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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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1st Aug 2022, 11:31 am | #1 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,511
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350Hz Variac
I have just bought, but not yet received, a Variac rated at 120v 6A '350 Cycles Min'.
I am hoping to use this to drop the output of a 115v 1600Hz rotary converter to 80v. I'm gambling on it being happy at 1600Hz, but does anyone have a view? I'm surmising it was intended for use at 400Hz. Andy |
1st Aug 2022, 4:14 pm | #2 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,194
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Re: 350Hz Variac
Hi Andy, probably from an aircraft 400Hz system, these were also usually 115V.
If you only need a fixed voltage ratio then it is quite simple to wind a small transformer to do this that will b e pretty compact Ed |
1st Aug 2022, 6:12 pm | #3 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,511
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Re: 350Hz Variac
Ed,
Agreed, but I've already bought this Variac! It will have the advantage of being able to set the 80v accurately. But is there any problem using a transformer at four times the frequency it was designed for which I haven't thought of? (Clearly operating at a lower frequency than designed for would be a no-no). Andy |
1st Aug 2022, 6:24 pm | #4 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,194
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Re: 350Hz Variac
Hi Andy
A much smaller core can be used and the T/V figure is also reduced. Ideally thinner lams should be used to reduce the iron losses, but if a reasonable size will perform without problems Ed |
1st Aug 2022, 6:59 pm | #5 |
Dekatron
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
Posts: 5,339
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Re: 350Hz Variac
My early 1940's edition of Sandretto's book on aircraft radio mentions that, as well as 400 Hz, AC frequencies of over 1000Hz were in use at that time. In some cases the frequency was a function of engine speed. Possibly this accounts for the "minimum" frequency marking.
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1st Aug 2022, 8:26 pm | #6 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,082
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Re: 350Hz Variac
Quote:
Going down in frequency - it's not necessarily a no-no, your 350Hz Variac should be quite happy at 50Hz if you reduce the input voltage by a factor of 7 (you'd probably get away with a factor of 5, as 350/400Hz transformers are rarely limited by core saturation). |
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1st Aug 2022, 10:47 pm | #7 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,511
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Re: 350Hz Variac
Many thanks for the replies, I'm feeling optimistic that it should be succesful.
Andy |
2nd Aug 2022, 10:17 am | #8 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,511
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Re: 350Hz Variac
kalee20, thinking further about your comment regarding operating at lower frequency if the voltage is reduced, I guess this makes sense as the turns per volt will be brought back to the design case.
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2nd Aug 2022, 11:46 am | #9 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,082
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Re: 350Hz Variac
No, the number of turns is constant (unless you start unwinding it!) so reducing the voltage definitely changes the turns per volt.
What does stay constant is the volt x time product, where volts is the average voltage over a half-cycle, and time is the period of that half-cycle. Reducing frequency increases the period, so volts must be reduced to keep the product the same. As long as this stays at the design value, all is good. |
2nd Aug 2022, 10:11 pm | #10 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,511
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Re: 350Hz Variac
OK, I've found a manual online for this Variac - mine is the M5.
This says it is designed for use up to 1200Hz, so use at 1600Hz isn't too excessive at all. Andy |