electric solenoids

[new guy]

On a hydraulic elevator the elevator is raised and lowered by opening solenoid activated valves. The solenoid has an electromagnetic coil that just sits over (around) a cylindrical spring loaded mechanical valve. When energized it just raises the valve off it's seat. The metal valve sits in the center of the coil. A coworker says that you cannot energize the coil if it is not sitting over the metal valve. if you energize the coil with nothing in it's center you will burn out the coil, damage it. Is this true, and if so, why?

[Robert Reed]

If we are talking AC solenoids and if the metal is soft iron or something similar, it forms an iron cored inductor. The naked coil alone may not have enough impedance (inductive reactance) to maintain a low enough current to prevent burnout. However with the addition of an iron core, the inductive reactance of that coil will be much higher, presenting a higher impedance and thus a safe operating current.

[Chris Smith]

No, the coil is wound for the values of a constant voltage and current and cant burn out with or without the magnetic’s of the metal involved.

.....but and only very unlikely but,.....

.......unless the metal parts just happen to be some kind very special heat sink designed into the coil system to extract heat from the unit,...... [wierd]

......again no,... NOT very likely.

[Crowbar]

They will typically have a laminated armature with shading coils, if they don't completely close or chatter excessively they will burn burn out in short order.

[Dean Huster]

Like motors, all solenoids have an inrush (or "starting") current. Until the solenoid armature is pulled into place by the magnetic force, that inrush current will be high -- 4 to 10 times the holding current, but it lasts for fractions of a second.

If, however, something impedes the action of the solenoid so that it can't be pulled fully into place, that current will remain high and can burn out the coil in short order. This is also why you don't want to have a solenoid (or contactor or motor starter) to be rapid cycled for a long period of time because the average current will increase and overheat the coil.

Dean

[shillyard]

Is this the same technology used for rail guns ?