Method for calculating the power consumption of electric motors, and industrial machinery

Abstract

To provide a power consumption calculation method for an electric motor that can accurately calculate power consumption even when the rotation speed is zero.SOLUTION: An industrial machine (1) equipped with one or more electric motors (41) that are supplied with current and driven by an inverter (35) calculate a power consumption Wm [W] of the electric motor (41) using the following formula: Wm=2π*Tm*Rm / (ηm*ηp) / 60+Pw; where Tm is an output torque of the electric motor [Nm]; Rm is an electric motor rotation speed [rpm]; ηm is a motor efficiency of electric motor; ηp is a power factor; and Pw is an inverter switching loss [W].SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an industrial machine provided with an electric motor driven by an inverter, a calculation method for calculating the power consumption of the electric motor, and an industrial machine.

Background Art

[0002] There are various types of industrial machines driven by electric motors, such as an electric injection molding machine driven by an electric motor. The electric injection molding machine is provided with a converter and an inverter, and the three-phase AC power supplied from the factory is converted into DC power by the converter. Then, the DC power is converted into three-phase AC current of a desired current at a desired frequency by the inverter and supplied to the electric motor. That is, the electric motor is driven.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

[0004] Patent Document 1 proposes a method for calculating the power consumption of an electric motor provided in an injection molding machine. According to the method described in Patent Document 1, the power consumption of the electric motor is calculated based on the current detected in the inverter, that is, the servo amplifier, and the rotational speed detected by the rotary encoder. The output torque of the electric motor is required for the calculation, and this is also calculated based on the current.

Summary of the Invention

Problems to be Solved by the Invention

[0005] The method for calculating the power consumption of the electric motor described in Patent Document 1 can be calculated from the current and the rotational speed. Therefore, there is an excellent feature that it is not necessary to detect the voltage in the servo amplifier and it is not necessary to provide a voltmeter. However, problems to be solved can also be found. Specifically, there is a problem that the power consumption is calculated as zero when the electric motor is not rotating. For example, in the molding cycle of an electric injection molding machine, there is a pressure holding process. In the pressure holding process, it is necessary to generate torque by an injection shaft servo motor that drives the screw and maintain a high resin pressure. Since torque is generated, power is naturally consumed. However, since the screw hardly moves forward, the rotational speed becomes zero. In the power consumption calculation method described in Patent Document 1, there is a problem that the power consumption becomes zero at this time.

[0006] In the present disclosure, there is provided a method for calculating the power consumption of an electric motor that does not require a voltmeter and can accurately calculate the power consumption consumed in the electric motor even when the rotational speed of the electric motor is zero.

[0007] Other problems and novel features will become apparent from the description of this specification and the accompanying drawings.

Means for Solving the Problems

[0008] In the present disclosure, in an industrial machine including one or more electric motors driven by supplying current by an inverter, the power consumption W m [W] is calculated by the following calculation formula. W m = 2π · T m · R m / (η m · η p ) / 60 + P w However, T m : Output torque [Nm] R m : Rotational speed [rpm] of the electric motor η m : Motor efficiency of the electric motor η p : Power factor P w : Switching loss of the inverter [W] Output Torque T m This is the current I supplied to the electric motor. m Detect [A] and obtain from it. And the switching loss P w The switching loss P w and the aforementioned current I m Based on the table showing the relationship, the current I m To obtain it from. [Effects of the Invention]

[0009] This disclosure relates to the rotational speed R of an electric motor. m Even if it is zero, power consumption W m It can be calculated accurately without ever becoming zero. [Brief explanation of the drawing]

[0010] [Figure 1] This is a front view showing the injection molding machine according to this embodiment. [Figure 2] This is a power supply system diagram for the electric motor in the injection molding machine according to this embodiment. [Figure 3A] This graph shows the relationship between the current and output torque of a servo motor. [Figure 3B] This graph shows the relationship between the rotational speed and power factor of a servo motor. [Figure 3C] This graph shows the changes in terminal voltage and terminal current when a power semiconductor performs switching operation. [Figure 3D] This graph shows the relationship between the servo motor current and the switching loss in the inverter. [Figure 4] This is a power consumption screen of the servo amplifier according to this embodiment. [Modes for carrying out the invention]

[0011] The following describes specific embodiments in detail with reference to the drawings. However, the embodiments are not limited to those described below. For clarity, the following descriptions and drawings have been simplified as appropriate. In each drawing, the same elements are denoted by the same reference numerals, and redundant explanations have been omitted where necessary. Also, hatching has been omitted in some parts of the drawings to avoid clutter.

[0012] The method for calculating the power consumption of an electric motor according to this embodiment, which will be explained in detail later, is a calculation method used in industrial machinery equipped with an electric motor. Industrial machinery includes various types such as presses, extruders, and lathes. In the following explanation, an injection molding machine will be used as an example of industrial machinery to explain this embodiment.

[0013] <Injection molding machine> The injection molding machine 1 according to this embodiment, as shown in Figure 1, includes a mold clamping device 2, an injection device 3, an ejection device 5, etc. The injection molding machine 1 is equipped with a controller, or control device 4, and the mold clamping device 2, injection device 3, ejection device 5, etc. are controlled by the control device 4. The control device 4 is provided with a monitor 4a on which various screens are displayed.

[0014] <Mold clamping device> The mold clamping device 2 comprises a fixed platen 7 fixed to the bed B, a movable platen 8 slidably mounted on the bed B, and a mold clamping housing 9. The fixed platen 7 and the mold clamping housing 9 are connected by a plurality of tie bars 11, 11, ..., and the movable platen 8 is slidably mounted between the fixed platen 7 and the mold clamping housing 9. A mold clamping mechanism, specifically a toggle mechanism 13 in this embodiment, is provided between the mold clamping housing 9 and the movable platen 8. The fixed platen 7 and the movable platen 8 are each provided with a fixed mold 15 and a movable mold 16. Therefore, when the toggle mechanism 13 is driven, the molds 15 and 16 are opened and closed. The ejection device 5 for ejecting the molded product is provided on the movable platen 8.

[0015] <Injection device> The injection device 3 comprises a heating cylinder 19, a screw 20 located inside the heating cylinder 19, and a screw drive device 22. The heating cylinder 19 is supported by the screw drive device 22, and the screw 20 is driven by the screw drive device 22 in both the rotational and axial directions. The heating cylinder 19 is provided with a hopper 23 and an injection nozzle 24.

[0016] The injection device 3 is advanced so that the injection nozzle 24 touches the fixed mold 15. A command from the control device 4 heats the heating cylinder 19, supplying injection material from the hopper 23 and rotating the screw 20. The injection material melts and is sent to the tip of the screw 20, i.e., it is metered. Once the injection material has been metered, the control device 4 controls the screw drive device 22 to drive the screw 20 axially. As the screw 20 moves forward, the injection material is pushed forward, i.e., the injection material is injected into the molds 15 and 16.

[0017] <Power supply system> The injection molding machine 1 according to this embodiment is driven by a servo motor, or electric motor. The power supply system will now be described. As shown in Figure 2, the injection molding machine 1 according to this embodiment is equipped with a converter 30. The converter 30 is connected to the factory's three-phase AC power supply 31 and also to a DC voltage line 33 provided inside the injection molding machine 1. Multiple inverters, or servo amplifiers 35, 36, 37, 38, ... are connected to the DC voltage line 33. Each of the servo amplifiers 35, 36, 37, 38, ... is equipped with a servo motor 41, 42, 43, 44, ... Specifically, the injection shaft servo motor 41, the plasticizing shaft servo motor 42, the mold opening / closing shaft servo motor 43, the ejection shaft servo motor 44, ...

[0018] The three-phase AC power from the three-phase AC power supply 31 is converted to DC power by the converter 30 and supplied to the DC voltage line 33. The DC power is then converted to three-phase AC power of the desired frequency and current by servo amplifiers 35, 36, 37, 38, ... and supplied to each of the servo motors 41, 42, ... This drives the servo motors 41, 42, 43, 44, ... In addition, regenerative power is recovered from the servo motors 41, 42, 43, 44, ... and converted to DC power by the servo amplifiers 35, 36, 37, 38, ... If the converter 30 consists of a PWM converter, the regenerative power can also be returned to the three-phase AC power supply 31.

[0019] Each of these servo amplifiers 35, 36, 37, 38, ... is equipped with a current sensor 46, 47, 48, 49, ... to measure the current supplied to the servo motors 41, 42, 43, 44, .... Each servo motor 41, 42, 43, 44, ... is also equipped with a rotary encoder 51, 52, 53, 54, ... to detect the rotational speed of the servo motors 41, 42, 43, 44, .... The current and rotational speed are sent to the control device 4 (see Figure 1).

[0020] <Background of the Invention> Before describing the method for calculating the power consumption of an electric motor according to this embodiment, the background of the present invention will be explained. In recent years, as exemplified by the so-called SDGs (Sustainable Development Goals), resource conservation and reduction of energy consumption have been requested in various fields, and the same demands apply to industrial machinery. In industrial machinery equipped with electric motors, it is desirable to review the power consumption of the electric motors. In reviewing the power consumption of electric motors in industrial machinery, it is necessary to accurately grasp the power consumption of each individual electric motor. This is because clarifying the power consumption of each electric motor through so-called "visualization" makes it possible to review the driving of each electric motor, which in turn leads to a reduction in the power consumption of the entire industrial apparatus.

[0021] Incidentally, electric motors have a current sensor in the inverter, so power consumption can be calculated using this. One calculation method is the power consumption calculation method described in Patent Document 1. However, according to this calculation method, as mentioned above, power consumption is calculated as zero when the electric motor is not rotating. When "visualizing" the power consumption of each electric motor, it is problematic that power consumption is shown as zero even though it is being generated.

[0022] Therefore, the inventors investigated a calculation method that can accurately calculate power consumption even when the electric motor is not rotating. The inventors focused on the switching loss of the inverter as the power generated when the electric motor is not rotating. Switching loss is the power loss that occurs when power semiconductors such as IGBTs that make up the inverter are switched, and it occurs as long as the inverter generates current, even when the electric motor is not rotating. Generally, switching loss is the servo amplifier efficiency η sa As described in Patent Document 1, it is treated as a coefficient multiplied by the power. The inventors completed the present invention by reviewing this treatment of switching loss.

[0023] <Method for calculating the power consumption of an electric motor according to this embodiment> The method for calculating the power consumption of the electric motor according to this embodiment is performed in the control device 4 (see Figure 1), and the control device 4 calculates the power consumption W using the following formula. m Calculate [W]. W m =2π·T m ·R m / (η m ·η p ) / 60+P w (1 set) However, T m : Output torque of electric motor R m : Rotational speed of electric motor [rpm] η m : Motor efficiency of electric motors η p Power factor P w Inverter switching loss [W]

[0024] Switching loss P w It is treated not as a coefficient multiplied by the power, but as a component added to the power. This allows the rotational speed R of the electric motor to be calculated. m Even if it is zero, power consumption W m The formula is designed so that it never becomes zero. The individual elements that make up the formula are explained below.

[0025] Electric motor output torque T m is the output torque of the servo motors 41, 42, 43, 44, ... (see Figure 2), and the current I measured by the current sensors 46, 47, 48, 49, ... m The calculation is performed using the following method: Current I m and output torque T m The relationship differs depending on the model of the servo motor 41, 42, 43, 44, ..., but it is generally as shown in Graph 61 in Figure 3A. Graph 61 shows the relationship between current I m Effective value and output torque T m This shows the relationship between current I m If obtained, output torque T m This is obtained. The control device 4 receives current I for each servo motor 41, 42, 43, 44, ... m Effective value and output torque T m A table is provided to show the relationship, and the output torque T is calculated using linear interpolation, etc. m You just need to calculate this.

[0026] In the injection molding machine 1 according to this embodiment, the current I supplied to the servo motors 41, 42, 43, 44, ... m It is controlled in a relatively linear range 62, for example, in a range of 100 Arms or less in effective value. In this range 62, graph 61 can be considered as a linear function 63. Therefore, in the injection molding machine 1 according to this embodiment, the output torque T can be easily controlled. m The current I is being calculated. m This is obtained by multiplying by a coefficient corresponding to the slope of the linear function 63.

[0027] The rotational speed of the electric motor is the rotational speed of the servo motors 41, 42, 43, 44, ... and is detected by rotary encoders 51, 52, 53, 54, .... The motor efficiency of the electric motor is η m This represents the efficiency of the servo motors 41, 42, 43, 44, ... and is given as a coefficient specific to each model of servo motor 41, 42, 43, 44, ....

[0028] Power factor η p As shown in Graph 65 of Figure 3B, the rotational speed R of servo motors 41, 42, 43, 44, ... m It is given as a function of η. It differs depending on the model of the servo motor 41, 42, 43, 44, ..., and the control device 4 stores a table that shows this relationship. Based on this table, the control device 4 calculates the power factor η. p I am trying to obtain it.

[0029] Switching loss P w This refers to the power loss that occurs when switching power semiconductors such as IGBTs and MOSFETs. Figure 3C shows the changes in terminal voltage 67 and terminal current 68 between the terminals of a power semiconductor, for example, between the collector and emitter in an IGBT, and between the drain and source in a MOSFET. Assume that a voltage is applied to the gate at timing 70 and released at timing 72. In other words, switching occurs. Then, from timing 70, the terminal voltage 67 decreases and the terminal current 68 increases, and from timing 71 onwards, the terminal voltage 67 and terminal current 68 become constant. On the other hand, from timing 72, the terminal voltage 67 increases and the terminal current 68 decreases, and from timing 73 onwards, they become constant. In this switching, power loss occurs at the rise time 75 and the fall time 76.

[0030] The power loss of a power semiconductor can be obtained by the following equation. Power loss=(1 / 6)·I max ·V max ·(T r +T f )·fsw (2 sets) However, I max : Maximum value of terminal current 68 V max : Maximum value of terminal voltage 67 T r Startup time: 75 T f : Recovery time 76 f sw : Switching frequency Startup time T r and fall-down time T f We will obtain this through experimentation.

[0031] By the way, each inverter, or servo amplifier 35, 36, 37, 38, ... (see Figure 2), is equipped with three arms on the top and bottom, for a total of six power semiconductors. Therefore, the sum of these power losses equals the switching loss P. w It is possible to calculate this. By the way, the maximum value V of the terminal voltage that makes up the two equations max And, startup time T r And, fall time T f All of these can be considered to be constant values. Switching frequency f sw Although it can be varied in multiple patterns, even if it is varied, each can be considered to be a constant value. In that case, the power loss in equation 2 is the maximum value I of the terminal current. max It can be treated as a function of . In this case, the switching loss P for each servo amplifier 35, 36, 37, 38, ... w This is the current I flowing through servo amplifiers 35, 36, 37, 38, ... m This becomes a function of , as shown in graph 80 in Figure 3D. A table showing the relationship in graph 80 is provided in the control device 4 (see Figure 1), and the current I m Switching loss P w We just need to find that.

[0032] <Servo amplifier power consumption screen> Figure 4 shows the servo amplifier power consumption screen according to this embodiment. This screen is displayed on the monitor 4a of the control device 4 (see Figure 1). Power consumption W for each of the servo amplifiers 35, 36, 37, 38, ... of the injection shaft servo motor 41, plasticizing shaft servo motor 42, mold opening / closing shaft servo motor 43, ejection shaft servo motor 44, ... m The calculation is performed using a single formula and displayed as an instantaneous value. Furthermore, the power consumption per molding cycle is calculated and also displayed.

[0033] <Modification of the embodiment> This embodiment can be modified in various ways. For example, switching loss P w The calculation method can be modified. Above, the switching loss P w The current I of servo amplifiers 35, 36, 37, 38, ... m It was explained that the calculation should be performed assuming it is a function of P. However, for example, when IGBTs are used as power semiconductors, the switching loss P w Current I m It can also be simply given as a constant independent of the formula. On the other hand, for example, when using SiC as a power semiconductor, the switching loss P w Current I m When it is zero, it is considered zero, and the current I m When it is greater than zero, it can be given as a constant value. Power factor η p The calculation method for this can also be modified, and it may be given as a constant.

[0034] The present invention has been described in detail above based on embodiments, but it goes without saying that the present invention is not limited to the embodiments already described, and various modifications are possible without departing from the spirit of the invention. The multiple examples described above can also be implemented in combination as appropriate. [Explanation of symbols]

[0035] 1 Injection molding machine 2 Mold clamping device 3. Injection device 4. Control device 5 Ejection device 7 Fixed plate 8 Movable plate 9-type clamping housing 11 tie bars 13 Toggle mechanism 15 Fixed side mold 16 Movable mold 19 Heating cylinder 20 Screw 22 Screw drive mechanism 23 Hopper 24 Injection nozzle 30 Converter 31 Three-phase AC power supply 33 DC voltage lines 35, 36, 37, 38 Servo amplifier 41, 42, 43, 44 Servo motors 46, 47, 48, 49 Current sensors 51, 52, 53, 54 Rotary encoders B Bed

Claims

1. A method for calculating the power consumption of an electric motor in an industrial machine equipped with one or more electric motors that are driven by an inverter, Current I supplied to the electric motor ｍ [A] is measured, The aforementioned current I ｍ From the output torque T of the electric motor ｍ [Nm] is obtained, Power consumption W of the aforementioned electric motor ｍ [W] W ｍ =2π・T ｍ ・R ｍ / (or ｍ ·or ｐ ) / 60+P ｗ However, R ｍ : Rotational speed of the electric motor [rpm] η ｍ : Motor efficiency of the electric motor η ｐ Power factor P ｗ : Switching loss of the inverter [W] Calculated by, A method for calculating the power consumption of an electric motor, wherein the switching loss Pw is obtained from the current Im based on a table showing the relationship between the switching loss Pw and the current Im.

2. The power factor η ｐ The rotational speed R ｍ A method for calculating the power consumption of an electric motor according to claim 1, provided as a function of .

3. The method for calculating the power consumption of an electric motor according to claim 1 or 2, wherein the industrial machine is an injection molding machine.

4. One or more electric motors that are driven by an inverter that supplies current, A control device is provided, Current I supplied to the electric motor ｍ [A] is measured, The aforementioned current I ｍ From the output torque T of the electric motor ｍ [Nm] is obtained, Power consumption W of the aforementioned electric motor ｍ [W] W ｍ =2π・T ｍ ・R ｍ / (or ｍ ·or ｐ ) / 60+P ｗ However, R ｍ : Rotational speed of the electric motor [rpm] η ｍ : Motor efficiency of the electric motor η ｐ Power factor P ｗ : Switching loss of the inverter [W] Calculated by, The switching loss Pw is obtained from the current Im based on a table showing the relationship between the switching loss Pw and the current Im. The power consumption W of the electric motor ｍ An industrial machine that displays the above on the control device.

5. The power factor η ｐ The rotational speed R ｍ The industrial machine according to claim 4, which is given as a function of .

6. The industrial machine according to claim 4 or 5, wherein the industrial machine is an injection molding machine.

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