JP6158881B2 - Motor control method characterized by inverter protection method - Google Patents

Description

  The present invention relates to a method for controlling a motor driven by PWM control of an inverter, and although not limited thereto, the present invention relates to a motor control method suitable for a servo motor control method of an electric injection molding machine.

  Many industrial machines are provided with a motor for driving each device. For example, an electric injection molding machine is composed of a mold clamping device that clamps a pair of molds, an injection device that melts resin and injects it into the mold, and the injection device is an injection cylinder and the direction of rotation within the injection cylinder. And an axially driven screw, a plunger and the like. Each of these devices is driven by a motor such as a servo motor, and is driven by a three-phase alternating current generated by an inverter. Since the three-phase alternating current generated by the inverter is generated by so-called PWM control that variably controls the ON / OFF pulse width, the waveform of the three-phase alternating current is a pseudo sine wave. When the carrier frequency of the PWM signal, that is, the PWM frequency is small, the waveform of the three-phase alternating current becomes rough. However, when the PWM frequency is high, the waveform approaches a sine wave, and the motor can be driven with high accuracy. In other words, it can be said that it is preferable to increase the PWM frequency when emphasizing responsiveness in motor control. However, if the PWM frequency is large, the frequency of switching of elements such as IGBTs constituting the inverter increases. As a result, power is lost due to so-called switching loss, and the amount of heat generation increases, so that the element becomes hot and easily breaks. Generally, an electronic thermal is provided in a motor driving device to protect the inverter, and the temperature of the inverter is estimated from the current flowing through the motor. When the estimated temperature is higher than the predetermined temperature, the controller stops driving the inverter and protects the elements constituting the inverter. Since it is protected in this way, the elements of the inverter are not actually destroyed. However, when the PWM frequency is high, the amount of heat generated in the element is large, so that the inverter cannot be driven for a long time, and the motor cannot be driven for a long time. The PWM frequency for driving the inverter is determined at the time of design. In determining this, the responsiveness required for the motor and the continuous drive time required for the motor are taken into consideration.

Japanese Patent No. 3442340 JP 2014-20266 A

  In the conventional inverter, the PWM frequency is fixed, but Patent Documents 1 and 2 propose a method of driving with the PWM frequency variable. The motor control method described in Patent Document 1 is configured to make the PWM frequency variable according to the performance required for the motor. In other words, in controlling the motor, it is determined each time whether the motor to be driven by the PWM control method is to be prioritized when driving the motor. The PWM cycle is shortened when high control accuracy is required, and the PWM cycle is lengthened when torque is required. That is, the PWM frequency is increased when high control accuracy is required, and the PWM frequency is decreased when torque is required. The control method described in Patent Document 1 is intended for motor control devices such as machine tools, robots, injection molding machines, etc., but in these devices, the controller that controls the device is going to perform the operation, Or they are familiar with the planned process. Therefore, the controller autonomously determines whether the control accuracy or torque is required for the operation or process to be executed next, and can increase or decrease the PWM frequency each time. That is, even if an engineer does not make a special setting for the controller, the PWM frequency is automatically changed and the motor is driven under appropriate conditions.

  Patent Document 2 describes a motor control method for changing the PWM frequency of an inverter for the purpose of protecting the inverter. The inverter described in Patent Document 2 is provided in an electric compressor having a special structure. The electric compressor is driven by an electric motor driven by an inverter, and when the electric compressor is driven, the refrigerant is sucked by the compression mechanism. The inverter is cooled by the sucked refrigerant. In this electric compressor, since the suction of the refrigerant is not stable immediately after driving the electric motor, the cooling of the inverter becomes insufficient, and the temperature of the inverter tends to rise. On the other hand, when a predetermined time has elapsed from the start of driving of the electric motor, the suction of the refrigerant is stabilized and the temperature of the inverter is decreased, and is stabilized after reaching the predetermined temperature. In the method described in Patent Document 2, the temperature of the inverter is monitored during a predetermined period in which the temperature of the inverter is likely to rise immediately after the electric motor is driven, and the PWM frequency is reduced when the temperature is high. This reduces switching loss in the inverter, suppresses heat generation of the inverter, and protects the inverter.

  In the control methods described in Patent Documents 1 and 2, the PWM cycle is made variable in order to achieve a predetermined object. That is, in the method described in Patent Document 1, the PWM frequency is changed according to the performance required for the motor so that the motor can be driven under suitable conditions, and the method described in Patent Document 2 is used. In order to protect the inverter, the PWM frequency is reduced to suppress heat generation when the temperature of the inverter is high. That is, it can be said that both are excellent as control methods for achieving a predetermined purpose. However, problems to be solved are also found in the control methods described in Patent Documents 1 and 2. First of all, it is a common problem in these control methods, but the PWM frequency is changed not at the engineer's discretion, but at the controller that controls the inverter, so the PWM frequency changes at an unintended timing by the engineer. There is a problem of end up. When the PWM frequency changes, the control characteristics also change, which should affect the operation of the device driven by the motor, but the engineer cannot control such effects. As a specific example, when the motor that drives the screw of the injection device of the electric injection molding machine is controlled, if the PWM frequency changes at a timing that is not intended by the engineer, the control characteristics will change, resulting in molding defects. May occur. Next, when the literature is individually examined, it can be said that the control method described in Patent Literature 1 is problematic in that the protection of the inverter is not particularly considered. When the PWM frequency is changed, the heat generation amount of the element due to the switching loss changes as described above, but the change in the risk of destruction of the element caused by the temperature change is not particularly considered. The control method described in Patent Document 2 has other problems. That is, when the inverter temperature is high, the PWM frequency is reduced to suppress the amount of heat generated by the element. However, since the inverter temperature must be monitored, a temperature sensor is essential. That is, there is a problem that the cost is large.

  An object of the present invention is to solve the above-described problems. Specifically, an engineer can change a motor control method according to a driving condition required for the motor, and an inverter element can be changed. An object of the present invention is to provide a motor control method that can reliably protect against destruction due to heat generation and can be protected at a low cost.

で与えて前記インバータにおける温度の上昇分を推定することを特徴とするモータの制御方法として構成される。
請求項３に記載の発明は、電動射出成形機に設けられているモータを、請求項１または２に記載の制御方法によって制御し、成形サイクルを構成している各工程のそれぞれについて前記ＰＷＭ周波数を設定することを特徴とするモータの制御方法として構成される。 In order to solve the above-mentioned problem, the present invention detects the current supplied to the motor when the inverter is PWM-controlled and drives the motor, and detects the current supplied to the motor by using a predetermined arithmetic expression from the current in the electronic thermal. In the motor control method for estimating the temperature rise and stopping the PWM control of the inverter when the temperature rise exceeds a predetermined reference value, the PWM frequency, which is the carrier frequency of the PWM control, is set by the user. The motor can be switched, and the arithmetic expression is changed as the PWM frequency is switched.
According to a second aspect of the present invention, in the control method according to the first aspect, the current is sampled at a predetermined sampling period, and the calculation formula in the electronic thermal is the following formula:

The control method for the motor is characterized by estimating the temperature rise in the inverter.
According to a third aspect of the present invention, the motor provided in the electric injection molding machine is controlled by the control method according to the first or second aspect, and the PWM frequency is set for each of the steps constituting the molding cycle. Is set as a motor control method.

  As described above, according to the present invention, when the inverter is PWM-controlled to drive the motor, the current supplied to the motor is detected and the temperature rise in the inverter is estimated from the current in the electronic thermal using a predetermined arithmetic expression. The present invention is directed to a motor control method in which PWM control of an inverter is stopped when the temperature rise exceeds a predetermined reference value. That is, the control method for a general motor including an inverter and an electronic thermal is targeted. The PWM frequency, which is the carrier frequency for PWM control, can be switched by user settings, and the arithmetic expression is configured to change with the switching of the PWM frequency. Since the PWM frequency can be switched, it can be switched to a high frequency when responsiveness is required, and can be switched to a low frequency when a long continuous drive time is required. Since this switching is performed according to user settings, the control is not affected. When the PWM frequency is changed, the switching loss of the inverter elements increases and decreases, the amount of heat generated in the inverter changes, and the rise in the inverter temperature changes, but according to the present invention, the electronic thermal equation is Since the frequency is changed as the PWM frequency is switched, the temperature rise can be correctly estimated. Thereby, the elements of the inverter can be reliably protected. In implementing the present invention, there are no necessary sensors other than the sensors provided in a general motor, and the cost can be reduced. According to another invention, the current is sampled at a predetermined sampling period, and an arithmetic expression in the electronic thermal is given by a predetermined expression to estimate the temperature rise in the inverter. In this equation, when the PWM frequency is switched, only the variable k (N) needs to be switched. That is, the arithmetic expression can be easily switched, and the present invention can be easily implemented. According to another invention, the motor provided in the electric injection molding machine is controlled by the control method according to claim 1 or 2, and the PWM frequency is set for each of the steps constituting the molding cycle. It is configured as follows. In that case, the PWM frequency is increased and the resin pressure is maintained after injection in processes that require responsiveness, such as a mold clamping process for mold clamping, an injection process for injecting molten resin, and a protruding process for projecting a molded product. In a process that requires a long continuous driving time, such as a pressure holding process, a mold opening process, and the like, which can be performed, the PWM frequency can be reduced and a molding cycle can be appropriately performed.

It is a figure which shows the drive device of the motor which concerns on this Embodiment, and is a figure which shows the circuit of the inverter which supplies electric power to a motor, and the functional block of the controller which controls this inverter. It is a graph which shows the collector current of the IGBT which comprises the inverter, collector-emitter voltage, and the loss electric power in a collector. The figure which shows the characteristic of IGBT which comprises the inverter when supplying a three-phase alternating current to a motor from an inverter, The (a) is a graph which shows the relationship between the electric current supplied to a motor, and the temperature of IGBT, (A) is a graph which shows the relationship between the electric current supplied to a motor, and the continuous operation | movement possible time of IGBT.

  Hereinafter, an embodiment of the present invention will be described with reference to FIG. The motor control device 1 according to the present embodiment controls a brushless motor such as a servo motor, and can be provided in various devices such as machine tools and industrial machines. Below, the motor 2 provided in the injection device of the electric injection molding machine will be described as an example. The injection device includes a heating cylinder and a screw, and the screw is driven by a rotation drive mechanism that rotates the screw and a shaft drive mechanism that drives the screw in the axial direction. The motor 2 according to the present embodiment is provided in the shaft drive mechanism. When the motor 2 is driven, the screw is driven in the axial direction so that the molten resin measured in the heating cylinder can be injected.

  The motor 2 according to the present embodiment is composed of a servo motor, and is driven by receiving power from a predetermined power supply device. Although the power supply device is not shown in FIG. 1, it is composed of a PWM converter and rectifies a three-phase AC voltage supplied from a factory into a DC voltage. Such a DC voltage is supplied to the DC voltage lines P and N. A smoothing capacitor 4 is provided between the DC voltage lines P and N, and the ripple voltage is smoothed. Servo amplifiers that drive the motor 2, that is, inverters 6 are provided on the DC voltage lines P and N. The inverter 6 is driven by a controller, and the software for driving the controller and the inverter 6 executed on the controller is shown as a control unit 7 including a plurality of functional blocks in FIG. Although the control unit 7 will be described later, the motor control device 1 includes an inverter 6 and the control unit 7.

  As is well known, the inverter 6 comprises six insulated gate bipolar transistors, i.e., IGBTs t1, t2,..., And six diodes d1, d2,. Two IGBTs t1, t2,... Are arranged in series, three of which are provided in parallel and connected to the DC voltage lines P and N, respectively. These power transistors t1, t2,... Are provided so that current flows from the positive voltage line P to the negative voltage line N when turned on. The diodes d 1, d 2,... Are provided in parallel to the IGBTs t 1, t 2,. And the R line, the S line, and the T line are drawn out from the midpoint of each of the three rows of IGBTs t1, t2,... Provided in parallel, and these are connected to the motor 2. As is well known, the inverter 6 is driven by so-called PWM control in which the pulse width is variable, and when the IGBTs t1, t2,... The supplied DC voltage can be converted into a desired three-phase AC voltage, that is, a three-phase AC current, and the motor 2 can be driven by supplying the three-phase AC current from the R, S, and T lines. . The electric injection molding machine is provided with a plurality of motors, and each motor is driven by an inverter connected to the DC voltage lines P and N. FIG. Not shown.

  The control unit 7 that performs PWM control of the inverter 6 will be described. As described above, the control unit 7 includes a predetermined controller and software for controlling the controller, and includes a plurality of functional blocks. First, the target value of the rotational speed of the motor 2 given from the outside, that is, the speed setting is input to the speed controller 11 of the functional block. The speed controller 11 is configured to calculate a current command for driving the motor 2. The speed setting is set as a target value, and feedback control is performed so that there is no deviation from the actually measured rotational speed detected from the encoder 8 of the motor 2. The current command is being calculated. To be precise, the current command calculated here is a current command on the dq axis. The current command calculated by the speed controller 11 is input to the current controller 11. The current controller 12 is a functional block that calculates current command values in the U-phase, V-phase, and W-phase from the current command in the dq axis. At this time, the R detected by the current sensor 9 provided in the motor 2 is detected. , S, and T-line currents, that is, U-phase, V-phase, and W-phase currents are also converted and input as currents on the dq axis. Then, calculation is performed by feedback control so that there is no deviation between the current command on the dq axis and the current on the dq axis, and current command values for the U, V, and W phases are obtained. The U-phase, V-phase, and W-phase current command values calculated by the current controller 12 are input to the PWM signal generator 13. The PWM signal generator 13 converts the U-phase, V-phase, and W-phase current command values into PWM control signals, and drives the inverter 6 with the PWM control signals. The PWM control signal is an ON / OFF signal having a variable pulse width, and the pulse width is determined by the carrier frequency, that is, the PWM frequency.

  The motor control device 1 according to the present embodiment has two features that are unique to the present invention. The first feature is that the PWM frequency can be switched by the user's setting. The second feature is to estimate the temperature rise of each IGBT t1, t2,... In the electronic thermal 17, the arithmetic expression is switched at the same time when the PWM frequency is switched. First, functional blocks that realize the first feature will be described. The control unit 7 is provided with a control mode switching means 18 so as to switch the control mode according to a user setting. The control mode includes a responsiveness-oriented mode in which the motor 2 can be controlled with high responsiveness, and a driving time-oriented mode in which the motor 2 can be continuously driven for a long time, and one of them is selected. The motor control device 1 according to the present embodiment that drives the motor provided in the shaft drive mechanism of the injection device sets either the responsiveness emphasis mode or the drive time emphasis mode for each process constituting the molding cycle. it can. For example, in this embodiment, the responsiveness emphasis mode is set in the injection process, and the driving time emphasis mode is set in other processes such as the pressure holding process. The PWM frequency switching unit 15 is a functional block for switching the PWM frequency, and increases the PWM frequency when the control mode switching unit 18 is switched to the responsiveness-oriented mode, and decreases the PWM frequency when switched to the driving time-oriented mode. For example, the PWM frequency in the responsiveness priority mode is set to be twice the PWM frequency in the drive time priority mode. The PWM signal generator 13 generates a PWM control signal according to the switched PWM frequency, and drives the inverter 6.

  The second feature will be described. The motor control device 1 according to the present embodiment is also provided with an electronic thermal 17 similarly to the conventional motor control device, and estimates the temperature increase of each IGBT t1, t2,. The present embodiment is characterized in that an arithmetic expression for estimating the temperature rise is switched. The arithmetic expression will be described later. First, the cause of the temperature rise of each IGBT t1, t2,... Caused by power loss in the IGBT will be described. The IGBT includes a collector, an emitter, and a gate. When a predetermined voltage is applied between the collector and the emitter, when a voltage is applied to the gate as shown in FIG. When the gate is turned off, the collector current is cut off. If the gate current is accurately turned on, the collector current rises slightly after the ON timing and stabilizes after a predetermined short time indicated by reference numeral 21. Further, when the gate is turned off, it falls slightly behind the OFF timing, and is shut off after a predetermined short time indicated by reference numeral 22. The voltage between the collector and the emitter, that is, the collector-emitter voltage also changes as shown in FIG. The power loss in the IGBT, that is, the collector loss power is given by the product of the collector current and the collector-emitter voltage. The turn-on loss 23 which is a switching loss when the gate is ON, the steady loss 24 which is a resistance loss in a steady state, It consists of a turn-off loss 25 which is a switching loss at the time of OFF. The IGBT generates heat by such collector loss powers 23, 24, and 25, and the temperature rises. The collector current flowing through each of the IGBTs t1, t2,... Constituting the inverter 6 is any of the U-phase, V-phase, and W-phase currents. Further, when the PWM frequency is high, the frequency of gate ON / OFF increases, and the collector power loss due to switching loss also increases. Therefore, it can be said that the collector loss power in each IGBT t1, t2,... And the temperature rise in the inverter 6 correlates with the collector loss power in each IGBT t1, t2,. The relationship between the current flowing through the IGBT and the temperature of the IGBT is schematically shown in FIG. 3A. When the IGBT is driven at a large PWM frequency 27, the IGBT is driven more than at a small PWM frequency 28. The temperature rise is large. In the present embodiment, the electronic thermal 18 estimates the increase in the temperature of the inverter 6 according to the following arithmetic expression.

Current I _m is the U phase in this computing equation, V-phase, it may be a sampling of any of the phase current of the W-phase, a sampling of the current in the dq-axis obtained by converting these phase current Also good. The coefficient k (N) is a coefficient whose value varies depending on the PWM frequency N, and is determined by conducting an experiment in advance. Specifically keep the temperature sensor is provided to the inverter 6, to detect the temperature rise by driving the inverter 6 at a predetermined PWM frequency N _1. At this time, the current _{Im is} also sampled, and if these are substituted into the above equation, the coefficient k (N ₁ ) at the PWM frequency N ₁ is obtained. Similarly, an experiment is performed at another PWM frequency N ₂ to obtain a coefficient k (N ₂ ). The coefficient k (N) obtained in this way becomes a small value when the PWM frequency is small, and becomes a large value when the PWM frequency is large. In the control unit 7 according to the present embodiment, when the PWM frequency switching unit 15 switches the PWM frequency, the electronic thermal 17 switches this coefficient k (N) in the arithmetic expression. That is, the arithmetic expression is switched. Since the arithmetic expression is switched in this way, the temperature increase in the inverter 6 can be accurately estimated even when the PWM frequency is switched. If the estimated increase in the temperature of the inverter 6 exceeds the reference value, the drive of the inverter 6 is stopped and the inverter 6 is protected from thermal destruction as in the conventional case. In FIG. 3A, the relationship between the current flowing through the IGBT and the continuous operation time of the IGBT is schematically shown in a graph, but when the IGBT is driven at a large PWM frequency 30, it is driven at a small PWM frequency 29. Rather than this, the temperature rise estimated by the electronic thermal 17 becomes larger, and the continuous operation possible time is inevitably shortened.

The motor control method according to the present embodiment can be variously modified. For example, it has been described that the coefficient k (N) in the arithmetic expression is determined by experimentation for each of the different PWM frequencies. However, for example, determine the coefficient k _{(N a)} and experiments on one PWM frequency _{N a,} the coefficient corresponding to other PWM frequency _{N b} k _{(N b)} may be obtained as follows.
k (N _b ) = (N _b / N _a ) · k (N _a )
Other variations are possible. For example, only the point that the drive of the inverter 6 is stopped when the estimated temperature rise of the inverter 6 in the electronic thermal 17 exceeds the reference value has been described. However, the estimated temperature rise is monitored and is lower than the reference value. If the value is reached, an alarm may be issued.

1 Motor controller
2 Motor 4 Smoothing capacitor
6 Inverter 7 Control unit 8 Encoder 9 Current sensor 11 Speed controller 12 Current controller 13 PWM signal generator 15 PWM frequency switching unit 17 Electronic thermal 18 Control mode switching means

Claims (3)

When driving the motor by PWM control of the inverter, the current supplied to the motor is detected, and the temperature rise in the inverter is estimated from the current in the electronic thermal by a predetermined arithmetic expression, and the temperature rise In a motor control method for stopping PWM control of the inverter when the minute exceeds a predetermined reference value,
A method of controlling a motor, wherein a PWM frequency, which is a carrier frequency of the PWM control, can be switched by a user setting, and the arithmetic expression is changed with the switching of the PWM frequency. 2. The control method according to claim 1, wherein the current is sampled at a predetermined sampling period, and the calculation formula in the electronic thermal is given by

A method for controlling a motor, characterized in that an increase in temperature in the inverter is estimated.   A motor provided in the electric injection molding machine is controlled by the control method according to claim 1 or 2, and the PWM frequency is set for each of the steps constituting the molding cycle. Control method.

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