GB2100476A - Asynchronous motor speed control - Google Patents

Abstract

A variable speed electric motor is supplied selectably through one of a plurality of terminals (3', 4', 5') connected to different coils in dependence upon a sensed temperature (NTC) so that the speed of the motordriving, for example, a ventilating fan is increased as the temperature increases. <IMAGE>

Description

SPECIFICATION Asynchronous motor speed variation The present invention relates to a device for vary ing or controlling the speed of asynchronous electric motors.

As it is well known, in the air conditioning systems there is necessary to use variable speed electric motors in order to provide, at the output of the conditioning fans, predetermined different air flow rates, depending on the requirements.

Presently this need is met by using multiple speed electric motors, or by means of single speed electric motors, as coupled to a supply voltage varying device.

In the first case the available couple on the motor shaft varies depending on the position of a switch or selector effective to switch on and off corrsponding wires coupled to the electric motor (conventionaliy three wires).

This switch is generally a manually operated switch thereby as the speed of the motor is to be changed, it is necessary the presence of an operator.

On the other hand, the use of a single speed electric motor, associated with a device for varying the supply voltage thereof allows for the change of the motor speed to be obtained, depending on the envi ronmenttemperature, by means of a temperature probe or sensor.

This supply voltage varying device, in the presently known embodiments, may be of substantially two types, that is either electromagnetic or electronic.

The electromagnetic speed varying device, however, is of great size and very expensive.

The known electronic speed varying devices, on the other hand, may alter the voltage waveshape, and cause vibrations and noise to occur in the motor; moreover the presence of voltage peaks generates rf noise and provides a scarcely reliable system.

That same type of electronic speed changing device, moreover, consumes a lot of power, thereby heat is generated which is to be removed by expensive thermal dissipators.

Though several electronic apparatus are presently known for varying the supply voltage to electric motors, these apparatus are generally excessively expensive to be used in small power electric motors, either of the single phase or three phase type, for the applications thereinabove mentioned.

Thus the task of the present invention is to overcome the thereinabove mentioned drawbacks, by providing a device for varying the speed of asynchronous electric motors which does not generate any noise or vibrations in the apparatus which are driven by said motors.

Within the scope of the mentioned task, it is a main object of the invention to provide such a device for varying the speed of asynchronous electric motors which consumes a very reduced power, thereby providing a high efficiency.

Yet another object of the present invention is to provide such a speed varying device which does not include any thermal dissipators.

Yet another object of the present invention is to provide such a speed varying device which can be constructed in a modular form to be easily associated to the multiple speed electrically motors which are presently available on the market.

The task and objects thereinabove mentioned, as well as yet other objects which will become more apparent hereinafter, are achieved by a device for varying the speed of asynchronous electric motors, characterized in that it comprises, in combination, an electronic switch, a thermostatic probe or sensor, a multiple speed electric motor, a plurality of wires permanently coupled in a predetermined relationship to terminals coupled to different coils of said motor and coupling said electronic switch to said thermostatic probe for selectively supplying said multiple speed electric motor through one of the wires coupled thereto, depending on the environ menttemperature as detected by said probe or temperature sensor.

Further characteristics and advantages of the device for varying the speed of asynchronous electric motors according to the present invention will become more apparent from the following detailed description of an embodiment thereof, with reference to the accompanying drawings, where: fig. 1A illustrates a general diagram of the speed varying device according to the present invention; and fig. 1 B illustrates a possible schematic diagram, partially in block form, of the electronic circuitry associated with the speed varying device according to the invention.

With reference to the figures of the accompanying drawings, the electronic device for varying the speed of multiple speed electric motors according to the present invention comprises, in combination, an electronic switch 1, a thermostatic probe or sensor 2, a multiple speed motor 3 and a suitable set of wires 4, coupled in a fixed predetermined way, with different arrangements, to terminals in turn coupled or leading to the electric motor coils, thereby said electronic switch 1 is controlled by said temperature sensor or probe 2 and selectively supplies or ener gives the multiple speed motor 3 through one of the wires 4 coupled thereto.

More specifically said electronic switch 1 may be formed by using four layer silicon semiconductors, or by means of silicon controlled rectifiers or SCR's, driven by driving logic circuitry; alternatively, and as it is illustrated in a more detailed way in fig. 1 B, said electronic switch may comprise comparing means effective to be energized as the in put thereof reaches a predetermined or set threshold, in order to drive relay elements for switching on the desired speed of the motor, depending on the temperature detected by said sensor.

Referring now more specifically to fig. 1B a poss ible schematic diagram of the speed varying device according to the present invention is herein illustrated.

In particular, the speed varying device according to this embodiment of the invention comprises, as the main components thereof, three analogic threshold detectors, or comparators, which, for example, may be formed by three Schmitttriggers respectively indicated at T2, T3, which are coupled in parallel relationship and supplied by a common power supply 50.

The common connection point 51 of the inputs of the three triggers is coupled to one end of a resistor R which, together with the potentiometer P1 and the thermosensible element NTC, supplies at said point 51 a voltage which is proportional to the temperature and varies depending thereon. The resistor R, in other words, causes the ohmic resistance of the branch containing said element NTC to increase or decrease, thereby providing a different temperature threshold: to this end the resistor R may be electrically shorted by means of a switch SW1, or summer-winter switch.

As it will be apparent to those skilled in the art, the three different threshold triggers suppliestheirout- put pulse as a predetemined or set voltage threshold or temperature is reached; each output pulse, in turn, energizes the respective transistor Tr1, Tr2 and Tr3 which, as they are brought to the on state, energizes the related relay RL1, RL2 and RL3 which relays causes their contacts RL1, RL2, RL3 to respectively close thereby supply at the terminals 3', 4' and 5' the coils of the motor corresponding to the desired speeds.

It should be noted thatthe relay RL3 is provided with an auxiliary contact, also indicated at RL3, which, for example, may be used for directly driving a valve or a further relay.

In order to provide for a mutually exclusive ener gizing of the three relays RL1, RL2 and RL3 there are provided respective AND gates, indicated at 52 anc! 53 and coupled as shown, which are effective to disenergize RL1, RL2 respectively as the triggers T2 and T3 are respectively energized.

In the practice, as the temperature increases, the voltage atthe connection point 51 also increases: this increasing voltage causes the triggers T1, T2 and T3 to be sequentially energized which, on the other hand, are calibrated on different thresholds; the latter, in turn, through a respective enabling gate thereof, actuate, depending on the temperature, the respective relay RL1, RL2 and RL3 which, by means of the mentioned contacts thereof, provide to switch on the respective speeds of the motor.

Obviously, by closing the mentioned switch SW1, the resistor R may be shorted, thereby causing the voltage at the connection point 51 to decrease. Thus, by the provision of the mentioned summer-winter switch, the possibility is afforded of providing a summer-winter type of operation.

In the practice, as itwill become apparent from the above disclosure, the speed is either increased or decreased depending on the selected or set temper ature.

This embodiment of the invention, as thereinabove stated, also provides the auxiliary contact RL3 which is effec ive to directly drive a solenoid valve or a further elay, said auxiliary contact being in the normally closed position thereof.

In particular, atthe lower speed, it is possible to stop the motor and open said auxiliary contact, or it is possible to only open the auxiliary contact RL3.

From the above disclosure it should be noted that the speed varying device according to the present invention substantially consists of a static speed selector (either of the two or multiple path type) which is made of electronic components effective to allow for it to be easily coupled to a temperature responsive probe or sensor.

More specifically the device according to the present invention provides the following outstanding advantages: - absence of noise and vibration in the controlled apparatus, since the device operate by precisely sinusoidal waves, particularly in the relay embodiment; - absence of rf noise, in the case of the relay embodiment; - a low power adsorptoion or drain and accordingly a high system efficiency, with consequent power savings with respect to the presently known speed controlling devices; - a minimum number of heat developping components and the consequent elimination of the thermal dissipators, thereby a more compact and reliable structure is obtained.

The above advantages, in the practice, afford the possibility of fully meeting the requirements of the air conditioning apparatus, such as ventilating fans, rerrigerators and the like.

While the invention has been thereinabove illustrated with reference to a preferred embodiment thereof, it should be noted that it is susceptible to all modifications and variations all thereof fall within the spirit and scope of the invention itself.

For example those skilled in the art will be able, in the light of the preceding disclosure, of replacing the relay means for switching on the motor speeds by solid state controlled switches, such as SCR's and TRIAC's by providing suitable noise filtering networks.

Moreover, while the preceding description has made reference to a temperature responsive probe or NTC element, inserted in a potentiometric branch, this element, or a like element, may also be inserted in a bridge arrangement branch, driving operational amplifier means, effective to switch on and off the three speeds of the motor as respective set temperature values are reached.

Claims (9)

1. A device for varying the speed of asynchronous electric motors, characterized in that it comprises, in combination, an electronic switch, a thermostatic probe or sensor, a multiple speed electric motor, a plurality of wires permanently coupled in a predetermined relationship to terminals coupled to different coils of said motor and coupling said electronic switch to said thermostatic probe for selec tivelysupplying said multiple speed electric motor through one of the wires coupled thereto, depending on the environment temperature detected by said probe.

2. A device for varying the speed of asynchronous electric motors, according to the preceding ctaim, characterized in that said electronic switch is constructed by using four layer silicon semiconductors, or silicon controlled rectifiers, which are driven by logic controlling means.

3. A device, according to claim 1 or 2, characterized in that said thermostatic probe or sensor is series coupled with resistance means, effective to be electrically shorted, in order to provide awinter- summertype of operation of said device.

4. A device, according to claim 1 or claim 3 when appendantto claim 1, characterized in that said electronic switch comprises threshold comparing means which are calibrated to operate in a mutually exclusive manner for switching on the respective speeds of said electric motor as at the in put of a respective threshold comparing means there is present a predetermined voltage value.

5. A device, according to claim 1 or 2 or claim 4 when appendantto claim 1, characterized in that said thermostic probe or sensor consists of an NTC element.

6. A device according to claim 5, characterized in that said NTC element is inserted in a potentiometric branch.

7. A device according to claim 5, characterized in that said NTC element is inserted in a branch of a bridge arrangement.

8. A device, accoringto any of claims 1 to 7, characterized in that said electronic switch comprises at least an auxiliary contact, effective to directly drive valve and/or relay means.

9. A device for varying the speed of an asynchronous electric motor substantially as hereinbefore described with reference to the accompanying drawings.