350Hz Variac

[M0FYA Andy]

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

[Ed_Dinning]

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

[M0FYA Andy]

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

[Ed_Dinning]

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

[emeritus]

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.

[kalee20]

Originally Posted by M0FYA Andy

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).

Generally no - I've used a standard 50/60Hz Variac at 400Hz, no problems. It's just a lot bigger than it needs to be at the voltage and current ratings.

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).

[M0FYA Andy]

Many thanks for the replies, I'm feeling optimistic that it should be succesful.

Andy

[M0FYA Andy]

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.

[kalee20]

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.

[M0FYA Andy]

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