GB2323936A

GB2323936A - Mains flicker reduction

Info

Publication number: GB2323936A
Application number: GB9706606A
Authority: GB
Inventors: Richard Ashley Hamer; David J Meads
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1997-04-01
Filing date: 1997-04-01
Publication date: 1998-10-07
Anticipated expiration: 2017-04-01
Also published as: GB2323936B; GB9706606D0

Abstract

A high current mains powered device eg a photocopier is controlled so that the number of operations requiring sudden changes in power consumption that can be carried out within a given time are limited. Flickering of lighting circuits resulting from operation of the device is thus kept to within permitted levels. For example the number of times that a copy can be made when the device is cold within a given period is limited.

Description

MAINS FLICKER REDUCTION The present invention relates to mains flicker reduction and is more particularly, although not exclusively, concerned with a method 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 flicker 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 appiication 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 beinq off for several seconds, more than once an hour. Moreover, when a fuser system is controlled using bangbang technology, each onloff cycle (occurring several times a minute) causes unacceptable levels of fluctuations in the mains line voltage.

It is therefore an object of the present invention to provide an intelligent control system which overcomes the problems mentioned above.

In accordance with one aspect of the present invention, there is provided a method of controlling mains flicker induced by an electrical device, the method comprising:- defining operating conditions for the device; monitoring changes in operation of the device; detecting sudden changes In operation of the device; and adjusting operation of the device to compensate for the detected sudden changes to maintain the previously defined operating conditions.

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 flow chart illustratrng the mains flicker control method in accordance with the present invention; and Figure 3 is a graph illustrating an operating curve showing the relationship between time on, time off and power.

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.

The power consumption of electrical devices are often periodically and predictably changed, for example, a controlled heater which is switched between off, low power and high power.

The power consumption of electrical devices may also unexpectedly need to be switched on in response to an external stimulus, for example, the fuser system of reprographic apparatus wherein the system is switched to full power immediately prior to a copy run after the apparatus has spent a period in the standby mode.

'Flicker' is defined in the standard (EN 61000-3-3:1995) as the "inpression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time" A 'short term flicker indicator (Psi)' is defined as "the flicker seventy evaluated over a short period (in minutes); Pst = lis the conventional threshold i1 iiiv'tability" 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 dtm. From the standard, the Pst value corresponding to the voltage change d is defined as Pst = d/d,im, 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.For a given current draw, if the minimum time between current changes is 'f, then the regulations could be complied with by making one change every t seconds. This covers the instance where the power consumption of electrical devices are often periodically and predictably changed, for example, in the thermostatic control of a heater as discussed above.

However, mains flicker is determined, in accordance with EN 61000-3-3:1995, by observing an electrical device for ten minutes. This means that the regulations would also be complied with if two changes were allowed every 2t seconds, three every 3t, etc. up to m changes every mt seconds where mt < 10 minutes, that is, up to the observation period.

In accordance with the present invention, an intelligent control system is provided which monitors and limits the number of times that a high current mains powered device is switched between its 'on' and 'oW states in a predetermined time period, for example, the minimum observation time for mains flicker (ten minutes).

By allowing m changes per mt seconds, sudden changes of state of the electrical device, for example, caused by a fuser system being switched on to full power before a copy run, would comply with the regulations. However, whilst this produces unacceptable levels of mains flicker in the short term, for example, in the first 10s, it is compensated for by allowing less mains flicker than would normally be obtained later on, for example, in the remaining time of the period mt, and provides an acceptable average mains flicker level.

The method of the present invention allows a particular control cycle to consist of m voltage changes, made over a period of mt (where t is as defined previously). For example, if the power and responsivity requirements for a heater are known, and the heater is switched between low and high power, rather than on and off, then the power consumption can be altered by varying the ratio of time on to time off as shown by the operating curve in Figure 3. The amount of time the heater is on or off does not matter as long as the complete cycle (that is, the time on plus the time off) satisfies the minimum average time between changes. In this particular example. m = 2, that is, two voltage changes (on then off) is allowable in double the normal cycle time, t.

A flow chart illustrating the operation of the method in accordance with the present invention is shown in Figure 2. The method involves monitoring an electrical device, for example, a high current device such as a heater, fuser system for reprographic apparatus, or other similar device, for its changes in current or power consumption.

The flow chart comprises two portions 10, 20 which respectively cater for predictable control requirements, that is, steady state conditions, e.g. a thermostatically controlled heater, and unpredictable or unexpected requirements, that is, sudden condition changes, e.g. a fuser system switching to full power prior to a copy run. In portion 10, the first step is to define the mean time t between changes, and the number of changes per cycle to provide a duration of mt seconds (block 12). In normal operation, the second step (as shown by block 14) is to allow m changes per mt seconds. This defines the predictable changes.

For unpredictable changes, as indicated by portion 20, the next step is to check for sudden change (block 22). If there is no sudden change, normal operation is maintained as shown by arrow 24. If there is a sudden change, shown by arrow 26, the operation of the device is adjusted (block 28) so that the average flicker level is acceptable within the minimum time period for flicker of ten minutes, that is, the control system spreads the change over the predetermined cycle time. A check is then made (block 30) to determine whether the cycle time, that is, m changes per mt seconds, has been re-established. If the cycle time has not been re-established, the operation is adjusted as shown by arrow 32. If the cycle time has been re-established, normal operation is resumed as shown by arrow 34. It will readily be appreciated that an intelligent control system may consist of any or all of these decision making elements.

Claims

CLAIMS:

1. A method of controlling mains flicker induced by an electrical device, the method comprising: defining operating conditions for the device; monitoring changes in operation of the device; detecting sudden changes in operation of the device; and adjusting operation of the device to compensate for the detected sudden changes to maintain the previously defined operating conditions.