IL26240A - Protective device for alternating-current motors against supply asymmetry - Google Patents

Description

n*nea»o npson laa Protective device or alternating-current motors against supply asymmetry z>"?2 mnsT *»j?n»> j-piaon toio TBCH ION RESEARCH AND DEVELOPMENT FOUNDATION LTD. -t- C. 27871 The present invention relates to a device for protecting alternatingr-current motors against supply asymmetry, and in particular fo protecting three-phase induction motors against single-phase operation.

Single-phase operation of three-phase induction motors ' is one. of. the main eauses of damage of such motors. Such single-* phase operation, is mostly due to fuse-blowing,, and it produces overheating and subsequent damage of the stator and sometimes of the rotor coils.

Various methods ai-e commonly used to prevent damage of motors caused by a non-symmetric power- supply* or example, one of these methods is based on comparing the voltages in the three lines supplying power to the motor,, and shutting off this power whenever there is an appreciable difference between these voltages. One deficiency of this method lies in the fact that a three-phase induction motor will act as a generator when powe in one of the phases is interrupted:, and will thus produce an induced voltage in this phase.: The measured Voltage difference between the three phases will therefore be relatively small, even though one phase receives no outside power at all. This method is therefore not very selective i differentiating between complete loss of one phase on the one hand and small voltage differences between the jphases often appearing during normal operation on the other,.

Another method is based on comparing the currents flowin in the three lines to the motor, and shutting off the power whenever there is an appreciable difference between them. This method is costly, since expensive current transformers and relays must usually be used to measure and compare the currents. In addition, the protective device must be by-passed during start-up of the motor until the currents in the three lines have had a and the motor could become damaged during this perioc^f one of ' the phases should have no power.

In addition, the above-described methods and most other conventional methods suffer from the disadvantage tliat they measure the voltage or current in the supply mains of the motor* When high-voltage motors are used, these methods all become very' 1 expensive, since the measuring device must be built to withstand these high voltages. ' It is an object of the present invention to provide a new and improved protective device for a multiphase alternating current motor against power in which the above referred to disadvantages are substantially reduced or overcome; According to the present invention there is provided a protective device for a multiphase alternating-current motor against power-supply asymmetry, comprising, in combination: a pick-ύρ coil mounted in proximity to the motor axis, a frequency-selective filter connected to said pick-up coil, said filter being tuned to one of the characteristic frequences of the motor whose signal level differs during symmetric conditions from that level during asymmetrical conditions, and switching means actuated by said filter, said switching means being adapted to interrupt the power supply of the motor whenever the signal reaching said switching means from said filter exceeds a predetermined value· For a better understanding of the present invention and to show how the same can be carried out in practice reference will be made to the accompanying drawings in which: Figure 1 is an electric circuit diagram illustrating one of the embodiments in which the invention may appear* and Figure 2 shows a typical twin-tee selective filter used with the embodiment of the invention described.

The invention is based upon measuring changes of leakage fields of the motor which appear whenever the suppl is non-symmetrical.' These changes are especially pronounced with respect to a voltage of frequency (2 - s)f $ where f is the frequency of the power supply and s is slip of the motor. The strength of this voltage increases by a factor of 30 or more during single-phase operation, as compared to normal three-phase operation. This voltage (after appropriate filtering from the total voltage induced in the piok-up coil 15) ie used to actuate a switching device cutting off the power supply to the motor in order to prevent damage.

Figure 1 shows a three-phase motor 10 being supplied by a three-phase power supply 11 by means of three mains 12, 13 and 14 respectively. A pick-up coil 15 is mounted next to the motor near the motor axle* In cases where the motor is designed with the invention in mind, the coll can even be mounted inside the motor housing. In cases where it is desired, to attach the device of the invention to an existing motor, the coil is preferably mounted outside the motor housing, since there will usually not be sufficient room inside the housing. It has been found that the pick-up coil is most sensitive if it has a diameter approximately equal to the inside diameter of the stator.

The pick-up coil 15 will pick up voltages at various frequencies induced by the leakage field of the motor.

Experiments have shown that whenever the motor supply is non- symmetric, here appears a frequency equal to (2r-s)f, wliS3?§ £ is facegueney of he powei1 supply (generally 50 or 60 cps), and s is the slip of the motor. Since the . slip of typical induction motors is usually very small (on the order of 0.01 to.0.04), this frequency is essentially equal to the second harmonic 2f. Thus, for a 50 cps power supply, this additional frequency appearing in the leakage field might be approximately 99 cps. While this frequency will be present to some extent even under normal conditions, its voltage as picked up by the pick-up coil will be negligibly' small compared to the other frequencies as long as the supply is symmetrical.

However, during one-phase operation, the voltage of this frequency will increase by a factor of 30 or more, whereas the voltages of the other frequencies will, change very little.

Accordingly, the invention uses a selective filter 16 connected to the coil 15, and so designed as to pass only frequencies in the neighbourhood of 2f (or 99 cps in the above example). This filter could, for example, be of the well-known twin- tee type, as illustrated in Fi . 2 in orde such filters is well, known in the art. The filter of Pig. 2 has "been designed so as to pass only frequencies close to 99 cps.

Filter 16 is connected to relay coil 17, which breaks contact .19, thus tripping a circuit breaker 18. Circuit breaker.18 normally connects the power supply 11 to the mains 12, 13 and 14.

In operation, whenever the -output..of filter ..16 exceeds a certain value due to asymmetry in the power supply, the relay coil 17 trips the circuit breaker 18, and thus disconnects the', power supply 11 from the motor mains 12, 13 and 14, thus preventing damage of the. motor. ' The filter! 16 could also include . an electronic; " ainplifier to .increase the voltage available for actuating the relay coil 17... The use of such an amplifier is advisable wherever the pick-up coil 15 or the motor 10 is small, so that the voltage . induced in the pick-up coil is very small. Such selective amplifiers are well known in the. art.

It will be obvious to those skilled in the art that various other types of switching elements can be substituted for' the relay coil .17 and circuit breaker- Ί8, such as transistors, reed relays, cold-cathode tubes, etc., without changing the principle of the invention.

Experiments were conducted, for example, on a typical 2 horsepower three-phase induction motor designed for 400 volt 50.eps operation and rotatin at 1000 r outside the motor housing concentric with the motor shaft. Aa long as the power supply was symmetric for the three ' phases, the pick-up coil picked up a 50 cps. signal of 12 mv strength, a signal at approximately 100 cps of 0.3 mv, and a il50 cps signal of 1 mv. When one of the three phases was cut I out from the power supply, the pick-up coil picked up a 50 cp: 'signal of 12 mv strength, a signal a . approximately 100, cps of 10 mv, and a 150 eps signal of 0.6 mv. Thus it is seen: that the 100 cps signal, increased over thirtyfold due to the missing phase, while the other signals were hardly affected. It is therefore obvious that the invention gives a reliable • indication of power-supply asymmetry; . ' ' The advantages of the invention will now be obvious.

Since the pick-up coil is not connected directly to the supply mains of the motor, the invention need no be built to withstand high voltages even in the „case of high-voltage motors. The invention is therefore extremely simple and inexpensive, especially if .only a selective filter without amplifier is used. Furthermore, the invention is effective even during the start-up period of the motor. Furthermore, the invention is extremely reliable. Furthermore, the invention can readily be attached to existing motors.

It is to be understood that the form of the invention here described is to be taken as a preferred embodiment.

Various changes may be made in the shape and arrangement of parts, and equivalent elements may be substituted for those described, all without departing from the spirit or scope of

Claims (6)

26240/2· :*-·. - 7 - CLAIMS , 1. A protective device for a multiphase alternating-current motor against power-supply asymmetry, comprising, in combination: a pick-up coil mounted in proximity to the motor axis, a frequency-selective filter connected to said pick-up coil, said filter being tuned to one of the characteristic frequences of the motor whose signal level differs during symmetric conditions from that level during asymmetrical conditions, and switching means actuated by said filter, said switching means being adapted to interrupt the power supply of the motor whenever the signal reaching said switching means from said filter exceeds a predetermined value. 2. A device of Claim 1, wherein said frequency-selective filter is adapted to substantially attenuate all voltages excepting those having a frequency of (2 - s)f where f is the frequency of the power supply and s is the slip of the motor. 3. A device of Claim 1, wherein said frequency-selective filter comprises a twin-tee network. 4. A device of Claim 1, wherein said frequency-selective filter includes amplifying means. 5. A device of Claim 1, wherein the pick-up Coil has a diameter substantially equal to the inside diaineter of the' motor stator. 6. A protective device for multiphase alternating-current motors against power-^supply asymmetry substantially as hereinbefore described by way of example and with referenoe to the accompanying drawings. 26240/2 - 7 - CLAIMS

1. Δ protective device for a multiphase alternating-current motor against power-supply asymmetry, comprising, in combination: a pick-up coil mounted in proximity to the motor axis, a frequency-selective filter connected to said pick-up coil, said filter being tuned to one of the characteristic frequences of the motor whose signal level differs during symmetric conditions from that level during asymmetrical conditions, and switching means actuated by said filter, said switching means being adapted to interrupt the power supply of the motor whenever the signal reaching said switching means from said filter exceeds a predetermined value.

2. A device of Claim 1, wherein 'said frequency-selective filter is adapted to substantially attenuate all voltages . excepting those having a frequency of (2 - s)f where f is the frequency of the power supply and s is the slip of the motor.

3. A device of Claim 1, wherein said frequency-selective filter comprises a twin-tee network.

4. A device -of Claim 1, wherein said freq ency-selective , filter includes amplifying means.

5. A device of Claim 1, wherein the pick-up coil has a diameter substantially equal to the inside diameter of the motor stator.

6. A protective device for multiptiase alternating-current motors against power-supply asymmetry substantially as hereinbefore described by way of example and with