JPH0219718B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a current detection circuit, and particularly to a current detection circuit that simultaneously detects the input and output current values and current phases of a power conversion device, and uses current to improve the power factor and efficiency of a load. Make it a detection circuit.

2. Description of the Related Art Recently, there has been an active movement toward variable speed motors using power conversion devices using semiconductors and the like to achieve energy-saving effects. With this purpose,
Various control devices for induction motors using power converters have been proposed, and the applicant has also proposed one previously (Japanese Patent Publication No. 55-45333). In this type of control device, a current detection circuit as shown in FIG. 1 is used in order to detect input and output current values and current phases of the power conversion device.

FIG. 1 is a block diagram showing a control device for an induction motor using a power converter. This control device drives the induction motor using an inverter, which is a power conversion device, and efficiently operates the induction motor by controlling the output voltage of the inverter according to the power factor of the load current. The following description of this control device will clarify the current detection circuit that is the prior art of the present invention.

In Fig. 1, pulse width modulation (hereinafter referred to as
The controlled AC output (referred to as "PWM") is supplied to the induction motor 3.

The alternating current is detected by current transformers 4A and 4B, rectified by a rectifier 5 and exchanged to direct current, and a level detector 6 detects the overcurrent level. The frequency reference set by the frequency setter 7 is input to the acceleration/deceleration control circuit 8, and when an overcurrent flows due to the output of the level detector 6, the acceleration/deceleration time of the acceleration/deceleration control circuit 8 is adjusted to control the current. Accelerate and decelerate while doing so.

The output f* of this acceleration/deceleration control circuit 8 is compared with a modulation triangular wave generator 9 as a voltage reference.
The PWM output is input to the waveform synthesis circuit 11 via the PWM circuit 10. On the other hand, the output f* of the acceleration/deceleration control circuit 8 is obtained by inputting an output proportional to the frequency of the inverter 2 to the waveform synthesis circuit 11 using a voltage/frequency converter (hereinafter referred to as a V/F converter) as a frequency reference, and converting this waveform. The inverter bridge 2 is driven by the output signal of the synthesis circuit 11, which is a drive signal that determines the frequency and voltage.

The current transformer 13 detects one phase of the alternating current, that is, the U-phase current, and the resistor 14 provided between e and f converts it into a voltage, and the zero level detector 15 generates a period signal O ₁₅ regarding the U-phase current. generate. and,
By detecting the phase difference between this period signal O ₁₅ and the U-phase voltage phase reference signal U* output from the waveform synthesis circuit 11, the power factor detection circuit 16 detects a signal corresponding to the phase of the alternating current. Generates a power factor signal _O16 . The power factor control circuit 17 thereby varies the input voltage of the PWM circuit 10 so as to approach the set power factor, and adjusts the voltage/frequency ratio (hereinafter referred to as "V/F ratio") of the inverter output. It is configured to control the load power factor.

In this way, in order to detect the magnitude and phase of the load current, the current detection circuit 20, as shown in FIG. and are detected separately. In such a system, the phase detection converter 1 is installed separately from the current transformers 4A and 4B that detect the load current.
3 and resistor 14 are required, which is disadvantageous in terms of space and cost.

Further, FIG. 2 shows the waveform of the voltage across the resistor 14 corresponding to the waveform of the U-phase current. The zero level detector 15 detects a period in which the polarity of the U-phase current is positive, that is, between zero cross points Z ₁ and Z ₂ , between zero cross points Z ₃ and Z ₄ ,...
The period signal _O15 is generated only during the period.

However, the waveform shown in FIG. 2 is a sinusoidal curve, and the polarity does not suddenly change before and after the zero crossing points Z ₁ , Z ₂ , Z ₃ , Z ₄ . Therefore, the maximum value V ₁
If the noise itself is small, or if noise is detected, errors will occur in detecting the positions of these zero-crossing points, and the accuracy in detecting the phase of the load current will deteriorate.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a current detection circuit that is advantageous in terms of space and cost, and has excellent phase detection accuracy.

In order to achieve the above object, the present invention comprises a current transformer for detecting the current of each phase in an AC line and a plurality of rectifying elements connected in a bridge, and the AC current detected by the current transformer is A voltage is applied between a rectifier to be rectified, a current value detection resistor connected between the output terminals of the rectifier, and one end of the current transformer and one end of the output terminal. The alternating current power supplied to the load is determined by detecting the phase difference between the voltage phase reference signal and the period signal from the zero level detector. The configuration includes a power factor detection circuit that generates a power factor signal corresponding to the power factor.

Hereinafter, one embodiment of the present invention will be described based on the accompanying drawings.

FIG. 3 is an explanatory diagram of a current detection circuit used in a control device for an induction motor using an inverter as shown in FIG. 1.

This current detection circuit, like the current detection circuit 20 shown in FIG. 1, simultaneously detects the load current value and current phase of the inverter, and is constructed as follows.

The circuit that detects the load current value of the inverter includes current transformers 24A and 24B that detect the load current, and a plurality of bridge-connected diodes (rectifying elements) D ₁
A full-wave rectifier 25 that is composed of ~ _D6 and converts the detected load current value I _L from the current transformers 24A and 24B into DC, and output terminals a and b on the DC side of the rectifier 25.
A current value detection resistor 25A provided in between, and a level detector 26 connected to this current value detection resistor 25A.
Be prepared. Here, the level detector 26 is composed of a comparator or the like. In the load current detection circuit configured in this way, current transformers 24A and 24B are used.
Detects the load current, converts this detected current I _L into direct current using the full-wave rectifier 25, and then detects it as a voltage proportional to the load current using the current value detection resistor 25A. The level detector 26 connected to this current detection resistor 25A is the level detector 6 in FIG.
Similarly, overcurrent etc. are detected and the detection signal is applied to the acceleration/deceleration control circuit 8 etc. in FIG. 1 mentioned above.

The circuit that detects the phase of the load current detects the potential between the connection point C (one end side of the current transformers 24A, 24B) in the full-wave rectifier 25 and the output terminal b on the one end side of the rectifier 25. a zero level detector 45 which generates a period signal O ₄₅ and this period signal O ₄₅
The power factor detection circuit 46 outputs a power factor signal corresponding to the power factor of the AC power supplied to the load based on the power factor signal _O46 . Here, the zero level detector 45 is composed of a comparator, etc., while the power factor detection circuit 46 is composed of an exclusive NOR circuit (hereinafter referred to as "XNOR") 46A and a filter 4.
6B.

In the circuit configured as described above for detecting the phase of the load current, the zero level detector 45 outputs the period signal _O45 by detecting the voltage between c and b in FIG. This period signal O ₄₅ is combined with a voltage phase reference signal U* (outputted from the waveform synthesis circuit 11 in FIG. 1) having a predetermined period.
By applying the output O _46A of this XNOR 46A to the filter 46B,
This filter 46B outputs a power factor signal O ₄₆ proportional to the power factor angle of the load. The power factor signal O ₄₆ output from this filter 46B is applied to the power factor control circuit 17 shown in FIG.

FIG. 4 is an explanatory diagram of the operation of the circuit for detecting the phase of the load current. The operation of the phase detecting circuit will be further explained based on this figure.

The U-phase current I _U is shown as in FIG. 4A, but the load current I _L detected by the current transformers 24A and 24B
is full-wave rectified by the rectifier 25, and the rectifier 25
A three-phase full-wave waveform output V _a as shown in Figure 4B is output from output terminals a and b of

Claims (1)

[Claims] 1. Detecting the current value of an alternating current supplied to a load from a single-phase or three-phase or more-phase alternating current line, and changing the polarity of the alternating current detected from any phase of the alternating current line. detects the zero cross point when the one-phase alternating current is detected, generates a signal that lasts for a period from the time when the zero cross point is detected to the time when the next zero cross point is detected, and is synchronized with the load current. A current detection circuit that generates a power factor signal corresponding to a power factor of AC power supplied to the load by detecting a phase difference between a voltage phase reference signal having a period and duration and the period signal, comprising: a current transformer for detecting the current of each phase in an alternating current circuit; a rectifier including a plurality of rectifying elements connected in a bridge and rectifying the alternating current detected by the current transformer; and an output of the rectifier. A period in which a voltage is applied between a current value detection resistor connected between the terminals, one end side of the current transformer and one end side of the output terminal, and the current in one phase of the AC power line has one polarity. a zero level detector that generates a period signal indicative of the power factor of the AC power supplied to the load by detecting a phase difference between the voltage phase reference signal and the period signal from the zero level detector; A current detection circuit comprising: a power factor detection circuit that generates a power factor signal corresponding to;