# B Power Line frequency

1. May 6, 2017

### oquen

Our street electric power lines have frequency of 50 Hz or 60 Hz.. what would happen if you increase the frequency higher to radio frequencies or microwaves.. would current still flow? and can you make it as antenna for emergency transmission?

2. May 6, 2017

### Staff: Mentor

Transformers are used in that power distribution system. They would stop working pretty quickly as you increased the frequency into the 100s of Hz or low kHz. In fact, they would fail pretty spectacularly by overheating, exploding and catching fire. Do not ask me how I know this...

3. May 6, 2017

### oquen

How do you know this?

Can anyone please explain why a transformer would overheat, explore and catch fire by increasing the frequency into the 100s of Hz or low kHz?

4. May 6, 2017

5. May 6, 2017

### Staff: Mentor

Pay particular attention to Dave's 2nd link, and search on Eddy Currents and Laminations.
I told you not to ask me this:
Let's just say that when a programmable power source is reset back to its default output frequency of 400Hz by accident, and the system being powered is designed for 50Hz power input, um, well, fire extinguishers were involved...

Last edited by a moderator: May 8, 2017
6. May 6, 2017

### davenn

eeekkkk

a mistake that you only do once

7. May 6, 2017

### Baluncore

The main reason why 50/60Hz is used is that it has a long wavelength, (5,500 km), so the local power distribution network can be treated as a lump rather than as an ever-changing terminated transmission line. The grid voltage is then fixed and each customer can decide how much power or current to use by selecting the resistance of their load. There is no need for dynamic local impedance matching if the frequency is below about 10 kHz. Ring mains would be difficult at RF because the phase where the ring closed would need a phase correction network and standing waves would form that would give users a varying voltage.

The thickness of the laminations in power transformers are selected so that the magnetic fields can reach the middle of the lamination before the AC field reverses. That is due to the skin effect. Thinner laminations must be used for higher frequencies to keep the transformer cost and weigh down. For frequencies above about 10 kHz iron powder or ferrite must be used.

Conductors used at higher frequencies need to be made from flat sheet metal or Litz wire to get sufficient area to carry the power. Litz wire is expensive and sheet metal is difficult to hang from poles.

50/60Hz is a good frequency for running cheap induction motors. It is also high enough to remove flicker from those old historical filament lamps. An arc at higher frequencies will not self extinguish, so switches become much more expensive.

Aircraft used 400 Hz because the alternators and transformers can be smaller and lighter than 50/60 Hz components. 400 Hz is in the middle of the audio band so there would be many complaints due to interference with phone lines.

50/60Hz does not radiate well as EM from the grid distribution lines. If power was distributed at a radio frequency, the “power” band used would be forever jammed with radio noise and radio would not be possible.

8. May 17, 2017

### Joseph M. Zias

By-the-way, some old mining towns in PA used 25 Hz. I remember seeing a radio designed for that power - big transformer for a small radio.

9. May 17, 2017

### cosmik debris

Skin effect on the power lines which raises the resistance of the lines is another factor.

Cheers

10. May 17, 2017

### diogenesNY

I am pretty sure that U.S. Navy ships and submarines also use 400Hz AC current.

diogenesNY

11. May 17, 2017

### marcusl

Switching power supplies operate at 25 kHz. Transformers can be wound on tiny toroidal cores that have low leakage and losses.

12. May 17, 2017

### Staff: Mentor

Yeah, and aviation uses that frequency as well, I think. It has advantages, unless you feed it into a 60Hz transformer...

13. Jun 6, 2017

### Joseph M. Zias

When I left the power supply industry we were using 100khz for most switching power supplies. Light weight - small size. EMC and EMI were the big battles but design and trial and error got us to meeting FCC requirements. I believe the Navy was switching from 400 Hz to 60 Hz because the 400 was sending out too much signal for others to pick up on. However, someone will have to research that a bit further.