GB2323982A - Mains flicker reduction - Google Patents

Abstract

Control of a mains powered high current element 130, such as a heater element in a washing machine or, particularly a fuser system for reprographic apparatus, is effected by use of a multi-tap transformer 110, whereby the element 130 can be switched between at least two non-zero power levels. An existing control system, which produces a duty cycle or phase angle control signal, may be modified by translating the existing control signal into a signal which controls a switch 140 to select the transformer tap giving the equivalent average power output from the fuser 130. Tap changing occurs at mains voltage zero crossings. The arrangement reduces the number of times the fuser is switched between off and on in a given time period, thereby reducing the amount of flicker in adjacent mains powered lighting.

Description

MAINS FLICKER REDUCTION The present invention relates to mains flicker reduction and is more particularly, although not exclusively, concerned with a method and control system for controlling high current elements or devices, for example, fuser systems in reprographic apparatus, to reduce the effects of switching on mains circuits.

It is well known that the application of a line voltage to a load which draws significant current, for example, a fuser system in a reprographic apparatus, can reduce the line voltage of equipment connected to neighbouring outlets when it is powered up from 'cold'. In particular, lighting units can suffer 'flicker' as the surge currents to the load cause dimming. This can be disturbing to occupants in a room or office in which the reprographic apparatus is located and also to occupants in adjacent or neighbouring rooms or offices. Legislation against this perceived is due to come into force in 1998 under EN 61000-3-3:1995.

British Standard EN 61000-3-3:1995 (equivalent to IEC 1000-3-3:1994) relates to the limitation of voltage fluctuations and flicker in low-voltage supply systems for equipment with rated current of or below 16A per phase and intended to be connected to distribution systems of between 220V and 250V at 50Hz line to neutral. Limits are specified for voltage changes which may be produced by a piece of equipment.

It is thought that current reprographic apparatus may not comply with the regulations because the application of line voltage to a load with a positive temperature coefficient, such as a cold filament In a fuser system, can result in a surge of current which is five times greater than the current used when the filament has reached its normal operating temperature. This occurs because the cold filament has a much lower electrical resistance than the same filament at its normal working temperature. This current surge will cause flicker and typically means that a fuser system in reprographic apparatus will not comply with the regulations if it is turned on, after being off for several seconds, more than once an hour. Moreover, when a fuser system is controlled using bangbang technology, each on/off cycle (occurring several times a minute) causes unacceptable levels of fluctuations in the mains line voltage.

One method of overcoming the problem of mains flicker is to use a proportional control circuit to control the operation of a high current element, the proportional control circuit providing a linear increase of current with time when the high current element is energised. However, such control circuits are costly as they utilise linear electrical elements.

It is therefore an object of the present invention to provide a method and control system in which switching between two or more voltage levels is controlled to substantially reduce the effects of mains flicker as described above.

It is another object of the present invention to provide a fuser system in reprographic apparatus in which this type of control replaces conventional arrangements wherein the fuser system is switched on and off quickly.

In accordance with one aspect of the present invention, there is provided a method of controlling mains flicker due to current changes in a high current element, the method comprising switching the high current element in response to control means, characterised in that the method includes switching the high current element between at least two non-zero power levels using a multi-tap transformer.

In accordance with another aspect of the present invention, there is provided a control system for controlling mains flicker comprising:- a high current element; control means for controlling operation of the high current element; and switching means for switching the high current element in response to the control means; characterised in that the switching means switches between two or more non-zero power levels and includes a multi-tap transformer connected across the high current element via a multi-position switching member.

The method and control system of the present invention has the advantage that mains flicker due to the operation of high current elements, for example, fuser systems in reprographic apparatus, is substantially reduced.

Moreover, the method and control system of the present invention can be adapted to operate with existing software and hardware. Furthermore, the present invention can be used to limit inrush currents into cold high current elements or devices, for example, the filaments of fuser systems, when they are switched on after being off or inactive for substantial periods of time. This is achieved by selecting an 'ultralow current' setting on the transformer, when starting the fuser system from cold.

Another benefit of the method and control system of the present invention, is that the fuser system always has some current passing through it during normal operation of the reprographic apparatus. This eliminates cold inrush currents.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings, in which: Figure 1 is a graph summarising the effect of the new legislation, EN 61000-3-3:1995, in terms of voltage change (expressed as a percentage) against the number of voltage changes per minute; Figure 2 is a graph illustrating voltage change with time for current bang-bang control techniques; Figure 3 is a graph illustrating voltage change with time for the method and control system of the present invention; and Figure 4 is a block diagram of a control system in accordance with the present invention.

Although the present invention is to be described with reference to mains flicker control for a fuser system in reprographic apparatus, it will readily be appreciated that the present invention is equally applicable to any other device wherein switching of the device produces mains flicker, for example, high current heater elements in washing machines.

'Flicker' is defined in the standard (EN 61000-3-3:1995) as the "impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time" A 'short term flicker indicator (Pst)' is defined as "the nicker severity evaluated over a short period (in minutes); Pst = 1 is the conventional threshold of irritability". The full test conditions for a piece of equipment is laid down in EN 61000-3-3:1995.

A Pst = 1 curve from the standard EN 61000-3-3:1995 is shown in Figure 1. This curve is applicable in the case of rectangular voltage changes of the same amplitude d separated by equal time intervals. The curve can be used to determine the amplitude corresponding to Pst = 1 for a particular rate of repetition - this amplitude is called dim. From the standard, the Pst value corresponding to the voltage change d is defined as Pst = d/d11rn, and the observation period, Tp, for Pst is 1 Omin.

In Figure 1, solid line 10 defines a boundary between a non-compliant region 20 and a compliant region 30. In particular, for fuser systems, the current change is limited to one current change of 16A per hour (not shown in Figure 1). It is to be noted that 1200 voltage changes per minute gives a 10Hz mains flicker. This corresponds to the minimum point 12 on the graph in Figure 1 where changes are perceptible and may be dangerous, for example, to sufferers of epilepsy.

In current bang-bang control techniques, the fuser system is switched between zero power and a high power mode by varying the duty cycle of a pulse waveform lasting several seconds. This is shown in Figure 2. Bang-bang control techniques involve many voltage changes at mains zero crossing points, as indicated by the arrows. In the example illustrated in Figure 2, there are ten of these voltage changes per second in both the low and high power modes. Referring back to Figure 1, it can be seen that an amplitude change of less than 0.3% is allowed with each on/off and off/on transition when there are ten such transitions per second as shown in Figure 2.

This corresponds to point 12 on solid line/l0 in Figure 1.

It is therefore desirable to made the duty cycle waveform last for longer than 0.1s (equivalent to 10 transitions per second) to take advantage of the higher current fluctuations which can occur when changes take place less frequently as illustrated in Figure 1. However, as the waveform period increases, the control becomes less flexible and the fuser system may become too cold or too hot during the onloff phases.

In accordance with the present invention, conventional methods of controlling the 'apparent' voltage across the fuser system, for example, using a low frequency pulse wave modulation (PWM) technique, are replaced with a step-down transformer and control algorithm which switches between various step-down ratios. In the conventional method illustrated in Figure 2, there are twenty on/off changes in 2s. With the control system of the present invention, a fluctuation in the amount of current drawn in a cycle would occur only when switching from low to medium or high power modes. By way of example, switching between two non-zero power modes Is shown in Figure 3, but it will readily be appreciated that the different power modes or levels available will depend on the choice of transformer.

In Figure 3, a pulse waveform which may be obtained in accordance with the control system of the present invention is shown. It can readily be seen that mains flicker has been improved in two ways. First, in a given period of 20s there will typically be, say, two current changes as opposed to the two hundred current changes with the conventional method described above.

Secondly, the current is switching between low and high power modes instead of on and off. As a result, both the frequency and level of flicker are substantially improved by the control system of the present invention. The result of this is that the number of voltage changes per minute are reduced and therefore the amplitude change d, which is allowed can be increased. For example in Figure 1, for six voltage changes per minute, d is just over 1.5%.

Figure 4 illustrates a block diagram of a control system 100 in accordance with the present invention. The control system 100 comprises a multi-tap transformer 110 connected to a mains supply 120 on one side thereof and across terminals 132, 134 of a fuser system 130 on the other side thereof via a multi-position switch 140. The transformer 110 includes four taps 112, 114, 116, 118 as shown. Each tap 112,114,116,118 is connected to a respective input terminal 142, 144, 146,148 of switch 140. It will readily be appreciated that, although transformer 110 and switch 140 are shown having four taps and four sets of terminals respectively, they may also have more than four sets or less than four sets according to the particular application.

Switch 140 includes four output terminals 152, 154, 156, 158 and switch elements 162, 164, 166,168 which make appropriate connection between the input terminals 142, 144, 146. 148 and the output terminals 152, 154, 156, 158. Switch element 164 is shown in the closed position, that is, making connection with input terminal 144 and output terminal 154. Its open position is shown in dotted form. Output terminals 152,154, 156, 158 are each connected to an output line 170 by way of respective connections 172,174,176,178 as shown.

Tap 112 of transformer 110 comprises a high output tap, and is connected through terminals 142, 152, switch element 162, and connection 172 to provide a high output for fuser system 130. In a similar way, taps 114, 116, 118 provide respective medium, low and very low outputs for fuser system 130. In Figure 4, the fuser system 130 is connected to a medium output.

If the transformer has only three taps, these may correspond very low, medium and high power settings. Similarly, if the transformer has only two taps, the power settings are medium and high. However, it will readily be appreciated that the differences between each pair of adjacent power levels will be chosen so that the amplitude change, d, meets the requirements of EN61000-33:1995.

As shown, terminal 132 of fuser system 130 is connected directly to transformer 110 by way of direct tap 180, and terminal 134 is connected to output line 170 of switch 140.

Switch 140 is controlled by switch control circuit 200 in accordance with information provided by software control of the reprographic apparatus (not shown) which generates a fuser control signal (step 300) so that the duty cycle or phase angle can be measured (step 400). These steps are known in the art and will not be discussed in detail here.

in accordance with the control system of the present invention, the transformer 110 and switch 140 set the power levels for the fuser system 130 in accordance with control signals from the switch control circuit 200. Switching between different power levels occurs at mains zero crossing points where the voltage is approximately zero. This minimises spikes in the mains waveform and the harmful effects if part of the transformer coil is shorted out during the switchover period, for example, if two outputs are momentarily selected at once. It will readily be appreciated that if only two power levels are utilised, a standard relay could be used to ensure break-before-make operation is obtained.

For the particular embodiment described with reference to Figure 4, the high output setting is used when the reprographic apparatus is being used for long jobs and the fuser system is required to be on for substantial periods of time. The low and medium settings are used to maintain the fuser system within a predetermined temperature range whilst the reprographic apparatus is in its standby mode. The very low setting is used when the fuser system has not been activated for a while and it is necessary to ensure that inrush currents caused by a cold fuser system are kept to a minimum.

If it is desired to use software control algorithms without alteration, additional circuits can be employed to measure the duty cycle of the fuser control signal for bang-bang control systems or the phase angle for phase angle control systems to determine the power level which is required.

However, although this may involve additional cost, it does mean that the software control system does not need modification when converting from bang-bang or phase control techniques.

Although the present invention has been described with reference to a control system 100 for a fuser system for reprographic apparatus, it will readily be appreciated that the control system can be used to control any other high current element. In such a case, the fuser control signal (step 300) could be any suitable control signal for the high current element to be controlled, the measurement of duty cycle or phase angle (step 400) could be any suitable error relating to the high current element to be controlled, and the switch control circuit 200 could be any suitable switch control circuit for the high current element.

Claims (8)

CLAIMS:

1. A method of controlling mains flicker due to current changes in a high current element (130), the method comprising switching the high current element (130) in response to control means (300, 400), characterised in that the method includes switching the high current element (130) between at least two non-zero power levels using a multi-tap transformer (110).

2. A control system for controlling mains flicker comprising: a high current element (130); control means (300, 400) for controlling operation of the high current element (130); and switching means (200) for switching the high current element (130) in response to the control means (300, 400); characterised in that the switching means (200) switches between two or more non-zero power levels and includes a multi-tap transformer (110) connected across the high current element (130) via a multi-position switching member (140).

3. A control system according to claim 2, wherein the multi-tap transformer means (110) has four output levels providing four power levels.

4. A control system according to claim 2 or 3 wherein the multi-position switch member (140) has four positions.

5. A fuser system for reprographic apparatus operating in accordance with a method according to claim 1.

6. A fuser system for reprographic apparatus including a control system according to any one of claims 2 to 4.

7. A method substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.

8. A control system substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.