JPS5829850B2 - Excitation circuit of electromagnetic flowmeter - Google Patents

Description

Detailed Description of the Invention The present invention provides an electromagnetic flowmeter that supplies an excitation current with a frequency lower than the commercial power frequency to an excitation coil of an electromagnetic flowmeter transmitter, and when obtaining a comparison signal proportional to the excitation current, the excitation current is This invention relates to an improvement in the excitation circuit of an electromagnetic flowmeter that detects the commercial power frequency state via a current transformer before it becomes a low-frequency current.

FIG. 1 is a circuit configuration diagram of an electromagnetic flowmeter that is the basis of the present invention, in which 1 is an electromagnetic flowmeter transmitter, 2 is a conduit through which the fluid to be measured passes, 3 and 4 are electrodes provided in the conduit, and 5 is an electromagnetic flowmeter transmitter. excitation coil,
6 is a signal amplifier that amplifies the flow rate signal generated between the electrodes 3 and 4, 7 is a divider circuit, 8 is a current transformer for detecting exciting current, 9 is a rectifier, 10 is a load resistor, 11 is a switch, 12 is a switch A drive circuit, 13 is a rectifier, and 14 is a commercial power source.

The configuration of this circuit is as follows.

The commercial power supply 14 is connected to the switch 11 and one of the current transformers 8.
It is connected to the rectifier 13 via the secondary winding 8a.

An excitation coil 5 is connected to the output end of the rectifier 13.

The electrodes 3 and 4 are connected to the input end of a signal amplifier 6, and the output end of the signal amplifier 6 is connected to a divider circuit 7.

The secondary winding 8b of the current transformer 8 is connected to a rectifier 9, and the output end of the rectifier 9 is connected to a load resistor 10.

The voltage across the terminals of the load resistor 10 is input to the divider circuit 7. The operation of this electromagnetic flowmeter is as follows.

The switch 11 is driven ON-OFF by a switch drive circuit 12 at a frequency lower than the commercial power frequency.

The AC voltage of the commercial power supply 14 is supplied to the rectifier 13 via the switch 11 and the primary winding 8a of the current transformer 8.
Full wave rectified.

Therefore, an exciting current flows through the exciting coil 5 when the switch 11 is turned on.

Even though the excitation current has a low frequency, the frequency of the current flowing through the current transformer 8 is the commercial power supply frequency,
The rectified value of the alternating current generated in the secondary winding 8b of the current transformer 8 is proportional to the excitation current flowing through the excitation coil 5.

On the other hand, the flow rate signal generated between the electrodes 3 and 4 is transmitted to the signal amplifier 6.
The signal is amplified by and supplied to the divider circuit 7.

An alternating current proportional to the excitation current generated in the secondary winding 8b of the current transformer 8 is rectified into direct current by a rectifier 9 and flows to a load resistor 10, and the voltage generated between the terminals of this load resistor 10 is divided as a comparison signal. The signal is supplied to the calculation circuit 7.

The divider circuit 7 divides the flow rate signal from the signal amplifier 6 by the comparison signal from the load resistor 10, compensates for the flow rate signal due to fluctuations in the excitation current, and obtains a highly accurate flow rate signal from the output terminal OUT. be.

In addition, in a low-frequency excitation type electromagnetic flowmeter, the flow rate signal and the comparison signal are generally sampled at the point where the excitation current is stable in the excitation state and non-excitation state (or positive excitation state and negative excitation state), and the excitation It has the function of canceling the DC component generated from electric circuits by subtracting the signal in the non-excited state from the signal in the non-excited state.

The electromagnetic flowmeter that is the basis of the present invention is characterized in that when it detects a comparison signal proportional to the excitation current, it uses a current transformer 8, and has the following advantages.

(a) Since the current flowing through the current transformer 8 is at the commercial power supply frequency even if the excitation current flowing through the excitation coil is at a low frequency, a highly accurate comparison signal can be easily obtained even with a small current transformer.

(b) Isolation between the commercial power source and the detected comparison signal is achieved by a current transformer.

(c) The comparison signal obtained has a relatively large value.

By the way, in such an electromagnetic flowmeter, when an attempt is made to detect a comparison signal proportional to the excitation current with higher precision, the following problem occurs near the point where the commercial power supply voltage reaches zero volts.

Due to the power supply side reactance (mainly transformer leakage reactance and line reactance) and resistance, when the commercial power supply voltage is around zero volts, it takes a little while for the current in the rectifier 13 to commutate from one diode to the other in the rectifier 13. It takes time.

During this commutation period, all the diodes in the rectifier 13 are conductive, and part of the current flowing through the excitation coil 5 is transferred to the rectifier 1.
3 and does not flow through the primary winding 8a of the current transformer 8.

Therefore, when the commercial power supply voltage is near zero volts, a part of the current flowing through the excitation coil 5, which bypasses the rectifier 13, is not detected by the current transformer 8, resulting in an error in the comparison signal.

Therefore, the present invention separately detects the current flowing through the exciting coil 5 when the commercial power supply voltage is near zero using a second current transformer, and transmits this detection signal to the commercial power supply 14 and the rectifier 13.
By adding it to the signal detected by the current transformer 8 (hereinafter referred to as the first current transformer) connected between the two, a more accurate comparison signal can be obtained.

FIG. 2 shows an electromagnetic flowmeter showing one embodiment of the present invention.
5 is a second current transformer, and 18 is a load resistor.

The configuration of this electromagnetic flowmeter is such that the following circuit is added to the electromagnetic flowmeter shown in FIG.

A primary winding 15a of the second current transformer 15 is connected in parallel to the exciting coil 5 via a diode 16.

The secondary winding 15b of the second current transformer 15a includes
A series circuit of a diode 17 and a load resistor 18 is connected, and a diode 19 is connected in parallel with this series circuit with a polarity opposite to that of the diode 17.

The load resistor 18 is connected in series with the load resistor 10 and connected to the divider circuit 7.

FIG. 3 is a waveform diagram showing the operation of the electromagnetic flowmeter shown in FIG.
3 is the excitation current waveform flowing through the excitation coil 5,
4 is the current waveform flowing through the first power transformer 8, 5 is the voltage waveform obtained between the terminals of the load resistor 10, 6 is the current waveform flowing through the primary winding 15a of the second current transformer 15, and 7 is the current waveform flowing through the primary winding 15a of the second current transformer 15. Secondary winding 15 of Karen 1-1-Lance 15 of 2
8 is the corrected voltage waveform obtained between the terminals of the load resistor 18; 9 is the comparison signal waveform obtained by adding the voltage generated between the ends of the load resistors 10 and 18; 10 is the waveform of the comparison signal obtained from the signal amplifier 6. This is a sample signal waveform that samples the flow rate signal from the load resistor 10.18 and the comparison signal from the load resistor 10.18.

In the electromagnetic flowmeter shown in FIG. 2, the voltage (waveform 5
) is not proportional to the excitation current (waveform 3) flowing through the excitation coil 5 when the commercial power supply voltage (waveform 1) is around zero volts.

Instead of flowing to the primary winding 8a of the first current transformer 8, this non-proportional component flows to the primary winding of the second current transformer 15 via the diode 16 as shown in waveform 6.

Here, the current flowing due to the back electromotive force of the excitation coil 5 flows to the primary winding 15a of the current transformer 15 of the main whistle 2, and the current flowing to the rectifier 13 is so small that it can be ignored.

The reason for this is that the primary winding 15 of the second current transformer 15
This is because current flows through a if there is a voltage higher than the forward voltage of one diode, but the rectifier 13 requires a forward voltage of two diodes.

The current flowing through the primary winding 15a of the second current transformer 15 (waveform 6) contains a DC component, but the current flowing through the secondary winding 15a includes a DC component.
Only the outside AC is transmitted to 5b.

The current becomes as shown in waveform 7, and the voltage across the terminals of the load resistor 18 becomes as shown in waveform 8.

Therefore, the voltage generated by adding the correction voltage (waveform 8) generated between the terminals of the load resistor 18 to the voltage generated between the terminals A of the load resistor 10 (waveform 5) is compared with the voltage of waveform 5 as shown in waveform 9. This results in a waveform close to excitation current waveform 3.

Therefore, by using the voltage obtained by adding the voltages between the terminals of both load resistors 10.18 as a comparison signal, an electromagnetic flowmeter capable of more precise flow measurement can be obtained.

Next, due to the low frequency characteristics of the second current transformer 15, the correction voltage obtained at the load resistor 18 includes an error component, but the value is sufficiently small compared to the obtained comparison voltage and the difference is negligible. explain.

When the rectifier 13 is a bridge circuit using diodes, the forward voltage of the diodes is approximately 1.

Therefore, when the commercial power supply 14 of 100 V 50 Hz is used, the pulse width of the current (waveform 6) flowing through the second current transformer 15 is about 90 μsec.

The correction voltage obtained between the terminals of the load resistor 18 is about 1% of the voltage obtained between the terminals of the load resistor 10.

Therefore, even if the correction voltage obtained between the terminals of the load resistor 18 includes an error component of, for example, about 5, the error with respect to the comparison signal is 0.05%, which is a very small value.

For this reason, the second current transformer 15 may be a small transformer.

Note that the function of the sample signal (waveform 10) is to sample the flow rate signal when the excitation current becomes sufficiently stable.

In the above embodiment, the switch 11 is 0N-O.
Although the electromagnetic flowmeter has been described in which a low-frequency rectangular excitation current is obtained using an FF, the present invention is not limited to this, and the present invention is not limited to this. It can also be applied to an electromagnetic flowmeter in which the rectifier 13 is supplied with a sinusoidal low frequency excitation through the rectifier 13.

Furthermore, the present invention is applicable not only to electromagnetic flowmeters that compensate for errors caused by fluctuations in excitation current by dividing the flow rate signal by a comparison signal, but also to electromagnetic flowmeters that maintain a constant excitation current based on the comparison signal. I can do it.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of an electromagnetic flowmeter that is the basis of the present invention.
FIG. 2 is an electromagnetic flowmeter showing an embodiment of the present invention, and FIG. 3 is a waveform diagram for explaining the operation of the embodiment in FIG. 1... Electromagnetic flowmeter transmitter, 3, 4...
Electrode, 5... Excitation coil, 6... Signal amplifier, 7... Division circuit, 8... First
Current transformer, 9, 13... Rectifier, 1
0.18...Load resistance, 11...Switch, 12...Switch drive circuit, 14...
...Commercial power supply, 15...Second current transformer.

Claims (1)

[Claims] 1. an AC power source, a first current transformer inserted between the AC power source and a rectifier that rectifies the output of the AC power source, and an excitation coil of an electromagnetic flowmeter transmitter connected to the output end of the rectifier; A series circuit of a rectifying element and a second current transformer is connected in parallel with the excitation coil, and the first
A signal generated in the secondary winding of the current transformer and a signal generated in the secondary winding of the second current transformer are added to obtain a comparison signal proportional to the excitation current flowing through the excitation coil. Excitation circuit for an electromagnetic flowmeter.