would indicate reducing heat transfer to the system being heated, due to rooms at temperature and/or TRV's isolating, and a lower motor speed can be utilised. Consequently the final window re-energises either pump speed 1 or 2, depending on which link is shorted, Link 1 or Link 2.

An LED across each transistor load will provide energised indication for each relay for diagnostic purposes.

4.4 Motor Interface This unit marries the control circuit output to the stator winding terminals. A command switch is incorporated so that automatic (thermo-electronic) operation can be over-riden and manual control selected if required. The switch is a two pole 4-way switch which facilitates isolation of an ac/dc transformer rectifier circuit when the automatic operation is de-selected.

Also contained within this unit is a starting capacitor, and the relays operated by the control circuit. The relays are not required to switch regularly or at high speed, they are required only to be reliable and cheap. Consequently solid state relays are not justified and standard electro-magnetic relays will be employed.

4.5 Power Supply A single rail power supply is favoured for low cost and simplicity. A dual power supply has been included in the design plan as an option.

Note: Present research suggests that ready made regulated ac/dc power supplies can be obtained cheaper than the cost of manufacturing a single demonstration prototype. Consequently the dc power for this project will be derived from bench sources, the purpose of this project being to prove the design of a control circuit.

SECTION 5 SPECIFICATION 5.1 The specification for the P-TEC prototype is as follows: 5.2 Demonstration System P-TEC.01(d)

AC Power Supply 240 V 50 Hz DC Power Supply 12 V Motor Type single phase 16-pole induction motor Motor Speeds (nominal) 800 rpm 1200 rpm 1900 rpm Load Current 160 mA 250 mA 380 mA Load Power 40 watt 70 watt 105 watt Temperature Sensor Range 0 to 100 degrees C Temperature References RV2 0 to 55 ( #5) deg C * RV3 55 to 65 ( #5) deg C * RV4 65 to 75 ( #5) deg C * RV5 75 to 90 ( #5) deg C * * These values arbitrarily set as required by individual heating systems, see 4.3 The values above are considered suitable for the thermodynamic response of the system examined and referred to in Section 2.3. When the water only circuit is required a very small pipework system is in use. The hot water reservoir thermostat is set to 550C, consequently the motor has been confined to the lowest speed for this duty cycle.

SECTION 6 COSTS 6.1 The following table lists all components and includes a circuit reference for Figure 11, supplier, unit and batch costs. Batch costs can be further substantially reduced, when bulk purchases for manufacturing purposes are placed.

Prices stated are those advertised in the October 1994 issues of the Maplin and RS Components catalogues.

6.2 TEMPERATURE SENSOR CIRCUIT

DESCRIPTION IDENTITY SPECIFICATION SUPPLIER PRICE (#) pu/batch Thermistor TH1 50k# at 25 C RS/856 DO-35 NTC -55 to 250 C 19 & 55 0.95/0.88 Op Amp 741 IC1 UA741CP RS/644 Vimax=#30V 305-311 0.51/0.25 Resistors +5% RS/563 carbon film 0.25w R1 120k2 131-508 0.03/0.02 R2 100kQ 131-491 0.03/0.02 Variable Resistor RS/571 3/8 in sq. Top RV1 500k# #10% 186-794 0.44/0.35 RV2 100k# +10% 186-772 0.44/0.35 6.3 CONTROL CIRCUIT

DESCRIPTION IDENTITY SPECIFICATION SUPPLIER PRICE (#) pu/batch Op Amp 741 IC2 IC3 UA741CP RS/644 IC4 IC5 Vimax=#30V 305-311 0.51/0.25 IC6 IC7 IC8 IC9 Variable Resistors RS/571 3/8 in sq. Top #10% RV3 50k# 186-766 RV4 RV5 10k# 186-744 0.44/0.35 RV6 20k# 186-750 Resistors #5% RS/563 carbon film 0.25w R3 R4 R5 2.2k# 131-299 0.03/0.02 R6 R7 R8 R9 R10 R11 R12 10k# 131-378 0.03/0.02 R13 R14 1% metal film 0.125w R15 R16 5.1k# RS/560 R17 164-362 0.05/0.04 Diodes D1 D2 D3 OA202 RS/579 D4 D5 D6 150 V 80mA D7 D8 D9 D10 D11 109-258 0.23/0.16 D12 D13 D14 D15 Transistors TR1 TR2 BC108 30m W RS/583 TR3 TR4 L@max 100 mA 293-533 0.22/0.19 Relays RL1 RL2 12V/5A 400# MAP/807 RL3 JM18U 1.29/ LED low current D16 D17 2-7mA 24m W RS/425 D18 Green 589-014 0.27/0.21 CONTROL CIRCUIT (continued)

Strip Board SRBP/Cu RS/513 0.1 in. 433-826 4.78/4.36 95 X 292 X 1.6 Capacitors input stability C1 C2 0.1yF ceramic RS/92 C3 C4 50V 124-178 0.26/0.21 CS C6 C7 C8 power supply by-pass C9 C10 1F tantalum RS/81 0.4/0.23 C11 C12 128-007 C13 C14 16V 6.4 MOTOR INTERFACE MODULE

DESCRIPTION IDENTITY SPECIFICATION SUPPLIER PRICE (#) pu/batch Rotary Switch SW1/2 2 Pole 4-Way MAP/794 0.99/ 300 Vac 5A continuous 150mA Box, Polypropylene RF Shielded RS/1171 120 X 65 X 40 501-569 2.97/2.72 Edge Strip Connector ESC 1 H Type RS/154 (nearest equivelant) 15-Way blade 481-960 5.31/4.78 Capacitor C17 2 F 400V RS/83 107-713 2.58/2.36 6.5 POWER SUPPLIES (Single Rail)

DESCRIPTION IDENTITY SPECIFICATION SUPPLIER PRICE (#) pu/batch Transformer T1 12V RS/966 207-649 4.75/4.28 Heatsink 4 C per W RS/816 401-497 2.44/1.9 Rectifier Bridge D18 D19 Diode lN4002 D20 D21 X 4 RS/578 VRRM 100V 261-154 0.05/0.03 Regulator, Fixed Reg MC7812CT RS/614 Voltage 641-623 1.01/0.3 Capacitors RS/76 C17 2200 F Electro 106-192 2.55/2.26 RS/80 C18 470nF disc 101-765 0.15/0.13 RS/70 C19 10 F Electro 107-397 0.07/0.05 Modular Power See Section Regulated MAP/43 Supply 4.5 page 17 Fixed Voltage 240/12V BZ83A 8.99/ 500mA 6.6 MATERIAL COSTS From the foregoing tables, a maximum costing for the P-TEC.01 prototype can be estimated.

() Temperature Sensor Circuit 1.64 Control Circuit 2.23 Motor Interface Unit 11.17 Power Supplies 11.17 Total 46.21 Batch Price 33.70 6.7 The following should be noted: - All prices can be reduced by batch purchase, and further reduced by bulk purchase.

- The power circuit will not be constructed, a ready made modular unit will make a significant reduction in cost.

For this project bench supplies will be used.

- Capacitor C17 is part of the redundant manual control unit and will be re-used.

- the required edge strip connector is not a standard pattern and will either need to be manufactured or a modification kit provided.

For this project the original connector will be modified.

6.8 The revised material cost, based on the changes listed in 6.7 above, is as follows: () One P-TEC.01 prototype 27.15 Batch price (no psu) 18.32 SECTION 7 TOOLS AND TEST REQUIREMENTS 7.1 Only standard range test instruments and tools are required.

7.2 Tools Availability Soldering Iron home/work Sidecutters/pliers/screwdrivers home/work PCB Drill home/work PCB Etching Equipment home/work Labelling Equipment home/work 7.3 List of Instruments Availability M ultimeters home/work Thermometer home/work Power Supplies work Tachometer work/college Oscilloscope work/college 7.4 Test Requirements The test procedure involves the testing of the individual "modules" of the system as defined by the block diagram, Figure 10. The temperature and control circuits will require only a 12 V power supply.

Final testing of the P-TEC system will require the use of 240 V ac, but only as connected within a standardised and proprietary connector to drive the load.

The stages of testing are defined within the critical path analysis, see Appendix I.

7.5 Areas of Uncertainty Calculations for component values are not yet finalised and are likely to require alteration during the test phases.

The final adjustment of voltage references RV2, RV3, RV4 and RV5 (temperature thresholds), will very much depend on the characteristics of individual heating systems. For the prototype the temperatures will be arbitrarily set, but will attempt to achieve those listed in 5.2.

The operation of each comparator stage is mutually exclusive, so only one relay at a time will be energised. Consequently there is no perceived need for hysteresis, however capacitors will be inserted across the power supply to each comparator Op Amp and the inputs, in order to eliminate any transients that might result in chatter, and to provide additional stability. An additional method of improving stability, is to latch the comparator output with a schmidt trigger. This will be an option reserved for the design review.

Similarly screening of the sensor cable and installation of the circuits within a metal box may be necessary to protect against radiated interference.

Claims (7)

1) An electronic device that automatically switches between motor windings on a multi-pole single phase ac motor.

2) An electronic device as claimed in 1), that drives a pump and circulates water around a water heating system, adjusting the flow rate of the driven pump according to the heat dissipation from the system.

3) An electronic device as claimed in 1) and 2) above, that detects heat dissipation by using a temperature sensor positioned on the water return line to the water heater/boiler.

4) An electronic device as claimed in 1), 2) and 3) that changes motor speed according to any increase or decrease in capacity of the water heating system.

5) An electronic device as claimed in 2), that in a dis proportionate two (or more) circuit system, eg hot water and central heating, always selects the lowest pump speed for the smallest circuit.

6) An electronic device as claimed in 4) and 5), that ensures that the lowest electrical power is used for circulation, at all load conditions.

7) An electronic device as claimed above that controls a pump which circulates water around a water heating system, adjusting the flow rate of the pump according to the heat dissipation from the system. No e::isting components of the system or its central programmer/controller (e::cept the plug), require alteration or replacement. The existing thermostatic and other controls of the system remain unaffected.

7) An electronic device as claimed above that can be fitted as an accessory to any single phase ac motor that employs pole switching as a means of changing speed.

Amendments to the claims have been filed as follows 1) An electronic device, that automatically switches between motor windings on a multi pole single phase ac motor and pump on a central heating system, in order to select alternative fi::ed motor speeds and change pump output to match the load on the system.

2) An electronic device as claimed in 1), that detects heat transfer in a hot water heating system by using a single temperature sensor, positioned on the return line before the water heater/boiler.

The detected temperature is identified within one of several "windows" and the motor speed changeover initiated by the progressive transition between windows.

3) An electronic device, as claimed in 1) and 2) above that changes motor speed according to any increase or decrease in the volumetric capacity of the water heating system. A reduction in capacity leading to a reduction in motor speed and pumping rate.

4) An electronic device as claimed in 2), that in a disproportionate two (or more) circuit system employing a diverting valve, eg hot water and/or central heating, always selects the slowest pump speed for the smallest circuit.

5) An electronic device according to the preceding claims, in which the lowest electrical power is used for water circulation under predetermined load conditions.

6) An electronic device according to the preceding claims that can be fitted as an accessory to any single phase ac motor that employs pole switching as a means of changing speed, and connects directly onto the existing terminals with a substitute plug.