JPS637194A - Protecting method for overload of variable speed motor - Google Patents

Abstract

PURPOSE:To eliminate an overload over the full speed range of an AC motor by varying a current limiting value in response to a rotating speed. CONSTITUTION:A current limiting command generator 8 compares speed/ allowable load current characteristic obtained in advance for an induction motor 40 with the input current (the input current of a motor 40) of an inverter 30 detected by a current detector 5 to be monitored. when the input current of the inverter 30 exceeds the allowable load current value with respect to an operating speed instructed by an operating speed command generator 7, a current limiting command is supplied to the inverter 30. Thus, when the operating speed is varied from high to low value, the current limiting value is controlled from large to small value in response to the variation.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for overload protection of variable speed electric motors.

[Conventional technology]

Conventionally, overload protection for a motor driven by a power converter such as an inverter has been carried out by monitoring a current detected by a current detector 5 with a protection device 6, as shown in FIG. When it exceeds the limit, the protection device 6 generates an overload signal to release the power switch 2 and stop the power converter 3.

1 is a power supply and 4 is an electric motor.

[Problem that the invention seeks to solve]

In this way, conventional motor overload protection compares the motor current with a certain overload level and performs a protective operation when the former becomes larger than the latter. For example, in the case of an induction motor that is driven at variable speed by a converter and whose load characteristics are square torque characteristics, it provides sufficient protection against thermal overload when the motor cooling air volume decreases due to low speed operation. The problem was that I couldn't do it.

This invention was made to solve the above problem.
The purpose of this invention is to develop an overload protection method for a variable speed motor that can provide overload protection over the entire speed range of an AC motor.

Means for Solving Problem C] In order to achieve the above object, the present invention determines the speed-permissible load current characteristic of an AC motor, and when the detected load current exceeds the above-mentioned permissible load current, the permissible load is determined. The configuration is such that the output voltage and frequency of the power converter are narrowed down until the current is below.

[Effect]

In this invention, when the operating speed is changed from high speed to low speed, the current limit value is controlled from a large value to a small value according to the change, so overload occurs over the entire speed range of the AC motor. Never.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, 30 is a variable speed variable voltage inverter device, which receives an inverter voltage command and an inverter frequency command created based on an operating speed command (signal) S, and controls the voltage/frequency ratio to be constant. and induction motor 4
0 with variable voltage and variable frequency power. 7 is an operating speed command generator, and 8 is a current limit command generator, which outputs the speed-allowable load current characteristics determined in advance for the induction motor 40 and the inverter input current (motor input current) detected by the current detector 5. The magnitude of the current IM is compared and monitored, and if the current IM exceeds the allowable load current value for the operating speed commanded by the operating speed command S, a current limit command (signal) is issued.
is supplied to the inverter device 30. The other configurations are the same as those shown in FIG. 2, and are designated by the same reference numerals.

In this configuration, when the load on the induction motor 4 increases while operating at a certain operating speed and its value exceeds the allowable load current value determined from the speed-allowable load current characteristic, the current limit command generator 8 Generates a current limit command.

In this embodiment, when the inverter device 30 receives this current limit command, it performs an operation to reduce the inverter output current and limits the motor input current IM to the allowable load current level or lower. Further, the inverter frequency command is automatically subtracted from the initial value, the speed of the induction motor 40 decreases, and it settles at a stable operating point where the speed-torque characteristic of the load and the torque generated by the induction motor 40 are balanced. When the load decreases and the current limit command disappears, the inverter device 30
The current limiting operation is canceled, and at the same time, the inverter frequency command also returns to its original value, and the speed of the induction motor 40 is restored.

〔Effect of the invention〕

As explained above, in this invention, since the motor current of the AC motor does not exceed the current value determined by the speed-allowable load current characteristic of the AC motor, filtering load protection can be realized over the entire speed range. It is possible to improve trustworthiness compared to the past.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an overload protection device implementing the overload protection method of the present invention, and FIG. 2 is a block diagram showing a conventional overload protection device for an electric motor. In the figure, 7 - operating speed command generator, 8 - current limit command generator, 30 - inverter device, 40 - induction motor. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] When driving an AC motor at variable speed using a variable voltage variable frequency power converter, the speed of the AC motor -
The present invention is characterized in that an allowable load current characteristic is determined, and when the detected load current exceeds the allowable load current, a current limit command is generated to narrow down the output voltage and output frequency of the power converter. Overload protection method for variable speed motors.