JPH01133596A - Variable frequency controller for ac motor - Google Patents

Abstract

PURPOSE:To effect accident preventing operation in an early period even in a device, in which the reduction of an output is connected to the accident, by a method wherein an abnormality alarm is outputted when an actual current becomes lower than the lowest current setting, effected by the output of a function generator. CONSTITUTION:The value of lowest current setting, outputted from a function generator 11, is the function of a frequency reference 6 and the opening degree 18 of a damper. An actual current from a current detector 12 is compared with the output of the function generator 11 by a comparator 13 and when the actual current is lower than the lowest current setting effected by the output of the function generator, an alarm is outputted from an alarm outputting device 14. According to this method, an abnormal output reduction may be detected in an early period and accident preventing operation may be effected in the early period even in a device, in which the reduction of the output is connected to the accident thereof.

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) The present invention is an AC type! The present invention relates to a variable frequency control device that performs variable speed control of an IJ machine.

(Conventional technology) A blower is a frequency control device for an AC motor.

For applications such as compressors and pumps that do not require particularly precise speed settings or rapid acceleration/deceleration, speed is often adjusted only by open-loop frequency settings without providing speed feedback.

Generally, the speed is adjusted in an open loop without detecting the actual speed of the motor. One of the malfunctions of this type of system is an abnormal drop in the control output, and even though the output is at a certain level based on one frequency, the inverter does not output enough frequency or one-volt pressure, and the motor For example, this failure can occur due to an abnormal drop in the output of the voltage control circuit, and may also occur if the electrical circuit between the frequency reference, voltage control circuit, and frequency control is cut off. This failure occurs. The main cause is disconnection of the electrical circuit. There may be a grounding accident on the Ti road, abnormal operation of the internal electronic circuit, etc. Such abnormal output drop failures are not accompanied by excessive output phenomena such as overcurrent or overvoltage, and therefore are difficult to detect in open loop control where the speed output is not directly compared with the speed reference. Depending on the application, such an abnormal decrease in output, that is, an abnormal decrease in motor speed, may lead to a serious accident. However, as mentioned above, it is difficult to detect an abnormal output drop early in the open-loop control method, so measures are only taken to increase the reliability of the control circuit and reduce the probability of failure, but the protection is incomplete. there were.

(Problems to be Solved by the Invention) The present invention provides an AC motor that improves protection by early detecting failures such as abnormal output drops in open-loop frequency control devices and issuing warnings. It is a frequency control device.

[Structure of the invention]

(Means for Solving the Problems) The present invention includes a frequency reference input to a frequency control device, a function generator that outputs a minimum current setting value for the given frequency reference, and a current detector that detects a motor current. , a comparator that compares the output of the function generator and the output of the current detector, and a variable frequency control device that issues a warning output when the actual current falls below the output of the function generator.

(Function) The motor speed at each frequency is almost proportional, and the minimum current of the motor at each speed is determined by the load characteristics. Therefore, when the actual current is lower than the minimum current setting by the output of the function generator, the control output This is detected by a comparator as an abnormal drop in

give false information.

(Embodiment) FIG. 1 is a configuration diagram of a variable frequency control device showing an embodiment of the present invention.

The main circuit rectifies the AC voltage supplied from the AC power supply 1 into DC using a rectifier 2, converts the smoothed DC using a smoothing reactor 3 into AC of the required frequency using an inverter 4, and supplies the AC voltage to an AC motor 5. do. In the control circuit, a voltage control circuit 7 and a frequency control circuit 8 operate in response to the frequency reference 6, and for voltage control, a phase control circuit 9 is operated based on the output of the voltage control circuit 7, and the necessary voltage output is output from the rectifier 2. Perform phase control to obtain At the same time, for frequency control, the inverter control circuit 10 is operated from the output of the frequency control circuit 8.

Ignition is performed to obtain the necessary frequency for the inverter 4. The electric motor 5 rotates at the required rotation speed following the output frequency of the inverter 4.

Then, the opening degree of the damper 17 is added as a parameter of the minimum current setting function for adjusting the air volume of the blower 15, thereby improving the accuracy of the minimum current setting. That is, the damper 17 provided in the conduit 16 of the blower 15 is provided with an opening detector 18, and the output of the opening detector 18 is added to the input of the function generator 11.

The minimum current setting value output from the function generator 11 is:

It is a function of the frequency reference 6 and the damper opening degree. When the blower load amount when the damper opening degree is θ is set as K(0), and when the blower 15 has a square load characteristic, the minimum current setting value becomes a function as shown below.

FIG. 2 shows an example of setting the load amount K(0) based on the minimum current setting value=K(θ)×(frequency reference output)2×constant damper opening degree.

In this way, for the air blower a15 whose load fluctuates depending on the opening degree of the damper 17, protection against an abnormal output drop is performed taking into account the opening degree of the damper 17. FIG. 2 is an explanatory diagram showing the relationship between the damper opening degree and the blower load amount.

〔Effect of the invention〕

According to the present invention, it is possible to early detect an abnormal output drop in a frequency control device for an AC motor that performs open-loop speed adjustment, and it is possible to take early accident prevention actions even in a device where a drop in output can lead to an accident. A frequency control device can be provided.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a frequency control device showing one embodiment of the present invention, and FIG. 2 is an explanatory diagram showing the relationship between damper opening and blower load amount. 6... Frequency base $ 7... Voltage control circuit 8
...Frequency control circuit 9...Phase control circuit 10.
...Inverter control circuit 11...Function generator 12.
...Current detector 13...Comparator 18
...Damper opening detector lil Figure 2

Claims (1)

[Claims] A variable frequency control device for an AC motor that performs speed adjustment by providing a primary variable frequency, comprising: a function generator that outputs the lowest current value of the AC motor as a function of a frequency reference; and a current detector that detects the motor current; a comparator that compares the output of the function generator and the current value detected by the current detector; and an alarm output circuit that issues an alarm when the output of the current detector falls below a minimum current value set by the function generator. A variable frequency control device for an AC motor.