Motor drive circuit and method of driving a motor

Abstract

A drive system for a brushless DC motor having a rotor includes at least one permanent magnet and a stator including at least one phase winding. The system has a drive circuit including a switch associated with the winding for varying the current passing through the winding; a rotor position sensor arranged to sense the position of the rotor; and a controller arranged to provide drive signals to control the switch. The drive system is further arranged to receive a temperature signal that has a value dependent upon the temperature of the at least one magnet of the rotor. The controller is arranged to vary the phase of the current passing through the winding relative to the rotor position dependent upon the temperature of the rotor magnet.

Claims

1. A drive system for a brushless DC motor having a rotor including at least one permanent magnet and a stator including at least one phase winding, the drive system comprising: a drive circuit including switch means associated with the winding for varying current passing through the winding; rotor position sensing means arranged to sense the position of the rotor; and control means arranged to provide drive signals to control the switch means; the drive circuit further being arranged to receive a temperature signal that has a value dependent upon the temperature of the at least one magnet of the rotor, characterized in that the control means is arranged to vary the phase of the current passing through the winding relative to the rotor position dependent upon the temperature of the rotor magnet by controlling the motor in a normal mode when the temperature of the rotor magnet is below a predetermined temperature and in an abnormal mode when the temperature of the rotor magnet is above the predetermined temperature.

2. A drive system according to claim 1 in which in the normal mode, the control means is arranged to advance the phase of the current passing through the winding as a function of the motor rotor speed.

3. A drive system according to claim 2 in which in the abnormal mode, the control means is arranged to advance the phase of the current passing through the winding as a function of the motor rotor speed and the temperature of the rotor magnet such that, under a defined range of operating conditions the control means is arranged to apply a reduced amount of phase advance.

4. A drive system according to claim 3 in which the defined range of operating conditions in which the phase advance is reduced corresponds to conditions in which the magnet would become permanently demagnetized if the phase advance is not reduced.

5. A drive system according to claim 2 in which the control means is adapted to advance the phase of the current passing through the windings relative to the rotor position when the motor speed reaches, or is in excess of, a maximum speed that can be achieved without phase advance.

6. A drive system according to claim 1 in which the control means operates in the abnormal mode when the temperature of the rotor magnet exceeds 100 degrees centigrade, or 120 degrees centigrade, or 140 degrees centigrade.

7. A drive system according to claim 1 in which the control means includes a d-q frame convertor that converts a current demand signal into a q-axis current component and a d-axis current component in the d-q frame of reference, and a phase advance calculator that determines the amount of phase advance that is to be applied and generates a phase advance value that is fed to the converter, the phase advance calculator being responsive to the rotor speed and the supply voltage.

8. A drive system according to claim 7 in which the phase advance calculator is responsive to the motor rotor magnet temperature signal at least when in the abnormal mode.

9. A drive system according to claim 1 wherein in the abnormal mode, the control means is arranged to multiply the phase advance with a scaling factor having a value between 0 and 1 so that a reduced phase advance value is generated.

10. A drive system according to claim 1 wherein in the abnormal mode, the control means is arranged to apply an absolute limit to the angle of phase advance that can be applied by the drive circuit so that the phase advance is kept below the predetermined absolute limit.

11. A drive system according to claim 1 wherein the control means produces a d-axis and q-axis current component that is dependent upon a current demand signal value and which represents the current to be applied to the motor, and in the abnormal mode the control means reduces the amount of phase advance by modifying the value of the d-axis current component.

12. A drive system according to claim 11 wherein the d-axis component is modified by passing the component through a limiter that limits the magnitude of the d-axis component.

13. A drive system according to claim 11 wherein the d-axis current component is passed through a scaling function that multiplies the d-axis component by a scaling factor when in the abnormal mode.

14. A drive system according to claim 1 wherein the amount of phase advance that is applied when in the abnormal mode is chosen such that the amount of demagnetization of the magnet does not exceed 1 percent, or 2 percent, or 3 percent, or up to 5 percent when the temperature of the rotor motor magnet is below a maximum agreed operating temperature.

15. A method of driving a brushless DC motor of the kind having a rotor including at least one permanent magnet and a stator including at least one phase winding, the method comprising the steps of: determining the position of the rotor of the motor; and applying a drive voltage to the phase that causes a current to flow in the phase, wherein the phase of the current passing through the winding relative to the rotor position is varied dependent upon the temperature of the rotor magnet by controlling the motor in a normal mode when the temperature of the rotor magnet is below a predetermined temperature and in an abnormal mode when the temperature of the rotor magnet is above the predetermined temperature.

16. A method according to claim 15 including the further step of operating the motor in the normal control mode when the temperature of the rotor magnet is below a first threshold level, whereby the control means is arranged to advance the phase of the current passing through the winding relative to the rotor position when the motor speed reaches or is in excessive of the maximum speed that can be achieved without phase advance, and operating the motor in the abnormal mode when the temperature of the rotor magnet is above a second threshold level, wherein in the abnormal mode an amount of phase advance is applied that is less than that which is applied when the control means is operating in the normal mode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) There will now be described, by way of example only, several embodiments of the present invention with reference to and as illustrated in the accompanying drawings of which:

(2) FIG. 1a is an overview of a embodiment of a drive system for a brushless permanent magnet motor in accordance with the present invention;

(3) FIG. 1b is a block diagram showing a suitable implementation for the current controller of FIG. 1a;

(4) FIG. 2 is graph showing motor Torque against drive current I when the motor drive current is in phase with the rotation of the motor;

(5) FIG. 3 is a plot of motor torque constant against motor drive current;

(6) FIG. 4a shows the variation in motor current and motor back emf over time in the motor when no phase advance is applied;

(7) FIG. 4b is a plot of motor current and motor back emf over time in the motor when a phase advance angle has been applied;

(8) FIG. 5 is a plot of phase advance against motor torque constant showing the effect of increasing phase advance on the motor torque constant;

(9) FIG. 6 is a set of traces of phase advance motor current against motor torque for different phase advance angles of 0 degrees, 30 degrees, 45 degrees and 60 degrees.

(10) FIG. 7a is a plot of motor flux, q-axis and d-axis current demand components in the d-q frame of reference when no phase advance is applied, showing also the back emf voltage

(11) FIG. 7b is a corresponding plot to FIG. 7a but for the case where a phase advance angle is applied that is greater than zero

(12) FIG. 8 is a plot of voltage against speed showing the maximum motor speed that can be attained without the use of phase advance for a given DC link voltage v1 or higher voltage v2;

(13) FIG. 9 is a plot of Torque against speed showing how phase advance allows increased motor speed to be attained traded against motor torque;

(14) FIG. 10 is a plot showing the relationship between phase advance angle and torque for a range of motor speeds up to and above the maximum rated motor speed;

(15) FIG. 11 is a block diagram showing the various parts of the drive circuit used to convert the torque demand signal into a current demand signal in the d-q frame, when running in a normal mode;

(16) FIG. 12 is a block diagram equivalent to FIG. 11 for a first modified drive circuit in an abnormal mode in which phase advance is limited;

(17) FIG. 13 is a block diagram equivalent to FIG. 11 for a second modified drive circuit in an abnormal mode in which phase advance is limited;

(18) FIG. 14 is a block diagram equivalent to FIG. 11 for a third modified drive circuit in an abnormal mode in which phase advance is limited;

(19) FIG. 15 is a block diagram equivalent to FIG. 11 for a fourth modified drive circuit in an abnormal mode in which phase advance is limited;

(20) FIG. 16(a) is a plot of maximum phase angle against temperature that can be used in the abnormal mode; and 16(b) is a plot showing the effect on D axis current;

(21) FIG. 17 is a plot showing the variation in gain applied to the phase advance in the abnormal mode as a function of temperature; and

(22) FIG. 18 is a plot showing an alternative variation in gain applied to the phase advance in the abnormal mode as a function of temperature, the entry and exit thresholds for the abnormal mode being chosen to be correspond to different temperatures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(23) Referring to FIG. 1 a drive system according to an embodiment of the present invention comprises a drive circuit 8, which is arranged to take power from a DC power supply 10, in this case a vehicle battery, via a wiring harness 12 represented as resistors Rh1 and Rh2. The drive circuit 8 uses this power to drive a three-phase AC permanent magnet motor 14 which is connected to a mechanical load 15, in this case the output shaft of an EPS system. The motor 14 is conventional and comprises three motor windings generally designated as phases a, b and c, connected in a star network, although there could be more than three phases and the three or more phases may be connected in an alternative topology such as a delta configuration. One end of each winding is connected to a respective terminal 16a, 16b, 16c. The other ends of the windings are connected together to form a star centre.

(24) The drive circuit 8 includes a switch means in the form of an inverter 18. The inverter 18 comprises three pairs of switches, typically transistors. Each of the pairs of switches comprises a top transistor and a bottom transistor connected in series between a positive line 20a and negative line 20b from the voltage source 10. Three outputs 21a, 21b, 21c are each tapped off from between a respective pair of transistors. The outputs 21a, 21b, 21c are each connected to the respective motor terminal 16a, 16b, 16c via current sensors 22.

(25) A control means, embodied as a programmable controller 30, is arranged to control the switches in the inverter 18 so as to provide pulse width modulation of the current to the motor windings a, b, c. The controller 30 therefore has six switch control outputs producing six switch control signals 6 to the control gates of the six switches.

(26) The controller 30 receives input signals from the dc-link sensors 28 indicating the instantaneous dc-link voltage ECU-dc.sub.link and current, from the current sensors 22 from which it can determine the three motor phase currents, from a speed sensor 32 on the motor output from which it can determine the motor speed, from a motor position sensor 34 from which it can determine the rotational position of the rotor of the motor 14 and (as will be explained later) a temperature signal from a temperature sensor. The controller also receives a torque demand input TD and is arranged to control the inverter 18 to drive the motor to produce the demanded torque TD.

(27) The switches in the inverter 18 are turned on and off in a controlled manner by the control circuit 30 to provide pulse width modulation of the potential applied to each of the terminals 16a, 16b, 16c, thereby to control the potential difference applied across each of the windings a, b, c and hence also the current flowing through the windings. This current is sensed by the current sensors 22. Control of the phase currents in turn controls the strength and orientation of the total current vector produced by the windings, and hence also the phase of the rotating current vector, relative to the phase of the magnetic field of the rotor as that rotates.

(28) FIG. 1a illustrates a suitable from of current control circuit 30 that may be used. It includes a current demand to d-q axis current converter 31, which includes a phase advance circuit 33 as will be explained later. The d-q frame is fixed relative to the rotor, and so to perform this conversion the position of the motor is used as a parameter along with the demanded current. This conversion process is well known to the person skilled in the art.

(29) The d-q axis current generated by the converter 31 is fed to a subtractor block 35 which is also fed with a measurement of the actual motor currents, iDQ, also in the d-q axis frame. The difference signal, or error signal, produced by the subtractor is fed into a PI controller 37, whose function is to drive the error signal towards zero in value, at which time it is assumed that the actual currents in the motor match the demanded currents. The output of the PI controller 37 is converted from the dq frame into actual phase voltages, which are then fed into the drive circuit that comprises a PWM controller 18 that generates PWM voltage waveforms for each phase that form the six current signals 6.

(30) In a practical system it is usual to have the current sensors 22 to measure the current in each of the phases, or a single current sensor 28 in the dc link which can be used to measure the current in each of the phases by sampling the current at controlled times in the PWM period of the controller 30.

(31) Referring to FIG. 2 and FIG. 4a, in the simple case where the phase of the current waveform is in phase with variation in time t of the rotor position and hence also in phase with the back emf produced in the motor, the torque T is directly proportional to the phase current I, and given by the equation:
T=K.sub.T I
where K.sub.T is the motor torque constant. This equation would be modified slightly for a salient machine, such as a buried magnet type motor, with the torque T then also being dependent in that case on an additional term that is dependent on the d-axis current.

(32) Under normal circumstances, K.sub.T is constant over all currents as shown in FIG. 3. This mode of operation is said to have zero phase advance, as the current waveform is in phase with the back emf. In the d-q frame, all the current will lie along the q-axis, 90 degrees offset from the rotor position but in phase with the back emf. There will no current along the d-axis.

(33) However, as is well known, if the current phase is advanced, as shown in FIG. 4b, so that it is no longer in phase with the back emf, but ahead of it by a phase advance angle .sub.adv then the motor output torque varies as:
T=K.sub.T I cos .sub.adv

(34) This produces an effective torque constant K.sub.T-effective that varies with the cosine of .sub.adv as shown in FIG. 5. As well as q-axis current there will now be a d-axis current component.

(35) For any phase advance angle .sub.adv the motor torque T is still proportional to the phase current I, but as .sub.adv increases, the effective torque constant decreases, and so the torque for any given current decreases as shown in FIG. 6. Increasing the phase advance angle can also enable the motor to operate at higher speeds and output power as will now be described in more detail.

(36) According to motor equivalent circuit theory, the applied phase voltage V.sub.ph is given by
V.sub.ph=E+I R.sub.ph+j I X
where E is the back emf, I is the phase current, and X is the synchronous impedance.

(37) These vector quantities can be represented in a motor phase diagram as shown in FIG. 7a. In this diagram the d-axis is aligned with the magnetic flux of the rotor and the q-axis is perpendicular to the d-axis. With zero phase advance, the back emf is in phase with the phase current I, and the voltage XI is 90 out of phase. The voltages V.sub.ph IR, E and XI can therefore be represented as shown. However the back emf increases with motor speed, and V.sub.ph is limited by the ECU-dc.sub.link voltage, typically to about two thirds of the dc link voltage. Therefore the phase voltage V.sub.ph cannot extend beyond the voltage limit circle, and there is a maximum motor speed, determined by the maximum back emf, above which the motor cannot be driven.

(38) However, referring to FIG. 7b, if a phase advance of angle .sub.adv is introduced, then the current is not in phase with the back emf. This varies the phase of the IR and XI voltage vectors with respect to the back emf as shown. The result of this is that the magnitude of the back emf can be increased, and the vector sum of the voltages E, IR, XI still equal the V.sub.ph limit as shown. This means that the maximum possible output speed and power of the motor can be increased for any given ECU-dc.sub.link voltage.

(39) Referring to FIG. 8, the result of this is that for a fixed maximum ECU-dc.sub.link voltage V.sub.1 there is a maximum motor speed .sub.1 which cannot be exceeded without phase advance. This is where the back emf, which increases with motor speed, equals the maximum possible ECU-dc.sub.link voltage V.sub.1. For higher motor speeds, phase advance must be used.

(40) A typical motor torque/speed curve can be plotted as shown in FIG. 9. As can be seen, with phase advance the operating area is greatly increased. Higher motor speeds can be obtained, and also higher torques for some motor speeds can also be achieved. A typical phase advance profile is shown in FIG. 10. The phase advance is maintained at zero up to a speed .sub.1, which is slightly lower than .sub.1 in FIG. 9, and then increased with increasing motor speed.

(41) This results in a maximum torque that is constant up to speed .sub.1 and then decreases at a constant rate with increasing motor speed.

(42) To exploit this effect, the controller includes a phase advance circuit which forms a part of the current controller 30 as shown in FIG. 1b and FIG. 11. The phase advance circuit is a calculator block that generates a phase advance angle value that is fed into the d-q converter. Typically, the converter subtracts (or adds) the phase advance angle from (or to) the motor rotor position signal it receives, and uses the new position signal as the basis for the conversion.

(43) As shown in FIG. 11, the phase advance calculator receives as inputs a motor rotor speed signal indicative of the speed of the rotor, and a motor drive voltage signal indicative of the drive voltage, ECU DC link.

(44) The applicant has appreciated that that many magnets have temperature dependant properties, including their ability to retain magnetic field once magnetized. As temperature is increased the field required to damage the magnet's magnetization is reduced. When the phase of the currents is advanced, this introduces potentially harmful d-axis current (which creates magnetic field which opposes the magnet).

(45) To prevent the phase advance currents from damaging the magnets, a modified control circuit 30 is provided that receives a signal indicative of the temperature of the motor rotor magnets. This can be provided by a temperature sensor that measures the temperature of the rotor magnets. Alternatively, an estimator may be provided that estimates the temperature from an indirect measurement of the rotor magnets. Dependent on the temperature the circuit 30 will operate in the normal manner described above, with phase advance being applied as a function or DC link voltage and motor speed, or in an abnormal mode in which the amount of phase advance is reduced in some way.

(46) The temperature value is with a threshold temperature value. If the temperature signals exceeds a predefined temperature value, such as 130 degrees centigrade, then the current controller moves from a normal mode as described above into the abnormal mode as will now be described.

(47) In the abnormal mode, the amount of phase advance that is applied, or can be applied, is reduced or limited to levels which will not harm the magnets, and therefore the magnets can survive higher temperatures without damage to the magnetization (and therefore without permanent damage to motor performance). In effect, the amount of d-axis current is limited or reduced to prevent permanent damage occurring.

(48) The reduction of the amount of phase advance can be applied in several different ways. The applicant envisages several options as shown in FIGS. 12 to 15 of the drawings. In each case, the only modification to the drive circuit shown in FIGS. 1a, 1b and 11 is to the d-q converter that converts the current demand signal into the q axis and d axis components that are fed to the subtractor block 35.

Abnormal Mode 1Limit Phase Advance

(49) In this mode, the converter 310 include a phase advance calculator that calculates the Phase advance as normal, but the advance angle is limited to a maximum phase advance angle, i.e. advance=MIN(advance, maximum advance angle) by a limiting block 310a which acts on the phase angle value before it is applied to the d-q converter. This limit is only applied when operating in the abnormal mode, and is not applied when in the normal mode. The modified (limited) phase angle is then fed to the converter.

(50) The Maximum phase advance angle that is permitted in the abnormal mode may be varied as a function of temperature as shown in FIG. 16a, being reduced linearly between the threshold at which the abnormal mode is entered up to a higher threshold, and then reduced linearly at a higher rate one above the higher threshold. The effect of this scaling on the maximum allowable d-axis current is shown in FIG. 16b. Provided the phase advance angle is below the limit, the same phase advance angle will be used in the normal and abnormal modes.

Abnormal Mode 2Scale Back Phase Advance Angle

(51) In this arrangement, as shown in FIG. 13, the Phase advance angle is calculated by the current converter block 311 as normal but the advance angle is scaled by a gain factor, i.e. advance=k*advance where k varies between 0 and 1. This scaling is only done when in the abnormal mode, and is not applied in the normal mode. Scaling is applied by a scaling block 311a that modifies the phase angle produced by the phase angle calculator, the scaled angle being fed to the d-q converter block.

(52) The scaling or Gain could be scheduled with speed or be a switch with hysteresis as shown in FIGS. 17 and 18 of the drawings respectively. When hysteresis is used, the threshold temperature for entry into the abnormal mode is chosen to be higher than a threshold temperature used to determine when to exit the abnormal mode and return to the normal mode.

Abnormal Mode 3Limit D Axis Current

(53) In this arrangement, shown in FIG. 14, the current converter 312 calculates current demands as normal, not dependent on the temperature. However, the D axis current component is then limited to a maximum magnitude, i.e. d axis current=MAX (d axis current, maximum D axis current) by passing it through a limiter 312a before feeding it with the q-axis component to the subtractor block 35. This limiting is only applied in the abnormal mode, and is not applied in the normal mode.

(54) Maximum D axis current could be scheduled with speed or be a switch with hysteresis as with advance angle approach as with Options 1 and 2.

Abnormal Mode 4Scale Back D Axis Current

(55) In this alternative scheme, shown in FIG. 15, the current demands are calculated as normal by the current converter 313 but the D axis current is scaled by a gain factor, i.e. d axis current=k*d axis current where k varies between 0 and 1. This is only done in the abnormal mode by passing the d-axis component through a scaling block 313a.

(56) Again, the maximum D axis current could be scheduled with speed or be a switch with hysteresis as shown in FIGS. 17 and 18.

(57) Of course, it is possible that any combination of these four modes could be used in a drive system to produce a reduction in phase advance in the abnormal mode.