JP2002186286A - Inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem that a conventional inverter with the output voltage of a converter fixed to maximum voltage required for variable-speed driving of a motor causes switching loss in an inverter during low-speed and low-torque operation. SOLUTION: An inverter control section 8 determines DC intermediate voltage large enough to perform variable-speed driving for a motor 5 based on a rotational speed detection signal or a motor speed command of the motor 5. The intermediate voltage is given to a converter control section 6 as a DC voltage control command, and the converter control section 6 regulates the output voltage of the converter 3 in accordance with the command, so that the DC intermediate voltage can be decreased at low-speed and low-torque operation, thus relieving switching loss in the inverter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter having a forward conversion section and an inverse conversion section for operating an induction motor or a permanent magnet type electric motor (motor) at a variable speed.
The present invention relates to an inverter that performs variable speed operation of a motor by performing M (pulse width modulation) control.

[0002]

2. Description of the Related Art FIG. The main circuit configuration uses the AC input of the forward converter 3 from the commercial power supply 1 through the reactor 2 and obtains a DC output from the forward converter 3. The inverse converter 4 uses the output of the forward converter 3 as a DC power supply and converts the output into AC power whose frequency and voltage are controlled, thereby driving the motor 5 at a variable speed.

[0003] The forward converter control section 6 provides an additional switch for pulse width control (chopper operation) or chopper operation for each switch element of the forward converter 3 so that the input current passing through the reactor 2 approaches a sine wave. In addition, the generation of harmonics on the power supply side and the deterioration of the power factor are prevented. The control by the forward converter control section 6 is based on the input current detected by the current sensor 9 and the DC intermediate circuit voltage detected at both ends of the capacitor 7 so that the input current becomes a sine wave and the DC voltage is
Above the peak value of the commercial power supply voltage indicated by the dashed line,
The gate signal to the switching element is controlled so that the DC intermediate circuit voltage can output the maximum voltage required by the motor.

The inverter control unit 8 detects the DC intermediate circuit voltage detected at both ends of the capacitor 7, the output current detected by the current sensor 10 and the rotation speed of the motor 5, and the output of the inverter 4 is variable. The voltage and frequency are controlled so as to follow the speed command, and PWM control is performed to improve the output waveform.

[0005]

As described above, the inverse converter controls the output frequency and voltage to operate the motor 5 at a desired rotation speed. However, the motor is operated at low voltage and frequency at low rotation speed and low load, and is operated at high voltage and frequency at high rotation speed and high load.

On the other hand, in the forward converter, a DC intermediate circuit voltage capable of outputting the maximum voltage required by the motor 5 is determined in advance, and the forward converter 3 is controlled so as to maintain this constant voltage.

[0007] As described above, the DC intermediate circuit voltage is set to the maximum voltage required by the motor, and the higher this voltage is, the higher the switching loss that occurs when the switching element of the inverter 4 turns ON and OFF. (Proportional to the product of voltage and current). In particular, when the inverter 4 is subjected to PWM control, the switching operation is performed with a carrier signal having a high frequency, so that the switching loss is further increased. This increase in switching loss reduces the efficiency of the device and raises the problem that the cooling function must be enhanced.

An object of the present invention is to provide an inverter capable of reducing switching loss in variable speed operation of a motor.

[0009]

SUMMARY OF THE INVENTION The present invention calculates a DC intermediate circuit voltage that is sufficient for an inverter to drive a motor at a variable speed, and adjusts the output voltage of a converter based on the calculated DC intermediate circuit voltage.
The present invention is designed to reduce the switching loss of the inverter, and is characterized by the following configuration.

A forward converter for converting an AC input current into a sine wave to convert it into DC power, and an inverse converter for driving the motor at a variable speed with an AC output whose frequency and voltage are controlled using the DC power as a DC intermediate voltage. Equipped inverter,
A DC intermediate circuit voltage sufficient for driving the motor at a variable speed is calculated, and a means for adjusting the output voltage of the forward converter to the DC intermediate circuit voltage is provided.

[0011]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. 2 is different from FIG. 2 in that the inverter control unit 8 drives the motor (induction motor or permanent magnet type motor) 5 at a variable speed based on a rotation speed detection signal of the motor 5 or a motor speed command. A sufficient DC intermediate voltage is obtained, and the obtained DC intermediate voltage is supplied to a forward converter control unit 6 as a DC voltage control command.
The point is to adjust the output voltage.

The operation of this embodiment will be described with reference to the schematic diagram of the DC intermediate circuit voltage pattern during acceleration in FIG. As shown in the figure, when the motor 5 is accelerated, the required voltage increases as the rotation speed increases. Further, since an acceleration torque is also required, a higher voltage is required.

The required DC intermediate voltage rises at a high rate of change with the start of acceleration of the motor, and when accelerated to a speed close to the motor speed command, decreases to a voltage required to maintain a constant speed. Becomes a low constant voltage.

From such a relationship, the inverse converter control unit 8
Detects the output current of the inverter 4 and the number of revolutions of the motor, calculates a DC intermediate circuit voltage sufficient to drive the motor at a variable speed, and gives the voltage to the forward converter control unit 6 as a DC voltage command.

The forward converter controller 6 adjusts the gate signal to the switching element of the forward converter 3 based on the voltage command, increases the output voltage of the forward converter 3 when the motor 5 starts accelerating, and sets the motor speed command. When the vehicle is accelerated to a speed close to the predetermined speed, the voltage is reduced to a voltage required to maintain the constant speed, and when the vehicle enters the constant speed state, the voltage is reduced to a low constant voltage.

Therefore, the DC intermediate voltage is variably controlled to a value close to the minimum voltage required for the variable speed operation of the motor 5, and the switching loss in the switching element of the inverter 4 can be reduced. . In particular, if the inverter 4 is PW
Switching loss due to the M control configuration can be greatly reduced.

The forward converter 3 is different from the conventional one in that
The difference between the voltage of the commercial power supply and the output voltage (DC intermediate voltage) increases, and the voltage controlled by the switch element increases. However, the switching frequency is lower than that of the inverter 4 and the switching by the reactor 2 is performed. The switching loss rarely increases due to the delay of the current with respect to the voltage at the time.

In the above embodiment, the case where the inverse converter calculates the DC intermediate circuit voltage sufficient for causing the motor to operate as desired by the inverter control unit 8 will be described. 6 or a dedicated arithmetic circuit.

[0019]

As described above, according to the present invention, a DC intermediate circuit voltage sufficient for the inverter to drive the motor at a variable speed is calculated, and the output voltage of the forward converter is adjusted based on the calculated DC intermediate circuit voltage. Therefore, by lowering the DC intermediate circuit voltage to a level required for the motor output, switching loss in the inverter is reduced, thereby increasing the power conversion efficiency of the inverter and simplifying the cooling mechanism.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of an inverter according to an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram of a conventional inverter.

FIG. 3 is a schematic diagram of a DC voltage pattern during acceleration of an inverter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Commercial power supply 2 ... Reactor 3 ... Forward converter 4 ... Inverter 5 ... Motor 6 ... Forward converter control part 7 ... Capacitor 8 ... Inverter control part 9, 10 ... Current sensor

Claims (1)

[Claims] 1. A forward converter for converting an AC input current into a sine wave to convert it into DC power, and an inverse converter for driving the motor at a variable speed with an AC output whose frequency and voltage are controlled using the DC power as a DC intermediate voltage. An inverter comprising: means for calculating a DC intermediate circuit voltage sufficient to drive the motor at a variable speed, and adjusting an output voltage of the forward converter to the DC intermediate circuit voltage. .