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Troubleshooting Control Relays
Control relays are used whenever such devices as motors or heaters consume large amounts of power.

Relays are also used to control equipment remotely and from several different areas around the equipment.

For newspaper presses, relays oversee a number of different tasks, including inch speed, impression, ink, dampeners and other circuits necessary to operate the machines.

Troubleshooting control relays or control relay circuits can be impossible if you don’t understand how the relay works. The relay is basically several switches that are operated by the magnetic action of a coil and armature.

The coil and armature, in turn, is a coil of insulated small-diameter wire wrapped tightly around a nonconductive core (normally the insulation on the wire is a coat of high temperature varnish.)

At the core

A ferrous material such as iron is placed in the core of the wire coil, which is called the stationary core and becomes magnetized whenever the coil is energized by a power source. (The power source is voltage that can be anywhere from 5 volts to 480 volts, depending on the voltage rating of the coil.) Another piece of iron is placed in close proximity to the stationary iron core, which is called the armature. Whenever the coil is energized by applying voltage to the coil, the stationary iron core inside the wire coil becomes magnetized, thus causing the armature to be pulled into contact with the stationary iron core.

A set of switches will then turn on or off whenever the armature is attached to that set of switches. If the switch is first off and subsequently turned on by the armature movement, then the switch is described as a normally open contact.

If the switch is on and then turned off by the armature movement, then the switch is described as a normally closed contact.

Diagnosing problems

We now have a good idea of how a relay works. We can use that knowledge to troubleshoot relay problems.

Illustration 1 is an example of a circuit that is used to operate a relay. When the switch is closed, the coil is energized by the 110-volt source, which magnetizes the stationary iron core. The armature is now drawn to the magnetized stationary core. The normally open contact is pulled closed by the armature. When the contact closes, the light will illuminate. If you try to duplicate this circuit, be sure that all of the components are rated for the power source that you will be using.

So what do you do when the light doesn’t come on when the switch is turned on? Here are a few areas to examine:

  • Check the power source with a voltmeter; in this case the voltage should measure 110 volts.
  • Measure the voltage at the light bulb. If there are 100 volts at this point and the light is still not illuminated, then the light bulb is burned out and will need to be replaced. If no voltage is measured at this point, the control relay will be the problem.
  • If the power source is okay, the switch is on and the light bulb doesn’t have voltage across it, then measure the voltage across the coil of the relay coil or look to see if the armature of the relay is in contact with the stationary iron core. If there is voltage to the wire coil, the armature will be in contact with the stationary core; that is if the wire within the coil is not burned out or the armature is not being blocked mechanically.
  • If the relay is functioning properly, then measure the voltage across the relay contacts. The voltage should measure zero because the contacts are closed. On the other hand, if voltage is measured across the contacts, it means that the contacts are burned or damaged in some way.
  • Using these basic steps will help you learn to troubleshoot relay controls. The only way to become proficient is to practice and to build some simple circuits.
  • If you have any questions concerning relays and how they work, please fee free to contact me.



    Frank Bourlon has more than 30 years experience in the newspaper industry. He is the executive director for Newspaper Production & Research Center. He can be reached @ 405-524-7774 or via email at nprc@flash.net.

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