Magnetic Permeability

[Mechineer]

I have been working on a motor brake that uses electromagetism to apply force to a clutch plate instead of using a mechanical spring. I have two brakes with identical geometries, the number of turns of the coils are comparable (2702 to 2685 turns), the same applied voltage, slightly different inductances (.311 to .263 Henries). The holding torque of the two brakes is significantly differnt.The only explanation I can figure is the magnetic permeability must be different or the difference in the inductances may be the culprit. Is there anything else that could cause this variation that I may be overlooking? How much influence does the magnetic permeability have on the force exerted on a ferrous material.

 

[ISUClone]

For the magnetic force you will need to look at amperage and curreent type. The amperage multiplied by the number of turns pretty close to a linear relation ship to the magnetic field intensity. If both coils get 5 amps there is an 85 (ampere turn) difference. The more amps ther greater the difference. Remember (amp turns) is not the magnetic field but it is a linear relation ship so looking at the relationship is an easy reference point.
Now if your input current is not pure DC the inductive difference will cause the current to be different in each coil. This will further increase the difference in amp turns on each coil.

 

[IRstuff]

Are the DC resistances the same?

Are there any possible differences in brake material that would change the operating temp of the coil?

TTFN

 

[Mechineer]

I am using the same DC power supply for both brakes. Also I only supply the electricity for a few seconds so there is not a chance of having significant heating of the wires to cause a resistance change.

 

[IRstuff]

You still haven't indicated whether the coils have the same resistance or not.

Also, are the two clutches made from exactly the same material and were treated identically? Is it possible that one of the clutches was magnetized? TTFN

 

[drdick]

Do the EMs have a ferrous core? Are they identical? Is the mounting identical? Are there ferrous mounting components that add "yoke effect" in one case and not the other?

It would help a great deal to know the flux density established in some fixed gap between the electromagnet and the clutch plate. Thickness of the ferrous clutch plate (and yoke components) can make more difference than permeability if they are nearing magnetic saturation.

In addition to items already mentioned, are the clutches used independentently, i.e., no series or parallel connection?