JPS5833484B2 - Excitation circuit of electromagnetic flowmeter - Google Patents

Description

[Detailed description of the invention] The present invention relates to an excitation circuit for an electromagnetic flowmeter.

One type of electromagnetic flowmeter is one in which the excitation current flowing through the excitation coil is an intermittent current having a frequency lower than the frequency of the commercial power supply.

This type of electromagnetic flowmeter has less zero point fluctuation and 90%
Since zero noise is small, there is an advantage that highly accurate flow rate measurement can be performed.

By the way, the excitation current of this type of flowmeter is normally rectified from a commercial power supply, and the rectified power is controlled intermittently by a semiconductor switch element or the like and then supplied to the excitation coil.

On the other hand, since the flow rate detection signal is proportional to the excitation current value, when the excitation current changes, the flow rate detection signal also changes.

For this reason, a signal proportional to the excitation current is extracted from the excitation circuit and a ratio is calculated between the excitation current signal and the flow rate detection signal to remove fluctuations in the flow rate detection signal due to fluctuations in the excitation current.

The calculation of the ratio between the excitation current signal and the flow rate detection signal is performed in the electronic circuit of the converter.

For this reason, it is necessary to insulate the electronic circuit from the commercial power source serving as the excitation power source.

For this reason, conventionally, as shown in FIG. 1, AC power from a commercial power source 1 is supplied to a rectifier circuit 3 via an isolation transformer 2, and the rectified output of the rectifier circuit 3 is connected to an exciting current signal detection resistor 4 and a current intermittent control switch. The excitation current signal is supplied to the excitation coil 7 of the flow meter transmitter 6 via the element 5, and the excitation current signal obtained across the resistor 40 is supplied to the converter 8. It is insulated by

However, the isolation transformer 2 has disadvantages of being large in proportion to the amount of power consumed by the excitation coil 7, increasing its weight, and being expensive.

For this reason, a method of omitting the isolation transformer 2 as shown in FIG. 2 may also be considered.

That is, the commercial power supply 1 is directly connected to the rectifier circuit 3, and a signal corresponding to the excitation current flowing through the excitation coil γ is taken out by the current transformer 9 and supplied to the converter 8.

Even in this case, since the excitation current flowing through the excitation coil I has a frequency lower than the commercial power supply frequency, the current transformer 9
If an attempt is made to accurately extract a signal corresponding to the excitation current without distortion, the core will have a large shape and will be about the same size as the isolation transformer 2 described in FIG. 1.

In any case, the excitation circuit device is large and heavy, which increases the cost of the device itself (and because it is large and heavy, it requires labor during manufacturing, as well as transportation and installation costs. etc. have the disadvantage of being expensive.

For this reason, there is a need to make the excitation circuit of an electromagnetic flowmeter that uses a low frequency excitation method smaller and lighter, and to manufacture it at a lower cost.

In response to this demand, DC power is first converted into an AC signal higher than the commercial power supply frequency, and this AC signal is supplied to a rectifier circuit via an isolation transformer.The rectifier circuit then obtains DC power again, and this DC power is controlled. It is conceivable to have a structure in which the excitation coil of the flowmeter oscillator is intermittently supplied with an excitation current at a frequency lower than the commercial power supply frequency by a switch element serving as the means.

However, this structure has the disadvantage that the switching element requires a large current capacity, for example, a semiconductor switching element such as a transistor.

An object of the present invention is to provide an excitation circuit for an electromagnetic flowmeter that does not have the problems of the prior art described above.

That is, according to the present invention, the excitation current can be turned on and off by a control means with a small current capacity, and the entire excitation device can be made smaller and lighter, and can be manufactured at a lower cost.

Specific embodiments of the invention will be described below with reference to the drawings.

In FIG. 3, parts corresponding to those in FIGS. 1 and 2 will be described with the same reference numerals.

1 indicates a commercial power source.

Commercial power from the commercial power source 1 is rectified by a rectifier circuit 3a, smoothed by a capacitor 10, and DC power is obtained between terminals 11-11.

That is, in the present invention, for example, the DC power obtained in this way is supplied to the DC-AC converter 12, and the DC power is supplied to the DC-AC converter 12 at a frequency higher than the commercial power supply frequency, for example, 400H.
Convert to AC power of about 40KHz to 40KHz.

In this example, the DC-AC converter 12 includes a pair of transistors Q1.
Q2 and these transistors Q1.

A coupling transformer T that connects the Q2 phase to each other in positive feedback, a connection point between the transistors Qs and Q2, and an isolation transformer 2.
A charging/discharging capacitor C connected in series with the primary winding of '
1, and a time constant circuit 14 comprising a coil L1 connected between one end of the drive winding W of the coupling transformer T and one terminal of a DC power source, a resistor R1, and a capacitor C2.

With such a configuration, the transistor Q1. Q2 is alternately turned on and off by automatic oscillation, thereby charging and discharging the capacitor C1, and the charging and discharging current alternately flows to the primary winding of the isolation transformer 2' to generate AC power.

The alternating frequency is defined by the time constant of the time constant circuit 14, and is set to a frequency of about 400 Hz to 40 KHz, for example.

Incidentally, since such a DC-AC converter 12 is conventionally known, further detailed explanation will be omitted.

2' is an isolation transformer.

A main winding 15a through which a controlled current flows between the isolation transformer 2' and the DC-AC converter 12;
The main winding 15a of the saturable reactor 15 having a control winding 16 for controlling the inductance of the reactor is inserted.

By controlling the inductance of the main winding 15a, the output of the DC-AC converter 12 is controlled intermittently, and the current flowing through the exciting coil 7 is intermittently controlled.

That is, the main winding 15a of the saturable reactor 15 is inserted between one output of the DC/AC converter 12 and one end of the primary winding of the isolation transformer 2', and the main winding 15a of the saturable reactor 15 is A control means 19 intermittently supplies direct current obtained by rectifying AC power obtained from a transformer 11 provided separately from the transformer 21 in a rectifier circuit 18 .

Assuming that this control means is constituted by a switch, this switch 19 is controlled on and off by a control signal source provided in the converter 8 at a frequency lower than the commercial power frequency.

3b is a rectifier circuit.

The AC power obtained in the secondary winding of the isolation transformer 2' is rectified by the rectifier circuit 3b, smoothed by the capacitor 13, and then intermittently supplied to the excitation coil 7 by the switch element 5 at a frequency lower than the commercial frequency. Supplied.

A signal corresponding to the excitation current is taken out from both ends of the resistor 40 and supplied to the converter 8.

In this way, the converter 8 and the commercial power source 1 are isolated by the isolation transformer 2'.

As described above, according to the present invention, the AC power transmitted through the isolation transformer 2' is at the commercial frequency (50 or 60 Hz).
) Since the alternating current is higher, for example, 400 H2 to 40 KHz, the shape of the transformer 2' can be made smaller.

Further, when the switch 19 is off, no current flows through the control winding 16, so that the iron core is not magnetically saturated.

Therefore, the inductance of the saturable reactor 15 is large,
The AC power to be supplied from the DC-to-magnetic current converter 12 to the isolation transformer 2' is attenuated by the impedance of the saturable reactor and limited to a sufficiently small value.

When the switch 19 is turned on, the DC output of the rectifier circuit 18 flows through the control winding 16 and magnetically saturates the iron core, so the inductance of the saturable reactor 15 becomes smaller and a sufficient amount of AC power is supplied to the isolation transformer 7. be done.

Therefore, the rectified output is supplied to the excitation coil 7.

Therefore, if the switch 19 is controlled intermittently at a required frequency lower than the commercial frequency, the flow meter oscillator 6 can be excited at a low frequency.

Also, the switch 19 is connected to the saturable reactor 150 control winding 16
Since it is only necessary to control the current flowing through the switch intermittently, the current capacity can be small and an inexpensive semiconductor switch can be used.

In addition, in this circuit, fluctuations in the excitation current are detected by the resistor 4, and the ratio of the detection signal and the flow rate detection signal is calculated by the converter 8, and fluctuations in the flow rate detection signal due to the fluctuations in the excitation current are calculated. However, by stabilizing the excitation current with a stabilizing circuit, it is also possible to eliminate the need for the converter 8 to calculate the ratio with the flow rate detection signal.

FIG. 4 shows an embodiment thereof. In this example as well, the excitation current is controlled at constant current and the excitation current is controlled intermittently by the small capacity control means.

This control means is now composed of a switch 22 which is controlled on/off by a converter 8, a comparator amplifier 20, and a transistor Q3.

In such a control means, the excitation current detection resistor 4
The voltage generated in the comparison amplifier 200 is applied to the inverting input terminal of the comparison amplifier 200, and the reference voltage obtained at the Zener diode 21 is applied to the non-inverting input terminal of the comparison amplifier 20 through the switch element 22.

The non-inverting input terminal is connected to a common potential point through a resistor R2.

With this configuration, when the switch element 22 is on, a control current flows through the control winding 16 of the saturable reactor by comparing the reference voltage with the voltage generated in the resistor 4.

Therefore, a predetermined excitation current is supplied to the excitation coil 7.

When the switch element 22 is turned off, the potential of the non-inverting input terminal of the comparator amplifier 20 becomes the potential of the common potential point, so the potential of the inverting input terminal becomes higher than the potential of the non-inverting input terminal, and the output of the comparator amplifier 20 becomes It saturates in the direction of negative polarity and completely turns off the transistor Q3.

Therefore, the current flowing through the control winding 16 becomes zero, the impedance of the saturable reactor 15 becomes zero, the output of the DC-AC converter 12 is no longer supplied to the isolation transformer 2', and the exciting current becomes zero.

By controlling the excitation current intermittently by turning the switch element 220 on and off in this manner, the excitation current can be controlled intermittently by the small capacity switch element 22.

However, since the excitation current can be controlled to a constant value by the stabilizing circuit comprising the comparator amplifier 20, the Zener diode 21, and the saturable reactor 15, there is no need to calculate the ratio between the flow rate signal and the excitation current signal in the converter 8. ,
Therefore, a significant cost reduction can be expected by reducing the capacitance of the switching element and omitting the ratio calculation circuit.

As explained above, according to the present invention, the size and weight of the exciter can be reduced by making the isolation transformer smaller and lighter, and since a control means with a smaller electric capacity can be used, handling becomes easier.

Cost reductions can also be expected.

In the above description, the transistor Q is used as the DC-AC converter 12.
Although a self-oscillating circuit with t and Q2 was used, other types of, for example, separately-oscillating DC-AC converters may also be used.

A converter using an element such as a thyristor may also be considered.

Furthermore, although the above explanation describes an excitation method in which the excitation current is simply intermittent DC power, it is easy to apply this invention to the case where the excitation current is excited using a three-value intermittent signal such as positive-zero negative-zero positive. can be understood.

[Brief explanation of drawings]

Figures 1 and 2 are connection diagrams for explaining the excitation circuit of a conventional electromagnetic flowmeter, Figure 3 is a connection diagram showing one embodiment of the present invention, and Figure 4 is another embodiment of the present invention. FIG. 1: Commercial power supply, 2': Isolation transformer, 3a, 3b: Rectifier circuit, 5: Switch element, 7: Excitation coil, 12: DC-AC converter.

Claims (1)

[Claims] 1. A DC power supply, a DC-AC converter that converts the DC power supplied from the DC power supply into AC having a frequency higher than the commercial frequency, an isolation transformer to which the output of the DC-AC converter is supplied, and the insulation transformer. A rectifier circuit that rectifies the output of the transformer and converts it into DC power, an excitation coil that receives the DC output of the rectifier circuit and generates a magnetic field in a flow path through which the fluid to be measured flows, and an output side of the DC-AC converter. The flow flows through the main winding of a saturable reactor that is inserted between the primary side of the isolation transformer and controls the excitation current given to the excitation coil, and the control winding that controls the inductance of the main winding of the saturable reactor. An excitation circuit for an electromagnetic flowmeter, comprising control means for intermittently controlling current at a frequency lower than the commercial frequency.