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Single Phase VFD

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barry995

New Member
I have been working on a 1-Phase VFD design that I think is ironed and I welcome any coments.

I will be using a simple light dimmer as my POT to vary the voltage and I include a sense line to my Frequency generator to vary the frequency. I decided to use a 555 Timer instead of a sq wave generator to generate a frequency signal, then the signal goes into a D flip flop to ensure 50% duty cycle (this however divides my frequency by 2) so the top end of my frequency range should be 120Hz. I then drive 2 pairs of Power MOSFETs. I should end up with variable voltage and frequency.
 

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Roff

Well-Known Member
I can't read your schematic very well, but it appears you don't understand Ohm's law.
The current through a resistor creates a voltage drop across that resistor equal to the product of the current and the resistance.

V=I*R

The most glaring example is the pot in series with the H-bridge. Another is the voltage divider in front of the 7808 (although this might work).
The other problem is that your H-bridge won't work as designed. There are several recent threads here detailing the shortcomings of your H-bridge design.

EDIT: I missed the part about the light dimmer, but I don't think I have seen one that works on DC, unless it is simply a pot, in which case my comments about Ohm's law still stand.

What are you going to drive with your VFD?
 
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Hero999

Banned
I don't see how a light dimmer will work any way.

For lower frequencies you need to reduce the duty cycle to provent core saturation of the motor; also the duty cycle should never reach 50% as the core saturation will also occur.
 

barry995

New Member
I guess I should clarify a few things. I am using a sold state rectifier not designing my own and the dimmer I have is an old style pot, not a modern PWM that has zero crossing detection etc. I redrew they way I want to connect it in my circuit to vary the DC voltage supplied to my output FETs, hopefully it is a little clearer. Thanks Hero for catching my duty cycle error, I had it in my head that I wanted a 50% duty cycle. I am actually getting a severely reduced duty cycle out of my 555 timer at low frequency. I think if i eliminate the flip-flop and with some tuning I should be there.
 

Sceadwian

Banned
Does anyone else smell smoke?
 

Hero999

Banned
barry995 said:
I guess I should clarify a few things. I am using a sold state rectifier
What sort of rectifier are you using then?

Valve?
Mechanical commutator?

I hope it's a solid state rectifier with schottky or silicone diodes.

not designing my own and the dimmer I have is an old style pot, not a modern PWM that has zero crossing detection etc.
That dimmer won't work on DC, it's designed for AC!

I redrew they way I want to connect it in my circuit to vary the DC voltage supplied to my output FETs, hopefully it is a little clearer.
Did you actually look at the datasheets for the components you're planning you use?

Look at the datasheet for the LM78XX series regulators - they will be cooked on 160V DC!

Thanks Hero for catching my duty cycle error, I had it in my head that I wanted a 50% duty cycle.
Your motor is designed to work on 50Hz or 60Hz (we would know the frequency if you filled your location in) AC with an RMS voltage of 120V. Your square wave will produce an RMS of 160V which is far too high. Excessive currents will flow in your motor leading to core saturation and eventually smoke, fire and a dead motor!

I am actually getting a severely reduced duty cycle out of my 555 timer at low frequency. I think if i eliminate the flip-flop and with some tuning I should be there.
The duty cycle needs to be reduced even more at lower frequencies because your AC motor is not designed to be run at anything lower than the rated frequency therefore more current will flow at lower frequencies; to prevent this you need to reduce the duty cycle considerably. This is done by pulse position modulation; you keep the pulse lengh equal but increase the time between pulses.

The chances are your single phase motor won't work with a variable frequency dirve because the starting mechinism is probably frequency dependent. You might have some luck starting it at full speed then reducing it down to a lower speed and maybe you could go as slow as half the speed but no lower.

Why don't you buy a three phase motor and a variable speed drive that will work from a single phase supply? No offence intended but judging by your past posts you don't have enought experiance to attempt a project like this.
 
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