Hello,
I want to control the speed of a woodcutter that is equipped with an asynchronous three-phase motor 400V 6000 RPM 6KW.
From the searches I came to the conclusion that a VFD would successfully solve my problem so I decided to build one.
**broken link removed** seems to me the simplest solution for adjusting the frequency - I tested this circuit and obtained a three-phase sinusoidal signal.
This is the control circuit
View attachment 151886
This is the gate driver circuit
View attachment 151887
Please help me to better understand how the control circuit works and tell me if the electronic circuit is correct.
Basically, I have to get 6 Hi / Lo signals that will command the three-phase IGBT bridge.
Thank you
This circuit creates the signals. But to drive a motor you need a power stage. 6 IGBTs and 6 gate drivers. Or you can just buy a compact IPM where all of them is inside. You can then attach this signal generator to your IPM.
google for intelligent power modules.
I'm already stumbling upon the motor specification. What is it? 6000 rpm at 100 Hz? Industry standard motors are specified for either 50 or 60 Hz.
Please help me to better understand how the control circuit works
My gate driver circuit is part of the power stage - i have 3x gate drivers for 3x IGBT half bridges
I have 3 IGBT modules for the motor driver https://www.semikron.com/dl/service-support/downloads/download/semikron-datasheet-skm50gb12t4-22892000/
Simulation showing basic operation of SPWM through H-bridge. Each op amp contains a half-bridge in the output stage. Filtering is through low-pass LC second-order type.
Fundamental frequency =100 Hz. The 1700 Hz carrier is slow so as to allow all waveforms to be easily seen in scope traces. (Your carrier is a much higher frequency.)
The load calculates as 80 ohms. 400VAC has peaks of 560V. To make it easy I gave it 1/3 of your 6 kW spec.
View attachment 151912
Each op amp is fed 100 Hz sine wave, at inverted polarity from the other.
My inductor L value is high due to the slow carrier frequency. You may choose a lesser L value. (The aim is so it passes your desired Ampere flow.) Select C so it provides power factor correction (in addition to supplementing a filtering role). C usually carries almost as much current as the load.
At reduced sine frequencies, it's possible the motor draws greater current (which is typical behavior). It may be necessary to experiment with L & C values, to find a suitable combination for your system.
Hi,
If you adjust frequency you need to adjust voltage, too.
This means at 100Hz you need about doubled voltage.
With async motors this is not that criticle as with synchronous motors or BLDC motors.
Klaus
Ok. It is possible to increase the frequency and get 6000 rpm (100Hz) for a 2 pole motor. But notice that if you increase the frequency of the motor over the specification (50Hz/60Hz) , the motor will degrade power. Very rough estimation at 6000rpm a 6kW power may decrease to 3kW. But it is not linear. After some certain frequency, it may be that there is a saturation and the motor just stalls.
I have a 3 phase asynchronous motor running @20.000 rpm (333Hz). 0.25kW. (But the case looks like a 0.75kW motor). I fed it through a standard mitsubishi inverter capable of delivering 1000Hz.
You have a chance up to 6000rpm with a standart motor, but with remarkable power degradation.
After 10.000rpm the power decreases too much or the motor even stalls and you need to change the design of the motor (coils, stator etc.)
The available torque of an asynchronous motor will reduce roughly with the square of applied voltage. Running a motor with double frequency and same voltage gives about 1/4 of the nominal torque and thus 1/2 the mechanical power.
Related to post #3, you didn't tell if 400V operation uses star or delta connection.
Obviously it can be connected star or delta. The question is, which configuration is used for 400V, full power, in other words, is it a 230/400V or a 400/690V motor?
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I'm sure that it's cheaper to buy a Chinese VFD than to start developing your own. The post #1 control circuit makes no sense for me, by the way.
I understand it will drastically decrease the power to about 100Hz. How do we make this VFD as effective as possible? Would it be a solution to change the 6KW motor with an 11KW motor to get the 6KW at the top speed of 6000rpm?
Using a 11kW motor @6000 rpm is maybe not the most effective solution but it is a solution. If the motor has a belt pulley, you can try to change the speed mechanically. (by adjusting the diameter of the pulley).
Frequency inverters are really cheap. They cost for 7.5kW-11kW between 150-350 USD in aliexpress.com. Most can drive up to 400Hz.
https://www.alibaba.com/product-detail/High-performance-AC-drive-three-phase_60734357792.html?spm=a2700.7724838.2017115.44.77da4d33E3o2ah&s=p
Your motor's windings dictate whether you shall wire the power supplies as a delta or as a wye. Create the full 3-phase simulation to see if any short circuits occur.
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