Magnet Myths and Misconceptions

[verpies]

Everybody is speaking the same language in the world of electronics, why do you want to speak your own language, especially if you are wrong about something?  Eventually you start speaking the same language and using the same "code."

@Tinman
This does not mean that you cannot have your own ideas, nor that you cannot question the established science.
It just makes it simpler to communicate your ideas to others.

[synchro1]

Fields with negligible fringing

Some problems of practical importance can be solved when the air gap between the electromagnet and the work piece is small in comparison with the field cross section. This is the situation found in most electromechanical relays.

Basic construction of an electromechanical relay

Equation EFB gives the energy density (joules per metre cubed). Assuming that the field inside the air gap is uniform you can use EFB to get the total field energy simply by multiplying by the volume of the field, V


V = g × A

Equation FRV


where g is the gap length and A is the cross sectional area of the coil's core. The total energy is then


W = ( B2/(2 μ0)) × (g × A)

Equation FRT



We need the force on the armature. That is given by the rate of change of energy with gap length


F = dW/dg

Equation FRP



F =  B2 A / (2 μ0)

Maxwell's pulling
force formula


We next need to find the flux density,  B. It's assumption time again. Well designed relays use such high permeability material for the core and armature that most of the field strength produced by the coil will appear across the air gap between the core and the armature and we can ignore the  reluctance of the core, pivot and armature. Substituting equation TMH into equation TMD we get


B = Fm μ0 / g

Equation FRQ


Substituting into Maxwell's force formula


F = ( Fm)2 μ0 A / (2 g2)

Equation FRS


If you have ever tried to bring a piece of iron into contact with a magnet manually then you will quite literally have a feel for the g2 term!

Example: A relay has a coil of 1200 turns. The diameter of the coil core is 6 millimetres and the air gap is 1.8 millimetres. The spring exerts a force on the armature of 0.15 newtons at the part of it opposite the air gap. What coil current will operate the relay?

The core cross sectional area, A = π (0.006/2)2 = 2.83×10-5 m2. Substituting into equation FRS


0.15 = (1200 × I)2 4π×10-7 × 2.83×10-5 / (2 × (1.8×10-3)2)

Equation FRU


Therefore I = 0.138 amps. The flux density will be 0.116 teslas. This should be well below saturation for iron. As the gap closes, and g goes to zero, equation FRS predicts that the force on the armature becomes infinite. Of course it won't do so because our assumptions about the field production will go down the tubes first. Under those conditions it might be far harder to calculate the force precisely. One point to note, though, is that flux density is limited by saturation to below about 1.6 teslas. Maxwell's force formula therefore sets a limit on the force of one million newtons per square metre (about 100 tons).

[tinman]

You;ve got the idea.  To be a bit more pedantic:  A kg is a unit of mass that is the same anywhere in the universe.  On average, at sea-level on earth a kg is subject to 9.81N force towards the earths CoG due to gravitational acceleration.

[synchro1]

Above are equations that calculate the amount of current required to close the relay contacts. What we're dealing with is the "Pull Force" between the electromagnetic coil and the iron armature. The spring exerts a force on the armature of .15 Newtons. Therefore it will require I = 0.138 amps. The flux density will be 0.116 teslas to equal the .15 newtons of spring force.

In view of these mathematical relationships, it's absurd to maintain that "pull force of iron is no measure of field strength" as MileHigh falsely alledges. The "Pull Force" of .15 newtons from the coil on the iron armature is equal to 0.116 teslas.

[tinman]

But then he should have stated the fact that the coil resistances were significantly different.  I had no clue.  The last time I might have even looked at that would have been in the back of a textbook in 1980.

My apologies MH,i thought you would have know that the soft iron wire coil of same number of turns to that of the copper wire coil would have had more resistance. I will say though,i also didnt know that until i placed my ohm meter across both coil's. But it took me not 10 seconds to learn that on the bench-i feel it would have taken much longer than that by reading books.
Hands on is a much faster and precise way to learn MH,but i do agree that if you dont understand what you are seeing,then turn to the book's-or in our day and age,the internet.