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frequency converter
5

frequency converter

frequency converter

(OP)
why is it that the frequency converter input voltage is not the same for the output voltage and the reading is more higher than the input

RE: frequency converter

I respectfully submit that you have not furnished enough detail to allow discussion other than guesswork and speculation.  

RE: frequency converter

3
Nonetheless, in most cases like this it is because you do not have a meter capable of properly reading the harmonic rich output of a PWM inverter, which is the likely design of your frequency converter. The result is that your readings on the output are essentially meaningless. It takes a very expensive and sophisticated meter to properly interpret that signal, something most people do not possess.
 

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: frequency converter

A frequency converter can, and often does, change the output voltage as well as the output frequency. There doesn't need to be any relationship whatsoever between the input and output voltage or frequency.

RE: frequency converter

sibeen, I think you might be just a little to generous with your statement about input and output voltages.

For example, with the exception of VFD's with voltage doubler front ends, the effective PWM equivalent voltage that is sent to the motor (output of drive) cannot be higher than the input RMS voltage.

That is because the output equivalent voltage is limited by the DC bus voltage which, in turn, is limited by the peak value of the incoming AC voltage.  So, for example, if you put 460VAC into a VFD, the maximum output to the motor will not exceed 460V.  If, on the other hand, you supply a drive with 502VAC, you can send up to 502V to the motor.

RE: frequency converter

Are we assuming that this a VFD and not a static frequency converter, eg 50/60Hz to 400Hz or similar? Wayne's reply is on the money, although jraef's speculative answer might well be correct.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: frequency converter

DickDV, many years ago I put into service shedloads of frequency converters where the output voltage was higher than the input.

Putting a step-up transformer before the rectifier wasn't considered too difficult a challenge :)

RE: frequency converter

sibeen, are you now considering the transformer part of the inverter or have you just raised the inverter input voltage with the transformer?

And, yes, it seems we are assuming the inverter is an electronics-based probably PWM design.  Certainly, a mechanical frequency converter has no voltage limitations either up or down.

RE: frequency converter

(OP)
thanks guys but how can this happen that the output was higher than the input voltage. what will happen to the motor its true that I can vary the speed of motor but what about the voltage

RE: frequency converter

From your response, we can now assume you were indeed referring to a motor controller.

If you supply more details about exactly what you are working on or planning to work on, maybe we can be more specific as to how it is going to work in your situation.

If this is a general question on how inverter drives function, please indicate that as well, but this is not a Learning Center for the curios amateur, the primary function of this forum is to assist working engineers with getting over specific problems. If you are completely unaware of the functioning of an inverter drive, I suggest a thorough search of learning resources in whatever your native language is as a place to start, then if you are working on something and get stumped, come back and ask for help.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: frequency converter

(OP)
yes jraef Im amateur in my field and also my first job Im wondering why because our motor is 690V and the output of the controller is 970 but tne motor is still fuctioning. can you give me a resourses to read or website to search so i can study why

RE: frequency converter

2
The voltage you mention is the DC link voltage. Are you actually measuring it with a voltmeter? Or is it what you see on the display?

The actual motor voltage is composed of pulses with varying width that are 'cut' out of this DC voltage by the IGBTs in the drive. If you filter high frequency components away from this pulse train (Pulse Width Modulated, hence PWM), you will get the voltage representing the motor voltage, which is somewhere between zero and mains voltage, depending on speed an control mode. In scalar control, the voltage is roughly proportional to motor frequency and in vector control mode, it is usually lower than expected because vector control usually optimizes mis fully loaded.

If you measure motor voltage without any filter, you will see the sum of fundamental an all switching components and that usually means that you see a lot more voltage than mains voltage.

That does not mean that the inverter outputs more than mains voltage - only that you are using the wrong measurement method and also not interpreting the result correctly.

You are starting a long and fascinating journey. Be prepared to meet many more things that you don't understand at first. You will understand as you proceed.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: frequency converter

(OP)
thanks Skogsgurra for your sharing your wonderful knowledge you gave me. yes its clear to me now when i measure the voltage without filter it is more higher and and your right I'd see only in the display in the converter thanks again.

RE: frequency converter

(OP)
hi Skogsgurra can you please give additional website to improve my knowledge in electrical because Im working in ship  

RE: frequency converter

When you get back to shore this is a very good reference book on the subject. It's been recently revised too; my copy is somewhat older.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: frequency converter

Yes Scotty. That is a very good book. Graphically and typographically pleasing as well.

When you have digested the Control Technique's handbook, I recommend Werner Leonhard's 'Control of electrical drives' from Springer. It is little more demanding, but does explain things that Control Techniques doesn't go deeply into.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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