Solenoids are mechanical actuators that are controlled by the application of a large current, through a coil. This applied current creates a magnetic field that exerts force on a metallic plunger, in order to create linear motion. The movement of the plunger can be used to pull a wire or cable, move a linkage arrangement, or to open and close a valve assembly.
The Solenoids Coil consists of multiple turns of enameled wire (sometimes call Magnet Wire). All coils have an Impedence (Z) at a nominated frequency; however this is normally not specified by the Manufacturer, so becomes unimportant for Functional Test purposes unless you are in a position to compare multiple Solenoids. An LCR Meter is generally used for this type of evaluation. Stay clear of Wheatstone Bridges, as these are pretty much a thing of the past. They are functional but don't provide the accuracy and convenience of Test equipment utilizing digital Metering.
The Solenoids Coil also has a Resistance associated with it (measured in Ohms). By taking this measurement you are looking for continuity of the windings, and confirming that the coil is not open.
During normal Solenoid operation: Kinetic energy (from electron flow through the Coil) causes the Solenoid to heat up. With the addition of an adverse operating environment (with excessive heat) the wires insulative enamel can become tacky, and break down. When this occurs adjacent turns of wire can short to each other, and the coil is said to have a 'Shorted Turn'. A Shorted Turn will cause an infinite current to pass through the Switching device and thus cause catastrophic failure. The Switching device can be Electro-Mechanical (a Relay) or Electronic (Transistor, MOSFET, IGBT or SCR).
To find a Shorted Turn can be extremely difficult, particularly if the Solenoids coil has hundreds of turns.
One very effective method is to apply a High Speed Pulse across the coil, and count the number of Rings that are produced. Normally a good coil will tend to try and sustain oscillations when the energy of the Pulse is removed. A Shorted Turn will absorb all of the applied Energy from the Pulse, and thus produce dampened oscillations, and a low ring count.
To Test the functionality of a Solenoid (out of circuit) I would suggest using the following procedure:
1. Confirm that the plunger freely moves inside the assembly. Inspect for dust debris, and other binding agents.
2. Using a DVM or Analog Meter: Measure the Resistance of the Coil. The resistance is dependant on the number of turns. I would normally expect to see less than 10 Ohms when high current driver circuits are being used at DC voltages; however I have seen some 115VAC Solenoids with coil resistances up to 400 Ohms.
3. Using a Shorted Turns Tester: Measure the Ring count from the coil. A good coil will respond with 2 rings (or greater).
4. Apply an appropriate working voltage to the Solenoid, and confirm consistent operation.
David Willcocks is an Electronics professional, and Technical writer. For more information on Automated Test Equipment please visit http://www.imagineATE.com.
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