GB2162387A

GB2162387A - Speed control

Info

Publication number: GB2162387A
Application number: GB8512985A
Authority: GB
Original assignee: Mabuchi Motor Co Ltd
Current assignee: Mabuchi Motor Co Ltd
Priority date: 1984-05-22
Filing date: 1985-05-22
Publication date: 1986-01-29
Also published as: KR900003099Y1; JPS60249885A; GB8512985D0; KR850008585A; GB2162387B

Abstract

A speed control device for a d-c motor has a control circuit for controlling the motor speed by connecting a resistor circuit across the terminals of a d-c motor to detect terminal voltage. The resistor circuit has a plurality of resistor circuit components enabling selection of speed settings by changing over the resistor circuit components to adjust the resistance value of the resistor circuit, and thereby allowing the d-c motor to operate at a plurality of speed settings. A first non-linear resistor element diode 7 is connected in common with the resistor circuit to compensate for motor speed changes caused by temperature changes. A second non-linear resistor element diode 24 having a temperature compensating function is connected to at least one of the resistor circuit components. <IMAGE>

Description

SPECIFICATION Speed control This invention relates to speed control devices for direct current electric motors.

As we shall explain below with reference to Figs. 1 to 4 of the accompanying drawings, there have been a series of prior proposals for the speed control of d-c motors. As will become clear from the discussion with reference to the said Figures, the prior proposals have suffered from a number of difficulties, including what we regard as inadequate temperature compensation.

The present invention has arisen from our work seeking to provide better temperature compensation over a range of motor speed settings.

In accordance with the present invention, there is provided a speed control device for a d-c motor having a control circuit for controlling motor speed by connecting a resistor circuit across the terminals of a d-c motor to detect terminal voltage; said resistor circuit having a plurality of resistor circuit components enabling selection of speed settings by changing over said resistor circuit components to adjust the resistance values of said resistor circuit, and thereby allowing the d-c motor to operate at a plurality of speed settings; a first non-linear resistor element being connected in common with said resistor circuit to compensate for motor speed changes caused by temperature changes, and a second non-linear resistor element having a temperature compensating function connected to at least one of said resistor circuit components.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram illustrating a previously proposed motor speed arrangement; Figure 2 is a similar circuit diagram showing another previously proposed motor speed control arrangement; Figure 3 is a diagram of assistance in explaining the torque-speed characteristics of the arrangement shown in Fig. 2; Figure 4 is a similar circuit diagram of Figs. 1 and 2 illustrating an improved prior arrangement for motor speed control; Figure 5 is a schematic circuit diagram similar to the circuit diagrams of Figs. 1, 2 and 4 illustrating an embodiment of speed control device constructed in accordance with the present invention; and Figure 6 is a diagram of assistance in explaining the temperature characteristics of diodes such as those included in the circuit of Fig. 5.

In each of the arrangements of Figs. 1, 2 and 4 a control circuit 1 in the form of an integrated circuit, such as that manufactured by Nippon Electric Co. Ltd and given the identification Pc1470H, controls a due motor 2 via a resistor circuit which, in Fig. 1, comprises resistors 9, 10 and 11 connected across the terminals of the motor. The power input is via an Rc circuit comprising a reactor 3 and a capacitor 4. The control circuit 1 controls the current flowing into the d-c motor 2 so as to keep the counter-electromotive force generated in the d-c motor 2 at a constant level as current flows from the terminal D to the terminal B.In the example shown in Fig. 1, the current flowing into the terminal D is adapted to be proportional to the current flowing into the terminal C through a semiconductor diode 7, with the terminal A being used for starting the d-c motor 2.

In this circuit arrangement, the speed (revolution) of the d-c motor 2 can be adjusted by changing the resistance value of the variable resistor 1 6. In order to operate the d-c motor 2 in two or more revolution settings, instead of a single revolution setting, it is necessary to set a plurality of resistance values in the series combination of the variable resistor 1 6 and the resistor 11.For instance, the d-c motor 2 can be operated at two speed settings by introducing an additional series combination of a variable resistor 1 7 and a resistor 1 2 in parallel with the resistor 11 (Fig. 2) in such a fashion that either the series combination of the variable resistor 1 7 and the resistor 1 2 or the resistor 11 can be connected to the d-c motor circuit through changeover of a switch 22, as shown in Fig. 2.

Revolution setting in this circuit configuration can be accomplished in the following manner. A first revolution setting is set by changing over the switch 22 to the side of the resistor 11, and adjusting the variable resistor 16, while a second revolution setting is set by changing over the switch 22 to the side of the series combination having the variable resistor 17, and adjusting the variable resistor 1 7. This circuit configuration, however, suffers from the problem of temperature compensation attributable to the temperature characteristics of the d-c motor 2, as will be described below.

In Fig. 3, the ordinate represents motor revolution and the abscissa motor torque, and the straight lines (1) and (2) represent the torque-revolution characteristics corresponding to the two speed setting at which the device shown in Fig. 2 may be operated at Talc. Assuming the speed setting for the straight lines (1) and (2) at the rated torque of TO are N, and N2, the variable resistors 1 6 and 1 7 are adjusted to obtain the speed settings N1 and N2 respectively, as described above.Assuming the respective terminal voltages of the d-c motor, set at the speed settings N1 and N2, are VM1 and VM2, the torque-revolution characteristics corresponding to the terminal voitages VM1 and VM2, when the temperature T is constant at Ta, are represented by the parallel two straight lines (1) and (2) in Fig. 3.

In general, with increases in temperature, the speed characteristics of a d-c motor show a tendency for motor revolution to increase and the maximum load value to decrease under no load due to changes in magnetic flux resulting from the temperature characteristics of the magnet, and to changes in rotor resistance resulting from the temperature characteristics of the conductors. With decreases in temperature, the characteristics show the reverse tendency.

Consequently, the straight lines (1) and (2) representing the characteristics at speed settings of N, and N2 and at a temperature of T,"C are shifted toward straight lines (1)b and (2)b if temperature changes from Ta C to Tb C (Tb > ta). In order to keep the revolution of the d-c motor 2 constant even at such temperature changes, it is necessary to shift the straight lines (1)b and (2)b toward the straight lines (1)b and (2)b, as shown in Fig. 3. The diode 7 is used to achieve this. However, the change AVF in the voltage across the terminals of the diode 7 due to a temperature change is unilaterally determined in accordance with the temperature change.

However, the change AVMl in the motor terminal voltage for the speed setting N1 is not equal to the change AVM2 -in terminal voltage for the speed setting N2. Consequently, when the changes in motor terminal voltage are AVM1 and AVM2, over-compensation for the speed setting N2 would result by setting the change AVF in the voltage across the terminals of the diode 7 so as to match with the change AVMl in the motor terminal voltage. To cope with this over-compensation, an additional means is needed for compensating for the difference between the changes AVM1 and AVM2 in motor terminal voltage.

Fig. 4 shows an example where the aforementioned shortcomings are improved to ensure proper temperature compensation for each speed setting. In the example shown in Fig. 4, thermistors 20 and 21 are used as means for compensating for the difference between the changes AVM1 and AVM2 in motor terminal voltage, and speed settings are changed over by changing the combined resistance of the parallel-connected speed-setting circuit component. The control circuit shown in Fig. 4 is essentially similar to the control circuit shown in Fig. 1, except that the terminals C and A are combined in common, with the result that the semiconductor diode 8 takes a slightly different position in the circuit.A capacitor 6 is connected across the resistor circuit which here comprises a resistor 1 3 connected in series with the parallel connection of a resistor 14 and a variable resistor 1 8. A change-over switch 23 connects or disconnects a further parallel circuit comprising thermistor 21 which is connected in series with the parallel connection of a resistor 15 and a variable resistor 1 9. The employment of thermistors in the circuit of Fig. 4 involves a significant labour cost because of the need for simultaneous control both of the resistance values and the temperature coefficients of the thermistors employed.

As we shall explain below with reference to Figs. 5 and 6, we have now found that a d-c motor can be operated at a plurality of speed settings while achieving temperature compensation without causing over-compensation for each speed setting.

Fig. 5 shows a schematic circuit diagram for an embodiment of speed control device in accordance with the present invention. For convenience like components in the circuit of Fig. 5 are given like reference numerals to the corresponding components in the circuit of Fig. 2. In the embodiment shown in Fig. 5, switch 22 is provided so as to change over from a series combination of a diode 24, a resistor 25 and a variable resistor 1 7 connected to a series combination of a variable resistor 1 6 and a resistor 11, to the shortcircuiting thereof.In the example shown in Fig. 2, on the other hand, a series combination of a variable resistor 1 7 and a resistor 1 2 is connected in parallel with a resistor 11, and switch 22 is provided to change over either of the resistor 11 or the series combination of the variable resistor 1 7 and the resistor 1 2.

Other circuit configurations, such as a configuration in which the series combination of the variable resistor 1 7 and the resistor 12 shown in Fig. 2 is replaced with a series combination of a diode 24, a resistor 25 and a variable resistor 17, may be employed within the scope of the present invention.

We shall now explain the operating principles behind the circuit configuration shown in Fig.

5.

Since the voltage VCD across the terminals C and D of the control circuit 1 is controlled at a constant level at any time, when the switch 22 is thrown to the side of the terminal 1 to shortcircuit the series combination of the diode 24, the resistor 25 and the variable resistor 17, the terminal voltage VM1 of the d-c motor 2 is expressed by the following expression: VM1 = VCD + VF + R1 IR1 (11) where VF is the forward terminal voltage of the diode 7, 1R1 is the current flowing in the resistor 9, R1 is the resistance value of the resistor 9, and the current, IR1 has the following relationship; IR1 IM/K + a (12) where K is a constant, 1M is the motor current, and a is a variable defined by the following expression:: VCD + VF a=ss+ (13) RA where RA is the combined resistance of the variable resistor 16 and the resistor 11, and ss is a constant.

Now, substituting the expressions (12) and (13) above into the expression (11) yields: R, R1 VM1 = (1 + ) VF + (1 + ) VCD RA RA 1M +R, ( +ss) + ss) (14) K As the change in the current 1M with temperature change is much smaller than the change in the forward voltage VF of the diode 7, the expression (14) may be substituted by the following expression ( 1 5) R, V (1 + ) VF + ss (1 5) RA (p' = a constant) As is evident from expression (15), the terminal voltage VM of the d-c motor is approximately proportional to the forward voltage, VF of the diode 7.

Fig. 6 shows the voltage vs. current characteristic curve for a diode at different temperatures.

As is apparent from the figure, when the current I is constant (lo), the forward terminal voltage of the diode decreases as temperature increases from Ta 'C to Tb C. Temperature compensation can be accomplished by taking advantage of the relationship between these characteristics of a diode and the expression (15). When temperature rises, for example, the forward voltage VF decreases accordingly, and the terminal voltage VM also decreases because the terminal voltage VM is almost proportional to the forward voltage VF of the diode 7. Consequently, the decrease in terminal voltage VM is offset by the increase in revolution due to temperature rise, which is a characteristic of the d-c motor, leading to the sustained operation of the d-c motor at the set speed.

Next, we shall describe what happens when the the switch 22 is thrown to the side of the terminal H to connect the series combination of the variable resistor 16 and the resistor 11 to the series combination of the diode 24, the resistor 25 and the variable resistor 17, that is, where speed setting is changed over from the setting N, to the setting N2 at which temperature recompensation is also needed.

In this case, too, the following expressions hold, as described earlier.

VM2 = VCD + VF + R1 IR1 (21) Ill = IM/K + a (22) where a' is a variable defined by the following expression.

VCD + VF + Vf a' = ss + ---------- (23) RB where V, is the forward voltage of the diode 24, and RB is the combined resistance of a circuit comprising the resistors 11 and 25, and the variable resistors 16 and 1 7. As described earlier, by introducing the expressions (22) and (23) into the expression (21), VM2 in the expression (21) is given by R1 R1 I M2 R1 ss VAA2=(1+ ---) Vcp +( --- + ---) RB K RB R, R, +[(1 + ) VF - Vf] (24) RB RB As before, having regard to the relative magnitude of the changes with temperature, the expression (24) may be simplified as below:: R, R1 VM2 = [(1 + ----) Vf]- ----V# + ss" (25) RB RB (ss is a constant) That is, assuming that the voltage change appropriate for maintaining motor revolution at the speed setting N2 is AVM2, the expression (25) is characterised by the addition of a term of - (R1/RB)Vf when compared with the expression (15). And the difference A in voltage, which represents an over-compensation.

= (1 + R1/RB) AVF - AVM2 is offset by this term.

We use the same means as used for temperature compensation, i.e., a diode, as means for temperature recompensation. This makes it easy to use diodes having similar temperature characteristics, and to construct a stable circuit that is generally less sensitive to temperature changes. In a closed circuit comprising a series combination of the diode 7, the C-D terminals of the control circuit 1, the variable resistor 16 and the resistor 11 and a series combination of the diode 24, the resistor 25 and the variable resistor 17, the relationship between the terminal voltage, VF of the diode 7 and the terminal voltage Vf can be in a simple and clear-cut form since VF + VCD + ( VRB) + ( Vf) and a recompensation circuit is connected in series.This makes it easy to estimate how the difference between the temperature characteristics of the diodes 7 and 24 influences temperature recompensation, and thus to adopt a combination of the diodes 7 and 24 having the optimum relationship for the two speed settings between the terminal voltage VF of the diode 7 and the terminal voltage Vf of the diode 24.

The changes in terminal voltage, AVM1 and AVM2 with temperature of the terminal voltage of the d-c motor at the speed setting N1 and the speed setting N2 are estimated below, using actual values.

Assuming that R1 (the resistance value of the resistor 9) = 300 ohms, RA = 150 ohms, and RB = 600 ohms, then: AVMi = AVF(1 + 300/150) = 3AVF, and AVM2 = l\V,(1 + 300/600) - AVf.300/600) = AVF.

Note that the diodes 7 and 24 in the above case are of the same type. It follows from this that AVMl = 3AVM2, that is, AVM2 during low-speed rotation is 1 /3AVMi during high-speed rotation.

Claims

1. A speed control device for a d-c motor having a control circuit for controlling motor speed by connecting a resistor circuit across the terminals of a d-c motor to detect terminal voltage; said resistor circuit having a plurality of resistor circuit components enabling selection of speed settings by changing over said resistor circuit components to adjust the resistance values of said resistor circuit, and thereby allowing the d-c motor to operate at a plurality of speed settings; a first non-linear resistor element being connected in common with said resistor circuit to compensate for motor speed changes caused by temperature changes, and a second nonlinear resistor element having a temperature compensating function connected to at least one of said resistor circuit components.

2. A speed control device according to Claim 1, wherein said control circuit is adapted to control voltage applied to said d-c motor so as to keep said motor terminal voltage at a predetermined value.

3. A speed control device according to Claim 1 or Claim 2, wherein said first and second non-linear resistor elements comprise semiconductor diodes.

4. A speed control device according to Claim 3, wherein said first and second non-linear resistor elements consist of semiconductor diodes having the same or substantially similar temperature characteristics.

5. A speed control device according to any preceding claim, including a speed control switch adapted to connect (or to disconnect) a circuit including said second non-linear resistor element and at least one said resistor circuit components to(from) the circuit including said first non-linear resistor element.

6. For a d-c motor, a speed control device substantially as hereinbefore described with reference to and as shown in Figs. 5 and 6 of the accompanying drawings.