How to use current transformer to measure three-phase electricity?

hau30729

How to use current transformer to measure three-phase electricity? [Copy link]

 

I bought an AC current transformer with a ratio of 10A:5mA. I want to measure the total power consumption of three-phase electricity. The questions are as follows:
1. A full-wave or half-wave rectifier is required to convert the measured AC current into DC current, and then use a resistor to convert it to voltage, and output it to the ADC through an amplifier circuit. Or should the value from the current transformer be amplified before rectification. Then use a resistor to convert the voltage to read the ADC? Is the above design direction correct?
2. Is the final three-phase power consumption calculation P=220V or380V * current per phase? Then add up the 3 groups to get the total power consumption? Or is the value calculated for a single phase the total power consumption?

maychang

"I bought an AC current transformer with a ratio of 10A:5mA. I want to measure the total power consumption of three-phase electricity."

To measure the total power consumption of three phases, the voltage should be measured at the same time. The voltage and current of each phase are multiplied by the power factor, and the total power consumption is obtained by adding the three phases.

maychang

1. A full-wave or half-wave rectifier is required to convert the measured AC current into DC current, and then use a resistor to convert it into voltage, and then output it to the ADC through an amplifier circuit for reading. It is better to first amplify the value from the current transformer and then rectify it.

Do not rectify first, as this will cause a large error.

Your current transformer is used for very small instruments. If it is a larger current transformer, it will explode if the secondary circuit is open.

maychang

2. The final three-phase power consumption calculation is P = 220V or 380V * current per phase? Then add up the three groups to get the total power consumption?

The voltage is multiplied by the current, not just the power consumption of this phase, but also by the power factor.

maychang

Or is the single-phase calculated value the total power consumption?

For a three-phase symmetrical system, the power measured in a single phase multiplied by 3 is the three-phase power, because the three-phase voltage and current are exactly the same.

But for a three-phase unsymmetrical system, the three phases must be measured (in fact, two phases are often measured because the vector sum of the three phases is zero).

hau30729

maychang posted on 2020-3-24 17:51 1. A full-wave or half-wave rectifier is required to convert the measured AC current into DC current, and then convert it into voltage with a resistor, and output it to the amplifier circuit through...

This means that I should first amplify the value measured by the transformer, then rectify it, and then convert it into voltage through a resistor?

The object under test is a 1HP motor, so a relatively small transformer is used.

maychang

hau30729 posted on 2020-3-24 18:02 This means that I should first amplify the value measured by the transformer and then rectify it. Then convert it to voltage through a resistor? The object to be measured is 1 ...

For a 1hp three-phase motor, the starting current may be greater than 10A. If you only want to measure the power consumption during stable operation after starting, you don't need to consider the current during starting, and you only need to measure the current during stable operation.

maychang

hau30729 posted on 2020-3-24 18:02 This means that I should first amplify the value measured by the transformer and then rectify it. Then convert it to voltage through a resistor? The object to be measured is 1 ...

"It is first amplified and then rectified. And then converted to voltage through a resistor?"

Don't rectify. I said on the 2nd and 4th floors: current (effective value) multiplied by voltage (effective value) multiplied by power factor is power dissipation.

After rectification, the phase difference between voltage and current is lost and cannot be found.

maychang

hau30729 posted on 2020-3-24 18:02 This means that I should first amplify the value measured by the transformer and then rectify it. Then convert it to voltage through a resistor? The object to be measured is 1 ...

I don't know what you are going to use to calculate power consumption. If you are using a microcontroller, there are many microcontrollers with built-in ADCs, and the price is not expensive. What you want to measure is the industrial frequency. Even if you measure 12 points per cycle, it is only 600 points per second. The ADC in the microcontroller is generally capable of doing this.

The MCU ADC measures voltage, so you must first convert the current transformer secondary current signal into voltage, and then pass it to the MCU ADC. At the same time, the MCU ADC also measures the voltage transformer secondary signal. The instantaneous values of the current signal and the voltage signal are multiplied to automatically get the power factor. Then the measured instantaneous power is averaged, which is the average active power within a power frequency cycle. Note: Both the voltage signal and the current signal have negative values.

hau30729

maychang posted on 2020-3-24 18:24 For a 1hp three-phase motor, the starting current may be greater than 10A. If you only measure the power consumption during stable operation after starting, you don't need to consider the current during starting...

You are right to measure the current after stable operation.

hau30729

maychang posted on 2020-3-24 18:33 I don't know what you are going to use to calculate power consumption. If you are using a microcontroller, there are many models of microcontrollers with built-in ADC, and the price is not expensive. What you want to measure...

The maximum output I get is 5mA AC current. If it is not rectified and converted into voltage, it should also be an AC voltage? Can the microcontroller read it directly in this way?

"You must first convert the current transformer secondary current signal into a voltage, and then pass it to the microcontroller ADC. At the same time, the microcontroller ADC also has to measure the voltage transformer secondary signal."
I don't quite understand this.

Current transformer (should be what I am going to use now), what does the secondary signal of voltage transformer refer to? (You replied on the second floor that you need to measure voltage at the same time, so does that mean I need to buy a voltage transformer?)

If we do not purchase a voltage transformer separately and use a fixed value (220 or 380) for calculation, based on your experience, is there a way to estimate whether the actual power consumption will be very different?

(Sorry...I don't know about this

maychang

hau30729 posted on 2020-3-24 22:43 It is correct to measure the current after stable operation.

Then there is no need to consider that the current exceeds the range (which may be the ADC of the microcontroller) when the motor starts, but a protection circuit is required to prevent the chip from being damaged when the transformer is overloaded.

maychang

hau30729 posted on 2020-3-24 22:43 maychang posted on 2020-3-24 18:33 I don't know what you are going to use to calculate the power consumption. If you are using a single-chip microcomputer, many single-chip microcomputers have built-in AD ...

The maximum output I get is 5mA AC current. If it is not rectified and converted into voltage, it should also be an AC voltage? Can the microcontroller read it directly in this way?

Of course it is an AC voltage. Add a DC bias to it so that the signal is never negative, and the microcontroller can read it directly.

maychang

hau30729 posted on 2020-3-24 22:43 maychang posted on 2020-3-24 18:33 I don't know what you are going to use to calculate the power consumption. If you are using a single-chip microcomputer, many single-chip microcomputers have built-in AD ...

"You must first convert the current transformer secondary current signal into a voltage, and then pass it to the microcontroller ADC. At the same time, the microcontroller ADC also has to measure the voltage transformer secondary signal."
I don't quite understand this.

The ADC built into the microcontroller can only receive voltage signals, not current signals. Therefore, the current transformer signal must be converted into voltage before being passed to the microcontroller. The output of the voltage transformer is originally voltage, so it does not need to be converted and can be directly passed to the microcontroller.

maychang

hau30729 posted on 2020-3-24 22:43 maychang posted on 2020-3-24 18:33 I don't know what you are going to use to calculate the power consumption. If you are using a single-chip microcomputer, many single-chip microcomputers have built-in AD ...

In your reply on the second floor, you said that the voltage needs to be measured at the same time, so does that mean I need to purchase a voltage transformer?

Yes, you have to buy another voltage transformer.

This is the case where the power measurement requirements are not high. You can use only one current transformer and one voltage transformer to measure the single-phase power and multiply it by 3 to get the three-phase power. If the power measurement requires high accuracy, you cannot simply measure the single-phase power and then multiply it by 3.

If the power measurement requirement is very low, for example, an error of 60% is not a problem, you can use 380V directly without using a voltage transformer, and then substitute a similar power factor value such as 0.75.

maychang

hau30729 posted on 2020-3-24 22:43 maychang posted on 2020-3-24 18:33 I don't know what you are going to use to calculate the power consumption. If you are using a single-chip microcomputer, many single-chip microcomputers have built-in AD ...

If we do not purchase a voltage transformer separately and use a fixed value (220 or 380) for calculation, based on your experience, is there a way to estimate whether the actual power consumption will be very different?

When the load of the motor (the machine towed by the motor, such as wheels, etc.) is very light, the error can reach more than 100%, and when it is unloaded, the error may reach more than 300%.

For a three-phase induction motor, the idling current (only friction between bearings and fans) is about one-third or even one-half of the full-load current, which is not very small. However, the active power is very small (the power factor is very poor).

hau30729

maychang posted on 2020-3-25 07:26 If you do not purchase a voltage transformer separately, use a fixed value (220 or 380) to calculate. Based on your experience, this is a way to evaluate the difference between the actual power consumption and the actual power consumption...

I think I understand what you said.

1. Measure the voltage and current of two phases (with requirements for accuracy) and multiply them by the power factor to get the power consumption
. 2. The measured secondary current needs to be amplified and then DC biased to be sent to the ADC for easy reading.

I will put the circuit I understand up here and please give me some guidance when you have free time.

maychang

hau30729 Published on 2020-3-25 10:05 I should have roughly understood what you said. 1. Measure the voltage and current of two phases (with requirements for accuracy) and then multiply by the power factor to get my power consumption. 2. Need...

The measured secondary current needs to be amplified and then DC biased to be sent to the ADC for easy reading.

1. I don't know what "secondary phase" is. I guess you mean the secondary of the current transformer, or the secondary winding of the current transformer.

2. It is not necessary to amplify the current first and then add a DC bias. It may be simpler to add a DC bias to the secondary of the current transformer, so that the current-to-voltage conversion (that is, the "amplification" you mentioned) can also use a single power supply.

maychang

hau30729 Published on 2020-3-25 10:05 I should have roughly understood what you said. 1. Measure the voltage and current of two phases (with requirements for accuracy) and then multiply by the power factor to get my power consumption. 2. Need...

Measure the voltage and current of two phases (with requirements for accuracy) and multiply by the power factor to get my power consumption.

For an approximately balanced system such as a motor, measure the current and voltage of one phase, calculate the power, and multiply it by 3. The result is very close to the power consumption of the motor. The error of this measurement (measuring only one phase) is not too large. Compared with the method of measuring the current and multiplying it by 380V, the error is much smaller (directly multiplying it by 380V without considering the power factor at all).

maychang

hau30729 posted on 2020-3-25 10:05 I should have roughly understood what you said. 1. Measure the voltage and current of two phases (with requirements for accuracy) and then multiply by the power factor to get my power consumption. 2. Need...

"Measure the voltage and current of two phases (with requirements for accuracy) and multiply by the power factor to get my power consumption."

For induction motors, the power factor is not a constant. The power factor varies with the motor load. When the motor is fully loaded, the power factor is generally 0.8 or slightly higher (it may be lower, and small motors often have a lower power factor). When the motor is unloaded, the power factor may be only 0.3 or even lower.