GB1590286A - Electric motor control circuits - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO ELECTRIC MOTOR CONTROL CIRCUITS (71) We, THE METTOY COMPANY LIMITED, a British company of 14 Harlestone Road, Northampton NN5 7AF, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to electric motor control circuits and is an improvement in or modification of the circuit disclosed in our U.K. Patent Specification No.

1,528,573.

Our U.K. t Specification No. 1,528,573 relates to an electric motor control circuit for an electric motor having a stator winding and a permanent magnet rotor provided with a plurality of rotor poles of alternate polarity, the circuit comprising a switching device for supplying current pulses to the stator winding of an electric motor, a monostable device for controlling the operation of said switching device, a feedback timing connection connected from the output to the input of said monostable device for producing a feedback signal for controlling the minimum period of said monostable device, and means responsive to the position of the rotor of the motor for producing a timing signal, the feedback and timing signals being combined at the input to said monostable device and arranged to control operation of the device whereby the current pulses from the switching device are delayed with respect to said timing signal by an amount depending on the speed of the rotor.

The specific description of our abovementioned specification discloses a single stator coil in which the direction of current flow is reversed. Such a stator coil has a number of advantages but it also has a number of disadvantages, for example the switching device for the stator coil has to be carefully designed.

The present invention provides an electric motor control circuit according to any one of claims 1, 2, 4, 5 or 6 of the claims of Specification No. 1,528,573 wherein the stator winding comprises two coils each arranged to be alternately connected to a source of voltage upon operation of the switching device.

In order that the present invention be more clearly understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of one form of electric motor control circuit; and Figure 2 is a circuit diagram of another form of electric motor control circuit.

The circuit shown in Figure 1 is a modification of the circuit shown in Figure 1 of our specification No. 1,528,573 but the operation thereof is essentially the same. A detailed disclosure of the operation of the circuit will therefore be given only for the stator winding part of the circuit up to transsistor Tr3.

From the drawing, it will be seen that the stator winding comprises two coils 21, 22 each driven by transistor Tr3 via a separate transistor Tr4, TrS respectively. When the transistor Tr4 is conductive, current flows through coil 21 and when transistor Tr5 is conductive current flows through coil 22.

The coils are wound and disposed on a former (not shown) in such a way that current flow through coil 21 is in the opposite direction to the current flow through coil 22 so that alternating magnetic poles are produced at the ends of the former.

If necessary a resistance can be connected in series with one or both of the coils.

Figure 2 shows a modification to the circuit shown in Figure 1 and which will operate in the same way as the circuit of Figure 1 but has a slightly different construction in that the Schmitt trigger of the timing circuit of Figure 1 is replaced by an amplitude-difference device comprising the transistors Trl 1, Trl2. The characteristics of this device are basically simiIar to the Schmitt trigger but the feedback required is slightly different in that the timing feedback loop comprises two separate paths taken from the outputs of transistors Tr3 and Tr4, combined at one input of the amplitudedifference device.

In more detail, a timing circuit comprises the amplitude-difference device comprising transistors Trll and Trl2 and further comprises resistors R2, Rs, capacitor Cl and two selectable variable resistors Rx, Ry. Current pulses derived from the drive transistor Tr4 are fed to one side of the capacitor Cl the other side of which is connected to the base of transistor Trl 1. The collector of transistor Tr3 is also connected to the base of the transistor Trll via one of the two resistors Rx, Ry.The two transistors Trll and Trl2 with a common emitter resistor R1 are connected as a long-tailed pair and the base of the transistor Tr12 is connected to the junction between resistors R2, R3 connected in series across the supply rails. Transistor TR12 has a collector resistor R6, the voltage across which is used to drive the base of transistor Tr3. A rotor position sensor in the form of a reed switch RS is connected to the base of the transistor Trll.

The timing circuit is thus in the form of a pair of potential dividers. One potential divider comprises resistors R2 and R3, and the other divider comprises capacitor C1 and one of two selectable variable resistors Rx, Ry. The first potential divider biases one input of the amplitude-difference device formed by the transistors Trll and Trl2 to a fixed potential, while the other divider, which is driven by voltage pulses from transistors Tr4 and Tr3 produces the second input to the amplitude-difference device via the feedback paths.

It is to be noted that the two feedback paths are in anti-phase and the combination of the transistors Trll, Trl2, Tr3 and Tr4 together with the feedback paths behaves like a monostable circuit in the sense that when the input to Trll is triggered Iby the closing of the reed switch, regenerative action takes place via C1, and the circuit will stay in this state until C, has charged (or discharged), regardless of the state of the reed switch.

Output pulses, from transistors Tr4 and Tr3 are applied to the amplitude difference device. In response to the start of one of the output pulses, the voltage which appears at the base of transistor Trll will rise at a rate which is determined by the time constant of the circuit consisting of capacitor C1 and one of the resistors R, Ry. Accordingly, the point in time at which the potential on the base of transistor Tri 1 in relation to that at Tr12 is sufficient to switch transistor Tr3 will be delayed by a time T after the switching of the transistors Tr4 and Tr5.Because the inputs to the bases of transistors Trll and Tr12 are equal, this time interval will be constant, and largely independent of variations of the supply voltage and for practical purposes the time interval is constant. If the motor is running at the desired speed, this time interval will be equal to the time delay between successive operations of the reed switch; that is to say, the speed of the motor is such that successive transitions of poles across a given pole face occur at intervals which are equal to the delay time to which the circuit has been set.

The reed switch modifies the timing of the circuit as described above at low speed in the manner disclosed in our Patent Specification No. 1,528,573.

The preferred stator arrangement is a former wound with coils 21, 22 and provided at each end with a plate of a metallic material such as is disclosed in our Patent No. 1,434,192. The present control circuits can be used with the record player construction as disclosed in our Patent No.

1,434,193.

WHAT WE CLAIM IS: 1. An electric motor control circuit according to any one of claims 1, 2, 4, 5 or 6 of the claims of Specification No. 1,528,573, wherein the stator winding comprises two coils each arranged to be alternately connected to a source of voltage upon operation of the switching device.

2. A circuit according to claim 1, wherein said switching device comprises two semiconductor switching devices, one in series with each coil.

3. A circuit according to claim 1 or 2, wherein the monostable device comprises an amplitude difference device for comparing voltage pulses derived from said switching device with a reference voltage level.

4. A circuit according to claim 3, wherein the amplitude difference device has a feedback timing connection comprising a parallel connection of two resistors, a capacitor, and switch means are provided for selectively connecting one of said resistors in series with said capacitor.

5. A circuit according to any one of the preceding claims wherein a resistance is connected in series with one of the coils.

6. A circuit according to any one of claims 1 to 4, wherein a resistance is connected in series with both of the coils.

7. An electric motor control circuit substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. from the outputs of transistors Tr3 and Tr4, combined at one input of the amplitudedifference device. In more detail, a timing circuit comprises the amplitude-difference device comprising transistors Trll and Trl2 and further comprises resistors R2, Rs, capacitor Cl and two selectable variable resistors Rx, Ry. Current pulses derived from the drive transistor Tr4 are fed to one side of the capacitor Cl the other side of which is connected to the base of transistor Trl 1. The collector of transistor Tr3 is also connected to the base of the transistor Trll via one of the two resistors Rx, Ry.The two transistors Trll and Trl2 with a common emitter resistor R1 are connected as a long-tailed pair and the base of the transistor Tr12 is connected to the junction between resistors R2, R3 connected in series across the supply rails. Transistor TR12 has a collector resistor R6, the voltage across which is used to drive the base of transistor Tr3. A rotor position sensor in the form of a reed switch RS is connected to the base of the transistor Trll. The timing circuit is thus in the form of a pair of potential dividers. One potential divider comprises resistors R2 and R3, and the other divider comprises capacitor C1 and one of two selectable variable resistors Rx, Ry. The first potential divider biases one input of the amplitude-difference device formed by the transistors Trll and Trl2 to a fixed potential, while the other divider, which is driven by voltage pulses from transistors Tr4 and Tr3 produces the second input to the amplitude-difference device via the feedback paths. It is to be noted that the two feedback paths are in anti-phase and the combination of the transistors Trll, Trl2, Tr3 and Tr4 together with the feedback paths behaves like a monostable circuit in the sense that when the input to Trll is triggered Iby the closing of the reed switch, regenerative action takes place via C1, and the circuit will stay in this state until C, has charged (or discharged), regardless of the state of the reed switch. Output pulses, from transistors Tr4 and Tr3 are applied to the amplitude difference device. In response to the start of one of the output pulses, the voltage which appears at the base of transistor Trll will rise at a rate which is determined by the time constant of the circuit consisting of capacitor C1 and one of the resistors R, Ry. Accordingly, the point in time at which the potential on the base of transistor Tri 1 in relation to that at Tr12 is sufficient to switch transistor Tr3 will be delayed by a time T after the switching of the transistors Tr4 and Tr5.Because the inputs to the bases of transistors Trll and Tr12 are equal, this time interval will be constant, and largely independent of variations of the supply voltage and for practical purposes the time interval is constant. If the motor is running at the desired speed, this time interval will be equal to the time delay between successive operations of the reed switch; that is to say, the speed of the motor is such that successive transitions of poles across a given pole face occur at intervals which are equal to the delay time to which the circuit has been set. The reed switch modifies the timing of the circuit as described above at low speed in the manner disclosed in our Patent Specification No. 1,528,573. The preferred stator arrangement is a former wound with coils 21, 22 and provided at each end with a plate of a metallic material such as is disclosed in our Patent No. 1,434,192. The present control circuits can be used with the record player construction as disclosed in our Patent No. 1,434,193. WHAT WE CLAIM IS:

1. An electric motor control circuit according to any one of claims 1, 2, 4, 5 or 6 of the claims of Specification No. 1,528,573, wherein the stator winding comprises two coils each arranged to be alternately connected to a source of voltage upon operation of the switching device.

2. A circuit according to claim 1, wherein said switching device comprises two semiconductor switching devices, one in series with each coil.

3. A circuit according to claim 1 or 2, wherein the monostable device comprises an amplitude difference device for comparing voltage pulses derived from said switching device with a reference voltage level.

4. A circuit according to claim 3, wherein the amplitude difference device has a feedback timing connection comprising a parallel connection of two resistors, a capacitor, and switch means are provided for selectively connecting one of said resistors in series with said capacitor.

5. A circuit according to any one of the preceding claims wherein a resistance is connected in series with one of the coils.

6. A circuit according to any one of claims 1 to 4, wherein a resistance is connected in series with both of the coils.

7. An electric motor control circuit substantially as hereinbefore described with reference to the accompanying drawings.