Are cheap VFD's typically noisier?

I just purchased a new cheap Chinese VFD to try out. I also have a higher quality Japanese VFD. I compared both VFD's running the same 3hp 60hz 1750 rpm motor. The Japanese VFD ran the motor very smoothly and quiet, with only minor VFD noise from the motor. The cheap Chinese VFD ran the motor with torsional vibration at most speeds, but was pretty smooth at full speed. It caused more VFD motor noise at all speeds and was significantly noisier at some speeds. The motor being tested was sitting on the floor in a quiet area. I suspect that someone hooking up the cheap VFD to a motor on a heavy machine in a noisy area would not be able to notice the vibration or motor noise. The cheap VFD has fewer parameters to play with, like carrier frequency. The noise isn't a big deal to me, but I don't care for the vibration. It is a torsional vibration that feels like power interruptions, only not as severe. Is this noise and vibration typical of all cheap VFD's?

bill1608 said:

The cheap VFD has fewer parameters to play with, like carrier frequency. The noise isn't a big deal to me, but I don't care for the vibration. It is a torsional vibration that feels like power interruptions, only not as severe. Is this noise and vibration typical of all cheap VFD's?

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No way to change carrier frequency?
You probably have one of those boxes that have a foot print less than the size of a slice of bread. Why
are they trying to make those things so small? They have that box tuned to sink a certain amount of heat.
A higher carrier frequency will generate more heat.

Reason:
The transition times from high-to-low or low-to-high of V and I are when the power is generated.
More transitions in a unit of time create more heat. Also if the switching devices are relatively slow the
V and I waveforms are in the transition zones longer, contributing to more heat.

Vibration? Using a cheapo import VFD at maximum ratings is something I wouldn't do. They lie and push limits.
Harmonics not being filtered most probably. Weak software algorithms or parts being pushed to perform in-line math.

The positive aspect of these small things is that it is amazing that they work at all.
Without more info I ask if both VFD's are the same size and Hp rating.

I'm not sure, I can't hear the "VFD motor noise" over the sound of the jet-turbine fan on my middle of the road Hitachi VFD.

Hitachi is quality... especially compared to "Huanyang" (or whatever name is stamped on them these days).

A whine is typical of the carrier frequency. Torsional vibration is more typical of a "non-fatal" fault with one of the phases, possibly one of the 6 IGBTs is bad, but OPEN, not shorted.. Sometimes the motor will sound like it wants to jump off the mountings.

Problem with the cheapo chinese VFDs is that a lot of them seem to have been stolen from the "reject" pile, and sold out the side door of the factory. So the one you get may work, with a minor fault, or it may flat be bad out of the box. Some of them are apparently perfectly fine...maybe those went out the correct dock door...... You don't always get what you pay for, but its pretty common to NOT get what you did NOT pay for......

JST is right.

Many cheap ones are fine.
Some are not.

You cannot know in advance.

But .. I use (sell) good chinese stuff, and have very good results.

These are not the cheapest, but they are very cost-effective.
No VFDs, all my stuff is ac brushless servo.

I played with the VFD some more. I guess I am not worried about any noise at this point, I am more concerned about the lack of smoothness. It is not as smooth running as my good VFD, kind of jerky at some speeds. I think the frequency which best illustrates it in a sound file is at 20hz. I have an MP3 file, but I don't think I can attach it here.

JST said:

You don't always get what you pay for, but its pretty common to NOT get what you did NOT pay for......

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That is some fine double talk. George Orwell would like it.

bill1608 said:

I played with the VFD some more. I guess I am not worried about any noise at this point, I am more concerned about the lack of smoothness. It is not as smooth running as my good VFD, kind of jerky at some speeds. I think the frequency which best illustrates it in a sound file is at 20hz.

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Different VFD's do not use the same parts/algorithms. So your comparison between two units which you have not given any specifications
is like saying this orange is sweeter than another orange from a totally different tree. The cheap VFD specification is most probably
a reflection of the people and country where it was produced.

I would try the changing the base carrier frequency at a speed that you find "jerky".
A little playing around with it and you might find the "sweet spot" that is not too bad for a range of speeds.
Not totally acceptable as the specification is concerned, but almost barely usable like most Chinese junk.

I would not waste any more time with it, save the igbt block though, and the 400 or 450volt capacitors. Some other parts are worth saving, if you're able to desolder them without thinking twice about it.

If one of the igbts has failed open, the motor might run, but you will be sending a net dc current through the motor. I am surprised if it would work at all. i can't tell you off the top of my head if it would still start in the same direction each time.


Jerkyness and vibration at something other than the motor frequency means there is something wrong with the pwm generators and the code, this could manifest as a fried motor sometime in the future.

Many of those darn things do not HAVE an "IGBT block" They have individual IGBTs each set with it's own little insulator and a screw through the "mounting hole". Great construction for a HIFI amplifier in the 1970s, not so good for a VFD in 2016....

"Superunknown", IIRC, posted pics of one he took apart to fix a bad trace. (maybe it was a video). Shows the above construction.... It was a Huanyang, I believe, or the same thing without the name on it..

http://www.practicalmachinist.com/v...around-troubleshooting-finally-repair-268342/

The jerkiness is most probably due to a control loop that is constantly banging back and forth.

Why would you buy a VFD from a Communist country which murdered close to 80 million of their people.
There is no innovation just copy-make-cheap mentality. A land where only the generals get rich.