Motor control apparatus

Abstract: To provide a motor control apparatus capable of preventing large accidental switching surges and suppressing switching surges that hamper efforts to improve the efficiency of PWM control inverters. The motor control apparatus has a PWM control inverter and axial position detector 3 which detects the position of the rotary axis of motor 14. Gate drive circuit 1 of the PWM control inverter comprises decoder circuit 4, correction gate circuit 5, and logical AND circuit 6. Said correction gate circuit 5 sets a time difference at the change point for PWM signals and at the change point for commutation signals. This time difference enables switching surges to be controlled and therefore improve the efficiency of the PWM control inverter.

Description: This invention relates to a motor control apparatus that suppresses the switching surge from pulse width modulation (PWM) control inverters. In recent years, PWM control inverters (inverters) have been widely used for driving and/or controlling motors. In such PWM control inverters control devices are used such as power transistors, MOSFET and insulated Gate Bipolar Transistor (IGBT) (hereinafter referred to as power devices). A conventional motor control apparatus (PWM) control inverter circuit is shown in Figure 16. As shown in the figure, in this motor control apparatus PWM circuit 2 transmits PWM signal 20, in which the pulse width is controlled so that a specified current flows, to gate drive circuit 1 of the inverter. Decoder circuit 4 transmits commutation signal 22 to logical AND circuit 6, based upon detection signal 21 that is transmitted by axial position detector 3 of motor 14. Logical AND circuit 6 turns the corresponding power devices Tr1, Tr2, Tr3, Tr4, Tr5 and Tr6 according to PWM signal 20 and commutation signal 22 and thus drives motor 14. Diodes 7. 8, 9, 10, 11 and 12 are connected to power devices Tr1, Tr2, Tr3, Tr4, Tr5 and Tr6 respectively. Direct current power supply 13 is connected to the input side of the inverter. In the inverter of the aforementioned conventional motor control apparatus, when the aforementioned power device is switched, a switching surge ΔV = L.dI/dt is generated, where L denotes the wire inductance, and I denotes the current that flows between positive pole bulk 15 and negative pole bulk 16. In that case, the switching surge can exceed the voltage tolerance of the power device, thus damaging the power device. Therefore, conventionally the aforementioned switching surge is suppressed by reducing the aforementioned wire inductance, and by providing a snubber circuit that absorbs ΔV into the phases of the aforementioned inverter. Please see for example, Japanese Patent Application No. H6-284517. In the prior art as described above the efficiency of the aforementioned inverter can be increased by increasing the aforementioned switching speed and by reducing the switching loss of the aforementioned power device. However, this increases the dI/dt: thus resulting in an increase in the aforementioned switching surge ΔV. This requires that the power device be a highly voltage-tolerant device. However, the higher the voltage tolerance of the power device the greater is the attendant loss. Thus, even if the switching speed is increased, the requirement for highly voltage-tolerant power devices defeats the purpose. Conventional techniques employed to circumvent this problem include reducing the magnitude of the wire inductance and providing a snubber circuit, as stated above. However, these techniques are subject to the limitations of the inverter's size and physical shape. Furthermore, the switching surge can increase accidentally: especially when the switching by PWM signal 20 and the switching by commutation signal 22 overlap in the power device. It is difficult to reduce the voltage tolerance of the power device in order to prevent damage to the power device from an accidental switching surge. Thus, there are difficulties in increasing the efficiency of the inverter due to switching surges and especially due to larger switching surges that occur accidentally. The present invention is intended to solve these problems. Thus an object of the present invention is to provide a highly efficient motor control apparatus. According to the present invention, there is provided a motor control apparatus that controls the operation of a motor by means of a PWM control inverter, characterised in that the motor control apparatus is provided with a correction means having a function of setting, during motor operation, a time difference at a PWM signal change point and a commutation signal change point. The motor control apparatus of the present invention controls the operation of a motor by means of a PWM control inverter, and comprises a PWM control inverter (an inverter) and an axis position detector that detects the position of the motor's rotary axis. The aforementioned PWM control inverter comprises a plurality (six) of power devices, a gate drive circuit, and a PWM circuit. It is also provided with a correction means with a function of setting a time difference at PWM signal change points and commutation signal change points. The correction means correct delays, shortens or elongates at least one of the PWM signals and commutation signals. Examples of correction means include a correction gate circuit (to be described later) and control means Embodiments of the present invention will now be described with reference to the accompanying drawings, of which: Figure 1 is a block diagram showing a configuration example of the motor control apparatus of the present invention (the PWM control inverter circuit for driving the motor under the present invention); Figure 2 is a time chart showing the basis for the present invention; Figure 3 is another time chart; Figure 4 is a graph demonstrating the basis for the present invention; Figure 5 is a block diagram showing a configuration example of the correction gate circuit of the present invention; Figure 6 is a time chart showing the signals in the correction gate circuit of Figure 5 when the commutation signal changes from the low level to the high level when the PWM signal is on; Figure 7 is a time chart showing the signals in the correction gate circuit of Figure 5 when the commutation signal changes from the low level to the high level when the PWM signal is off; Figure 8 is a block diagram showing another configuration example of the correction gate circuit of the present invention; Figure 9 is a time chart showing the signals in the correction gate circuit of figure 8; Figure 10 is a block diagram showing another configuration example of the correction gate circuit of the present invention; Figure 11 is a time chart showing the signals in the correction gate circuit of Figure 10 when the commutation signal changes from the low level to the high level when the PWM signal is on; Figure 12 is a time chart showing the signals in the correction gate circuit of Figure 10 when the commutation signal changes from the low level to the high level when the PWM signal is off; Figure 13 is the main part of a flowchart (the main routine) showing the operation of the control means in the present invention; Figure 14 is a flowchart (a subroutine) showing the operation of the control means during commutation signal interrupt processing in the present invention; Figure 15 is a conceptual diagram explaining the gate signal switching of the present invention; and Figure 16 is a block diagram showing a conventional motor control apparatus (PWM control inverter circuit). The following is an explanation of the embodiments of the present invention with reference to the drawings. Although in the following embodiments power devices Tr1, Tr2, Tr3, Tr4, Tr5 and Tr6 of the PWM control inverter (inverter) are described as MOSFET devices, the description should not be construed as limiting the power devices Tr1-Tr6 to MOSFET devices. In addition to MOSFETs, power transistors and IGBT's can be used as power devices Tr1-Tr6. Figure 1 is a block diagram showing an example configuration of the motor control apparatus of the present invention (the motor driving PWM control inverter circuit based on the present invention). It should be noted that figure 1 corresponds to figure 16, in which items that have virtually identical configurations are assigned the same reference numbers and their explanation is omitted. As shown in figure 1 in contrast to the conventional motor control apparatus, this embodiment is provided with correction gate circuit 5 as a correction means with a function of setting, during motor operation, a time difference at change points for PWM signals and change points for commutation signals. In the context of this document, the gate signal switching time Tg for the power devices Tr1-Tr6 during motor operation refers to the time that the gate voltage (the gate-source voltage in the MOSFET) for the power device Tr1-Tr6 changes from VG1 to VG2 through threshold voltage Vth, as shown in figure 15. The change point for a PWM signal or a commutation signal, as mentioned above, refers to a point in time when the gate voltage passes through Vth, as shown in figure 15. In the following, the term "gate signal" refers to the aforementioned gate voltage. When the gate voltage of the aforementioned power device is VG2, the gate signal is said to be on; when the gate voltage is at VG1, the gate signal is said to be off. The following explains the appropriateness of the above scheme with reference to the time chart in figure 2. For the sake of simplicity the discussion below only explains the points in time when the direction of motor current changes from phase U to phase W and from phase V to phase W. In this case, gate signal 25 of power device Tr1 changes from on to off (Figure 2(1)), gate signal 27 of power device Tr3 changes from off to on (Figure 2 (2)), and PWM signal 20 is transmitted to gate 30 of power device Tr6. The switching surge mentioned above is proportional to both wire inductance L and the quantity dI/dt. The quantity dI/dt is generated by the switching of power devices Tr1-Tr6. The larger the number of power devices that are switched, the greater is the quantity dI/dt. Thus, when gate signal 30 of power device Tr3 is at position (3) in figure 2, the change point for the commutation signal and that for the PWM signal coincide, thus increasing the size of dI/dt. In conventional PWM control inverts, this phenomenon occurs accidentally and accidentally increases the switching surge. Therefore, a time difference is provided at the change point for said PWM signal and that for said commutation signal so that gate signal 30 of power device Tr6 will be at lines (4) or (5) in figure 2. This prevents the large, accidental switching surges that are described above. In the second embodiment, the conventional motor control apparatus is provided with correction gate circuit 5 with a function of setting a time difference for the change point for a PWM signal and that for a commutation signal during motor operation, as well as a function of turning the change point for said PWM signals during "off" time. The following is explained with reference to the time chart in figure 3. For the sake of simplicity, the discussion below only explains the points in time when the direction of motor current changes from phase U to phase V and from phase U to phase W. In this case power device Tr1 remains "on", and the power device to which PWM signal 20 is transmitted changes from power device Tr4 to power device Tr6. If commutation signal 22 corresponding to power device Tr4 is transmitted as in figure 3(1), and commutation signal 22 corresponding to power device Tr6 is transmitted as in figure 3(2), when PWM signal 20 that is transmitted from PWM circuit 2 is on figure 3(3), the gate signals of power devices Tr4 and Tr6 assume the states shown in figure 3(4) and figure 3(5), respectively. This causes power devices Tr4 and Tr6 to undergo switching. However, if said PWM signal 20 is off (Figure 3(6)) gate signals 28 and 30 of power devices Tr4 and TR6 during commutation assume the states indicated by Figure 3(7) and (8). In this case, power devices Tr4 and Tr6 will not switch. Therefore, a time difference is set at the change point for said PWM signal, and at that for said commutation signal, and the change point for said commutation signal is defined as the time when said PWM signal is off. This prevents any large accidental switching surges as described above. In particular, if power devices Tr1, Tr3 and Tr5 are constructed as the upper arm, and power devices Tr2, Tr4 and Tr6 as the lower arm, the greater the effect of switching of the lower arm compared to the switching of the upper arm, as described in Embodiment 1, the greater is the effect of the present invention, as illustrated by the present embodiment. Figure 4 shows an example of actual measurements based upon the present embodiment. The horizontal axis of the graph shown in figure 4 plots the time difference at the change point for the PWM signal and that for the commutation signal (also simply referred to as the "time difference"), normalised by said gate signal switching time Tg. The vertical axis plots the maximum voltage that appears at positive pole bulk 15 and negative pole bulk 16, normalised by the DC power supply voltage. The motor current Im is normalised by the motor current that is required for rated motor output. As illustrated in the figure, the switching surge can be reasonably suppressed by setting the aforementioned time difference at 0.5 Tg or greater, preferably at 0.8 Tg or greater and optimally at Tg or greater. As described above, the aforementioned large, accidental switching surges can be prevented and switching surges can be inhibited by setting a time difference at the change point for said PWM signal and that for said commutation signal, and by setting the change point for said commutation signal to the time when said PWM signal is off. The following is an explanation of Embodiment 3. Figure 5 is a block diagram showing a configuration example of correction gate circuit 5 in the motor control apparatus for the present invention. Figure 6 is a time chart showing the signals in correction gate circuit 5, shown in figure 5, when commutation signal 22 changes from the low level (synonymous with the signal being off)to the high level (synonymous with the signal being on) when PWM signal 20 is on. Similarly, figure 7 is a time chart showing the signals in correction gate circuit 5, shown in figure 5, when commutation signal 22 changes from the low level to the high level when PWM signal 20 is off. The following explanation is based on figures 5, 6 and 7. Correction gate circuit 5 in this embodiment is provided with a function of setting a time difference, during motor operation, at the change point for PWM signals and at the change point for commutation signals, as well as a function of setting the change point for said commutation signal at the time when a PWM signal is off. Specifically, as shown in Figure 5, correction gate circuit comprises PWM signal delay circuit 31, which delays PWM signal 20, and commutation signal delay circuit 32, which delays commutation signal 22, based on the signals from PWM signal delay circuit 31. In this case, the signals from commutation signal delay circuit 32, namely corrected commutation signal 24, are used as commutation signals. First, the following explains the situation in which commutation signal 22 changes from the low level (synonymous with the signal being off) to the high level (synonymous with the signal being on) when PWM signal 20 is on. As shown in figures 5 and 6, the PWM signal 20 which is transmitted from PWM circuit 2 is delayed by the time Td1 (Td1 ≥ 0.5 Tg) and by PWM signal delay circuit 3 and becomes delayed PWM signal 33 (figure 6). This delayed PWM signal 33 is used as a clock signal and commutation signal 22 is converted into corrected commutation signal 24 by commutation signal delay circuit 32 (figure 6). It should be noted that, in this embodiment only commutation signals are corrected. By the transmission of this corrected commutation signal 24 and delayed PWM signal 23, which is the same as PWM signal 20, to logical AND circuit 6, the aforementioned accidental large switching surges can be prevented and switching surges can be suppressed when the motor operates by satisfying the condition: O < Ton < Tpwm - 0.5Tg - Td1 given the fact that if the PWM signal cycle is denoted as Tpwm and the PWM signal on time as Ton, the time difference described in Embodiment 2 is greater than or equal to 0.5Tg. The following explains the case where computation signal 22 changes from the low level to the high level when PWM signal 20 is off. As shown in figures 5 and 7, as commutation signal 22 changed from the low level to the high level when aforementioned PWM signal 20 was On, PWM signal 20 which is transmitted from PWM circuit 2 is also delayed by time Td1 by PWM signal delay circuit 31 and becomes delayed PWM signal 33 (figure 7). This delayed PWM signal 33 is used as a clock signal, and commutation signal 22 is converted into corrected commutation signal 24 by commutation signal delay circuit 32 (figure 7). It should be noted that, in this embodiment, only commutation signals are corrected. Both corrected commutation signal 24 and delayed PWM signal 23, which is the same as PWM signal 20, are transmitted to logical AND circuit 6. In this embodiment, corrected commutation signal 24 changes from the high level to the low level when both delayed PWM signal 33 and commutation signal 22 are at the low level (when they have reached the low level) as shown in figures 6 and 7. Timers and counters can be used as a means of generating a time delay (Td1). In this case, the time delay can be changed by switching the circuits or by setting a count in the counter. For suppressing the switching surge more reliably, the aforementioned time Td1 should be Td1 ≥ 0.5 Tg, preferably Td1 ≥0.8 Tg and optimally Td1 ≥ Tg. To create a time difference greater than or equal to 0.8 Tg. the aforementioned PWM signal on time, Ton, should be 0 < Ton < Tpw - 0.8 Tg-Td1. To create a time difference greater than or equal to Tg, Ton should be 0 < Ton < Tpwm - Tg - Td1. Because it only uses delay circuits, this Embodiment offers the advantages of a simple circuit configuration and the ability to set accurately specified time differences. The following is an explanation of Embodiment 4. Figure 8 is a block diagram showing another configuration example of correction gate circuit 5 in the motor control apparatus of the present invention. Figure 9 is a time chart showing the signals in correction gate circuit 5, shown in figure 1. The following explanation is based on figures 8 and 9. Correction gate circuit 5 in this embodiment provides a time difference, during rotor operation, at the change point for PWM signals and at the change point for commutation signals, and has the function of setting the change point for said commutation signal at the time when a PWM signal is off. Specifically as shown in Figure 8, correction gate circuit 5 comprises prohibition gate circuit 34, which adjusts the output of PWM signal 20, and commutation signal delay circuit 32, which delays commutation signal 22. In this case, prohibition gate circuit 34 is configured so that it disables the output of PWM signal 20 by means of commutation signal 22. Further, the signals from the prohibition gate circuit 34, namely corrected PWM signals 23, are used as PWM signals; and the signals from commutation signal delay circuit 32, namely corrected commutation signals 24, are used as commutation signals. As shown in figures 8 and 9, commutation signal 22 is delayed by time Td2 (Td1 ≥ 0.5 Tg) by commutation signal delay circuit 32 and becomes delayed and corrected commutation signal 24 (Figure 9). The PWM signal 20 transmitted from PWM circuit 2 becomes in prohibition gate circuit 34, corrected PWM signal 23 that is forced to become an off signal, triggered by commutation signal 22, at least during time Tm (Tm ≥ Td2 + 0.5 Tg) from the change point for commutation signal 22 (Figure 9). Specifically, as shown in Figure 9, when commutation signal 22 changes from the low level to the high level when PWM signal 20 is on, corrected PWM signal 23 is turned off by prohibition gate circuit 34 using commutation signal 22 (in this embodiment, the rise of commutation signal 22) as a trigger. The corrected PWM signal remains at the low level until a pulse after PWM signal 20 arrives. Although not shown in the figure, if PWM signal 20 remains at the high level after the passage of time Tm, corrected PWM signal 23 turns on and reaches the high level. Also as shown in figure 9, if commutation signal 22 changes from the high level to the low level when PWM signal 20 is off, corrected PWM signal 23 is changed by prohibition gate circuit 34 to the low level during time Tm from the change point (in this embodiment the fall change point for commutation signal 22) for commutation signal 22 and turns on after the passage of time Tm. By transmitting corrected PWM signal 23 and corrected commutation signal 24 to logical AND circuit 6, it is possible to prevent the accidental large switching surges discussed above and to suppress switching surges. For suppressing a switching surge more reliably, aforementioned time Td2 should be Td2 ≥ 0.5 Tg, preferably Td2 ≥ 0.8 Tg, and most optimally Td2 ≥ Tg. For suppressing a switching surge more reliably, aforementioned time Tm should be Tm ≥ Td2 + 0.5 Tg, preferably Tm ≥ Td2 + 0.8 Tg, and optimally Tm ≥ Td2 + Tg. This embodiment offers the advantages of a simple circuit configuration, the ability to set a specified time difference accurately, as described above, and of not requiring the aforementioned time Ton condition because the PWM signal is unilaterally turned off. The following is an explanation of Embodiment 5. Figure 10 is a block diagram showing another configuration example of correction gate circuit 5 in the motor control apparatus of the present invention. Figure 11 is a time chart showing the signals in correction gate circuit 5, shown in figure 10, when commutation signal 22 changes from the low level to the high level when PWM signal 20 is on. Figure 12 is a time chart that shows the signals in correction gate circuit 5, shown in figure 10, when commutation signal 22 changes from the low level to the high level when PWM signal 20 is off. The following explanation is based on figures 10, 11 and 12. Correction gate circuit 5 in this embodiment is provided with a function of setting a time difference, during motor operation, at the change point for PWM signals and at the change point for commutation signals, as well as a function of setting the change point for said commutation signal at the time when a PWM signal is off. Specifically, as shown in Figure 10, correction gate circuit 5 comprises prohibition gate circuit 34, which adjusts the output of PWM signal 20 (the output corrected PWM signal 23), a commutation signal delay circuit 32 (the first commutation signal delay circuit) that delays commutation signal 22, and a commutation signal delay circuit 35 (the second commutation signal delay circuit) which further delays the commutation signal (corrected commutation signal 24) that was delayed by commutation signal delay circuit 32. In this case, prohibition gate circuit 34 is configured so that it disables the output of PWM signals by means of commutation signal 22 and enables the output of PWM signal 20 by means of the signals from commutation signal delay circuit 35. Further, the signals from the prohibition gate circuit 34, namely corrected PWM signal 23, are used as PWM signals; and the signals from commutation signal delay circuit 32, namely corrected commutation signal 24, are used as commutation signals. First, the following describes the case where commutation signal 22 changes from the low level to the high level when PWM signal 20 is on. As shown in figures 10 and 11, commutation signal 22, transmitted by decoder circuit 4, is input into prohibition gate circuit 34 as well as into commutation signal delay circuit 32, and emerges as corrected commutation signal 24, which is delayed by time Td3 by commutation signal delay circuit 32 (Figure 11). The corrected commutation signal 24 is changed by commutation signal delay circuit 35 into a re-delayed signal 36 delayed by time Td4 (figure 11). Re-delayed signal 36 is input into prohibition gate circuit 34. The PWM signal 20, transmitted from PWM circuit 2, becomes, in prohibition gate circuit 34, corrected PWM signal 23. Corrected PWM signal 23 is forced to become an off signal, triggered by aforementioned commutation signal 22 and re-delayed signal 36, at least between the change point for commutation signal 22 and the change point for re-delayed signal 36 (Figure 11). By transmitting both corrected PWM signal 23 and corrected commutation signal 24 to logical AND circuit 6, it is possible to prevent the accidental, large switching surges as discussed above and suppress switching surges. In the case of the timing shown in figure 11, the time difference between change point 37 for corrected commutation signal 24 and change point 38 for corrected PWM signal 23 will be Td3; similarly, the time difference between change point 37 for corrected commutation signal 24 and change point 39 for corrected PWM signal 23 will be Td4. Next, the following describes the case where commutation signal 22 changes from the low level to the high level when PWM signal 20 is off. As shown in figures 10 and 12, similar to the case where commutation signal 22 changes from the low level to the high level when PWM signal 20 is on, as described above, PWM signal 20, transmitted from PWM circuit 2, becomes, in prohibition gate circuit 34, corrected PWM signal 23. Corrected PWM signal 23 is forced to become an off signal, triggered by aforementioned computation signal 22 and re-delayed signal 36, at least between the change point for commutation signal 22 and the change point for re-delayed signal 36 (figure 12). Both corrected PWM signal 23 and corrected commutation signal 24 are transmitted to logical AND circuit 6. In the case of the timing shown in figure 12, the time difference between change point 37 for corrected commutation signal 24 and change point 40 for corrected PWM signal 23 will be greater than or equal to Td3, and the time difference between change point 37 for corrected commutation signal 24 and change point 41 for corrected PWM signal 23 will be Td4. Aforementioned time Td3 should be Td3 ≥ 0.5 Tg, preferably Td3 ≥ 0.8 Tg, and optimally Td3 ≥ Tg. Similarly time Td4 should be Td4 ≥ 0.5 Tg, preferably Td4 ≥ 0.8 Tg and optimally Td4 ≥ 0.5 Tg. In this case, times Td3 and Td4 can be either equal or different. This embodiment offers the advantages of a simple circuit configuration, the ability to set a specified time difference accurately, as described above, and of not requiring the aforementioned time Ton condition because the PWM signal is unilaterally turned off. Because this embodiment is capable of generating corrected PWM signals 23 for which PWM signal 20 is off (at the low level) only for the necessary and sufficient interval it offers the advantage of superior PWM commutation. In the embodiment described above, a time difference is set at the change point for a PWM signal and at the change point for a commutation signal by corrected gate circuit 5. However, in the present invention a control means (not shown in the figures) can be provided as a correction means in place of correction gate circuit 5, and this control means can be used to set a time difference at the change point for a PWM signal and at the change point for a commutation signal. In this case, for example, in the specified memory (not shown in the figures) that is built into the control means a program can be pre-stored or recorded to set a time difference at the change point for a PWM signal and at the change point for a commutation signal during motor operation. Thus said control means executes the program to control the operation of gate drive circuit 1, PWM circuit 2, and other components, in order to set the aforementioned time difference. Such control means can be a microcomputer. Such memory can be ROM. EEPROM and similar non-volatile memory. The following is an explanation of Embodiment 6. Figure 13 is the main part of a flowchart (the main routine) showing the operation of the control means. Figure 14 is a flowchart (a subroutine) showing the operation of the control means when it performs commutation signal interrupt processing. The following explanation is based on these flowcharts. As shown in figure 13, when an instruction voltage associated with the target value for the voltage to be applied to motor 14 is input, a PWM instruction value is read according to the instruction voltage (step 201). Subsequently, PWM processing (a PWM subroutine) is executed (step 202). The PWM processing replaces the said PWM instruction value with PWM data, reads the PWM data after replacement, outputs a PWM signal with a specified time ratio based on the PWM data that is read out, and then returns to the main routine. After the above step, the commutation signal interrupt processing is enabled (step 203). Although not shown in the flowchart of figure 13, the commutation signal interrupt processing is disabled in specified steps following step 203. The commutation signal interrupt processing is executed when a commutation signal interrupt instruction is in input when the aforementioned commutation signal interrupt processing is still enabled. The commutation signal interrupt instruction is input at specified times based on detection signal 21 that is transmitted from axial position detector 3 of motor 14. As shown in figure 14, first, the commutation signal interrupt processing determines whether or not the PWM signal is on (at the high level) (step 301). In above step 301, if the PWM signal is determined to be on, the PWM signal, i.e. only the PWM pulse in question, is turned off (to the low level) (step 302). After that, the timer (not shown in the figures) is activated, and a standby state is effected until a pre-set amount of timer time passes (step 303). The timer time is a specified amount of time greater than or equal to 0.5 Tg. This ensures that the length of time when the PWM signal falls until the commutation signal rises, i.e. the time from the change point for the PWM signal to the change point for the commutation signal, is greater than or equal to 0.5 Tg. After that, the output commutation signals is started, thus changing the commutation signals from the low level to the high level (step 304). Then, the timer is activated, a standby state is effected until a pre-set amount of time passes in accordance with the timer (step 305). The time is a specified amount of time greater than or equal to 0.5 Tg. This ensures that the length of time from when the commutation signal rises until the pulse following the PWM signal rises, i.e. the time from the change point for the commutation signal to the change point for the PWM signal, is greater than or equal to 0.5 Tg. Subsequently, the output of PWM signals is enabled (step 306). In the above step 301, if the PWM signal is determined to be off, the output of PWM signals is disabled (step 307). Then, commutation signals are output, namely, commutation signals are changed from the low level to the high level (step 308). After that, the timer is activated, and a standby state is effected until a pre-set amount of time passes in accordance with the timer (step 309). The time is a specified amount of time greater than or equal to 0.5 Tg. This ensures that the length of time when the commutation signal rises until the PWM signal rises, i.e. the time from the change point for the commutation signal to the change point for the PWM signal, is greater than or equal to 0.5 Tg. After that, the output of PWM signals is enabled (step 310). After either step 306 or 310, control returns to the main routine. Thus, as in Embodiments 4 and 5, this embodiment sets a time difference at the change point for the PWM signal and at the change point for the commutation signal, and sets the change point for the commutation signal as the time when the PWM signal is off. In this manner, the embodiment can prevent large accidental switching surges and suppress switching surges. Moreover, because the timer bases standby time (the commutation signal delay time, etc) to be set at greater than or equal to 0.5 Tg and a time difference is set at greater than or equal to 0.5 Tg, the embodiment is capable of reliably suppressing switching surges. In the case of a rewritable program, this embodiment allows appropriate modifications of the program. In this case, either the entire program or parts of it, such as the pre-set amounts of time, can be modified. Moreover, the omission of correction gate circuit 5 in this embodiment simplifies the circuit configuration. Although in this embodiment the standby time is set at greater than or equal to 0.5 Tg, it can be set at greater than or equal to 0.8 Tg or even at greater than or equal to Tg in order to enhance reliability. Although in this embodiment the program is stored in memory that is built into the control means, for the purposes of the present invention only, the program could be stored in a magnetic recording medium, an optical recording medium, a magneto-optical recording medium, or other recording media; or on an IC memory card or other storage media. Although the motor control apparatus of the present invention is described in terms of the configuration examples shown in the figures, the present invention should not be construed as being limited to those examples. For example, for the purposes of the present invention the time difference between the change point for PWM signals and that for commutation signals can be set to be less than 0.5 Tg. As described above, according to the motor control apparatus of the present invention, the provision of a correction means with the function of setting a time difference between the change point for PWM signals and the change point for commutation signals during motor operation prevents any large, accidental switching surges and can suppress switching surges. Consequently, the voltage tolerance of power devices can be reduced and their switching speed can be increased. As a result, the efficiency of inverters can be increased. Thus the present invention contributes to improvements in equipment performance and reliability. The suppression of switching surges, moreover, helps to prevent electromagnetic interference and either eliminates or reduces the need for electromagnetic wave-shielding means.

Claims: A motor control apparatus for controlling a motor by means of a PWM control inverter (1-5), characterised in that said inverter includes correction means (5) for setting a time difference (Td1-Td4) between a PWM signal change point (38,39,40,41) and a commutation signal change point (37). The motor control apparatus of claim 1, in which said correction means sets said commutation signal change point whilst said PWM is at a low level. The motor control apparatus of claim 1 or 2, in which said correction means corrects at least one of said PWM signals and commutation signals to set the time difference. The motor control apparatus of any one of claims 1 through 3 in which said correction means comprises a commutation signal delay circuit (32,35) and a PWM signal delay circuit (31). The motor control apparatus of claim 4, in which said commutation signal delay circuit is arranged such that it delays said commutation signal based upon signals from said PWM signal delay circuit to provide said commutation signals for the PWM control inverter. The motor control apparatus of any one of claims 1 through 3, in which said correction means includes a prohibition gate circuit (34), which adjusts the output of PWM signals, and a commutation signal delay circuit (32,35). The motor control apparatus of claim 6, in which said prohibition gate circuit is arranged such that it disables, by means of said commutation signals, the PWM signals to provide PWM signals for the PWM control inverter, and in which the prohibition gate circuit uses the signals from said commutation signal delay circuit as commutation signals. The motor control apparatus of any one of claims 1 through 3 in which said correction means comprises a prohibition gate circuit that adjusts the output of PWM signals, a first commutation signal delay circuit (32) which delays commutation signals, and a second commutation signal delay circuit (35) which further delays the commutation signals which are delayed by said first commutation signal delay circuit. The motor control apparatus of claim 8, in which said prohibition gate circuit is arranged such that, by means of said commutation signals, it disables the output of said PWM signals and enables the output of said PWM signals by means of the signals from said second commutation signal delay circuit and in which the prohibition gate circuit uses the signals from said first commutation signal delay circuit as commutation signals. The motor control apparatus of any one of claims 1 through 3, in which said correction means is a control means, which is provided with a memory for storing a program to set a time difference at PWM signal change points and commutation signal change points during motor operation, and in which the correction means is arranged so that said time differences are set by the execution of said program by the control means. The motor control apparatus of claim 10, in which said control means is provided with a function of determining whether said PWM signal is on or off and performs processing based on the result of the determination. The motor control apparatus of claim 11, in which said control means, when said commutation signal is input, turns the PWM signal off if said PWM signal is on; and if, while said PWM signal is off, the output of said commutation signals is commenced, enables the output of said PWM signals, after the output of said commutation signals is commenced. The motor control apparatus of claims 1 through 12, in which said time difference is 0.5 Tg or greater if the gate switching time of the power device of said PWM control inverter is defined as Tg.

Inventors: KANNA SHIGEO, UETAKE AKIHITO, KANNA, SHIGEO, UETAKE, AKIHITO
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1996
Main classification: H02P6
IPC codes: H02P6