Transformer: Excitation Frequency and Turns Ratio

[MadHattah]

Another transformer question: if I have an oil-filled transformer coil and I calculate the turns ratio with voltage in / voltage out at 60 Hz, why, when I increase the frequency to 23 kHz would the turns ratio appear to change? Is it because of the inductance of the primary coil? My boss' question, but I didn't have a good answer for him.

 

[berkeman]

Another transformer question: if I have an oil-filled transformer coil and I calculate the turns ratio with voltage in / voltage out at 60 Hz, why, when I increase the frequency to 23 kHz would the turns ratio appear to change? Is it because of the inductance of the primary coil? My boss' question, but I didn't have a good answer for him.

Welcome to the PF. I think we'll need a little clarification on your question. If you start at 60Hz, it is a decrease to 23Hz, not an increase. In the general case, the turns ratio does not depend on the excitation frequency. The total number of turns may depend on frequency, depending on the type of transformer and its use. Also,l 50/60Hz transformers are not generally made to be used at other frequencies -- they are optimized for 50/60Hz, and will experience large losses at frequencies much away from 50/60Hz.

 

[MadHattah]

Excuse me, I should have said inductive reactance. I think I've answered my own question. The reactance of an inductor increases with frequency, so it's the reactance of the primary coil itself that lowers the voltage in, therefore, the voltage out would reduce by the turns ratio. Thanks for your help! :)

 

[BackEMF]

...If you start at 60Hz, it is a decrease to 23Hz, not an increase...

Funny, that's what I read too, but he actually did say 23 kHz - "kilohertz".

 

[berkeman]

Funny, that's what I read too, but he actually did say 23 kHz - "kilohertz".

Oh wow, you're right! Glad I wasn't the only one.

MadHattah, do NOT try to run a 60Hz transformer up in frequency. There's a good chance it will smoke. Don't ask me how I know that. The losses are huge when you get above the intended operating frequency of 50/60Hz transformers.

 

[MadHattah]

Hey, wow, I thought my posts were just disappearing. Anyway, the transformer we're using is from a car, and is usually operated in the 200 - 1000 Hz range, apparently we are using it from 600 to 1000 Hz not 23 kHz (that's a different unit with a different transformer). It seems to me that the transformer is optimized to be used mostly at the lower end of its range with occasional jumps to higher frequencies (i.e. accelerating in your car), but we're using them at the higher range occasionally jumping higher than the single kHz it's designed for. But even so, the confusion comes from the fact that at low frequency we get a certain turns ratio based on voltage out over voltage in, but at higher frequencies the ratio changes. I really think that it's because of the reactance of the primary coil at the higher frequency dropping voltage, maybe? So there's less voltage available to induce magnetic flux?

 

[berkeman]

At higher frequencies, you may be getting less AC voltage out due to core losses being higher. At lower frequencies, you could get less AC voltage out because of core saturation.