What happens if the inverter is connected to a transformer at the back end?

乱世煮酒论天下

What happens if the inverter is connected to a transformer at the back end? [Copy link]

 

I have a question in mind. If the inverter output is connected to a transformer, can the function of the transformer be realized? I have a few questions to ask.
1. The voltage waveform of the inverter output is a PWM wave, and the current waveform is an approximate sawtooth sine wave. What effect will such a waveform have on the transformer when added to the primary side of the transformer?
2. The PWM voltage waveform of the inverter is an approximate sine waveform. The principle of the transformer requires a sinusoidal change in the magnetic flux change. It is mainly the current of the inverter that affects the transformer. Under such a large harmonic current waveform, the temperature rise of the transformer will be very high. How will this determine the choice of the transformer?
3. The three-phase output of the inverter should be star-connected according to the star-delta connection method, and is the power taken from one source point?

qwqwqw2088

What happens if the inverter is connected to a transformer? The inverter is usually used to control the motor.

There will be high-order harmonics in the later stage, which will cause great harm to the transformer and thus affect the motor.

The author's recent posts have focused on the issue of frequency converters. You should be exposed to many frequency converters at work. You can try to test it.

乱世煮酒论天下

qwqwqw2088 posted on 2024-2-28 07:57 What will happen if the inverter is connected to the transformer? The inverter is usually used to control the motor. The inverter will have high-order harmonics, which will be very dangerous to the transformer...

Thanks for your answer. I have just started to learn about inverters. I have been thinking about many problems recently and have seen many things that I don't understand. The motors are all 380V, and the inverters are 380, 660, 1140, etc. If it is a 660 or 1140 inverter, it cannot be directly connected to the motor. The method is to connect it to a step-down transformer for transfer and then connect it to the motor. There are some things in this process that I don't understand.

maychang

[What happens if the inverter is connected to a transformer at the back stage? ]

No matter what, it won't matter.

In addition to DC motors, AC motors, etc., the textbook "Electrical Machinery" always starts with transformers, which shows that transformers and AC motors are originally the same type.

乱世煮酒论天下

The reason is that the voltage output by the inverter is a PWM wave, not the sinusoidal wave of the power grid. Of course I know that the inverter can be connected to a transformer in the later stage. There are only a few situations such as overcurrent, short circuit, overheating and burning. The main question is how to use it normally?

maychang

乱世煮酒论天下Published on 2024-2-28 20:12 The reason is that the voltage output by the inverter is a PWM wave, not a sine wave of the power grid. Of course, I know that the inverter can be connected to a transformer at the later stage, no matter what...

Your AC motor is originally connected to the industrial frequency AC power grid. The voltage applied to the AC motor is a sine wave when connected to the power grid and a PWM wave when connected to the inverter. Is your motor burned out?

maychang

乱世煮酒论天下Published on 2024-2-28 20:12 The reason is that the voltage output by the inverter is a PWM wave, not a sine wave of the power grid. Of course, I know that the inverter can be connected to a transformer at the later stage, no matter what...

[Of course I know that the inverter can be connected to the transformer at the later stage, but there are only a few situations such as overcurrent, short circuit, overheating and burning.]

You [of course] know?

lanhua

The voltage waveform output by the inverter is a PWM wave, and the current waveform is an approximate sawtooth sine wave. What effect will this waveform have on the transformer when added to the primary side of the transformer? I will test it when I have the chance.

乱世煮酒论天下

The situations I mentioned are the worst cases, but in practice I have seen the inverter output connected to the transformer and used like this?

maychang

[The voltage waveform output by the inverter is a PWM wave, and the current waveform is an approximate sawtooth sine wave]

The current waveform in the inverter is related to the load type. If the inverter has a resistive load, the current waveform is the same as the voltage waveform, which is also a PWM wave. A sawtooth current will only be generated when it has a resistive-inductive load.

maychang

[The voltage waveform output by the inverter is a PWM wave, and the current waveform is an approximate sawtooth sine wave]

The inverter is connected to a transformer, and the current in the transformer may not necessarily be a sawtooth wave.

maychang

[It is mainly the current of the inverter that affects the transformer. Under such large harmonics in the current waveform, the temperature rise of the transformer will be very high]

As mentioned above, the current waveform in the inverter is determined by the load. In other words, the load affects the inverter rather than the inverter affecting the load. You have no reason to say that the current waveform in the transformer contains many harmonics, and there is no reason to say that the temperature rise of the transformer will be very high.

振动试验仪器

I have a question. The working process of the frequency converter can be said to be the conversion of AC into DC voltage, and then the DC is inverted into the AC voltage of the required frequency for output. The output should be a sine wave, and the inverted AC voltage is described in the information as a sine wave.

The motor uses three-phase alternating current, and the voltages of the three phase lines have a phase difference of 120 degrees, so that a rotating magnetic field can be formed inside the stator of the motor, driving the rotor to convert into mechanical energy.

The voltage waveform output by the inverter is a PWM wave, which is a high-frequency square wave. Is there a phase difference between square waves of different phases?

maychang

If you design a transformer with a rated voltage of 220V and insist on connecting it to 380V, of course the transformer will heat up seriously. However, if a transformer with a rated voltage of 380V is connected to 220V, the heat will be less than when it is used at 380V.

maychang

The voltage used by the electrified railways in the United States and China is the same, both are 25kV single-phase AC, but the United States uses 60Hz and China uses 50Hz. Electric locomotives produced in the United States cannot be used directly on Chinese railways - although the track gauge is the same, the transformer designed for 60Hz will heat up when used on 50Hz. Conversely, it doesn't matter much if the transformer designed for 50Hz is used on 60Hz. The reason why the 60Hz transformer heats up when used on 50Hz is because the frequency is reduced, not because [the temperature rise of the transformer will be very high under such large harmonics in the current waveform].

振动试验仪器

Most motors are three-phase motors. The key to the working of a motor is that the three-phase electricity forms a rotating magnetic field. The magnetic lines of force cut the short-circuit ring of the rotor to generate current. The current also cuts the magnetic lines of force to generate force to drive the rotor to move. Therefore, the key to three-phase electricity is that there is a phase difference between the three-phase electricity to form a rotating magnetic field. In this way, whether the three-phase electricity is a sine wave should not matter much.

Another thing is that the magnetic material of the motor is silicon steel sheet, which is suitable for low frequency. What is the conversion efficiency of the square wave motor of several hundred Hz?

The three-phase 50Hz 380V power supply has a maximum motor speed of 3000rpm for a two-pole motor. The design, production and corresponding national standards of the motor should be mature products formulated decades ago. The motor bearings have a maximum speed limit, so the frequency of the inverter output is limited.

振动试验仪器

The vibrating power amplifier is a class D amplifier of the switching tube. After filtering, the power amplifier tube outputs the audio voltage.

I once repaired an amplifier. After the repair, I couldn't find a filter, so I directly connected the 50K square wave signal of the amplifier to a high-power speaker. The input signal was a song, and the speaker produced a nice sound. It's OK without filtering.