GB2127634A

GB2127634A - Apparatus for limiting current through a DC motor

Info

Publication number: GB2127634A
Application number: GB08325180A
Authority: GB
Inventors: Terrence George Gentle
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1982-09-21
Filing date: 1983-09-21
Publication date: 1984-04-11
Also published as: GB2127634B; GB8325180D0

Abstract

The starting or stall current through the windings of a DC motor is limited to a safe value by causing the current to flow through a reference resistor R6 and the emitter/collector path of a power transistor Q4. When the current exceeds the limit value, the voltage drop across the resistor renders a transistor Q2 conductive to draw base current from transistor Q4 and thus increase its impedance to limit the motor current to a preset value. After a chosen time of maximum current, an associated timer circuit R8, C2 causes a diode D3 to breakdown to render a transistor Q3 conductive to further increase the impedance of transistor Q4 to reduce the motor current to a very small value prior to de-energization. <IMAGE>

Description

SPECIFICATION DC motor control circuit This invention relates to apparatus for controlling the current flowing through the windings of a DC motor to a safe value when its internal impedance is either low or drops to a low value.

When the rotor of a DC motor is stationary, either on starting or when the load on the rotor becomes excessive and the motor stalls, its internal impedance falls to a value such that the resultant current through the windings would lead to unnacceptable overheating if continued for too long.

The present invention aims at providing apparatus for limiting the starting current of a DC motor to a chosen value, and for limiting the stall current to the same value for a chosen period before automatically reducing it to a very small value until the motor is de-energised.

Accordingly the present invention provides apparatus for limiting the current through the windings of a DC motor as claimed in the appended claims.

The present invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is a circuit diagram of one form of current-limiting apparatus of the present invention, and Figure 2 is a schematic graph of current against time to show the characteristics of the apparatus of the present invention both when the motor is starting and when it has stalled.

A 24V source represented by battery B supplies current to a motor M through a fuse FS in series with a reference resistor R6 and the emitter/collector path of transistor Q4, the path also including a reverse-blocking diode D2. As the current through the motor fluctuates, corresponding alternations would take place in the voltage of point A if there were no compensating factors. The full 24V is applied across a series combination of Zener diode D1 and resistor R1. Between these two components is connected the collector of transistor Q1 and a resistor R2 coupled between the collector and base of Q1, which is coupled to the opposite side of the voltage source through resistor R4, so that the voltage of the base is dictated by the values of R4, R2 and the reference voltage developed across diode D1.

The emitter of Qi is connected to the emitter of a second like transistor Q2 of which the emitter is also coupled to the voltage source through a resistor R5, its collector is coupled to one side of a resistor R3, and its base to point A. The combination of resistors R4 and R5, transistors Q1 and Q2 and resistors R1 and R2 and capacitor C1 act in known fashion as a so-called 'longtailed pair' to produce a temperaturecompensated reference voltage across resistor R4. Thus the voltage excursions of point A are compared with the reference voltage across the resistor R4 in order to enable the apparatus to decide when the current through motor M is in any danger of becoming excessive.

Coupled across resistor R6 and transistor Q4 is an RC combination of resistor R8 and capacitor C2 having a known time constant. The midpoint of this combination is connected through a voltage-reference diode D3 to the base of a transistor Q3 and to the other side of the voltage source through resistor R7. The emitter/collector of transistor 03 is connected to the base of transistor Q4.

A capacitor C1 is coupled between the supply bus and the base of transistor Q4 to reduce the high frequency gain of the transistor.

In order to describe how the present invention works, let us assume that the motor M has run up to its design speed so that it is drawing a steady current through resistor R6 and the emitter/collector of transistor 04. Assume also that the steady current is below the designed current limit value. Under these conditions, the transistor 02 is biassed off, and transistor Q4 is biassed on, so that its impedance is at a minimum. When the motor M stalls, the resultant reduction in the impedance of its windings leads to the current through R6 rising, which causes the voltage of point A to fall. This enables transistor Q2, which starts to draw current from the base of transistor Q4, leading to its voltage dropping and hence the impedance of the emitter/collector path starting to rise, in order to counteract the fall in impedance of the motor windings.

Referring to the characteristic curve of Figure 2, assuming that the motor has started to stall at time t4, at time t5 the motor impedance has reached its minimum but the current through the windings is limited to that indicated by chaindotted line 2 by the series impedance of resistor R6 and transistor Q4. In one typical example, this stall current is limited to about 350 mA.

After this stall current has been reached, the voltage across diode D3 starts to increase at a rate dictated by the time constant of resistor R8 and capacitor C2, which is chosen to be of more than about four seconds for reasons to be discussed below. When this chosen period has elapsed, represented by point t6 on Figure 2, the voltage across diode D3 causes it to break down and pass current to transistor Q3. This immediately switches on and draws still more current from the base of transistor 04, so that its impedance rises even further and causes the current through it and resistor R6 to drop down to a very small value at time t7, at which the current remains until the apparatus is de-energised.

When the motor is first energised at time tO, the growth of current with time through it is represented by the curve from tO to t1 in Figure 2.

If it were not for the apparatus of the present invention, the current would rise to a peak and then fall away to the steady state value shown by the horizontal curve, the variation of current being represented between times tl and t3 by the broken curve. However, when the apparatus of the present invention is in position, at time tithe current has reached the maximum value of about 350 mA for the reasons discussed above. The motor thus takes longer to accelerate to its operating speed, as represented by the portion of the curve from times t1 to t2 to t3.The time constant of the R8.C2 combination is chosen to allow the limited starting current to continue for a time sufficient to accelerate the motor until the increase of its impedance causes the starting current to fall off, as represented by the falling curve between times t2 and t3. When this increase in impedance occurs, this leads to a concomitant reduction in the impedance of transistor Q4, which causes the voltage across diode D3 to stop increasing, so that it fails to break down. Various current flows and voltage drops occur until the circuit stabilises under its steady-state conditions with the motor running at its operating speed and drawing a current significantly less than the preset limit.

It will be appreciated by those skilled in the art that the voltage of point A can be controlled by means other than the long-tailed transistor pair described above, but what is common to all versions of the present invention is that there is an increase in the base current of transistor Q4, followed by a voltage drop, as the current through reference resistor R6 starts to rise towards a chosen limit.

A further advantage of the R8.C2 combination is that the motor may draw its maximum current for short periods, such as may be caused by the load connected to the motor rising transiently, because of dirt etc. As long as these increases in load last for only a short time, the motor is continuously energised, whereas if the load imparted, be it by dirt or a deliberate obstruction, rises sufficiently high to stall the motor, and to keep it stalled for longer than the chosen time interval, then the present invention comes into operation and first limits the stall current to an acceptable value and then reduces it even further to a trickle value which is in no danger of overheating the motor however long it continues.

The motor of the present invention may be designed to drive a load, such as the offsetting catch tray of a plain paper copier, at random intervais. Alternatively, in other embodiments, the motor may be rotated in opposite directions so as to reciprocate an object, such as the paper tray of a plain paper copier. In either of these cases, or any other case, the present invention permits the object being moved to hit a permanent or movable obstruction so as to stall the motor for an indefinite period without its being overheated or otherwise damaged, before the motor is restarted to drive its load in the same or opposite direction, until it next stalls.

It will thus be seen that the present invention provides a simple and automatic apparatus for limiting the current through a DC motor at small cost and quite safely.

Claims

1. Apparatus for limiting the current through the winding of a DC motor, including a reference resistor and the emitter/collector of a power transistor in series with the winding and a voltage source; means for causing current to be drawn from the base of the power transistor, to increase its impedance, when the voltage across the reference resistor starts to rise, the total impedance in the motor circuit limiting its stall or starting current to a chosen value; a control transistor having its emitter and collector in series with the voltage source and the base of the power transistor, and having its base connected through a voltage-regulator diode to the mid-point of a capacitor/resistor combination having a known time constant and connected between the source and the motor winding, so that after the maximum current has been flowing for a predetermined time, the diode breaks down to switch the control transistor on and thereby increase the impedance of the power transistor to such a value that the maximum current is further limited to a very small value.

2. Apparatus as claimed in claim 1, in which a point between the reference resistor and the power transistor is connected to a voltage reference circuit.

3. Apparatus as claimed in claim 2, in which the reference circuit includes a long-tailed pair of transistors.

4. A current limiter circuit substantially as described herein with reference to the accompanying drawing.