before learn about Troubleshooting, the question is not if a control circuit will fail, but when it will fail. When a control circuit malfunctions, one of the primary responsibilities of an industrial electrician is to troubleshoot and fix it. To repair or replace a damaged component, it is required to first identify which component is at problem. The voltmeter, ohmmeter, and ammeter are the three major tools used by an electrician to diagnose a circuit.
Troubleshooting tools
measurement devices
Voltmeters and ohmmeters are commonly seen in the same meter (Figure 49–1). These meters are known as multimeters because they can measure a variety of electrical values. Plunger voltage testers are used by certain electricians because they are not vulnerable to ghost voltages. Because of feedback and induction, high impedance voltmeters frequently indicate some amount of voltage. Plunger voltage testers have a low impedance and just a few milliamperes to function. The downside of plunger type voltage testers is that they cannot be used to test low voltage control systems, such as 24 volt systems.
Figure 49–1
Ammeters are typically clamp-on devices (Figure 49-2). Both analogue and digital meters are widely used. Clamp-on ammeters have the benefit of not requiring the circuit to be disrupted in order to insert the meter into the line.
Figure 49–2
Safety Precautions for Troubleshooting
It is frequently required to troubleshoot a circuit when it is powered on. When this is the case, safety should always come first. The electrician should be outfitted in flame retardant clothes and use safety glasses, a face shield, and a hard helmet while deenergizing or energizing a control cabinet or motor control center module. Motor control centres used in industry may often discharge enough energy in an arc-fault event to kill a person thirty feet distant. Another guideline to follow while energizing or de-energizing a circuit is to always stand to the side of the control cabinet or module. When opening or shutting the circuit, do not stand in front of the cabinet door. A direct short situation might blast the cabinet door off.
After opening the cabinet or module door, the power should be tested with a voltmeter to ensure that the power is turned off. To ensure that the power is turned off, utilise the check, test, check procedure:
1. Test the voltmeter against a recognized voltage source to ensure it is working properly.
2. Check the circuit voltage to ensure it is turned off.
3. Recheck the voltmeter on a recognized voltage source to ensure that it is still operating properly.
Voltmeter Basics for Troubleshooting
Remember that electrical pressure is one definition of voltage.
A voltmeter measures the potential difference between two points in the same manner as a pressure gauge measures the pressure difference between two places. Figure 49-3 shows a circuit that assumes a voltage of 120 volts exists between L1 and N. When the leads of a voltmeter are connected between L1 and N, the meter reads 120 volts.
Figure 49–3
The voltmeter measures electrical pressure between
two points.
Assume that the voltmeter’s leads are connected across the lamp (Figure 49–4).
Figure 49–4 The voltmeter is connected across the light source.
Question 1:
Considering the lamp filament is in good condition, would the voltmeter read 0 volts, 120 volts, or any figure between 0 and 120 volts?
Solution:
The voltmeter would show 0 volts. The switch and bulb are connected in series in the circuit seen in Figure 49-4. The voltage drop across all circuit components must match the applied voltage, which is one of the basic criteria for series circuits. The voltage drop between each component is related to the component resistance and the quantity of current flow. There is no current flow through the lamp filament and no voltage drop in this scenario since the switch is open.
Question 2:
Is it 0 volts, 120 volts, or any number between 0 and 120 volts if the voltmeter is connected across the switch as indicated in Figure 49-5?
Figure 49–5 The voltmeter is connected across the switch.
Solution:
The voltmeter would show 120 volts. Because the switch is an open circuit, the resistance at this point is infinite and millions of times larger than the resistance of the light filament. Remember that voltage equals electrical pressure. The only current flowing through this circuit is that of the voltmeter and the light bulb (Figure 49–6).
Figure 49–6 A current path exists between the voltmeter and the light bulb.
Question 3:
If the total or applied voltage in a series circuit must equal the sum of the voltage drops across each component, why is there no voltage drop across the light filament but all across the voltmeter resistor?
Solution:
Because the voltmeter’s current is going through the lamp filament, there is some voltage loss across it. The voltage drop across the filament, on the other hand, is so modest in comparison to the voltage drop across the voltmeter that it is commonly assumed to be zero. Assume the resistance of the light filament is 50 ohms. Assume the voltmeter is a digital device with a resistance of 10,000,000 ohms. The overall resistance of the circuit is 10,000,050 ohms. The total circuit current is 0.000,011,999 amperes (120/10,000,050), which is approximately 12 microamperes. The voltage drop across the bulb is measured in millivolts (50 12 A).
Question 4:
Assume the light filament is now open or burned out. Would the voltmeter in Figure 49-7 read 0 volts, 120 volts, or somewhere in between?
Figure 49–7 The light filament has been burnt out.
Solution:
The voltmeter would show 0 volts. If the lamp filament is open or burned out, there is no current channel for the voltmeter, and the voltmeter reads 0 volts. In order for the voltmeter to indicate voltage, it must be linked across both components, forming a full circuit from L1 to N. (Figure 49–8).
Figure 49–8 Both components are linked to the voltmeter.
Question 5:
Assume that the lamp filament is not open or burned out, and that the switch has not been switched off or on. Would a voltmeter wired across the switch show 0 volts, 120 volts, or a value between 0 and 120 volts (Figure 49-9)?
Figure 49–9 The switch is turned on or closed..
Solution:
The voltmeter would show 0 volts. The contact resistance is extremely low now that the switch is closed, and the lamp filament suddenly has a significantly greater resistance than the switch. Almost all of the voltage loss will now be visible across the light (Figure 49–10).
Figure 49–10 Almost majority of the voltage drop occurs across the light.
Test Procedure Example 1
When troubleshooting a circuit, the technique to be used is determined by the type of problem. Assume, for example, that an overload relay has tripped multiple times. The first step is to establish what conditions could be causing this issue. If the overload relay is of the thermal variety, a source of heat is most likely to blame. Make a mental note of what can cause the overload relay to overheat:
1. An excessive amount of motor current.
2. Extremely hot weather.
3. Fragile relationships.
4. Incorrect wire diameter.
If the motor has been functioning normally for some time, the improper wire size may most likely be deleted. That would be a thing to consider if it is a fresh installation.
Because overload relays are designed to disconnect the motor from the power line if the current demand becomes too high, the motor should be checked for excessive current. The first step is to read the nameplate on the motor to ascertain the usual full load current. The next step is to calculate the overload relay’s percentage of full load current setting.
EXAMPLE:
A motor nameplate shows that the motor’s full load current is 46 amps. The service factor of the motor is also indicated on the nameplate.
According to the National Electrical Code, the overload should be configured to trip at 115% of the entire load current. Overload heaters should be rated at 52.9 amps (46 x 1.15).
The next step is to use an ammeter to measure the motor’s operating current. Typically, this is performed by monitoring the motor current at the overload relay (Figure 49–11).
Figure 49–11 A clamp-on ammeter is used to measure motor current.
Each phase’s current should be monitored. If the motor is working properly, the readings may not be precisely the same, but they should be near to the full load current value and somewhat similar to each other. Phase 1 has a current flow of 46.1 amperes, phase 2 has a current flow of 45.8 amperes, and phase 3 has a current flow of 45.9 amperes in the example depicted in Figure 49-12. These figures show that the motor is in regular operation.
Figure 49–12 The ammeter measurements show that the motor is running normally.
Because the ammeter indicates that the motor is running normally, additional heat sources should be explored. Check all connections after shutting off the power to verify they are tight. Loose connections can create a lot of heat, and loose connections near the overload relay can trigger the relay.
Another factor to consider is the temperature outside. If the overload relay is positioned in a hot region, the extra heat may cause the overload relay to trip early. In this instance, bimetal strip type overload relays (Figure 49-13) can frequently be set to a higher setting to compensate for the problem of ambient temperature.
Figure 49–13 Overload relays of the bimetal strip type can be adjusted to a higher current value.
If the overload relay is the solder melting kind, the heater size must be increased to compensate, or some form of cooling device, such as a tiny fan, must be installed. If a source of heat cannot be detected, the overload relay most likely has a mechanical failure and should be replaced.
Assume the ammeter shows an abnormally high current measurement on all three phases. Phase 1 has a current flow of 58.1 amperes, phase 2 has a current flow of 59.2 amperes, and phase 3 has a current flow of 59.3 amperes in the example depicted in Figure 49-14.
Figure 49–14 The ammeter values show that the motor is overloaded.
Remember that this motor’s full load nameplate current is 46 amps. These numbers show that the motor is overburdened. The motor and load should be examined for mechanical issues such as a faulty bearing or a brake that has been engaged.
Assume the ammeter shows one phase with normal current and two phases with abnormally high current. Phase 1 has a current flow of 45.8 amperes, phase 2 has a current flow of 73.2 amperes, and phase 3 has a current flow of 74.3 amperes in the example depicted in Figure 49-15.
Figure 49–15 Ammeter values show that the motor’s winding is shorted.
Two phases with abnormally high current suggest that the motor’s winding is most likely shorted. If two phases have normal current and one phase has an abnormally high current, it is a solid sign that one of the phases has gotten grounded to the motor’s casing.
Example 2 of a Test Procedure
Figure 49-16 depicts a reversing starting circuit with electrical and mechanical interlocks. It is important to note that double acting push buttons are used to disconnect one contactor when the start button for the second contactor is pressed. Assume that if the motor is running forward and you press the REVERSE pushbutton, the forward contactor de-energizes but the reverse contactor does not. The motor will resume in the forward direction if the FORWARD push button is pushed.
Figure 49–16 Reversing starter with interlocks contacts.
To begin troubleshooting this issue, make mental notes of potential causes of this condition:
1. The reverse contactor coil is broken.
2. The typically closed F auxiliary contact has a short circuit.
3. The FORWARD push button’s typically closed side is now open.
4. When pressed, the typically open side of the REVERSE push button does not complete a circuit.
5. The mechanical connection between the forward and reversing contactors is faulty.
Make mental notes of any situations that might not be the source of the problem:
1. The STOP button is depressed. (If the STOP button was pressed, the motor would not move ahead.)
2. The overload contact is not closed. (Once again, if this were true, the engine would not run forward.)
To begin testing this circuit, use an ohmmeter to see if there is a complete circuit route across particular components.
When using an ohmmeter, check sure the power is turned off in the circuit. In most control circuits, removing the control transformer fuse is a good method to do this. When the REVERSE push-button is pressed, the ohmmeter may be used to examine the continuity of the reverse contactor coil, the normally closed F contact, the normally closed part of the FORWARD push-button, and the normally open section of the FORWARD push-button (Figure 49–17).
Figure 49–17 Using an ohmmeter to check for continuity on components.
The ohmmeter can be used to test the starting coil for a complete circuit to see whether the winding has been burnt open, but it cannot be used to see if the coil is shorted. To make a final conclusion, it is usually essential to turn on the circuit and measure the voltage across the coil. Because the REVERSE push button must be closed in order to do this measurement, it is usual practice to connect a fused jumper across the push button if no one is available to hold the button closed (Figure 49–18).
Figure 49–18 Testing to find out if voltage is applied to the coil.
Figure 49-19 depicts a fused jumper. When utilizing a fused jumper, power should be turned off before connecting the jumper across the component. Power can be restored to the circuit after the jumper is in place.
If voltage emerges across the coil, it means the coil is faulty and should be changed, or the mechanical interlock between the forward and backward contactors is faulty.
Figure 49–19 When troubleshooting, a fused jumper is often used to complete a circuit.
Example 3 of a Test Procedure
Figure 49-20 depicts the next circuit to be described. This circuit allows the motor to be started at any of three speeds, with a 5 second time delay between accelerating from one to the next. Regardless of which speed push button is touched, the motor must begin at its slowest setting and gradually increase to the specified speed. The time delay for acceleration to the next speed is supposed to be provided by eight-pin on-delay timers.
Figure 49–20 A wound rotor induction motor with three speeds.
Assume that when you hit the THIRD SPEED push button, the motor starts at its slowest speed. The engine accelerates to second speed after 5 seconds but never to third speed. Begin by generating a mental list of the situations that might create this difficulty, as in the previous examples:
1. Contactor S2 is faulty.
2. TR2’s timed contact did not close.
3. Timer TR2 is faulty.
4. The contacts CR2 or S1 linked in series with timer TR2 did not shut.
To begin troubleshooting this circuit, press the THIRD SPEED push button and let the motor to accelerate to the second speed. After the motor has achieved third speed, attach a voltmeter across the coil of the S2 contactor for at least 5 seconds (Figure 49–21).
Figure 49–21 Checking for voltage across S2 coil
Assume the voltmeter displayed a value of 0 volts. This means that no power is being delivered to the coil of the S2 contactor. The next step is to check for voltage between timer TR2 pins 1 and 3. (Figure 49–22). If the voltmeter reads 120 volts, it implies that the usually open timed contact has not closed.
Figure 49–22 Checking the voltage between TR2 timer pins 1 and 3.
Check for voltage across timer TR2 if timed contact TR2 has not closed (Figure 49–23). This may be accomplished by measuring the voltage across the timer’s pins 2 and 7. If a voltage of 120 volts is available, the timer is receiving power, but contact TR2 remains open. This indicates that the timer is faulty and should be replaced.
If the voltage across timer coil TR2 is zero, use a voltmeter to verify whether contact CR2 or S1 is open.
Figure 49–23 Checking for voltage across the TR2 coil
Troubleshooting entails working rationally through a circuit. Without a working grasp of schematics, troubleshooting a circuit is nearly difficult. You can’t tell what a circuit is or isn’t doing unless you know what it’s supposed to perform in normal operation. It takes time and effort to develop good troubleshooting strategies. In general, it is easier to work your way backward through the circuit until the problem is recognized. In this circuit, for example, contactor S2 delivered the final step of motor acceleration.
Starting at contactor S2 and working backward to determine which component was responsible for no power being transferred to S2’s coil was much easier and faster than starting at the beginning of the circuit and working through each component.
