GB2117196A

GB2117196A - Detecting faults in a multi phase AC supply

Info

Publication number: GB2117196A
Application number: GB08206204A
Authority: GB
Inventors: Ronald Charles Adey
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-03-03
Filing date: 1982-03-03
Publication date: 1983-10-05

Abstract

A switching circuit, interposable between a multi phase AC supply and a motor-controlling circuit to switch off the motor should a fault occur in the supply, comprises a relay (RL) operable by any of the live supply lines (R, Y, B) to switch the control circuit in one sense, and a semi- conductor switching arrangement (D4, ZD1, VT1) for shorting out the relay (RL) in the event of a supply fault. The switching arrangement comprises a transistor (VT1) of which the base is connected through a rectifier (D4), and a zener diode (ZD1), to the null point (X) of three resistors (R2, R3, R4) connected in a star configuration to the live supply lines (R, Y, B). Under correct supply conditions, the potential between the null point (X) and a neutral line (N) is substantially zero, but should one or more of the live supply lines fail the null point (X) becomes unbalanced and the transistor (VT1) switched on to short- circuit the relay (RL). <IMAGE>

Description

SPECIFICATION Detecting faults in multi phase supplies This invention relates to devices for detecting faults in multi phase AC supplies, for example failure of one or more of the supply lines.

The invention is particularly concerned with a problem which frequently arises in star connected motors operated from a three phase supply. If one of the live supplies should fail whilst the motor is running, e.g. due to blowing of a fuse, the motor will continue to run on the two remaining supplies.

resulting in overheating and possible damage to the motor. Various arrangements for protecting the motor have been proposed but they all suffer from drawbacks; some tend to be sluggish in operation (as in the case of thermal cutouts) and some are disposed to fail to operate altogether, in certain circumstances, owing to the fact that back feed through the motor still produces a voltage on the unsupplied terminal. Also, thermal cutouts do not prevent the motor attempting to start under fault conditions.

One aim of the invention is to provide a device which is capable of responding rapidly and reliably to supply faults.

The invention provides means for detecting faults in a multi phase AC supply, comprising conductive elements connected together in a star configuration for connection to the live supply lines, the arrangement being such that the potential between the common point of the elements and the neutral line or earth is substantially zero under the correct supply conditions, and the common point being connected to sensing-and-switching means for sensing a rise in that potential due to a fault on the supply and switching a control circuit in response to that rise in potential.

In its simplest form the sensing-and-switching means may consist only of a relay operable by a rise in the stated potential, the coil of the relay being connected between the common point of the elements and the neutral line or earth.

Preferably the sensing-and-switching means includes a relay operable by one or more of the live supply lines to switch a control circuit in one sense, and override means for overriding the relay in response to a rise in the stated potential so as to switch the control circuit in the opposite sense.

The override means preferably comprises a semi-conductor switching arrangement for shorting out the relay in response to the rise in potential. Such an arrangement is capable of very rapid response. This switching arrangement is preferably so designed as to respond only when the stated potential exceeds a predetermined threshold, thus ensuring that small variations in the supply which normally occur do not operate the device.

In a preferred application the device is used to protect a motor operated from the supply by switching a control circuit including the coil of a motor-control contactor so that the contactor switches off the motor in response to a fault on the supply.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of the device, and Figure 2 is a circuit diagiram which illustrates an application of the device.

Referring to Figure 1, three connection points R, Y and B are provided for connection to the live lines of a three phase supply, and a further point N is provided for connection to either the neutral line or earth, whichever is convenient.

The incoming supply is half-wave rectified by diodes Di-D3 and the rectified output current is fed through a resistor Rl,the coil of a relay RL, and a light-emitting diode T1 L1, all connected in series. A smoothing capacitor C1 is connected across the relay coil and the LED to prevent relay chatter. Two normally-open contacts RL1 of the relay RL are connected between a pair of terminals FFR for switching an external control circuit.

Three resistors R2-R4 of equal value are connected to the incoming live lines in a star (wye) configuration. The common connection point X of the resistors (the null point) will produce the algebraic summation of the three supply voltages and will therefore be at, or somewhat near, zero potential with respect to the neutral or earth connection N when the supply lines are operating normally, that is, when they are all of equal voltage and 1 200 out of phase with each other.

This null point X is connected to the base of a transistor VT1 via a rectifier D4, a resistor R5, a zener diode ZD1 and a further resistor R6. A smoothing capacitor C2 is connected between the junction of zener diode ZD1 and resistor R6 and the neutral or earth point N. The collector-emitter junction of the transistor VT1 is connected across the coil of relay RL and the LED, with its emitter connected to point N.

In use, the connection points R, Y, B and N are connected to the supply to be monitored. Provided all three live supply lines are healthy, diodes Di-D3 supply current through resistor R1 to the coil of relay RL to operate it and close contacts RL1. However, if there is a fault on the line which, for example, results in one or more of the supply lines being dead, the rectified output voltage is lower than it should be and the relay fails to operate.

The current flowing through the relay coil can be monitored by the LED which lights up whenever the relay is energised.

As already explained, under normal supply conditions null point X is at zero potential. Thus, no current flows into the base of transistor VT1 and it remains switched off.

Once the relay RL has been energised its magnetic characteristics are such that if one, or even two, of the live supply lines fail subsequently, the resulting drop in current through the coil is insuffcient to cause the relay to drop out and open contacts RL1. However, in such an eventuality the null point X becomes unbalanced and its potential rises with respect to the neutral or earth point N.

The resulting AC waveform at point X is half-wave rectified by D4 and, provided the rectified potential exceeds the zener voltage of diode ZD1, current flows through the base-emitter junction of transistorVrl and switches on the transistor.

Thus, the coil of relay RL is short-circuited and contacts RL1 open.

The zener voltage of diode ZD1 is chosen so that potentials appearing at point X as a result of normal supply variations do not exceed the zener voltage, which is typically 1 Ov.

In the event that all three live supply lines fail simultaneously the output from rectifiers D1--D3 falls to zero and the relay RL drops out.

To summarise, relay contacts RL1 can only close if all three live supply lines are operating normally, and failure of any one or more of these supply lines will cause the contacts to open virtually instantaneously.

Figure 2 illustrates how the device may be used to protect a squirrel cage motor M. The three incoming live supply lines Li-L3 are connected via a three pole isolator switch SW1, fuses Fi-F3, contacts CTi-CT3 of a motor-control contact CT, and thermal cutout elements TCI--TC3. The coil of the motor-control contactor CT is operated from one of the live supply lines Li , from which power is taken at a point immediately following fuse F1 to supply the contactor via a normally closed stop switch SW2, a normally open starter switch SW3 and the normally closed contacts TC of the thermal cutout.

The contactor coil is returned to the neutral line N'. A further pair of contacts CT4 of contactor CT are connected across the starter switch SW3.

Considering first a case in which the protective device described above is ommitted, closure of starter switch SW3 energises the contactor to close contacts CT1--CT3 and so feed power to the motor M. At the same time contacts CT4 close to hold the contactor energised after the starter switch is released. If one of the supply lines should fail (e.g. if one of the fuses Fi-F3 blows) the motor M will continue to run until the thermal cutout operates to de-energise the contactor. This may take several seconds, during which the motor could overheat and burn out. Even if the failure occurs on line Ll,the contactor may still remain energised by back feed through the motor.

If the device (designated 3PFFR in Figure 2) is now connected between fuses Fi-F3 and contacts CT1--CT3 with terminals FFR in series with the coil of contact CT, as shown, the contactor cannot be energised to start the motor unless all three lines are healthy allowing contacts RL1 to close. Failure of any one or more of the supply lines will cause contacts RL1 to open and de-energise the contactor with little or no delay.

Even if only one supply line fails and there is a back feed from the motor, the null point will still be sufficiently unbalanced to cause contacts RL1 to open.

In many situations it is necessary to interlock several motors to ensure that one motor cannot - run without the others. This is usually achieved by placing a set of auxiliary contacts of one motor contactor in series with the contactor of the next motor, but if, for example, the fuse blows in one supply line to a motor its contactor may be held in by back feed through the motor, as described.

Thus, the next motor will continue to run. This situation can be prevented by providing each motor with a device as described, so that the respective contactor drops out as soon as the supply tails, and switchesbff the next motor in the chain.

The device is capable of protecting motors of 1/6 h.p. to over 1 000 h.p.

Claims

1. Means for detecting faults in a multi phase AC supply, comprising conductive elements connected together in a star configuration for connection to the live supply lines, the arrangement being such that the potential between the common point of the elements and the neutral line or earth is substantially zero under the correct supply conditions, and the common point being connected to sensing-and-switching means for sensing a rise in that potential due to a fault on the supply and switching a control circuit in response to that rise in potential.

2. Means according to claim 1 wherein the sensing-and-switching means comprises a relay operable by a rise in said potential, the coil of the relay being connected between the common point of the elements and the neutral line or earth.

3. Means according to claim 2 wherein said sensing-and-switching means comprises a supply line relay operable by one or more or the live supply lines to switch a control circuit in one sense, and override means for overriding the supply line relay in response to a rise in said potential so as to switch the control circuit in the opposite sense.

4. Means according to claim 3 wherein said override means comprises a semi-conductor switching arrangement for shorting out said supply line relay in response to the rise in potential.

5. Means according to claim 4 wherein said switching arrangement is arranged to respond only when potential exceeds a predetermined threshold.

6. Means for detecting faults in a multi phase AC supply substantially as hereinbefore described with reference to the accompanying drawings.

7. A switching circuit which can be interposed between a multi phase AC supply and a control circuit to switch said control circuit upon occurence of a fault in the supply, said switching circuit comprising a plurality of conductive elements connected together in a star configuration, for connection to the live supply lines in such a manner that the potential between the common point and the neutral line or earth is substantially zero under the correct supply conditions, and switching means connected to the common point of said elements for switching said control circuit in response to a rise in the potential at the common point.

8. A protection circuit for an electric motor adapted to be operated from a multi phase AC supply, said protection circuit comprising a switching circuit according to claim 7 interposed between the supply and a motor-controlling circuit to switch off the motor in response to a fault on the supply.