Why the DC Solenoid Coil will Burned Out?

A DC solenoid  opens and closes when the DC current is applied to the coil.  The Solenoid coil consists of an enamel coated copper wire winding on the bobbin and usually some magnetically permeable metal frame or housing enclosure, which may be visible or contained in molding.  There may or may not be a thermal cutoff or fuse in electrical series with the winding.  More complex solenoid coils have diode bridges that rectify the DC current,  and even circuits which can control periodic inrush of current.  The combination of the copper coil, the magnetically permeable surrounding material, and plunger inside the pressure boundary of the valve are collectively known as the “solenoid”.

Part 1 :  Whey the Solenoid Coil will “burn out”, and consequently failure of a solenoid to operate, it may cause by the following scenarios:

1.1 Overheated Copper Wire Enamel of the poor quality

This generally applies to a solenoid coil that does NOT have a fuse or thermal cutoff.  If the copper wire, for whatever reason, exceeds the temperature rating by enough to melt away the enamel coating and allow the strands of copper to touch, the number of loops or turns is effectively reduced.  The shortage number of turns may result in a lower electrical resistance. When the current increases because the voltage remains constant.  Temperature then exponentially increases as more and more turns are lost and power increases.  Failure occurs when the copper wire or soldered joint melts and breaks the circuit,  there is zero current.  The solenoid would  return to the “failed” state, i.e. closed if “fail closed”.

1.2 Using a solenoid  in below scenarios may cause this to happen:

Too high of a surrounding ambient temperature for the duration that the solenoid coil is held energized, A high surrounding ambient and/or high fluid temperature, exceeding catalog ratings. Holding a solenoid energized for long periods of time when it is not rated to be energized indefinitely (continuous duty).

1.3 Cycling time a solenoid coil is overwork in a period of time, which may cause excessive temperatures from inrush current on a DC solenoid coil with programmed inrush.

In a DC solenoid, a jammed plunger that is prevented from contacting the pole piece will draw more current (near inrush levels). This may over heat the solenoid coil.

In an DC solenoid, energizing a solenoid coil without a plunger in the solenoid effectively draws more than the intended current.

Part 2 : Tripped Fuse or Thermal Cutoff

When a solenoid coil is built with a fuse or thermal cutoff , the design intent is to prevent surface temperature spikes due to some of the causes listed above already.  One reason for designing these in is to achieve a UL-1203 hazardous location certification and operating temperature code.  The fused solenoid coil design can essentially remove all chances of the coil surface ever getting hot enough to ignite a surrounding flammable gas by using fuses.

Different types of fuses are available.  Thermal cutoffs simply sever the circuit when they reach a certain temperature.  Other fuses may trip due to amperage exceeding a certain value.

Since the fuse in embedded in the potting compound or sometimes a welded canister, once tripped, the solenoid coil is no longer usable, even after it cools.

Part 3 :  The cause of Solenoid Coil burn out Conclusion:

Usually, a solenoid coil is a very simple item to replace on a solenoid.  However, it is important to understand WHY a solenoid coil burned out before replace a new one.  For example, if the plunger was jammed inside the solenoid and a technician simply replace a new solenoid coil to  the burned out one, the new solenoid coil would almost surely burn out as well.