GB2165705A

GB2165705A - Starter noise reduction devices for fluorescent lamp ballast inductors

Info

Publication number: GB2165705A
Application number: GB08426272A
Authority: GB
Inventors: David John Martin
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-10-17
Filing date: 1984-10-17
Publication date: 1986-04-16
Also published as: GB8426272D0

Abstract

A ballast indicator for a fluorescent lamp circuit has an axial sleeve of non-magnetic conducting material for shielding leakage flux from a fluorescent lamp housing to prevent unwanted noise being induced into the lamp body during starting. The sleeve may be an extrusion of aluminum or a wrapped copper foil and may contain one or more apertures. <IMAGE>

Description

SPECIFICATION Starter noise reduction devices for fluorescent lamp ballast inductors This invention relates to fluorescent lamps with a choke ballast and an electronic starter of the type which passes unidirectional (rectified) heating current through the ballast and tube heaters during the start sequence. With most types of unidirectional starter the DC component of the heater current results in saturation of the ballast inductor core. The corresponding reduction of the ballast impedance over part of the current conduction period gives rise to a much increased heater current as shown in Fig. 1. Advantage can be taken of this increase over the normal heater current to obtain a shorter heating time and hence a faster start.However, a disadvantage of allowing the ballast to saturate is that a large pulsed leakage magnetic field around the inductor is generated because the flux is no longer constrained to follow the magnetic circuit of the core once it saturates. If the ballast inductor is mounted in a lamp housing fabricated in light gauge steel, as is commonly the case, the leakage flux finds a magnetic path through the housing. Fig. 2 shows a typical inductor construction and location in a lamp housing and Fig. 3 shows a cross section of the inductor and housing with the magnetic field of the leakage flux. The magnetism and attendant forces introduced cause movement of the housing which is manifest as a very noticeable and undesirable level of 50 Hz pulsed audible noise during starting.

Two well established effects relevant to magnetic flux deflection can be applied to control the leakage flux. They are the eddy current effect and the shorted turn effect. As will be seen the shorted turn effect is the most significant in this application.

Fig. 4 illustrates the eddy-current effect. A field at right angles to a non magnetic conductive sheet gives rise to induced circulating currents in the sheet which generate a field such as to oppose the incident flux field, thereby reducing the total flux through the sheet.

Fig. 5 shows the shorted turn effect which is similar to the eddy-current effect in that current induced by an incident field within a loop of non magnetic conductive material oppoises the incident flux. Provided the loop has a very low resistance, most of the incident flux is excluded from passing inside the loop.

Applying these effects to reduce the leakage flux from a saturated ballast inductor, the first and obvious improvement is to place a conductive non magnetic sheet between the inductor mounting face and the lamp housing.

This gives two improvements. First a gap is introduced between the metal of the inductor and the lamp housing and secondly the eddycurrent effect reduces the amount of flux reaching the lamp housing on the mounting face side. This still leaves a considerable amount of flux in the sides of the lamp housing as shown in Fig. 6. A further conductive sheet can be placed above the inductor but this does not contribute much because there is already a large air gap and there is still a ready path for flux in the sides of the housing as shown in Fig. 7.

Further single plates at the sides of the inductor contribute little to impede the flux because the plates would be inside a loop field rather than at right angles to the main flux path (dotted in Fig. 7). A shorted turn around the whole inductor is more effective but not very convenient to apply.

According to this invention though, a complete sleeve of non-magnetic material placed conveniently axially around the inductor does in fact provide for a shorted turn encompassing the sides of the inductor. This is because the induced current flows along the sides of the sleeve, then over the top and bottom of the sleeve at one end, back along the opposite side and over the top and bottom of the other end. This is shown in Fig. 8. Thus a simple sleeve provides for eddy-current screening top and bottom, and gives shorted turn flux deflection for the leakage flux in the critical path to the sides of the lamp housing.

In a practical embodiment of this invention, a convenient and inexpensive rectangular cross section of aluminium extrusion is placed around the ballast inductor. The dimensions of the embodiment are shown in Fig. 9 and it will also be seen that with the high length to width ratio of the particular ballast, it is not necessary to have the sleeve covering the whole length.

Another embodiment for a situation where the avaiiable cross-section of the lamp housing is not much bigger than the unmodified ballast inductor, it is possible to wrap the inductor with a suitable foil (e.g. copper) as shown in Fig. 10.

In a further embodiment, if for reasons of cost, weight or cooling the full sleeve is not appropriate it is possible to remove material in the middle of the top of the sleeve either completely (Fig. 11) or by forming holes (Fig.

12) without significantly affecting the flux reaching the housing.

The sleeve method of leakage flux control can also be applied to inductors having windings at right angles to the type as shown so far. Fig. 13 shows such an arrangement and the sleeve in this case simply forms a shorted turn around the unwanted leakage path thus confining most of the flux within the sleeve.

The embodiments described reduce the starting noise of the ferro-magnetic fittings in which they were placed to negligible levels thus providing a much more subjectively acceptable performance.

Claims

1. A sleeve of non-magnetic conductive material axially surrounding the ballast inductor in a fluorescent lamp fitting in order to reduce mechanical vibration, and hence audible noise, in the body of the fitting caused by leakage magnetic flux from the inductor during starting of the lamp.

2. A sleeve as claimed in claim 1 wherein the sleeve forms part of the ballast inductor assembly.

3. A sleeve as claimed in claims 1 and 2 where part of the top centre or bottom centre of the sleeve material has been removed.

4. A sleeve as claimed in claim 1 formed by wrapping a number of turns of conductive foil in the shape required to fit round the ballast inductor.