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vikashkumar
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VFD

09/11/2015 12:52 AM

How exactly VFD frequency changes ie what happen inside vfd that it changes frequency?
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Tornado
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#1

Re: VFD

09/11/2015 2:06 AM

The firing rate of the SCRs or IGBTs that switch the current from the DC bus, done by electronic timing circuitry.

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vikashkumar
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#2
In reply to #1

Re: VFD

09/11/2015 2:34 AM

Hello Tornado,

suppose input voltage we are applying to VFD is 415V. 585VDC will be converted by rectifier and given to inverter.how much voltage exactly motor get to run?

Could you please explain more on how VFD changes frequency?
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IdeaSmith
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#3
In reply to #2

Re: VFD

09/11/2015 3:49 AM

I find Wikipedia pretty useful to answer questions like this!

https://en.wikipedia.org/wiki/Variable-frequency_drive

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Tornado
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#4
In reply to #2

Re: VFD

09/11/2015 3:59 AM

It's like turning a switch on/off so many times a second; how many times determines the frequency. The average output voltage depends on the proportion of on to off times.

If the VFD is operating at normal line frequency, its output voltage will match the original line voltage (at least approximately). For lower frequencies, the output voltage is typically lowered proportionately. This is an oversimplified description, and there may be variations.

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vikashkumar
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#5
In reply to #4

Re: VFD

09/11/2015 6:12 AM

Hello Tornado,

Whether firing angle is fixed for particular frequency? Like 18 degree firing angle for 45Hz and 45 degree firing angle for 20Hz? Could you please help me in regards that?

Regards

Vikash
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Tornado
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#6
In reply to #5

Re: VFD

09/11/2015 6:45 AM

No. All of a sudden you have leaped from excessively naïve questions to technically sophisticated ones, with seemingly no inkling of the difference. However, in this context a lesser firing angle probably leads to a larger average voltage per cycle, which matches the previously mentioned voltage-frequency proportionality. You need to draw on a graph what happens, and look at the area under the plotted wave form for each case.

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Fredski
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#7
In reply to #4

Re: VFD

09/11/2015 8:12 AM

you mean the light bulb in my room is actually turning on and off 120 times a second??
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Andy Germany
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#10
In reply to #2

Re: VFD

09/12/2015 3:50 AM

I think firstly, you need to educate yourself better on this subject as it appears you don't understand even the good and simple answers you have received....

Secondly, maybe you should explain EXACTLY why you are asking such questions. They appear to homework, if true, you should not be posting here, but simply talking to your tutor....

I have a feeling that you will not find what you are looking for here.....

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brich
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#11
In reply to #10

Re: VFD

09/12/2015 6:12 AM

took the words right out of my mouth!

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vikashkumar
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#12
In reply to #10

Re: VFD

09/13/2015 10:47 PM

Hello Andy

I think you have not gotten exactly, what i have asked, first do some study on that then only you will able to understand what i had asked.
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JRaef
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#8

Re: VFD

09/11/2015 11:16 AM

It's not about firing angles. A modern VFD is rectifying to DC at full peak voltage, then the pseudo-AC output is created via PWM (Pulse Width Modulation) of the DC out to the motor through transistors firing at a very high rate. Looking at a single phase, a transistor connected to the positive rail of the DC bus, and referenced to Zero voltas, is fired to vary the width of the pulses and creates a varying amount of RMS AC, both up and back down, to create a half sine wave. Although the pulses are DC, the fact that the motor circuit is inductive means that the inductive time constant makes the motor REACT to the varying RMS voltage AS IF it were an AC RMS curve. Then by using a different transistor on that same phase connected to the negative rail and referenced to zero, the other half of that sine wave is created. By varying the width of the pulses and the gaps between them, you can control the amount of RMS AC voltage getting to the motor.

Then by simply changing the timing of how OFTEN you change from positive to negative, you can control the frequency getting to the motor as well. Controlling both the voltage and the frequency together means you can maintain the same ratio of voltage and frequency, called the V/Hz ratio, that the motor was originally designed for. Maintaining that V/Hz ratio allows you to attain rated torque from the motor at any speed, so the motor can turn the load slower without over heating and/or losing torque.

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Legolaz
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#9
In reply to #8

Re: VFD

09/11/2015 1:48 PM

Snappy as usualGA.

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