JP2000069760A - Inverter having reduced switching loss - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce switching loss by making constant the number of carriers per period when inverter frequency satisfies conditions higher than a specified value and below a maximum current limit value, thereby lowering the carrier frequency. SOLUTION: A three-phase inverter 7 is constituted by connecting three arms 6-1, 6-2, 6-3, each comprising switching elements 4, 5 connected in series, in parallel with a battery 1. The switching elements 4, 5 are operated by a gate drive circuit 13 and a control circuit 14 comprising a microcomputer performs processing including current limitation and generation of operational signal for the inverter 7 by judging detection signal received from current sensors 9, 10 and a rotation sensor 11. Carrier frequency is decreased by performing control for making constant the number of carriers per period of the inverter frequency when the r.p.m. of an AC motor 8 exceeds a specified value below a maximum current limit value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter for an electric vehicle equipped with an AC motor, and more particularly to control for reducing switching loss.

[0002]

2. Description of the Related Art In order to reduce the switching loss of an inverter, as disclosed in Japanese Patent Application Laid-Open Nos. 59-139871 and 59-139870, as shown in FIG. During one period, two-arm modulation for continuous ON / OFF state is performed.

FIG. 2 shows a modulation waveform of one arm.
(A) shows a modulation waveform when the accelerator is slightly opened, (b) shows a modulation waveform when the accelerator is opened in the middle, and (c) shows a modulation waveform when the accelerator is fully opened.

[0004] Periods 3 and 6 of one cycle of the inverter mean that the element on the (+) side and the element on the (-) side of the arm are made conductive. This means that the battery voltage and 0
Outputs the V voltage, that is, performs two-arm modulation. Switching is not performed for one-third cycle per arm, thereby reducing switching loss.

There is also an advantage that the temperature rise can be made uniform by making the conduction ratio of the element on the (+) side and the element on the (-) side of the arm the same during one cycle.

However, two thirds of the switching loss remains. The switching loss is proportional to the frequency of the carrier. Therefore, when the frequency of the carrier wave is reduced, there are problems such as that the line voltage waveform of the AC motor is not a sine wave, the ripple of the current is increased, and the current capacity of the element must be increased, and noise increases.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an inverter having a reduced switching loss for determining an allowable carrier frequency with respect to the frequency of the inverter or the number of rotations of the motor. The object is to make the carrier frequency variable.

[0008]

According to the present invention, there is provided a device (or program) for comparing a current value from a current detector of an AC motor with a maximum current limit value, and a rotation speed (or a frequency of an inverter) of the AC motor. And a carrier frequency relationship table.

There is provided software program means for detecting that the frequency of the inverter (or the number of revolutions if a tachometer is provided in the AC motor) exceeds a predetermined value.

When the frequency of the inverter satisfies the condition of not less than the maximum current limit value and not less than the specified value, there is provided means for keeping the number of carrier waves per cycle constant.

[0011]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

FIG. 3 is a general AC motor control circuit diagram. The (+) side switching element 4 and the (−) side switching element 5 are connected in series from the (+) power supply line 2 of the battery 1.
(-) Connect to power supply line 3. An arm 6-1 is formed by connecting the (+) side switching element 4 and the (−) side switching element 5 in series. Arm 3-1, 6-2, 6-3
The three-phase inverter 7 is connected in parallel. From each arm, the current sensors (A), (B) 9 and 10 are set on two arms connected to the AC motor 8.

The rotation sensor 11 for detecting the rotation of the AC motor 8 is sometimes set. The accelerator 12 changes the opening of the accelerator pedal into an electric signal. The gate drive circuit 13 is a signal for turning on / off the switching element. The control circuit 14 has a microcomputer and performs processing such as inputting the signals of the current sensors 9 and 10 and the signal of the rotation sensor 11 to limit the judgment and generate an operation signal of the inverter.

FIG. 1 shows the operating characteristics of the present invention. The horizontal axis shows the rotation speed, and the vertical axis shows the current value (solid line) and the inverter frequency (dotted line).

The carrier frequency is reduced from a value N1 whose rotation speed is larger than N0, which is equal to or less than the maximum current limit value.
From 2, it is assumed that the number of carriers per one cycle of the inverter frequency is constant.

FIG. 4A shows an arm output waveform at N2 in FIG. 1, and FIG. 4B shows an arm output waveform at N3 in FIG. From N2 to N3 in FIG. 1, a desired inverter waveform can be obtained by changing only the period of the carrier. There is also an advantage that software processing is simplified. Rotation speed 0 in FIG.
The carrier frequency is a constant 10 KHz from to N1. The rotation speed is set to 10 KHz or less from N1 to N3.

[0017]

As described above, according to the present invention, the switching loss can be reduced by lowering the carrier frequency.

[Brief description of the drawings]

FIG. 1 shows an operation characteristic diagram of an inverter used in an AC motor control circuit according to an embodiment of the present invention.

FIGS. 2A to 2C show modulation waveform diagrams up to the maximum voltage utilization.

FIG. 3 shows an AC motor control circuit of the present invention.

4 (a) and (c) show the arm output waveform diagrams of FIG. 1. FIG.

[Explanation of symbols]

1 ... battery, 2 ... (+) power line, 3 ... (-) power line,
4 ... (+) side switching element, 5 ... (-) side switching element, 6 ... arm, 7 ... inverter, 8 ... AC motor, 9 ... current sensor (A), 10 ... current sensor (B),
11: rotation sensor, 12: accelerator, 13: gate drive circuit, 14: control circuit.

Continuing from the front page (72) Inventor Nobuo Inoue 2520 Oji Takaba, Hitachinaka City, Ibaraki Prefecture Inside the Automotive Equipment Division of Hitachi, Ltd. 5H007 AA06 BB06 CA02 CB04 CB05 CC23 DA03 DB12 DC02 DC07 EA14 5H576 AA01 BB02 CC02 EE18 EE19 GG02 GG04 HA03 HB01 JJ03 LL22

Claims (1)

[Claims] 1. A three-phase inverter having three arms,
In a two-arm modulation inverter in which each arm outputs a battery voltage of 0 V in a 1/6 cycle and a line voltage of the inverter becomes a sine wave by pulse width modulation,
An inverter with reduced switching loss, characterized in that the carrier frequency is variable when the motor speed or the inverter frequency is above a specified value below the maximum current limit value.