GB2342723A

GB2342723A - Electrical appliance testing apparatus

Info

Publication number: GB2342723A
Application number: GB9822462A
Authority: GB
Inventors: Andrew Gordon Sellars
Original assignee: Edgcumbe Instruments Ltd
Current assignee: Spirent Systems Ltd
Priority date: 1998-10-15
Filing date: 1998-10-15
Publication date: 2000-04-19
Anticipated expiration: 2018-10-15
Also published as: GB2342723B; GB9822462D0

Abstract

Apparatus to test electrical appliances comprising a capacitor and a means to charge it connected in parallel. Terminals 10, 12 are used to connect the appliance L across the capacitor C, which is discharged through the appliance. An internal load R is connected via a switch S with the capacitor and a control means 20 closes the switch at an instant during the discharge of the capacitor which increases the rate of discharge of the capacitor. This enables the appliance to be safely disconnected and the next appliance under test to be quickly connected.

Description

Electrical Test Apparats The present invention relates to an apparatus for use in testing electrical appliances.

In the manufacture of some electrical appliances, e. g. electric kettles, it is known to test the electrical insulation of the appliance by applying a predetermined high voltage to the appliance for a period of time. The test apparatus typically includes a generator which comprises a step-up transformer having a capacitor connected across its secondary winding via a rectifier: the capacitor is accordingly charged via the rectifier to store a high dc voltage of several kV.

The appliance to be tested is connected across the capacitor and forms an external load through which the capacitor discharges. However, whilst the capacitor charges quickly, in a linear or ramp manner, it discharges quite slowly if the external load, which the appliance presents to the circuit, remains of high impedance. The appliance cannot be safely disconnected from the test apparatus, for connection of the next appliance to be tested, until the capacitor charge has decayed to zero.

We have now devised a circuit arrangement which overcomes the above-noted difficulty.

Thus, in accordance with the present invention, there is provided an apparatus to testing an electrical appliance, the apparatus comprising a generator for applying a dc voltage across the appliance, the generator comprising charge storage means, means for charging the charge storage means, the appliance being connected or connectable across the charge storage means for discharge of the charge storage means through the appliance, a load connected via a switch to the charge storage means, and control means for closing the switch at an instant during the discharge of the charge storage means.

By switching the load (internal load) to the circuit to the charge storage means during the discharge of the charge storage means, the rate of discharge of the charge storage means is substantially increased. In this way, the charge storage means (preferably a capacitance) is fully discharged in a rapid manner.

The control means may be arranged to monitor the discharge current flowing through the appliance and close the switch, for connecting the internal load in circuit, when this current falls to a predetermined value or to a predetermined proportion of its initial value. Alternatively, the control means may be arranged to monitor the voltage across the capacitor or other charge storage means and close the switch once this voltage falls to a predetermined level or to a predetermined proportion of its initial level.

Two or more internal loads may be provided, with respective switches, and connected into circuit by the control means: different such loads may be connected in, possible at different times, according to the sensed current or voltage.

An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIGURE 1 is a diagram of the voltage generator circuit of a test apparatus in accordance with the present invention; FIGURE 2 is a waveform diagram to show the voltage variation on the capacitor of the prior art voltage generator circuit; and FIGURE 3 is a waveform diagram to show the voltage variation on the capacitor of the capacitor of the voltage generator circuit of the present invention.

Referring to Figure 1, there is shown the pulse generator circuit of an apparatus for testing an electrical appliance. The generator circuit comprises a step-up transformer T having a capacitor C connected across its secondary winding via a rectifier D. Terminals 10,12 are connected to the capacitor C, enabling an external load L (the electrical element of the appliance being tested) to be connected across the capacitor.

The circuit so-far described corresponds to the prior art arrangement. In use, the generator applies a high voltage level to the appliance: Figure 2 shows the variation in the capacitor voltage. Thus, the capacitor C is charged via the rectifier D to store a dc voltage of several kV: it will be noted that the capacitor charges in a linear or ramp manner and accordingly quite quickly (of the order of 2 seconds in the waveform which is shown). The ac supply is then switched off, or reduced such that the capacitor then discharges through the external load L, but providing the impedance of the appliance remains high (i. e. its insulation does not break down), the charge on the capacitor will decay only slowly.

In accordance with the present invention, the generator circuit further includes a resistive load R connected across the capacitor C via a switch S. The circuit further comprises a control means, indicated diagrammatically at 20, to monitor the current i flowing to the external load L, and close the switch S at a selected instant.

Initially the switch S is open. However, when the external load current i has reduced to a predetermined level, the control means 20 acts to close the switch S and so connect the internal resistive load R across the capacitor: the impedance of the internal load R is much smaller than the impedance of the external load, so that the capacitor C now completes its discharge quickly. Figure 3 shows the variation of voltage across the capacitor and it will be noted that, by switching in the internal load R at an appropriate instant, the capacitor can be made to discharge in a substantially linear or ramp manner, corresponding to its charging profile. The arrangement ensures that the capacitor is fully discharged quickly (in about 2 seconds, in the example shown). It will be appreciate that, in the event that the insulation of the appliance being tested breaks down, the external load current will remain high whilst the capacitor is discharged rapidly: the internal load R is accordingly not switched into operation.

Claims

Claims 1) An apparatus for testing an electrical appliance, the apparatus comprising a generator for applying a dc voltage across the appliance, the generator comprising charge storage means, means for charging the charge storage means, the appliance being connected or connectable across the charge storage means for discharge of the charge storage means through the appliance, an internal load arranged to be connected, via a switch, to the charge storage means, and control means for closing the switch at an instant during the discharge of the charge storage means.
2) An apparatus as claimed in Claim 1, wherein the control means is arranged to monitor the discharge current flowing through the appliance and to close the switch, to connect the internal load in circuit with the charge storage means, when this current falls to a predetermined proportion of its initial level.
3) An apparatus as claimed in Claim 1, wherein the control means is arranged to monitor the discharge current flowing through the appliance and to close the switch, to connect the internal load in circuit with the charge storage means, when this current falls to a predetermined level.
4) An apparatus as claimed in Claim 1, wherein the control means is arranged to monitor the voltage across the charge storage means and to close the switch, to connect the internal load in circuit with the charge storage means, when this voltage falls to a predetermined proportion of its initial level.
5) An apparatus as claimed in Claim 1, wherein the control means is arranged to monitor the voltage across the charge storage means and to close the switch, to connect the internal load in circuit with the charge storage means, once this voltage falls to a predetermined level.
6) An apparatus as claimed in any preceding claim, comprising two or more internal loads, each having a respective switch, and arranged to be connected in circuit with the charge storage means by the control means.
7) An apparatus as claimed in Claim 6, wherein different loads are arranged to be connected in circuit with the charge storage means according to the level of monitored current or voltage.
8) An apparatus substantially as herein described with reference to Figures 1 and 3 the accompanying drawings.