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Q & A: Twisted Wires

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Most recent answer: 09/06/2016
Q:
I am making a probe for a 20T magnet in a He3 system. I understand that the wires connecting everything have to be twisted otherwise a current will be induced. Why is that current induced if the wires are not twisted? -Thanks
- Anonymous
A:

The wires leading to your magnet form a closed loop, which is necessary of course since all electronic circuits must form such closed loops in order to function. The reason you'll get an induced current in these wires is that your dealing with a magnetic field. That field passes through the closed loop of wire, creating what's known as a "flux", and how much flux there is depends on how strong the field is times how large the area of your wire loop is.

Flux = Magnetic Field * Area of Loop

The danger comes in while you're changing the magnetic field. As you increase (or decrease) the strength of the field, you're causing the amount of flux passing through the wire loop to change, and this induces a electromotive force, E.M.F., in the wire. Basically, a voltage develops around the loop that is proportional to the rate of change of the flux, and pointing opposite to the way the current is changing.

E.M.F. = -Change in Flux / Change in Time
(if you use the right units)

This voltage difference causes unwanted current to flow by Ohm's law:

Current Induced = E.M.F. / Resistance of the wire loop

The reasons that twisting the wires gets rid of this effect are; first, that it effectively decreases the area of the closed loops by breaking it up from one large loop into several smaller ones (the twists). This decrease in loop area decreases the amount of flux through the loop. And second, the E.M.F.s induced in each successive twist alternate sign (one positive, one negative), canceling each other out so that no current flows.

This same technique is used anywhere that stray magnetic fields may be a problem, like in experiments running near large power supplies., or where electric lines carrying data need to traverse long distances, such as phone and ethernet lines.

Also, I'm very impressed, 20 Tesla is a VERY large magnet. Good luck, and keep anything else magnetic far away from it!

Hope this helps,

Tim


(published on 10/22/2007)

Follow-Up #1: twisting away emf

Q:
Thanks for the twisted wire answer. I wonder if emi from motor wires would be reduced by twisting the three wires?
We are trying to shield the motor wires, but find that there are differences to the radiated EMI from one motor set of wires to the next. would twisting them make the results a bit more consistant?
- dale (age 62)
charlottesville,va
A:
Sorry for the delay in getting to this. Sure, twisting the leads is a very good idea. It should work for 3-phase circuits just like for 2-phase ones.

Mike W.

(published on 10/22/2007)

Follow-Up #2: twisting 3 wires

Q:
Related to the practice of twisting pairs of wires to reduce the effect of induced currents... can you twist more than just two wires? For example, I have a instrument situation where a common reference conductor is used for two devices (A and B). To measure on device A, I measure between the common wire and wire A while leaving B as open. (vise versa for measuring device B) In this scenario is it effective to twist all 3 wires?
- Mike (age 27)
Madison, WI, USA
A:

You can certainly twist the three wires to avoid electromagnetic pickup. You might get some cross-talk between A and B, especially if you wanted to measure them simultaneously. If that turns out to be a problem, you could add another reference wire and use two twisted pairs. Probably that's not needed.

Mike W.  


(published on 09/06/2016)

Follow-up on this answer.