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TROUBLESHOOTING ELECTRICAL CONTROL CIRCUITS

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Troubleshooting electrical control circuits is defined as the logical and orderly process to determine the fault or faults in an electrical control circuit. It requires you to follow specific and systematic procedures. First make sure the problem is not something simple such as an unplugged device or a tripped circuit breaker. Should the problem be more complex, check with your supervisor for company policies and procedures. You should have technical knowledge about the circuit on which you are working. If you are unfamiliar with any malfunctioning circuitry, report this to your supervisor and do not blindly proceed. Some form of aid is nearly always available and seeking this help not only allows the problem to be repaired correctly and promptly but may also insure that damage to equipment, or worse, injury to yourself and to others, does not occur.

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Electrical Safe Work Practices

Do not take another person's word that a circuit is de-energized. Instead, check the circuit yourself. Obtain the proper service literature before troubleshooting. Remove any tripping hazards and make sure your work area is clean. Stay away from grounded objects or standing water when checking voltage. Do not lean over any equipment to check a circuit while resting one hand on a grounded object. Practice good judgment and use a logical and orderly process when troubleshooting. Understand the function of the devices and circuits and how they work. Familiarize yourself with the use of test equipment and tools. Check your work carefully to make sure nothing is overlooked.

Personal Protective Equipment

Wear personal protective equipment (PPE) including rubber or composition-soled shoes to insulate you from the ground. Leather soles may conduct electricity. Insulating mats also provide protection by increasing resistance between your feet and electrical ground. Other insulators include rubber aprons, insulated blankets and rubber gloves. Wear a hard-hat and safety glasses of non-conductive material. Jewelry can conduct electricity or get caught in machinery. Follow all of your facility's procedures regarding PPE and jewelry.

Reporting Safety Hazards

If you know of a safety hazard within your facility, you should report it to the appropriate personnel. Report any unusual odors, such as those from overheated circuitry, burned insulation, or leaking material. Do not modify a piece of equipment solely for convenience, or attempt to defeat a safety feature. Instead, get specific approval from appropriate personnel to repair the malfunctioning equipment. With few exceptions, electrical circuits must be de-energized before work can begin, and a proper lockout/tagout procedure is required.

Troubleshooting Equipment

Multimeters and clamp-on ammeters are test devices widely used in troubleshooting. Both may have a digital or analog display. Multimeters allow you to make a variety of electrical measurements with the same instrument. Most multimeters can be used to measure resistance, voltage, and current. Many digital multimeters can measure frequency and capacitance. A clamp-on ammeter measures current flow on an energized system. It has a metal core, also known as a set of "jaws," which opens to clamp around a live conductor. Its display is controlled by a dial, which you can set for different ranges of current flow. Some models are designed for AC only, DC only, or both AC and DC.

The Troubleshooting Sequence

Determine the symptoms of failure by gathering as much information as you can and then verify the symptoms. Localize the source of the trouble and narrow down the causes. Do not assume you know where the problem originates. Isolate the malfunction to a specific circuit or device. Confirm your diagnosis and complete a final analysis.

Gathering Information

Gather as much detailed information as possible. Speak to the person who reported the problem. Ask questions and take notes. Find out if the circuit has had problems in the past. If it has, try to discover what those specific problems were. Although this information can be helpful it may not be of any use in solving the immediate problem so do not get stuck wasting too much of your time here. Find out when the problem occurred, where it occurred, and how the operation now is different from operation prior to the problem. Record the complete list of symptoms (and possibly the name of the source of information), so that you can refer to them as you proceed and, if a report is to be recorded.

Verifying the Symptoms

Once you have written down the complete list of symptoms, verify those symptoms by performing an electrical or mechanical operational check. You may also have the equipment operated manually, which may require the circuit to be de-energized. Make sure there are not any additional symptoms that may have been overlooked. If there are, be certain to record them in your notes.

Verifying a De-energized Circuit

Before measuring the resistance of a circuit, make sure it has been locked and tagged out and confirm it is de-energized (three-step voltage check). To do this, set the multimeter to measure volts and check it on a known live circuit to verify that the meter functions properly. Locate the power source, identify the leads, and confirm that the circuit is de-energized. A de-energized circuit will register zero volts. Check the meters functionality again on a live circuit.

Visual Inspections

Sometimes visual signs or warnings can explain the malfunction of a control circuit. Perform a visual inspection on the equipment before you do anything else. Visual inspections can identify any loose connections, broken wires, poor solder joints, blown fuses, or tripped circuit breakers. If a visual inspection indicates a defect, you should not assume that it is the cause of the malfunction. Instead, complete all of the troubleshooting steps. Should you visually locate a defect, consult your facility guidelines for repair or replacement procedures.

Non-visual Inspections

Use your sense of smell, hearing and touch to perform non-visual inspections while troubleshooting. Overheated resistors, transformers, coils, windings, and insulating materials can often be detected by their odor. Listen for a frying noise that can indicate overheating circuitry. Overheated devices can often be located by touch. If any parts feel too warm, there may be a defect in the device or in the circuit wiring.

AC versus DC Contactors

Although AC and DC contactors both operate on the same principle of solenoid action, there are some major differences. A DC solenoid contact assembly may have only one set of contacts, while an AC contactor may have several sets of contacts. In a DC contactor it is necessary to break only one side of the line, while in three-phase AC circuits you are required to break all three current paths, creating the need for several sets of contacts. For multiple contact control, the T-bar assembly allows several sets of contacts to be activated simultaneously.

Another distinguishing difference between DC and AC contactors is their magnetic assembly. In a DC contactor the magnetic assembly is made of solid steel. Laminations are unnecessary in a DC coil since the current is traveling in one direction at a continuous rate and does not create eddy current problems.

The other major differences between AC and DC contactors are the electrical and mechanical requirements necessary for suppressing the arcs created in opening and closing the contacts under load.

The Contacts

The most common causes of failure are bounce, arcing, and welding. Bounce occurs when the contacts close quickly and literally bounce before settling. Arcing occurs when the contacts close too slowly or when the contacts are misaligned, causing electricity to jump from one contact to the other. This results in pits in the contact surface. Welding occurs when the contacts are melted together and they won't separate. On a contactor there are often two types of contacts, the main and the auxiliary. The main contacts deliver power to the motor. The auxiliary contacts operate control devices that usually, but not always, are related to motor operation.

Contact Construction On High Amperage Contacts

How contacts are designed and from what materials they are made depend upon the size, current rating, and application of the contactor. When contactors use double-break contacts, they are usually made of a silver cadmium alloy. In large contactors with single-break contacts, contacts are frequently made of copper due to the lower cost.

Because of the nature of copper contacts, the single-break contacts are designed with a wiping-sliding action to remove the copper oxide film, which forms on the copper tips. This wiping action is necessary because copper oxide formed on the contact when not in use is a nonconductor and must be eliminated to maintain good circuit conductivity.

In most cases the slight rubbing action and burning that occur during normal operation will keep the contact surfaces clean enough for good service. Copper contacts that seldom open or close, however, or those being replaced, should be cleaned to reduce contact resistance, which is often the cause of serious heating of the contacts.

General Purpose AC/DC Contactor Sizes and Ratings

Magnetic contactors, like manual starters and disconnect switches, are rated according to the size and type of load by NEMA. The tables below indicate the number size designation -- 00, 1, 2, 3, through size 9, for general purpose AC and DC contactors and also establish the current load carried by each contact in the contactor. Note that the rating is for each contact individually, not for the whole contactor. In other words, a three-pole contactor rated at 18 amperes is actually capable and rated for switching three separate 18 ampere loads simultaneously.

Troubleshooting Steps

  • Check the control voltage that powers the coil to ensure the problem is in the motor starter and not in the power source.
  • If the voltage is correct, turn all power off, making sure you follow your facility guidelines for lockout / tagout, safety equipment, and procedures.
  • Conduct a sensory inspection to look for obvious causes.
  • Check moving parts for binding.
  • Check the coil and contacts for continuity.
  • Remove and replace any defective parts.

Sensory Inspection

Verify all power sources to the motor starter are locked and tagged out and the equipment is de-energized. A sensory inspection can help you find obvious signs of failure. Look for signs of overheating. Do you smell any burned material? Next, look for discoloration or charring. Look for blistering. Is the unit hot? Without touching the unit, hold your hand near it. Look at the coil. Do you see any discoloration? Are there any broken or damaged wires? Do you see any rust or corrosion?

Testing the Armature

Check the armature for binding. You are simulating the action of a powered coil. It should move freely when you push it in. It should push in with spring resistance but it should not bind. When you remove your hand, it should immediately snap back to its original position. If the armature binds or does not move at all, an obstruction, dirt, rust, or corrosion could be the cause. Other problems that may affect the operation of the armature are:

  • Filings or shavings that block movement
  • A broken component
  • Welded contacts
  • Sticking or binding parts
  • A coil that is deformed because of overheating The most likely solution will be to clean or replace damaged components.

Checking Continuity on the Coil

To perform a continuity check on the coil, you will need to measure its resistance. Disconnect one of the leads to the coil. This way you're reading resistance through the coil only and not through the circuit. Select the resistance scale and zero the meter. Touch one probe to each terminal of the coil. Resistance of a properly functioning coil will be relatively low. If the resistance is reading out of tolerance to the manufacturers specification, then the coil is malfunctioning and should be replaced. If resistance is infinite, the coil wire is probably broken and the coil should be replaced.

Checking Continuity on the Contacts

Perform a continuity check on the contacts. Place one probe on one of the power lead terminals. Place the second probe on the terminal at the other side of the contact block. The contacts should start in the open position. In this position you should get a reading of infinite resistance. Push the armature in to close the contacts, in this position the meter should read near or at zero resistance. If you are still reading resistance then the contacts did not close properly. Damaged contacts or a mechanical problem inside the contactor could cause this problem. Repeat this procedure for the remaining power leads.

Next check the auxiliary contacts. For an accurate check, lift one of the leads of the contact terminals, and then use the same technique that you employed in testing the main contacts for probe placement. Some auxiliary contacts may be normally open and some may be normally closed.

Disassembly of a Motor Starter

This should only be performed on a pulled MCC bucket, never while connected to the bus. First, make a sketch of your motor starter wiring layout. Label all wires so that you can reconnect them in the proper order. Taping the exposed leads back is a good safety measure. As you remove each part, place it on a flat surface in the order that it was taken off. When you reassemble the unit, replace the parts in the reverse order. First remove the overload relay. Remove the armature. Remove the magnet assembly. Remove the coil. Remove the contactor. Your magnetic motor starter should be completely disassembled.

Testing the Electromagnet

Inspect the armature, making sure the rivets are intact and not loose. Check for signs of crimping, binding, or discoloration. The movable contact bar should move freely against the seated contacts. The contacts should make a good connection and should be evenly aligned. Look for any abrasion, dirt, corrosion, or rust. Look for melting or other damage to any plastic parts. Look for cracked or broken parts.

Testing the Contacts

When examining the contacts, look for signs of surface damage. Look at the movable contacts. Do you see pitting, discoloration, or wear? Since pitting is the result of excessive arcing, damaged contacts are sometimes symptomatic of a larger problem. Check the stationary contacts for pitting, discoloration, and wear.

The Interlock

A device is considered interlocked if its operation is affected by the action of a related device. Interlocks are safety devices, which protect a motor. Three-phase motors can be made to change direction of rotation by interchanging any two motor leads to the main power source. Interlocks protect the motor from being energized to rotate in both the forward and reverse directions at the same time. Changing a motors operating direction takes two coil-operated contactors, one for forward, one for reverse. The coil-operated contactors use mechanisms to keep the reverse contactor from engaging when the forward is engaged as well as the opposite, which is to keep the forward contactor from engaging when the reverse is engaged. Interlocks may be either mechanical or electrical or both.

How a Mechanical Interlock Works

The mechanical interlock physically keeps the second set of contacts from closing when the first is closed. When the first set of contacts is engaged, a lever moves the second set of contacts to keep them from closing accidentally. If you try to push the second set of contacts closed, the armature will be blocked from closing. A mechanical interlock provides redundant protection when used in connection with push button interlocks, or auxiliary contact interlocks.

How an Electrical Interlock Works

The electrical interlock controls power to the starter coils providing protection. Electrical interlocks do this with a set of auxiliary contacts. When power is applied to the forward coil, the main contacts are closed providing power to the motor in the forward direction. At the same time, the forward auxiliary contact is opened, breaking any potential connection to the reverse coil. Conversely, when power is applied to the reverse coil, the main contacts close, turning the motor in the reverse direction. This opens the reverse auxiliary contact, breaking any potential connection to the forward coil.

Testing the Electrical Interlock

Hook one of the multimeter probes to the L2 or grounded lead of the control circuit. Touch the other probe to a point where you can read continuity through the forward auxiliary normally closed contacts and the reverse coil. Your meter should read low resistance. Push the forward armature in. The forward auxiliary contacts should open. The meter should show infinite resistance. Now touch the probe to a point where you can read continuity through the reverse auxiliary normally closed contacts and the forward coil. Your meter should read low resistance. Push the reverse armature in. The reverse auxiliary contacts should open. Again, the meter should show infinite resistance. In both of these instances notice that the auxiliary contacts open as the main contacts close.