GB2497648A

GB2497648A - AC asynchronous motor having relay for controlling start-capacitor

Info

Publication number: GB2497648A
Application number: GB1221858.2A
Authority: GB
Inventors: Niels Christian Weihrauch
Original assignee: Secop GmbH
Current assignee: Secop GmbH
Priority date: 2011-12-16
Filing date: 2012-12-05
Publication date: 2013-06-19
Anticipated expiration: 2032-12-05
Also published as: GB2497648B

Abstract

A single-phase AC asynchronous motor has a main winding 4, an auxiliary winding 5, a start-capacitor CS, a positive temperature coefficient (PTC) component 18 and a relay 9 inserted in series with the start-capacitor. To prevent contact welding in the relay, the PTC component bypasses the contact pads 14, 15, 16, 17 of the relay enabling simultaneous charging of the start and run CR capacitors when the relay is in an open state. To extinguish an arc which may nevertheless occur between the contact pads, the relay is provided with two relay-switches 12, 13 arranged in parallel, with at least one of the relay-switches having an associated rectifier diode 29, 20 ensuring conductance through that relay-switch can occur only in one direction. A control structure may be provided comprising a coil 10 and an armature 11 which is movable by the magnetic field generated by the current through the coil.

Description

AN AC ASYNCHRONEOUS MOTOR

TECHNICAL FIELD

The invention relates to a single phase electric asynchronous motor for connection to an AC power supply. Particularly, the invention relates to a motor with a main winding, an auxiliary winding, a start-capacitor, a Positive temperature coefficient component (PTC), and a relay for controlling use of the start-capacitor.

The invention further relates to a method of operating a motor, a relay for an electric AC asynchronous motor, and to the use of such a relay for preventing or reducing an arc between contact pads of a relay-switch for a motor.

BACKGROUND

Traditional AC asynchronous motors include either a potential relay as illustrated in Fig. 1 or a current relay.

In motors with a run-capacitor and with a start-capacitor, the traditional current relay cannot be used, and such motors are fitted with a potential relay as illustrated in Fig. 1.

The electrical potential relay is a "normally-closed-type" relay and the run-capacitor and the start-capacitor are therefore charged simultaneously when the motor is turned on. When the motor gains speed, the potential over the relay increases. This opens the relay contact pads, and the motor is then operated without the start-capacitor.

The current relay is cheaper than the electrical potential relay. The current relay is a "normally-open-type" relay, and when the motor is started, it typically takes about 15-25 ms before the current in the main winding generates sufficient magnetisation of the armature and before the armature has moved sufficiently to close the contact pads of the relay.

In this very short period of time right after the motor is started, in the following referred to as t=0, the run-capacitor is charged with no charging of the start-capacitor. Unfortunately, when the relay contact pads closes, it is loaded by electrical charging caused by electrical discharging of the run-capacitor, i.e. when the run-capacitor discharge into the start-capacitor and thereby counterbalance the difference in electrical potential between the run-capacitor and the start-capacitor. The contact pads of the relay-switches may in worst case weld together and the relay is thereby destroyed.

SUMMARY

It is an object of embodiments of the invention to provide a robust AC asynchronous motor and a relay for such a motor.

According to a first aspect, the invention provides a motor where the PTC and the relay are connected in parallel between the power supply and the start-capacitor, and where the relay comprises at least two relay-switches arranged in parallel between the power supply and the start-capacitor and a control structure which can perform an opening or closing sequence in which all relay-switches are moved to an open or to a closed state, where the relay-switches are arranged in parallel between the power supply and the start-capacitor, and where at least one of the relay-switches has an associated rectifier whereby conductance through that relay-switch can occur only in one direction between the power supply and the start-capacitor.

Due to the PTC, both capacitors are charged simultaneously which reduces the arc over the contact pads.

Since, however, there is an electrical potential across the PTC, and since the time constant R(0f the pTc)*capacitancecor the start-capacitor) restricts the charge and therefore delays the voltage build-up on the start-capacitor compared to that of the run-capacitor, there will be a small voltage different between the voltage on the start-capacitor and the voltage on the run capacitor which can cause an arc between contact pads of the relay.

Another aspect relates to the opening of the relay-switches when the start-S capacitor is no longer needed. At this point in time, there is a current through the relay-switches, and the opening of the switches must interrupt this occurring current. When the relay-switch opens, the arc may cause burning of the pads and thereby reduce the lifetime of the relay-switches. Due to the rectifier, an arc will, however, be extinguished as soon as the AC-signal reverses and the conduction through the rectifier therefore becomes impossible. That means that the rectifier takes over the interruption of the current and the risk of destroying the contact pads of the relay by melting is reduced or eliminated.

Since wear on the contact pads is related to the activation and deactivation of the start-capacitor, the motor and relay according to this invention is particularly advantageous in connection with motors having frequent start/stop characteristics, e.g. motors for compressors in refrigerators, freezers, heat pumps, and similar devices.

Since wear is particularly related to a voltage difference between the start-capacitor and a run-capacitor, the invention is particularly relevant in motors operating both with a start-capacitor and a run-capacitor.

If the relay-switches open at different points in time, that one which opens later will be subject to an arc, whereas the earlier relay-switch will be protected by the current running in the later relay-switch.

If only one of the relay-switches has an associated rectifier, only half of the contact openings, in average, will be positively influenced by the rectifier. In this case, the relay-switch which does not have an associated rectifier remains unprotected and will be subject to an arc if it is opened later than the other relay-switch To ensure that the relay-switch is always protected by the rectifier, each relay-switch which has an associated rectifier may advantageously be arranged to be opened later than relay-switches which have no associated rectifier in the opening sequence.

S As an alternative, it may be desirable if all relay-switches have associated rectifiers whereby conductance through the relay-switches can occur only in opposite directions between the power supply and the start-capacitor. This will protect the contact pads of both relay-switches and thereby provide a further increased expected lifetime of the relay-switches. By opposite directions is herein meant that half of the relay-switches can conduct from a phase of the power supply to the start-capacitor and the other half can conduct from the start-capacitor to the phase of the power supply, i.e. in case of two relay-switches, one in each direction.

The control structure may particularly operate both relay-switches simultaneously e.g. based on a current through the main winding or based on an electrical potential across the auxiliary winding. As an example, the switches may be closed when the current through the main winding is large and the motor therefore requires a starting torque from the auxiliary winding, and the switches may be opened as the motor has gained speed and the current through the main winding has therefore decreased.

Since the problem associated with the arc over the contact pads during opening of a relay-switch is particularly relevant for control structures operating based on a current through the main winding, the invention may particularly relate to such control structures, e.g. to a control structure comprising a coil and an armature which is movable by a magnet field generated by a current through the coil and where the coil is inserted between the power supply and the main winding to conduct the current through the main winding such that the relay-switches are operated between the open and closed states depending on a current through the main winding.

S

As it will be disclosed in further details later, the armature may be mechanically connected to any number of relay-switches such that all relay-switches are moved between the closed and open states simultaneously under influence of a

magnet field working on the armature.

The mentioned coil introduces an inductance which dampens the impact of the discharge of the run-capacitor into the start-capacitor. To further protect the contact pads of the relay switches, the coil may be located in serial connection between the run-capacitor and the relay-switches.

When the motor is turned on, the PTC is cold and therefore in a low resistance state with a resistance in the range of only a few ohm, e.g. S -25 Ohm. In this state, the bypass of the relay charges the start-capacitor such that it is charged simultaneously with the charging of the run-capacitor. Typically after 15-25 ms when the magnetising of the armature becomes sufficient, the relay closes and the resistance through the relay-switches becomes lower than the resistance in the PTC. The start-capacitor is maintained active via the relay until the motor operates at higher speed. When the rotational speed of the motor increases, the current in the main winding will be reduced, and the relay-switches will open. At this time, the PTC will typically still be cold and therefore be low ohm resistant.

The PTC therefore takes over conduction to the start-capacitor, and the start-capacitor therefore continues operational. however, after a short period of time, the PTC will become warm and therefore change to high resistance mode and the start-capacitor will disconnect slowly compared to the relay which opens and disconnects very fast. Had the start-capacitor been disconnected by the relay, the disconnection would have been abrupt and associated with an arc which produces electromagnetic noise. The same applies relative to Electromagnetic Compatibility (EMC). After this, the motor will continue in operation only with the run-capacitor until the motor is switched off.

The PTC may be a traditional PTC or it may be an e-PTC.

If the PTC is traditional, it will be kept warm by a small leakage current which causes a loss of typically 2 watt. Irrespective this is traditionally an unwanted loss, it may be desirable in connection with the run-capacitor and the start-capacitor as specified in claim 1. The reason for this is that the electrolytic capacitor typically increases its life-time when being "exercised" by the steady charging and de-charging at low power due to the forming of aluminium oxide in S the electrolytic chemistry of the start-capacitor.

If a motor which is fitted with a relay of traditional kind without the PTC is started within a traditional start/stop cycle for a compressor e.g. for a refrigerator etc, e.g. within 5 minutes, the start-capacitor will still be charged with a random voltage depending on the sinusoidal disconnection point. This generally causes an unpredictable opening and closing potential between the contact pads of the relay. In worst case, the opening and closing potential may be doubled by the remaining charge on the start-capacitor. This creates increased wear on the contact pads. In traditional motors, the start-capacitor is sometimes discharged by a separate bleed resistor arranged in parallel to the start-capacitor, c.f. Fig. 2. If the PTC according to the invention is traditional, i.e. if it has a leakage current, it improves this situation since its leakage current ensures discharging of the start-capacitor during intervals where the motor is not active and the unwanted situation with unpredictable potential and increased wear on the contact pads is therefore reduced or eliminated and the resistor shown in Fig. 2 can be omitted.

An electronic PTC, sometimes referred to as an e-PTC electronically disconnects and therefore has no leakage current. By use of an e-PTC, power can therefore be saved. In this case the resistor shown in Fig. 2 should preferably be used.

The PTC is connected such that it bridges the contact pads and the associated rectifiers and thereby enables charging of the start-capacitor when the relay switches are open.

In a second aspect, the invention provides a method of operating a single phase AC motor which is connected to an AC power supply and which comprises a main winding, an auxiliary winding, a start-capacitor, a relay, and a Positive temperature coefficient component, where the PTC and the relay-switches are connected in parallel between the power supply and the start-capacitor, and where the relay comprises a first and a second relay-switch arranged in parallel between the power supply and the start-capacitor, the method comprising providing a rectifier such that conductance through one of the relay-switches can S occur only in one direction between the power supply and the start-capacitor.

In a third aspect, the invention provides a relay for a single phase AC motor, the relay comprising a first and a second relay-switch arranged in parallel between a power supply connector and a start-capacitor connector, at least one of the relay-switches having an associated rectifier whereby conductance through that relay-switch can occur only in one direction between the power supply connector and the start-capacitor connector.

The control structure, relay-switches and rectifiers may generally be configured as described relative to the motor according to the first aspect of the invention.

In a fourth aspect, the invention provides the use of the above mentioned relay for reducing or preventing generation of an arc between contact pads of a relay-switch in an electrical motor, particularly in a motor with frequent starts/stops e.g. a compressor for a refrigerator or heat pump.

In a fifth aspect, the invention provides a compressor with a motor or relay as described herein.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of example with reference to the following figures in which: Fig. 1 illustrates a diagram of a motor with an electrical potential relay (prior art); Fig. 2 illustrates a diagram of a motor according to the invention;

S

Figs. 3 and 4 illustrate diagrams of alternative embodiments of motors according to the invention; and Figs. 5 and 6 illustrate time! current diagrams describing a start and stop sequence for the auxiliary winding of the motor;

DESCRIPTION OFAN EMBODIMENT

Further scope of applicability of the present invention will become apparent from the following detailed description and specific examples. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

Fig. 1 illustrates a diagram of a single phase AC asynchronous motor 1 with an electrical potential relay. Such a motor exists already. The motor is connected to an AC power supply 2, 3. The motor comprises a main winding 4, an auxiliary winding 5, a run capacitor (Cr), a start-capacitor (Cs) and an electrical potential relay 6 which relay-switches between an open state with no conduction between the contact pads 7, 8 and a closed state with conduction between the contact pads 7, 8 depending on an electrical potential over the auxiliary winding S. The start-capacitor and the run-capacitor are connected in parallel between the phase 2 of the AC power supply, and the auxiliary winding S when the relay is in the closed state, and the contact pads 7, 8 are inserted in serial with the start-capacitor when the relay is closed. The relay therefore connects or disconnects the start-capacitor depending on the electrical potential over the auxiliary winding 5.

Fig. 2 illustrates a diagram of a motor according to the invention. In this case, the relay 9 is a current relay with a coil 10 inserted in serial with the main winding 4 such that the armature 11 is lifted against gravity by the magnetic field generated by the current in the main winding.

The armature 11 is connected to two contact elements 12, 13 which connects or disconnects the associated contact pads 14, 15, 16, 17 depending on the position of the armature 11. The contact element 12 and the associated contact pads 14, 15 forms the first of the at least two relay-switches and the contact S element 13 and the associated contact pads 16, 17 forms the second of the at least two relay-switches according to the invention.

This type of relay is cheaper than the electrical potential relay which is disclosed in Fig. 1. As already discussed, however, this current relay would suffer from the electrical load of the run-capacitor when it discharges into the start-capacitor if it was not protected by the PTC 18 and rectifiers 19, 20 according to this invention.

As illustrated in Fig. 2, the motor according to the invention comprises a PTC 18 inserted such that it bypasses the relay and enables simultaneous charging of the start capacitor CS and run capacitor CR, when the relay is in the open state.

In the illustration, the PTC is traditional, i.e. it has a leakage current which gives a loss of 2 watt which keeps the PTC warm. The PTC could have been an e-PTC which electronically disconnects to avoid the leakage current and loss. The PTC could also have been an m-PTC which mechanically disconnects to avoid the leakage current and loss.

The rectifier 19 ensures one-way conduction across the contact pads 14, 15 in the direction from the power supply to the start-capacitor and prevents conduction in the opposite direction. The rectifier 20 ensures one-way conduction across the contact pads 16, 17 in the direction from the start-capacitor to the power supply and prevents conduction in the opposite direction.

The resistor 21 is optional and ensures discharging of the start-capacitor when the start-capacitor is disconnected.

When the main-switch 22 closes, the motor starts. This event is referred to in the time/current description with reference to Figs. 4 and 5 and it is considered to occur at time 0. For use in the diagrams where time is along the abscissa and current is along the ordinate, Fig. 2 comprises indications of names of currents at different locations. The current from the main-switch 22 to the first junction 23 is named i1, the current from the junction 23 to the relay-switches and to the PTC is named i2, the three currents from the relay-switches and from the PTC to the junction 24 are named i3, i4, i5, the current from the junction 24 to the start-capacitor CS is named i6, and the current from the junction 23 to the junction 25 is named i7.

In the motor illustrated in Fig. 2, the coil 10 is in serial connection with the main winding, and further located between the run-capacitor and the relay-switches.

Fig. 3 illustrates a motor with an alternative coupling of the coil 10, i.e. the coil is still in serial connection with the main winding but not located between the run-capacitor and the relay-switches.

Fig. 4 illustrates a motor with an alternative layout of the relay-switches. Due to the mechanical layout where the triangular bridge forms three individual switches, the relay-switch becomes simple and reliable since the need for two separate switch elements has been overcome.

Figs. 5 and 6 illustrate the currents in ampere as a function of time starting from t=0 when the main-switch connects power to the motor. At time 0 i1, i2, and i7 starts essentially immediately. At this time, the relay-switches are open and the auxiliary winding is powered via the run-capacitor and via the start-capacitor which is charged via the PTC. At time=25 ms at 50 Hz the armature has been magnetised and moved such that the relay-switches closes. Due to the rectifiers only one of i3 and i4 contributes at a time, and the resulting i5 becomes a complete sinus-shaped current. Even a cold PTC has much higher resistance than the relay-switches and due to this difference in resistance, i5 becomes zero.

Fig. 6 illustrates the continued time/current diagram at a time=x where the relay-switches open. At this time, the motor rotates and the current in the main winding has dropped to a lower level whereby the magnetic field in the coil is insufficient for maintaining the armature lifted and thus the relay-switches to the open position. Due to a resistance in the PTC there will be an electrical potential over the relay-switches. Due to this electrical potential, an arc forms between the contact pads 14, 15 and the contact element 12. Since the arc conducts a current which is essentially identical to i3 before the relay-switches were opened, i3 is illustrated to continue un-amended after the opening of the relay-switches.

At time=y, the AC-current passes zero and changes direction. Accordingiy, the rectifier 19 prevents further conduction and i3 drops to zero and extinguish the arc. At this point, y, in time, the relay-switches are already opened and because the current was zero when the contact pads 16, 17 opened and the distance between the contact pads 16, 17 is now larger, a new arc will not be ignited. The duration of the arc can at most be half a period. In 50 Hz that corresponds to 10 ms. In Fig. 6 the arc is less than 10 ms.

As a result, conduction to the start-capacitor, i.e. i5, becomes equal to that provided through the PTC, i.e. equal to i5. When the PTC becomes warm, the resistance increases and i5, i6 therefore reduces essentially to zero. The motor is thereafter operated by the main winding supported by the auxiliary winding when driven essentially only through the run-capacitor except for the loss of 2 watt which will occur if the PTC is traditional.

Claims

<claim-text>CLAIMS1. A single phase AC asynchronous motor (1) for connection to an AC power supply (2, 3), the motor comprising a main winding (4), an auxiliary winding (5), a run-capacitor (CR), a start-capacitor (CS), a relay for controlling use of the start-capacitor, and a Positive temperature coefficient component (18), where the PTC and the relay are connected in parallel between the power supply and the start-capacitor, and where the relay comprises at least two relay-switches (12, 13, 14, 15, 16, 17) and a control structure (9, 10, 11) which can perform an opening or closing sequence in which all relay-switches are moved to an open or to a closed state, where the relay-switches are arranged in parallel between the power supply and the start-capacitor, and where at least one of the relay-switches has an associated rectifier (19, 20) whereby conductance through that relay-switch can occur only in one direction between the power supply (2) and the start-capacitor (CS).</claim-text> <claim-text>2. A motor according to claim 1, where each relay-switch which has an associated rectifier is arranged to be opened later than relay-switches which have no associated rectifier in the opening sequence.</claim-text> <claim-text>3. A motor according to claim 1 or 2, wherein both relay-switches (12, 13, 14, 15, 16, 17) have associated rectifiers (19, 20) whereby one relay-switch can conduct only in one direction whereas the other relay-switch can only conduct in the opposite direction.</claim-text> <claim-text>4. A motor according to any of the preceding claims, wherein the control structure (9)is adapted to operate all relay-switches (12, 13, 14, 15, 16, 17) simultaneously.</claim-text> <claim-text>5. A motor according to claim 4, wherein the control structure (9) operates the relay-switches (12, 13, 14, 15, 16, 17) based on a current through the main winding (4).</claim-text> <claim-text>6. A motor according to claim 4, wherein the control structure operates the relay-switches (12, 13, 14, 15, 16, 17) based on an electrical potential over the auxiliary winding (5).</claim-text> <claim-text>7. A motor according to any of the preceding claims, where the control structure (9) comprises a coil (10) and an armature (11) which is movable by a magnet field generated by a current through the coil and where the coil is inserted between phase (2) of the power supply and the main winding (4) to conduct the current through the main winding such that the relay-switches are operated between the open and closed states depending on a current through the main winding.</claim-text> <claim-text>8. A motor according to claim 7, where coil (10) is in serial connection between the run-capacitor (Cr) and the relay-switches (12, 13, 14, 15, 16, 17).</claim-text> <claim-text>9. A method of operating a single phase AC motor which is connected to an AC power supply (2, 3) and which comprises a main winding (4), an auxiliary winding (5), a start-capacitor (Cs), a relay (9), and a Positive temperature coefficient component (PTC), where the PTC and the relay are connected in parallel between the power supply (2) and the start-capacitor (CS), and where the relay (9) comprises a first and a second relay-switch (12, 13, 14, 15, 16, 17) arranged in parallel between the power supply (2) and the start-capacitor, the method comprising providing a rectifier (19, 20) such that conductance through one of the relay-switches can occur only in one direction between the power supply (2) and the start-capacitor (CS).</claim-text> <claim-text>10. A relay for a single phase AC motor, the relay comprising a first and a second relay-switch arranged in parallel between the power supply connector and a the start-capacitor connector, at least one of the relay-switches having an associated rectifier whereby conductance through that relay-switch can occur only in one direction between the power supply connector and the start-capacitor connector.</claim-text> <claim-text>11. A relay according to claim 10, comprising a control structure which can operate both relay-switches simultaneously.</claim-text> <claim-text>12. A relay according to claim 11, where the control structure comprises a coil and an armature (11) which is movable by a magnet field generated by a S current through the coN.</claim-text> <claim-text>13. use of a relay according to any of claims 10-12, for reducing or preventing generation of an arc between contact pads of a relay-switch in an electrical motor.</claim-text> <claim-text>14. A compressor with a motor according to any of claims 1-B.</claim-text>