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Open Roads Forum  >  Tech Issues

 > Converter to change modified sine wave to sine wave?

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Bit Bucket

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Posted: 07/01/12 10:01am Link  |  Print  |  Notify Moderator

Wayne Dohnal wrote:

MSW output voltage regulation:
The inverter has a DC-to-DC converter that steps the 12 volt input up to about 140 volts. The 140 volts is then switched positive and negative onto the AC output to form the output waveform. The DC-to-DC converter is unregulated. The output voltage is directly proportional to the input voltage. The device that controls the switching to make the AC waveform (microprocessor or PWM controller) has no control over the DC-to-DC converter's output voltage. It does, however, have full control over switching this DC voltage to make the AC output, and regulates the output at 120 volts RMS by controlling the width of the positive and negative pulses. When the inverter's DC input is at the top of it's input voltage range, the AC waveform has relatively tall and narrow + and - pulses. As the inverter's input voltage goes lower, the AC output pulses become shorter and wider, maintaining 120 volts RMS all the time. One side-effect of this is that microwave ovens, being peak voltage sensitive, have more cooking power when the battery powering the inverter is fully charged, and the cooking power goes down as the battery discharges, even though the RMS voltage is constant.


This is introducing the newer style of MSW inverters that operate like a variable frequency drive for motors.

Turning a high switching speed PWM waveform into a sinewave is a different problem than turning the square wave into one.

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Posted: 07/01/12 11:14am Link  |  Print  |  Notify Moderator

Same here, like the effect of a toroid transformer on a stepped psuedo sine wave :-)

Cedarhill

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Posted: 07/01/12 11:36am Link  |  Print  |  Notify Moderator

One of the first things any competent engineer has to do is abandon the idea that everything that is a fact has to be intuitive.

wa8yxm

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Posted: 07/01/12 03:29pm Link  |  Print  |  Notify Moderator

Will I share it: NO, because it's not practical.
However I will tell you the design considerations and problems.

What you need is a resonate filter, Not a low pass but a band pass with a very narrow band, (I know of an engineer who designed a band pass filter with a 1 HZ bandwidth, Not quite that tight please)

The problem is: The filter's resonate frequency is determined IN PART by the load.. So it next to impossible to design one for use in this case.

But the key thing is resonance. I do not know if a saturated core transformer (Such as Magnetek used in the 6300) would do it.


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Bit Bucket

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Posted: 07/01/12 07:11pm Link  |  Print  |  Notify Moderator

wa8yxm wrote:

So it next to impossible to design one for use in this case.


...this is what I suspect.

Cedarhill

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Posted: 07/02/12 06:29am Link  |  Print  |  Notify Moderator

You don't need a bandpass filter if the lowest frequency in the spectrum is the one you want to keep.

wa8yxm

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Posted: 07/02/12 10:30am Link  |  Print  |  Notify Moderator

One of the differences between a low pass, and a band pass, is that if you design a band pass filter with a high enough "Q" (Narrow bandwidth) it will ring like a bell.. And as you know if you strike a bell (A real bell) with a hammer the sound can continue for quite some time (Depending on the size and quality of the bell).

Now imagine if you kept hammering on that bell,

A low pass filter.. Does not ring like that (There may be a tiny amount of ring but not enough to make a difference) So you need a nice tight bandpass, very high Q, in short,, A bell.

Wayne Dohnal

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Posted: 07/02/12 03:09pm Link  |  Print  |  Notify Moderator

I found a source that calculated the power content of the harmonics in a square wave at 19%. The THD of a square wave is 45%, and the THD of 3-level MSW (the common one MSW type) is ~24%. I'd hazard an intuitive guess that the power content of the MSW harmonics is in the 10% ballpark, meaning a good filter would result in at least a 10% power loss. Aside from all the other issues, said filter would have to dissipate a pretty good chunk of heat.


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wa8yxm

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Posted: 07/02/12 04:16pm Link  |  Print  |  Notify Moderator

True Wayne, True, as I said, it could be done, it would NOT be practical.

Cedarhill

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Posted: 07/02/12 08:24pm Link  |  Print  |  Notify Moderator

wa8yxm wrote:

One of the differences between a low pass, and a band pass, is that if you design a band pass filter with a high enough "Q" (Narrow bandwidth) it will ring like a bell.. And as you know if you strike a bell (A real bell) with a hammer the sound can continue for quite some time (Depending on the size and quality of the bell).

Now imagine if you kept hammering on that bell,

A low pass filter.. Does not ring like that (There may be a tiny amount of ring but not enough to make a difference) So you need a nice tight bandpass, very high Q, in short,, A bell.


Take a hint from someone who has actually done this kind of work for a living. You need a little more knowledge and a little less technical sounding lingo. This is not kHz or mHz circuits we are talking about. This is 60 hertz.

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