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Home > Archive > Electrical Engineering > April 2006 > Home made VFD questions/help
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Home made VFD questions/help
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| I have a mini-lathe with an induction motor 0.8HP, it has a bulge on top
housing a capacitor.
Starting cap? Running cap?
I like to home-build a VFD as simple as possible. This is just an hobby,
nothing fancy but the approach must be sound.
I have skills in switching regulators, PFC etc. but not a good grasp on the
motor technology and behavior when driven by a synthesized waveform.
I like to receive, from the experts on this NG, comments and suggestions on
the validity of my basic design which is as described:
------------------------------
Bridge Rectify 120V to ~160V Peak rectifier with sufficiently large cap.
5000uF @ 250V
Invert by means of an "H" bridge with max duty cycle of 67% to produce the
same VxSec as a 120V sine.
Average Voltage (VxSec/0.5T) is what determines peak flux.
I believe this is what the Motor "wants".
From this point either increase frequency to 90-120Hz at max duty cycle of
63% or reduce linearly frequency and Duty Cycle down to perhaps 2Hz and
2.2% Duty Cycle.
The pulse duration is constant at about 5.6mS in the 2-to-60 Hz region
giving a constant V/Hz
and will fall to 2.8mS at 120Hz maintaining a 67% duty cycle giving fixed V.
My questions:
Is the basic plan safe for the Motor?
Is a rectangular wave OK, bipolar pulses at constant amplitude variable Duty
cycle?
Have I left out something? What?
All figures are back of the envelope, at this point I am looking for a proof
of concept.
Thank you for your help.
Mauro Gaetano
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| MG wrote:
> I have a mini-lathe with an induction motor 0.8HP ...
>
> I like to home-build a VFD as simple as possible.
> From this point either increase frequency to 90-120Hz at max duty cycle of
> 63% ...
> My questions:
> Is the basic plan safe for the Motor?
Operating the motor and connected load above its intended speed
(i.e., above 60 Hz) invites centrifugal failure.
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| JohnR66 2006-04-28, 12:21 am |
| "Roby" <roby@no-address.net> wrote in message
news:OtudnZTP46aaXc3ZnZ2dnUVZ_tOdnZ2d@adelphia.com...
> MG wrote:
>
>
>
>
> Operating the motor and connected load above its intended speed
> (i.e., above 60 Hz) invites centrifugal failure.
Not sure this would work with a single phase motor. The capacitor was
selected for the 60Hz supply for one.
The capacitor gives a phase shift that helps the motor start and increases
efficiency while runing.
John
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| electrician@electrician2.com 2006-04-28, 3:21 am |
| According to my recent call to allan bradley there ain't no single
phase VFD's.
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"MG" <nomail@please.net> wrote in message
news:cRW3g.1408$Vn.577@newsread4.news.pas.earthlink.net...
>I have a mini-lathe with an induction motor 0.8HP, it has a bulge on top
>housing a capacitor.
> Starting cap? Running cap?
>
> I like to home-build a VFD as simple as possible. This is just an hobby,
> nothing fancy but the approach must be sound.
> I have skills in switching regulators, PFC etc. but not a good grasp on
> the motor technology and behavior when driven by a synthesized waveform.
>
> I like to receive, from the experts on this NG, comments and suggestions
> on the validity of my basic design which is as described:
> ------------------------------
> Bridge Rectify 120V to ~160V Peak rectifier with sufficiently large cap.
> 5000uF @ 250V
>
> Invert by means of an "H" bridge with max duty cycle of 67% to produce the
> same VxSec as a 120V sine.
> Average Voltage (VxSec/0.5T) is what determines peak flux.
> I believe this is what the Motor "wants".
>
> From this point either increase frequency to 90-120Hz at max duty cycle of
> 63% or reduce linearly frequency and Duty Cycle down to perhaps 2Hz and
> 2.2% Duty Cycle.
>
> The pulse duration is constant at about 5.6mS in the 2-to-60 Hz region
> giving a constant V/Hz
> and will fall to 2.8mS at 120Hz maintaining a 67% duty cycle giving fixed
> V.
>
> My questions:
> Is the basic plan safe for the Motor?
> Is a rectangular wave OK, bipolar pulses at constant amplitude variable
> Duty cycle?
> Have I left out something? What?
>
> All figures are back of the envelope, at this point I am looking for a
> proof of concept.
>
> Thank you for your help.
>
> Mauro Gaetano
>
Thanks to all for setting me straight. Bad idea, I guess I am stuck with
changing belt slot.
Mauro
>
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