GB2110892A

GB2110892A - A control system for a brushless electric motor

Info

Publication number: GB2110892A
Application number: GB08136243A
Authority: GB
Inventors: T Nicol
Original assignee: Jones Walter & Co
Current assignee: Jones Walter & Co
Priority date: 1981-12-01
Filing date: 1981-12-01
Publication date: 1983-06-22
Also published as: GB2110892B

Abstract

A control system for a brushless three phase electric motor has a power control section (A), an oscillator section (O), a divide-by-six circuit section (B), and an electronic switching section (C) for switching the current in the phases of the motor. The switching section (C) includes three identical sets of circuits, each circuit having a pair of power transistors (e.g. TR6, TR7) whose emitter-collector paths are connected in series across a D.C. supply. Also connected across the D.C. supply is a circuit including transistors (e.g. TR4, TR5) for controlling the conduction of the power transistors. The transistors (e.g. TR4, TR5) are controlled by the divide-by-six circuit section (B), such that a cyclic flow of current will occur in the associated power transistors (e.g. TR6, TR7). Thus all three pairs of power transistors (TR6, TR7, TR10, TR11, TR14, TR15) are sequentially controlled from the divide-by-six circuit section (B) such that the current in the windings of the three phase motor is sequentially switched. <IMAGE>

Description

SPECIFICATION A control system for brushless motors The present invention relates to a control system for brushless n phase electric motors.

One such control system has been disclosed in our Patent Specification No. 1,510,627. The system disclosed in this prior Specification is one which relies on logic and relates to an electronic commutator system for sequentially switching the current in the windings of an electric motor. The system includes means for switching the current in each winding on and off sequentially, the end of each period of conduction being fixed in relation to a respective predetermined angular position of the armature of the motor, whilst the beginning of each period of conduction is controlled so as to occur not earlier than a respective predetermined time after the beginning of a preceding period of conduction in the same or another winding.

According to the present invention there is provided a control system for a brushless n phase electric motor, including for each phase: a pair of electronic switches connected in series across a D.C.

supply; means associated with each electronic switch for turning said switch on and off; and means for sequentially causing all the electronic switch control means to operate in a predetermined sequence to switch the current in the n phases of the motor according to the required sequence for operation.

Preferably said electronic switches are power transistors.

Means may be provided to limit the flow of current in the power transistor stages due to either overloading or a short circuit occurring.

Moreover means may be provided for holding the system in a safe latched condition in the event that system remains in the current limit mode for more than a predetermined time.

Means may also be provided for ensuring that no two power transistors of a pair can be conductive together under normal operating conditions.

The control system may readily be adapted to operate with a brushless D.C. motor.

Moreover, variable speed applications are readily available as well as closed loop applications where a constant speed can be obtained.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein: Figure 1 is one preferred form of control system for a brushless three phase electric motor; Figures 2A and 2P are waveform diagrams which assist in the explanation of the system shown in Figure 1; and Figure 3 is a part circuit diagram showing the addition of extra components for the case of a brushless D.C. motor, where the oscillator and divide-by-six binary component circuit are omitted.

Referring first to Figure 1, it should first of all be noted that the basic control system can be used for a variety of applications, there including: (a) Brush less D.C. - electronic commutator.

(b) Brushless D.C. - variable speed with electronic commutator.

(c) Brushless D.C. - closed loop speed control with electronic commutator.

(d) Induction driven A.C. motors.

Accordingly, the basic circuit will be described in broad terms before dealing with the individual modified embodiments.

As shown in Figure 1, the basic control system for brushless motors includes: (a) a power control section A.

(b) an oscillator 0.

(c) a divide-by-six binary component circuit B.

(d) an electronic switching section C for switching the current in the three phases of the motor.

The motors to be driven by such a control system are all three phase and whether star or delta connected, they have a current step characteristic on the line which is shown in greater detail in Figures 2A and 2B, where the former shows the current feed per phase and the latter the motor current per phase.

The three phase motor is not shown but is connected across the outputs OtP1; O/P2 and O/P3.

The electronic control section C thus includes three identical sets of circuits for switching the current in the winding of the three phase motor.

The first circuit includes a pair of power transistors TR6 and TR7 whose emitter - collectors paths are connected in series across a 28 volt D.C. supply. Also connected across the D.C. supply is a circuit comprising in series, resistors R27 and R28, a NPN transistorTR4, a PNP transistorTR5, resistors R29 and R30. The junction between the two resistors R27 and R28 is connected to the base electrode of the power transistor TR6, whilst the junction between the two resistors R29 and R30 is connected to the base electrode of the power transistor TR7. The transistors TR4 and TR5 have their emitter electrodes interconnected and connected to the positive rail of the D.C. supply through a transistor TR2 in the power control section A.The base electrodes of the transistors TR4 and TR5 are interconnected and connected to an appropriate output terminal of the divide - by - six circuit B through a resistor R1 9.

The second and third circuits are arranged in identical manner to the first circuit described above.

The output terminals O/P1; O/P2 and O/P3 are connected to the respective junctions between the power transistors TR6 and TR7; TR10 and TRi 1; and TR14 and TR15.

The oscillator section 0 includes an operational amplifier Q the output of which is applied to the divide - by - six binary component circuit B. A feedback resistor R12 is connected between the output and a first input thereof. Series feedback resistors R13 and R14 are connected between the output and a second input of the operational amplifier Q.

The oscillator 0 receives its power supply from the positive supply rail across a series chain comprising a resistor R8; a capacitor C4 and a resistor R9. A zener diode D5 is connected in parallel with the capacitor C4to provide a stabilized voltage supply to the oscillator. The oscillator elements include series resistors R10 and RIl connected in parallel with the zener diode D5 and a capacitor C5 connected between the second input to the operational amplifier Qand the junction between resistors R9 and R11.

The power control section A includes transistors TR1 and TR2, resistors R1 to R7, capacitors Cl and C2, diodes Dl and D3, and zener diodes D2 and D4.

The diode Dl which prevents the 28 volt D.C. supply from being connected the wrong way around and the resistor R4 is connected in series between the positive terminal and the emitter electrodes of the power transistors TR6, TR10 and TR14. The capacitor Cl is connected between the negative rail and the junction between the diode Dl and the resistor R4.

The zener diode D2 and the resistor R1 are connected in parallel with the capacitor Cm. cross the zener diode D2 there is connected a series chain comprising the resistor R2, the diode D3, and the resistor R3. The base electrode of the transistor TRI is connected to the junction of the diode D3 and the resistor R3. The transistorTRl is in series with the resistor R5 across the two power supply rails to the power transistors. The capacitor C2 is connected across the base and collector electrodes of the transistor TR1. Across the emitter and collector electrodes of the transistor is connected the resistors R6 and R7 in series, the junction of which is connected to the base electrode of the transistor TR2.The zener diode D4 is connected across the positive and negative supply rails at the end of the power control section A. The combination of the elements in the power control section A is for transient protection and for limiting the current to the windings of the three phase motor The operation of the control system shown in Figure 1 will now be explained in greater detail. Let us suppose that the three windings X, Y and Z of the motor are connected in star, and that at a particular instant under consideration, the windings X and Z are connected to the positive rail through power transistors TR6 and TR14 respectively and that the winding Y is connected to negative rail through the power transistor TRI 1. Under these conditions the current flow through the winding Y is twice that through the winding X or Z.The power transistors TR6 and TR14 are rendered conductive by means of the respective control transistors TR4 and TR12 being rendered conductive so that the collector current through the power transistors is represented by the first step in the current waveform shown in Figure 2A. On the other hand the power transistor TRi 1 is also rendered conductive by means of the control transistor TR9 so that the collector current through the power transistor Tri 1 is represented by the second or highest step in the current waveform shown in Figure 2A.

It will thus be appreciated that the conductivity of the upper set of control transistors TR4, TR8 and TR12 is controlled by three sets of waveforms, the sets being output from the divide -by - sx binary component circuit B and off - set 120 in phase relative to one another. Likewise the conductivity of the lower set of control transistors TR5, TR9 and TR13 is controlled by three sets of waveforms inverted with respect to those referred to above, and again off - set by 120 in phase with respect to one another.

Accordingly, at the next switching instant, the control transistorTR4 is rendered conductive which in turn renders the power transistor TR6 conductive.

The control transistor TR9 remains conductive, which keeps the power transistor TRl 1 conductive.

The power transistor TRl4 is switched off owing to the appearance of a negative waveform at the base electrode of its control transistorTR12 and the power transistor TRl5 is rendered conductive owing to the presence of an inverted waveform appearing at the base electrode of its associated control transistor TR13. Under these conditions the current through the winding X is twice that of the current through the winding Y orZ.

Accordingly over one period of collector current waveform of 2 power transistor as shown in Figure 2A, the magnitude of the current flowing in the windings of the motor can be represented by the following table, in which the maximum magnitude is represented by 2: Winding X 1 a 2 1 -1 -2 -1 Winding -2 -1 1 2 1 -1 Winding 1 -l -2 -l 1 2 Thus, it will be seen that the current in each winding for a complete cycle is that shown in Figure 2B. The actual magnitude of the current in the windings is defined by the supply.It should be noted that the resistors R28, R29, R32, R33, R36 and R37 all have an equal value of resistance, and that the base currents into the power transistors are defined by the specific circuit arrangement which seeks to keep these to a minimum to achieve a fast switching time.

In this basic form, the speed of the motor is controlled by the frequency of the oscillator 0. This speed may be adjusted by means of the variable resistor R14.

A nurnber of modified or alternative versions of the above described system are possible in order to cater for the variety of applications listed hereinbefore.

These will now briefiy be described.

(a) For a brush less D.C. motor, the basic circuit is modified as shown in Figure 3. As shown in the part circuit diagram, the oscillator 0 and the divide - by six binary component circuit B have been omitted.

As shown in the circuit, resistors R23, R22 and R21 are provided between the positive rail of the power supply to the power transistors and the respective base electrodes of the pairs of control transistors TR4, TR5; TR8, TR9; and TR12, TR13. Terminals 1,2 and 3 are also connected to the base electrodes of the above referred to control transistor pairs, and supply the base electrodes of the control transistors with control signals picked up off equi - spaced Hall devices or opto - electronic sensors associated with the rotor and stator of the motor so as to provide rotor position information in the form of 1:1 duty cycle square waves, whereby the control transistors are switched in the appropriate sequence.

(b) Avariable speed application of the basic control system shown in Figure 1 can be achieved by modulating the conductivity of the transistor TR2.

Since the commoned emitter electrodes of the control transistor pairs are connected to the collector electrode of the transistor TR2, modulating the conductivity of the transistor TR2 will vary the current flow in the windings of the motor and hence its speed. In one preferred form the base drive to the transistor TR2 is modulated at 20 Khz. This has the effect of pulling the common emitter electrodes of the control transistor pairs high at the modulated frequency in order to reduce the current flow in the motor and hence its speed.

(c) The speed ofthe motor can be held at a constant predetermined value by utilizing the Hall effect devices placed in the rotor of the motor. From these we getthree signals of square waveform of 1:1 duty cycle displaced by 1200 electrical from one another. By using an additional circuit such as that disclosed in Patent Application No. 80.30164, and taking the output from the bistable circuit 22 in order to modulate the transistor TR2 (not shown), the speed of the rotor is detected by the Hall effect or opto - electronic devices can be compared with a desired preset speed in an electronic comparator.

The output from the electronic comparator can then be utilized to modulate the base drive to the transistor TR2 to either increase or reduce the speed of the motor so as to maintain the speed constant at the present value.

(d) In certain applications it may be desirable to limit the flow of current in the power transistor output stages, in order to prevent the power transistors from being damaged due to excessive current flow which might occur either if the motor were overloaded or if there was a short circuit. This can be achieved by the circuit shown in the power supply section A, in which a comparison is made between the current flows in the resistors R4 and R2, the resistor R4 being in series with all the power transistor circuits. The circuit is arranged such that if the current flow th rough the resistor R4 is excessive, there is sufficient drop thereacross to reduce the voltage atthe emitter electrode in comparison with the base electrode of the transistor TRi to render the transistor non - conductive.When the transistor TR1 is turned off, the voltage at the base electrode of the transistor TR2 is lowered which allows the transistor TR2 to conduct. Conduction of the transistor TR2 causes the voltage at the commoned emitter electrodes of the control transistor pairs to be increased towards the voltage of the positive supply raH. Under these conditions all the lower control transistors TR5, TR9 and TR13 are rendered conductive and hence all the lower power transistors TR7, Tor11 and TRIPS are rendered conductive thus reducing the current in the windings of the motor. This application is preferred for negative earth.

(er Further current limiting protection can be achieved by placing an equivalent resistor to the resistor R4 in the negative rail together with an equivalent circuit inclucF-ingtrnnsistorTR1-a nd TR2 - so as to cause all the upper power transistors TR6, TR10 and TR14to conduct in the event of an overload or short circuit occurring. This application is preferred for positive earth.

(f) Another optional feature included within the basic control circuit system shown in Figure 1 is a current limit mode for more than a predetermined time (e.g. 30 seconds) The circuit for this feature includes a transistorTR3, diodes D6, D7 and D8, resistors R15, R16, R20, R24, R25 and R26, and capacitors C6 and C10. The diodes D6, D7 and D8 have their anodes connected to the junctions between pairs of power transistors TR6, TR7; TR10, TR11; and TR14, TR15 respectively. The cathodes of these three diodes are commoned and connected to the negative rail through the resistor R26.The base electrode of the transistor TR3 is also connected to the commoned cathodes of the diodes through resistors R24 and R25 connected in series, the junction between these resistors being connected to the negative rail through the capacitor C10. The collector electrode of the transistor TR3 is connected to the stabilized voltage supply for the oscillator O through resistors R15 and R16 connected in series.

The junction between the resistors R15 and R16 is connected to pin 15 of the divide - by - six binary component circuit B. The capacitor C6 is connected between the emitter electrode of the transistor TR3 and the junction between the resistors R15 and R16, whilst the resistor R20 is connected across the base and emitter electrodes of the transistor TR3.

If now the current limiting mode remains in operation for more than the predetermined time (e.g. 30 seconds), because the power transistors TR7, TR11 and TR15 are all conductive together, the voltage at the commoned cathodes of the diodes D6, D7 and D8 falls towards that of the negative rail which causes the transistorTR3 to be turned off.

When the transistor TR3 is turned off, which happens immediately the circuit goes into the current limiting mode including start up, there isa delay of 30 seconds before the capacitor C6 has charged up to 50% of the stabilized oscillator supply voltage.

Thus when the voltage on pin 15 connected to the divide - by - six binary component circuit B has risen to 50% of the supply voltage, the counter is reset at a predetermined position in which the outputs to the commoned base electrodes of the control transistor pairs are low, so that the transistors TR5, TR9 and TR13 drive their respective power transistors TR7, TR11 and TR15 into full conduction. This in turn holds the commoned cathodes of the diodes D6, D7 and D8at substantially zero potential and so com pletes the latch. Once the short circuit or overload condition has been cleared, the system can be reset by disconnecting the 28 volts D.C. supply and then re-connecting.

(g) Another optional feature, shown in Figure 1, is the provision of capacitors C7, C8 and C9 which are connected between a common point and the base electrodes of respective control transistor pairs TR12, TR13; TR8, TR9; and TR4, TR5. Since power transistors and especially Darlington pairs have a large storage time where the collector current con fineesto flow after the base current Itasbeen-cut off, it is undesirable that there should be any point in the normal cycle where the two power transistors associated with the same phase (e.g. TR6 and TR7) are "on" together.Due to the presence of time constants associated with the capacitors a slight delay occurs in the switching on of a power transistor, thus ensuring that one of a pair in the same phase cannot be switched on until the other has fully turned off.

(h) In the variable speed application referred to above, it is desirable to provide commutation diodes D9 to D14 inclusive across respective power transis torsTR6,TR7,TR1O,TR11,TR14and TR15. These diodes also serve to protect the power transistors against transients on switching off, any such transients being safely conducted to the power rails thus protecting the base-emitter junction of a non-conductive power transistor.

One advantage of the above described embodiments which use the double stepped waveform to operate the control transistors, is that because the current through a particular power transistor is dropped to one half of its maximum value before the end of the cycle, the storage time associated with the power transistor is also reduced by half, thus enabling the switching speed to be doubled.

CLAIMS (Filed on 26.11.82) 1. A control system for a brushless n phase electric motor, including for each phase: a pair of electronic switches connected in series across a D.C.

2. Acontrol system according to Claim 1, wherein the electronic switches are power transistors.

3. A control system according to Claim 2, wherein means are provided to limit the flow of current in the power transistors in the event of either overloading or short - circuit conditions occurring.

4. A control system according to Claim a, where- in means are provided for holding the control system latched in a safe condition in the event that thesystem remains in the current limit mode for more than a predetermined time.

5. A control system according to any one ofthe preceding claims 2 ta 4, wherein the power transistor control means, includes for each pair of power transistors, a voltage divider chain connected across said D.C. supply, said chain comprising a first pair of resistors, a pair of transistors, and a second pair of resistors, the emitter - collector path of the two transistors being connected in series between the two sets of pairs of resistors across the D.C. supply, the junctions between the respective pairs of resistors being connected to the control electrodes of the respective power transistors.

6. A control system according to Claim 5, wherein the control electrodes of respective pairs of transistors for the n phases are connected to respective outputs of a diode by 2n binary circuit constitutingthe sequential electronic switch control means, whereby the power transistors can be operated in appropriate sequence to control the current supplied to the windings of said brushless electric motor.

7. A control system according to Claim 6, wherein said divide - by - 2n binary circuit is supplied through an oscillator having an operational amplifier.

8. A control system according to any one of the preceding claims 5 to 7, wherein the said pair of transistors are of opposite conductivity type and have their emitter electrodes interconnected and supplied with a predetermined reference voltage.

9. A control system according to any one of the preceding claims 2 to 8, wherein commutation diodes are placed across the collector - emitter paths of respective power transistors.

10. A control system according to Claims 3 to 8, wherein the means for limiting the flow of current in the power transistors, comprises a first resistor in series with the paralleled pairs of power transistors, a second resistor forming part of a voltage dividing circuit across the D.C supply, means for comparing the current flow in said two resistors, and means for effectively raising said reference voltage applied to the commoned emitter electrodes of the pairs of control transistors to the potential of the D.C supply in order to render all the same power transistors of said pairs fully conductive in order to reduce the current in the windings of the brushless motor.

11. A control system according to Claim 10, wherein said comparison means includes a transistor connected across the D.C. supply and having its control electrode connected in the voltage dividing circuit associated with the second resistor.

12. A control system according to Claim 10 or 11, wherein a third resistor is connected in the other current supply rail, and associated further comparison means are provided, whereby all the other power transistors of said pairs are rendered fully conductive so to further reduce the current in the winding of the brushless electric motor.

13. A control system according to Claims 4 and 8, wherein the means for holding the control system latched in a safe condition comprises a transistor operatively associated with the divide - by - 2n binary circuit, n diodes connected between a common point and the junctions between respective power transistors, the commoned point of said diodes being connected to the control electrode of said transistor, and a capacitor connected across the collector - emitter path of said transistor, the capacitor being short - circuited by the transistor when the current limiting mode occurs and being partially rechanged after said predetermined time to effect the binary circuit to hold the same power transistors of each pair fully conductive.

14. A control system according to any one of the preceding claims 2 to 13, wherein means are provided for ensuring that no two power transistors of a pair can be conductive together under normal operating conditions.

15. A control system according to Claim 14 and Claims 5 to 8, wherein n capacitors are connected between a common point and the commoned base electrodes of the respective pairs of control transis

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. it is undesirable that there should be any point in the normal cycle where the two power transistors associated with the same phase (e.g. TR6 and TR7) are "on" together. Due to the presence of time constants associated with the capacitors a slight delay occurs in the switching on of a power transistor, thus ensuring that one of a pair in the same phase cannot be switched on until the other has fully turned off. (h) In the variable speed application referred to above, it is desirable to provide commutation diodes D9 to D14 inclusive across respective power transis torsTR6,TR7,TR1O,TR11,TR14and TR15. These diodes also serve to protect the power transistors against transients on switching off, any such transients being safely conducted to the power rails thus protecting the base-emitter junction of a non-conductive power transistor. One advantage of the above described embodiments which use the double stepped waveform to operate the control transistors, is that because the current through a particular power transistor is dropped to one half of its maximum value before the end of the cycle, the storage time associated with the power transistor is also reduced by half, thus enabling the switching speed to be doubled. CLAIMS (Filed on 26.11.82)

1. A control system for a brushless n phase electric motor, including for each phase: a pair of electronic switches connected in series across a D.C.

tors of then phases of the system.

16. A control system according to any one of the preceding claims, wherein means are provided by holding the speed of the brushless electric motor constant by the utilization of Hall effect devices and the circuit disclosed in our Patent Specification No.

2,084,761.

17. A control system according to Claim 10, wherein the means which determine the value of the reference voltage applied to the commoned emitter electrodes of the pairs of control transistors is a transistor which has its conductivity modulated in order to achieve a variable speed control for the brushless electric motor.

18. A control system according to any one of the preceding Claims 2 to 5, for application with a brushless D.C. electric motor, wherein n resistors are provided between the positive supply rail and the commoned control electrodes of then pairs of control transistors.

19. A control system for a brushless n phase electric motor constructed substantially as herein described with reference to Figure 1 of the accompanying drawings.

20. A control system for a brush less D.C. electric motor constructed substantially as herein described with reference to and as illustrated in Figure 3 of the accompanying drawings.