GB2323983A

GB2323983A - Current inrush control

Info

Publication number: GB2323983A
Application number: GB9706607A
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
Also published as: GB9706607D0

Abstract

Current inrush into a load 20 is controlled by means of an active current limiting device 40 responsive to a control signal from a control device 60 which compares the peak current drawn by the load 20 with a predetermined value. The load current may be limited to the predetermined value for a predetermined time. The current limiting device 40 may comprise switched power resistors or a phase controlled triac. An input 64 to control device 60 allows the maximum peak current to be set- eg. 10 to 20% above the normal current. The load 20 may be a heater element in a washing machine or, particularly, a fuser system for reprographic apparatus.

Description

CURRENT INRUSH CONTROL The present invention relates to current inrush control 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 omces. 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 å 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.

Passive inrush current limiting devices are known, for example, devices having negative temperature coefficients. However, such devices have the disadvantage that the thermal iag in periods of cooling is too long. This means that they are therefore not effective for use in circuits where time intervals between inrush currents are shorter than the cooling period.

It is therefore an object of the present invention to provide a method and control system in which current levels in a circuit containing a high current device are controlled to be within predetermined limits thereby substantially reducing the effects of inrush currents as described above.

In accordance with one aspect of the present invention, there is provided a method of providing current inrush control to a circuit including a device to be operated, the method comprising:- determining the peak current in the circuit; comparing the peak current with a predetermined value; and limiting the current to the predetermined value for a predetermined time.

In accordance with another aspect of the present invention, there is provided a control system for controlling inrush currents in a circuit including a load, the system comprising:- current determining means for determining peak current values; current limiting means for limiting the peak current values to a predetermined value; and control means for controlling the current limiting means in response to signals from the current determining means.

The method and control system of the present invention have the advantage that inrush currents due to the operation of high current elements, for example, fuser systems in reprographic apparatus, is substantially reduced. In particular, 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.

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 block diagram of a control system in accordance with the present invention; and Figure illustrates the effects of current inrush control in accordance with the present invention.

Although the present invention is to be described with reference to current Inrush 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 on of the device produces a surge in current with an accompanying reduction in line voltage, for example, high current heater elements in washing machines.

In accordance with the present invention, it is desired to limit inrush currents, typicaily 500% greater than normal operating currents, when a high current element, for example, a fuser system in reprographic apparatus, is switched on, to suitable values which fall within the requirements of EN 61000-3-3:1995.

Figure 1 shows a block diagram of a current inrush control system 1 in accordance with the present invention. The control system 1 comprises an A.C. line voltage source 10 which is connected in series with a device 20 to be controlled, for example, a fuser system for a reprographic apparatus, a current measuring device 30 and an active current limiting device 40. The current measuring device 30 comprises a low impedance instantaneous current measuring device such as, for example, a current transformer or a Hall effect device. The current limiting device 40 may comprise, for example, switched power resistors or phase control using triacs. Output signals 32 from the current measuring device 30 are input to a peak detector circuit 50 which provides output signals 52 as an input to control device 60. The peak detector circuit 50 may comprise, for example, an electrical circuit to sample and hold the peak current. The control device 60 may comprise, for example, a microprocessor or analogue controller. Output 62 from control device 60 provides control signals for current limiting device 40. Input 64 to control device 60 allows the maximum peak current to be set. The maximum peak current may be set as a percentage above the normal current, for example, between 10 and 20% above the normal current.

Referring now to Figure 2, current from the A.C. line voltage source 10 is shown as trace (A). When a switch (not shown) is closed to operate the device 20, the current drawn from the system 1 is shown by trace (B). This has the effect on the line voltage trace (A) which is illustrated as trace (C). Here, it can readily be seen that the current increases as an initial surge which greatly exceeds the normal operating limits as indicated at the end of the trace.

Referring also to Figure 1, when the device 20 is switched on after a period in an 'off state, the control system 1 will initially enforce full current limiting as a default to ensure that there are no surges - even during the first cycle (trace (D) in Figure 2). The instantaneous current is measured by device 30 and its output 32 is monitored by the peak current circuit 50, which in turn provides the output signals for the control device 60. If the monitored current is within predetermined limits, the amount of current limiting applied by the current limiting device 40 will be decreased accordingly. After a few cycles, there will be no current limiting as the current reaches normal operation.

Claims

CLAIMS:

1. A method of providing current inrush control to a circuit including a device (20) to be operated, the method comprising: determining the peak current in the circuit; comparing the peak current with a predetermined value; and limiting the current to the predetermined value for a predetermined time.

2. A control system for controlling inrush currents in a circuit including a load (20), the system comprising: current determining means (30, 50) for determining peak current values; current limiting means (40) for limiting the peak current values to a predetermined value; and control means (60) for controlling the current limiting means (40) in response to signals from the current determining means (30, 50).

3. A method substantially as hereinbefore described with reference to the accompanying drawings.

4. A control system substantially as hereinbefore described with reference to the accompanying drawings.