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Can A Ac Drive Give Output Voltage More Than The Supply Voltage.


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#1 VST

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Posted 09 May 2008 - 10:33 AM

Dear Sir,
All inverters are specified that Output voltage can be max equal to input voltage(Ac to AC converter). Is their any inverter available, which can provide more output voltage than the supply voltage.
Regards,



#2 marke

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Posted 09 May 2008 - 11:07 AM

Hello vst

Welcome to the forum.

A VFD operates by rectifying the incoming supply voltage converting it to DCvoltage. This DC voltage is then chopped up to convert it back to AC voltage. The maximum output AC voltage is limited by the maximum DC voltage which is limited by the AC input voltage, so the only way to get a higher output voltage is to include a transformer at the output or the input.

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Mark

#3 jraef

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Posted 11 May 2008 - 03:26 AM

There are, however, small VFDs, (under 2kW) for use in the North American market which can take 120VAC input and produce a 240VAC max output, so you can run a 230V motor from a 120V supply. What they do is to insert a "voltage doubler" circuit on the DC side of the rectifier to increase the DC bus voltage before the transistors, so the AC that the transistors produce is at twice the voltage of the input. It is a somewhat inefficient process, but on small motors the losses are still low. That however becomes a serious problem on larger power systems, so you don't typically see anyone doing that.
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#4 marke

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Posted 11 May 2008 - 03:31 AM

Good point, I had forgotten about that, but this is pretty ugly on supply side harmonic currents, but small drives are not particularly good anyway.

Best regards,

#5 VST

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Posted 12 May 2008 - 04:41 AM

QUOTE (marke @ May 11 2008, 09:01 AM) <{POST_SNAPBACK}>
Good point, I had forgotten about that, but this is pretty ugly on supply side harmonic currents, but small drives are not particularly good anyway.

Best regards,



Dear Marke and Jraef,
Thank You very much for your answers .
As you said Input RMS ac is convereted in to Dc And then converted again in to Ac . I assumes some percenatge of losses during this conversion???.
And hence the out put should be less than the input RMS value. I want to know&nbsp

#6 marke

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Posted 12 May 2008 - 07:36 AM

Hello VST
Yes, you are correct, the output voltage for a pure clean sinewave current is lower than the input due to the voltage drops in the circuit, but a small amount of 3rd harmonic can be introduced into the waveform to lift the effective voltage.

The best output voltage will be achieved in inverters that do not use DC bus chokes or AC Line reactors. This causes the rectifier to act as a peak rectifier and results in the maximum DC voltage.
If a DC bus choke is used, the rectifier will act more like an averaging rectifier.

Good question.

Best regards,




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