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#127172 05/09/01 02:32 PM
Joined: Oct 2000
Posts: 4,107
Likes: 3
From: "Francis" <>

Am a new user and thought this would be a good place to find the answer to this question. All of our calculations are for 60 Hz. What do you do if using other than 60 Hz, such as 25 or 400, or for VFD's where the frequency is variable? How does this affect ampacity?

Thanks for your thoughts.

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How many amps are we talking about on these conductors anyway?

The larger cmil conductors will definitely need derated at higher frequencies.

Joined: Oct 2000
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Broom Pusher and
What Dspark is referring to is an increase in Skin Effect with larger conductors at higher frequencies [Dspark, I'm just amplifying on your message, not being arrogant!].

What types of calcualtions are you intending to modify with different Hz?? I'm still trying to figure out how to do Voltage drop calcs [X and R figures] at 400 Hz and make them + 99.99% accurate!!

Anyhow, here's the deal:
If you lower the frequency, Inductive Reactance [XL] becomes the "dominate" type of Reactance.

If you increase the frequency, Capacitive Reactance [XC] becomes the "dominate" type of Reactance.

25 Hz has low skin effect problems on large conductors, but requires very large cores and coils for Inductive devices [AC motors, Transformers, Ballasts, etc.]. Line charging is much lower at 25 Hz than at 60 Hz, so there's less charge Capacitance between circuit conductors [a GFCI could be trip rated at maybe 1 or 2 milliamps without nuiscance trips].

25 Hz would have a totally different X vs R calculation for voltage drops.

The change in Hz would effect the overall KVA of a certain motor during many points along the HP output curves, and also non-loaded.

25 Hz would run a typical 2 pole Induction motor at maximum speed of 1500 RPMs unloaded [slip would bring it to 1425 RPMs for a 5% slip off synch Hz].

400 Hz would be exactly the opposite!
Skin effect will be dramatically increased on large conductors.
Cores and coils for Induction machines [motors, transformers, ballasts, etc.] can be made smaller, but primary / secondary separation increases [so does coupled RFI/EMI]. Inductive machine become more efficient with higher Hz.

Line charging increases big time as Hz increases, so GFCI [and the like] would need higher trip ratings [+20 milliamps and such].

Capacitive Reactance will become a factor at some points on a motor's power curves - especially at no-load [idle]. That would throw power factors into the Leading type areas, rather than the more familiar lagging type.

AC Induction motors would have a great max speed at 400 Hz! The same 2 pole AC Induction motor [let's use a Squirrel cage rotor type motor] that peaked at 1500 RPMs idle / 1425 RPMs full load at 25 Hz, would be 24000 RPMs idle / 22800 RPMs full load [5% slip].

Drawn amperage per KVA as regarding to KW will be common on all Hz. If you have a 1 KVA load connected to 100 VAC, it's going to draw the same 10 amps as it would at any other frequency.
The key is how much KVAR per KW is bouncing around the circuit.
Lower Hz, higher XL KVARs [normally],
Higher Hz, higher XC KVARs [normally],
All still delivering the same amount of KW [Horsepower, if driving motors].

If you are trying to adjust a voltage drop calculation - one which uses Reactance [X] and Resistance [R] instead of straight Resistance - you will need to adjust "Q" according to the Hz.

If I can somehow stumble across an existing formula, I'll add it to this thread.

Hope this make sense!!

Scott SET.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: May 2001
Posts: 176
This was general question that came up on our Forum with VFD's in mind, particularly the difference between large and small drives. It is understandable that sizing cables for the smaller drives, say up to maybe 75-100Hp may not cause any problem based on all of the known 60 Hz calculations and using NEC Tables 310. After reading the post on skin effect, I then realized there maybe more to this than first meets the eye. At the present time, I don't have an application in mind, although in the past we estimated the design of a project using large motors with VFD's. And there would be a tendency to disregard or overlook the effects of increased Frequency on the larger cables. Another use is in older main frame computers that operate at 400 Hz.
Thanks to all who helped me (the dummy) understand this subject a little better.
And no, I don't have any handy formulas for this type calculation. My first contact would be manufacturers of the VFD and the proposed cable for their input. I know Allen-Bradley and Belden have looked into it. And if someone does run across those majic formulas, please post.
I hope this may be of some help to other visitors or regulars to this site who may need this type info in the future. This is a very helpful site.
Thanks again.

I'm just amplifying on your message, not being arrogant!
What you did was fine; I couldn't possibly take offense at it.

I made my message short simply because if we were talking about only 50 amps, I didn't see any concern.

Even at this, I think we are assuming sinusoidal waveforms.

You didn't mention whether XL at low frequency would heat the conductor more than happens for the same amps at 60 Hz.

Intuitively I was ready to derate the conductors in both directions: for skin effect at 400 Hz and for heating at 25 Hz.

But since I didn't know, I was waiting for you to come along and explain. I'm glad you did.

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