FIELD OF THE INVENTION

The present invention concerns control circuits for driving electric motors, such as induction motors. More

particularly, the invention concerns control circuits incorporating power factor correction circuits, and methods of driving electric motors using power factor correction circuits .

BACKGROUND TO THE INVENTION

It is known for control circuits for motors to incorporate a power factor correction circuit. The power factor

correction circuit corrects the difference between the real power and the apparent power required by the motor which is largely caused by the inductive effect of the motor. If not corrected, the motor would appear to require more generating capacity than is in fact the case, and would not operate efficiently. A power factor correction circuit generates the required output supply from a DC-link voltage, based upon a current control signal (and usually other additional modifying signals) .

Known active power factor correction circuits include a booster circuit provided between a rectifier and a DC link. The booster circuit ensures that the DC link voltage is higher than the peak line voltage of the AC input. The DC link voltage is maintained at a constant level and the output current drawn from the DC link is at the same frequency and phase as the AC line voltage, thereby

providing an output with a power factor approaching unity. A problem with known control circuits incorporating power factor correction circuits is that they can generate excess heat. This is due to the high difference between the peak line voltage (say 260V) and the DC-link voltage (which may for example be 400V) which causes high currents to circulate in the power factor correction circuit, thereby increasing losses. However, lowering the voltage level of the DC-link (for example to 350V) will under certain operating

conditions cause the power factor correction circuit to saturate, and is not a practical solution to the problem of excess heat generation.

The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved control circuit for driving a motor, and method of driving a motor.

SUMMARY OF THE INVENTION

The present invention provides a control circuit for driving a motor, the control circuit comprising: a power factor correction circuit arranged to receive a DC-link voltage and a current control signal, and to output a control supply for the motor; and a voltage control circuit arranged to control the voltage level of the DC-link voltage, wherein the voltage control circuit is arranged to control the voltage level of the DC-link voltage to be dependent upon the higher of the voltage level required by the power factor correction circuit and the voltage level required by the electric motor . By controlling the DC-link voltage level in this way, the voltage level is kept sufficiently high to allow the power factor correction circuit and the motor to operate properly. However, as the voltage is not maintained at the highest level that may be required during operation, but rather is varied according to the actual requirements of the power factor correction circuit and the motor, overheating of the components of the circuit is avoided or at least reduced.

The voltage level required by the power factor correction circuit may for example be 5 to 10V higher than the peak line voltage. Of course, other voltages (typically fixed voltages) above the peak line voltage could be chosen in a particular embodiment of the invention.

The voltage level required by the motor will depend on various factors such as the type motor type, operating frequency and the required or desired motor torque.

Preferably, the voltage level of the DC-link voltage is the higher of the voltage level required by the power factor correction circuit and the voltage level required by the electric motor. This means the voltage level is as high as required for proper operation of the circuit, but no higher. Alternatively, the voltage level of the DC-link voltage may depend upon, but not be equal to the higher of those values; for example the voltage level could be a fixed amount above the higher of the values. This would allow a tolerance in the setting of the voltage level, so that impaired operation would be avoided if the required voltage level was not accurately determined or set. Often, the voltage level required by the motor is dependent upon the operating frequency of the motor. The voltage level required by the motor may be the voltage level required to provide sufficient torque. The operating frequency of the motor will generally affect the voltage level required to provide a particular level of. torque.

Preferably, the control circuit further comprises a

microcontroller that controls the voltage level of the DC- link voltage. The microcontroller may output a pulse width modulation signal indicative of a desired DC link voltage.

Advantageously, the DC-link voltage supply may be supplied to the load feed forward input of the power factor

correction circuit. In the case that the control circuit comprises a microcontroller, this allows the circuit to be operated at the voltage level of microcontroller rather than the voltage level of the power factor control circuit, giving a simpler and more accurate circuit that does not require level shifting (for example) .

In accordance with a second aspect of the invention there is provided a method of driving an electric motor using a power factor correction circuit, comprising the steps of:

providing a DC-link voltage to the power factor correction circuit; providing a current control signal to the power factor correction circuit; and driving the electric motor using the output of the power factor correction circuit, wherein voltage level of the DC-link voltage is controlled to be dependent upon the higher of the voltage level required by the power factor correction circuit and the voltage level required by the electric motor. Preferably, the voltage level of the DC-link voltage is the higher of the voltage level required by the power factor correction circuit and the voltage level required by the electric motor .

The voltage level required by the motor may be dependent upon the operating frequency of the motor.

The voltage level of the DC-link voltage may be controlled by a microcontroller.

The DC-link voltage supply may be supplied to the load feed forward input of the power factor correction circuit.

In accordance with a third aspect of the invention there is provided a control circuit for driving an electric motor substantially as herein described with reference to Figures 1 to 3.

In accordance with a fourth aspect of the invention there is provided a method of driving an electric motor substantially as herein described with reference to Figures 1 to 3.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of example only with reference to the accompanying figures of which:

Figure 1 is a circuit diagram of a control circuit for driving an electric motor in accordance with a first embodiment of the invention;

Figure 2 is a flow chart showing the operation of the

microcontroller of the circuit of Figure 1; and

Figure 3 is a circuit diagram of a control circuit for driving an electric motor in accordance with an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A control circuit for a motor in accordance with an

embodiment of the invention is shown in Figure 1. In an exemplary implementation of the invention, the electric motor is used in a water pump, although this is not

essential to the present invention.

The control circuit 1 includes an inner (fast) current control loop that is used to control the instantaneous line current (and thereby ensure that the power factor is high) and an outer (slower) voltage control loop that is used to control the DC link voltage.

The control circuit 1 comprises a microcontroller 2, which controls the operation of the circuit, and a power factor corrector 3, which provides the power factor correction. The power factor corrector 3 is in the present embodiment an L4981B chip from manufactured by ST Microelectronics™. The power factor corrector 3 comprises an analogue multiplier 4, which provides a signal with the required waveform based on a number of inputs, as described in more detail below. The signal from the analogue multiplier 4 is passed to an op-amp 5, which takes a current input 6 to provide a current control loop. The output of the op-amp 5 is passed to a pulse-width modulator 7. The output of the pulse-width modulator 7 is then passed to a gate drive 8, the output 9 of which is used to drives the insulated gate bipolar transistor (IGBT) switches of the power factor correction circuit . An AC line supply 10 is input to the microcontroller 2. The analogue multiplier 4 also takes the AC line supply 10 as reference current input, which defines the shape of the waveform output by the analogue multiplier 4. The AC line supply 10 also passes through a low-pass filter 11 and divider 12 to provide a line feed forward input to the analogue multiplier 4, to compensate for any line voltage changes .

The DC link voltage is measured and provided as a DC voltage signal 13 that is input to the microcontroller 2.

The microcontroller 2 provides a pulse-width modulation (PWM) signal to a low-pass filter 14. The duty cycle of the PWM signal output by the microcontroller 2 sets the desired DC link voltage. By way of example, a duty cycle of 0% may indicate that the DC link voltage should be set to 320 volts, a duty cycle of 50% may indicate that the DC link voltage should be set to 360 volts and a duty cycle of 100% may indicate that the DC link voltage should be set to 400 volts .

The PW signal is filtered by the low-pass filter 14 to generate a slowly changing signal indicative of the desired DC link voltage. The output of the low-pass filter 14 is used as input to an op-amp 16. The op-amp 16 also has an input receiving the measured DC link voltage 13, and its own output via a feedback loop 17. The op-amp circuit 16 implements the voltage control loop and sets the DC link voltage to the desired DC link voltage as defined by the output of the low-pass filter 14. This is achieved by the op-amp circuit 16 subtracting the measured DC-link voltage 13 from the low-pass filter output 14 and amplifying this difference as determined by the feedback circuit 17.

The analogue multiplier 4 takes the output of the op-amp 16 as a load feed forward input, which modifies the current output proportionally.

The microcontroller 2 also provides an enable signal to the power factor corrector 3. The output of the low-pass filter 14 is pulled down, via a diode 15, when the enable signal is low (i.e. when the power factor correction is disabled). The enable signal provided to the power factor corrector 3 allows proper start up and shut down of the power factor corrector 3, without high line current spikes. In

particular, the time constant of the low pass filter 14 via the diode 15 provides a "soft start" of the power factor corrector 3.

In use, the AC line supply 10 provided as reference current input to the analogue multiplier 4 defines the waveform shape of the output of the power factor corrector 3, with the line feed forward input to the analogue multiplier 4 compensating for any line voltage changes. The analogue multiplier 4 also receives a load feed forward input from the op-amp circuit 16.

The voltage control for the power factor corrector 3 is controlled by the microcontroller 2, which controls the DC voltage supply 13 using the op-amp 16. The microcontroller 2 controls the voltage level supplied as load feed forward input to the analogue multiplier 4, in other words the DC- link: voltage for the power factor corrector 3 as a whole, based on the current operating conditions. This is done by virtue of the software on the microcontroller 2, i.e. the microcontroller 2 is programmed to provide a suitable pulse- width modulation signal based on information it has

available on the current operating conditions, for example the frequency at which the motor is operating and/or the load upon it.

The operation of the microcontroller 2 is shown in more detail in Figure 2. In a first step 100, the

microcontroller 2 calculates the voltage level required by the power factor corrector 3. This calculation is based upon the peak voltage level of the AC line supply 10. For example, the DC-link voltage required by the power factor corrector may be set as the peak value of the line voltage , 10 plus a fixed difference (e.g. 10 V) . In alternative embodiments, the required voltage level may be a predetermined value based on known the properties of the power factor corrector 3 and the expected AC line supply.

In a second step 101, the microcontroller 2 calculates the voltage level required by the motor. This calculation is based on the known properties of the motor and its current operating conditions, including its frequency of operation. In an exemplary implementation of the invention, the voltage level calculated in step 102 is a fixed multiple of the motor frequency (such as 5.5 times the motor frequency) . In a third step 102, the microcontroller 2 sets the level of the pulse-width modulation signal at the appropriate level based upon the higher of the two levels calculated in the previous steps 100, 101, so that the output of the op-amp 16 has the appropriate voltage level. Thus, the DC-link voltage for the power factor corrector 3 is always

sufficiently high for proper operation, but is also not maintained at a higher level than is required so excess heat in the components of the power factor corrector 3 is avoided (or at least reduced) .

By way of example, in an exemplary implementation of the invention, the duty cycle of the PWM signal is set in step 102 as ((desired DC-link voltage ) -315 ) /86. Where the duty cycle is always between 0.0 and 1.0 As described above, the control circuit 1 includes an inner (fast) current control loop that is used to control the instantaneous line current (and thereby ensure that the power factor is high) and an outer (slower) voltage control loop that is used to control the DC link voltage. Figure 3 shows an alternative embodiment of the invention in which the voltage control loop used to set the DC link voltage is implemented entirely within the microcontroller. As in the previous embodiment, the control circuit 200 comprises a power factor corrector 3 with an analogue multiplier 4, the signal of which passes to an op-amp 5, which takes a current input 6 to provide a current control loop; the output of the op-amp 5 is passed to a pulse-width modulator 7, and the output of the pulse-width modulator 7 is passed to a gate drive 8, the output 9 of which is used to drive a motor. The analogue multiplier 4 takes an AC line supply 10 as reference current input to define the shape of the waveform output by the analogue multiplier 4, and with the AC line supply 10 also passing through a low- pass filter 11 and divider 12 to provide a line feed forward input to the analogue multiplier 4 to compensate for any line voltage changes. However, in contrast to the previous embodiment, the outer voltage control loop previously provided by the op-amp 16 is instead directly provided by a suitably programmed

microcontroller 201. As the voltage control loop is slower and not time-critical, the slower operation of the

microcontroller 201 (compared with the analogue electronics of the power factor corrector 3) is not a problem, and the accuracy of the circuit can, in fact, potentially be improved. (It will be appreciated, however, that the PWM reference frequency and the properties of the other

components in the circuit, in particular the low pass filter 14, will need to be varied as appropriate.)

Thus, Figure 3 shows an arrangement in which digital circuits are used to set the DC link voltage and analogue circuits are used to set the line current. It is possible, however, for both the DC link voltage and the line current to be controlled digitally. The microcontroller could be used to set both the DC link voltage and the desired line current. This would require a very fast microcontroller and this speed requirement is one reason why the line current at least is typically controlled using analogue circuits.

However, as microprocessor speeds increase, purely digital control becomes more attractive.

While the present invention has been described and

illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.