The circuit principle and modification and exchange method of frequency inverter transformer

The current transformers used in frequency inverter circuits, except for a few early products that used traditional transformers wound with through core inductor coils, are often integrated sealed current transformers made of Hall elements and pre current detection circuits (let’s call them electronic current transformers) in mature circuits. They are divided into standard and non-standard types, and the standard type uses specialized molded products in the market. For example, a 10A/1V current transformer generates a 1V signal voltage output for every 10A current in the circuit. Non standard type, designed and customized by the frequency converter manufacturer, cannot be used interchangeably. When damaged, it is generally necessary to replace the same model product provided by the original manufacturer. Of course, with deeper maintenance efforts, different models of current transformers can also be used for emergency repair or improvement, and later replaced.
Electronic current transformers often use some type of sealant for curing, which can cause damage and cannot be restored once removed. What kind of circuits are inside and whether they can be repaired or replaced, causing a lot of speculation. When I was repairing a Fuji frequency inverter, I replaced it with the main board of the TECO frequency inverter. When it was necessary to adjust the A/V ratio of the electronic current transformer, it had to be adjusted by the internal circuit of the transformer. Only then did I make up my mind and use a knife, a saw, and a lot of effort to dissect and map the internal circuits of the current transformers of these three types of frequency inverters. It was hard won.

An electronic current transformer is actually a circuit for a current/voltage converter. The Taian 7.5kW inverter current transformer circuit has a certain representativeness. The main body of the current transformer is also a circular hollow magnetic ring. The U, V, and W output lines of the frequency inverter pass through the iron core magnetic ring as the primary winding (small power models usually pass through multiple turns), and the magnetic ring generates magnetic field lines that vary in density with the output current of the frequency inverter. This magnetic ring has a gap in which a Hall element with four lead terminals is embedded. Hall elements are packaged in sheet form, and the magnetic field lines of the magnetic ring pass through the packaging end face of the Hall element, which is also known as the magnetic field line collection area (or magnetic induction surface). Hall elements convert changes in magnetic field lines into induced voltage outputs. The circuit consists of Hall elements and a precision dual operational amplifier circuit 4570. A constant current of mA (about 3-5mA) level must be added to the operation of the Hall element, and 4570A should be connected as a constant current source output mode to provide the mA level constant current required for the normal operation of the Hall element (the working current of the Hall element in this circuit is about 5.77 mA), which should be added to pins 4 and 2 of the Hall element; The induced voltage that varies with the output current at pins 1 and 3 of the Hall element is applied to the input terminals 2 and 3 of 4570b. Three pins are embedded in the reference voltage (zero potential point), and the change in input voltage of two pins is amplified and output by one pin (current detection signal). Electronic current transformers often have four terminal components, with two terminals supplying power to internal amplifiers of+15V and -15V, the other two terminals serving as signal output terminals, one terminal grounded, and one terminal serving as signal OUT terminal+ In addition to providing power for the dual operational amplifier IC4570, 15V and -15V are further stabilized by 6V to form a zero potential point introduced into the three pins of 4570. When the frequency converter is in a shutdown state, the ground measurement OUT point should be 0V. During operation, it will output an AC signal voltage below 4V in proportion to the output current.

After the electronic current transformer is damaged, it outputs a higher positive or negative DC voltage during static state (when the frequency inverter is shut down), which is mostly due to damage to the internal operational amplifier. Power on self-test of the frequency inverter, which displays a fault code (sometimes without a code in the manual), the frequency inverter will refuse to start or even parameter operation!
The current transformer circuit of TECO 3.7kW frequency converter uses a programmable operational amplifier chip. I have not yet found the model of this chip, but through modification tests, some characteristics of the circuit have been identified. According to the experiment, pin 2 is the constant current power supply terminal, pins 3 and 4 are the input terminals of the differential amplifier, and pin 13 is the signal output terminal. When short circuiting the solder gaps of pins 11, 12, and 13 step by step, the amplification factor shows a decreasing trend; When opening the circuit step by step, the amplification factor increases. This can adjust the amplification factor of the chip, making it easier to match frequency converters with different power outputs. I successfully applied the current transformer to a 45kW Fuji frequency Inverter by taking corresponding measures.
The voltage detection and current detection signals of the frequency converter may be applied by the program to control the output three-phase voltage and current – when the detection signal changes, the output three-phase voltage and current also change accordingly. When repairing or modifying the original circuit, be careful not to change the original circuit parameters. It is still recommended to use original accessories to repair the frequency converter while maintaining the original circuit form.