JP2000092892A - Motor controller - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a surge voltage, the biased magnetization and overexcitation of a step-up transformer, the variation of the neutral-point voltage of an inverter output, the electrolytic-corrosion damage of a motor bearing, and the leakages of the currents flowing through the stray capacitances of a motor and motor cable. SOLUTION: This controller has an inverter portion 2 for outputting the variable voltage of a variable frequency and for controlling a motor 4 by a pulse width modulation, a sine-wave filter portion 7 for outputting a sine-wave voltage converted thereinto from the output voltage of the inverter portion 2, and a step-up transformer 3 for outputting to the motor 4 the voltage stepped up thereto from the output voltage of the sine-wave filter portion 7. In this case, at least one of the primary and secondary windings of the step-up transformer 3 are subjected to star connections to ground the neutral points of the star connections.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a control device for driving an electric motor.

[0002]

2. Description of the Related Art FIG. 6 shows a prior art example. In the figure, 1 is a commercial power supply, 2 is an inverter section, 3 is a step-up transformer section, 4 is a motor to be controlled,
Is directly boosted.

[0003]

According to this conventional method, when the cable connecting the inverter 2 and the step-up transformer 3 and the cable connecting the step-up transformer 3 and the motor 4 are long, high-speed switching of the main circuit elements of the inverter can be achieved. Excessive surge voltage is generated due to the influence of the stray impedance of the cable. The surge voltage may deteriorate the insulation of the coil of the step-up transformer and the motor coil, resulting in burning. In addition, since the inverter 2 outputs a rectangular wave including a DC component, a demagnetization phenomenon occurs in the step-up transformer, and in particular, the step-up transformer becomes overexcited in a low frequency region of the output frequency from the inverter 2. there were. Further, when the shaft of the motor 4 is not grounded by an earth brush or the like, or when the load side is separated from the ground, such as a fan, the shaft of the motor is changed due to the fluctuation of the neutral point voltage of the inverter output. In such a case, there is a problem that a voltage is generated in the motor and electric erosion is generated on a bearing of the motor and the motor is damaged. This change in the neutral point voltage has also caused leakage current flowing through the floating capacitance of the motor or the motor cable. The present invention relates to the above-described surge voltage, demagnetization and overexcitation of the step-up transformer, fluctuation of the neutral point voltage of the inverter output, damage to the electric corrosion of the bearing of the motor, and leakage current flowing through the floating capacitance of the motor and the motor cable. An object of the present invention is to provide a motor control device that solves the problems.

[0004]

According to the present invention, there is provided an electric motor control apparatus comprising: an inverter for outputting a voltage having a variable voltage and a variable frequency for controlling the electric motor by pulse width modulation; and converting the output voltage of the inverter to a sine wave. And a booster transformer for boosting the output voltage of the sine wave filter and outputting the boosted output to the motor. At least one of the windings has a star connection, and a neutral point of the star connection is grounded. Further, in the control device for the electric motor, a housing of the inverter unit, a frame of the electric motor, a shield plate of the step-up transformer, and a neutral point of the star connection are grounded. In addition, in the control device for the electric motor, the sine wave filter unit may be a star-shaped or annular connection between a reactor connected in series with each phase of the output of the inverter unit and a line on the primary side of the step-up transformer. It consists of connected capacitors. Also, a common mode choke coil having a common iron core is connected in series with each phase of the sine wave filter unit.

[0005]

Embodiments of the present invention will be described with reference to the drawings. In addition, the same reference numerals are given to the same names, and redundant description will be omitted. FIG. 1 is a configuration diagram of a first embodiment of the present invention. In the figure, 1 is a commercial power supply, 2 is a PWM inverter section, 3 is a step-up transformer section, 4 is a motor to be controlled,
Reference numeral 7 denotes a sine wave filter unit that converts the output voltage of the PWM inverter unit 2 into a sine wave and outputs the sine wave, and 8 denotes a shield plate of the step-up transformer. The sine wave filter unit 7 includes a reactor 5 connected in series to each phase of the output of the inverter unit and a capacitor 6 connected in a star or ring connection between the primary side wires of the step-up transformer 3. The PWM waveform of the inverter output is converted into a sine wave by the sine wave filter unit 7. Thus, the output voltage of the inverter is boosted without harmonic components being removed and the surge voltage in the transformer or the motor as described above and without causing the overexcitation phenomenon or the demagnetization phenomenon in the transformer section 3, and is supplied to the AC motor. In the configuration of the step-up transformer 3, at least one of the primary winding and the secondary winding is star-connected. FIG. 1 shows an example in which the primary side has an annular connection and the secondary side has a star connection. In the grounding method, the neutral point of the star connection is grounded. FIG. 1 shows an example in which a single point ground including a grounding cable connected to a housing of an inverter, a shield plate of a step-up transformer, and a motor frame is provided. FIG. 2 shows an inverter output voltage, a motor input voltage, and a motor shaft voltage in a system using the driving device of the present invention. 2A shows the inverter output voltage, and FIG. 2B shows the input voltage of the motor terminal. The waveform (a) of the inverter output voltage is converted into a sine wave by the sine wave filter unit 7, and this sine wave voltage is boosted and applied to the motor, so that a surge voltage is generated at the motor terminal as shown in the waveform (b). do not do. In addition, since the neutral point of the star connection of the transformer is grounded, the fluctuation of the neutral point voltage on the secondary side of the transformer can be suppressed. FIG. 3 shows a second embodiment. 3 differs from FIG. 1 only in that a common mode choke coil (9) having a common iron core is connected in series with each phase of the sine wave filter unit. The common mode choke coil increases the common mode impedance of the inverter output, and can further suppress the common mode voltage, leakage current, and motor shaft voltage generated by the PWM inverter as compared with the first embodiment. This common mode choke coil may be connected anywhere between the output of the inverter unit and the step-up transformer. FIG. 3 shows an example in which a common mode choke coil is connected in series between the capacitor 6 and the step-up transformer. FIG. 4 shows a modification of the second embodiment, in which a common mode choke coil is connected in series between a reactor 5 and a capacitor 6. FIG.
Shows an example of a common mode choke coil. The windings of each phase all have the same number of polarities in the same direction and are formed on a common iron core magnetically coupled to each other. When the zero-sequence current is zero, the magnetic flux generated in the common iron core is canceled out, so that the impedance becomes zero with respect to this current.

[0006]

As described above, an inverter for outputting a variable voltage and a variable frequency for controlling a motor by pulse width modulation, and a sine wave filter for converting the output voltage of the inverter into a sine wave and outputting the sine wave. And a boosting transformer for boosting the output voltage of the sine wave filter and outputting the boosted voltage to the motor, wherein at least one of the primary winding or the secondary winding of the boosting transformer is connected to a star. Since the neutral point of the star-shaped connection is grounded and the neutral point voltage of the voltage applied to the motor causing the above-mentioned shaft voltage is the same as the ground, the shaft voltage is not generated. No leakage current flows. In addition, since a voltage waveform obtained by converting the inverter output voltage into a sine wave by a sine wave filter is boosted by a boosting transformer, a sine wave voltage is applied to the boosting transformer and motor terminals, no surge voltage is generated, and the bias transformer is demagnetized. A phenomenon or overexcitation is avoided. Therefore, the present invention has the following advantages as a drive device for an AC motor. (1) The inverter output voltage is converted into a sine wave by the sine wave filter, and no surge voltage occurs even when the connection cable is long. (2) At least one of the primary winding and the secondary winding of the step-up transformer is formed in a star connection, and the neutral point voltage applied to the motor by grounding the neutral point of the star connection is The ground becomes the same potential, no shaft voltage is generated, and no leakage current flows. (3) The drive device of the present invention is effective when used in a boost drive system in which a high-voltage motor is driven by a low-voltage inverter.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention.

FIGS. 2A and 2B are waveform diagrams when the first embodiment of the present invention is used, wherein FIG. 2A shows an inverter output voltage, FIG. 2B shows a motor input voltage, and FIG. 2C shows a motor shaft voltage.

FIG. 3 is a configuration diagram of a second embodiment of the present invention.

FIG. 4 is a modification of the configuration diagram of the second embodiment of the present invention.

FIG. 5 is a configuration diagram of a common mode choke coil used in a second embodiment of the present invention.

FIG. 6 is a configuration diagram of a conventional AC motor driving device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Commercial power supply 2 Inverter 3 Step-up transformer 4 Motor 5 Reactor 6 Capacitor 7 Sine wave filter 8 Shield plate 9 Common mode choke coil

Claims (4)

[Claims] An inverter unit for outputting a voltage having a variable voltage and a variable frequency for controlling a motor by pulse width modulation;
A motor control device comprising: a sine wave filter unit that converts an output voltage of the inverter unit into a sine wave and outputs the sine wave; and a boost transformer that boosts an output voltage of the sine wave filter unit and outputs the output voltage to the motor. A control device for an electric motor, wherein at least one of a primary winding and a secondary winding of a transformer is star-connected, and a neutral point of the star connection is grounded. 2. An inverter unit for outputting a voltage having a variable voltage and a variable frequency for controlling a motor by pulse width modulation;
A motor control device, comprising: a sine wave filter unit that converts an output voltage of the inverter unit into a sine wave and outputs the sine wave; and a step-up transformer that boosts an output voltage of the sine wave filter unit and outputs the voltage to the motor. A controller for a motor, wherein a housing of the unit, a frame of the motor, a shield plate of the step-up transformer, and a neutral point of the star connection are grounded. 3. The sine wave filter section comprises a star-connected or ring-connected capacitor between a reactor connected in series with each phase of the output of the inverter section and a primary side line of the step-up transformer. Item 3. The control device for an electric motor according to item 1 or 2. 4. The motor control device according to claim 1, wherein a common mode choke coil having a common iron core is connected in series with each phase of the sine wave filter unit.