August 1, 2014
Tips and tricks for troubleshooting dredge sensors and other electronic equipment.
by Jay Wise
Downtime on dredges due to electronic equipment failure varies based on the level of automation, age, hardware brand, size, use, and environment on each dredge. Additionally, downtime varies based on the response and level of competence to individual trouble issues. One thing is for certain: Downtime can last days or weeks as opposed to minutes or hours without the proper in-house skill set competence or spares parts.
Understanding control wiring diagrams
Wiring diagrams typically begin with the power and power-supply sections. Next, comes the actual PLC (Programmable Logic Controller) field inputs and outputs (see Figure 1), also known as I/O, or the relay ladder logic wiring for older relay style control systems. This is where you will find wire numbers to specific field devices (such as transmitter sensors, switches, thermocouples, push buttons, motor starters, VFD interconnections, etc.) that will guide you to the proper field terminal when doing your field voltage or milliamp (ma) current testing. Note the numbered lines to the left of each section on the page. These reference other sections in the drawing set. You will occasionally find a wire or device that continues to another area in the drawing set.
Using a multi-meter for troubleshooting
Multi-meters come in a variety of styles, but often carry similar features. Most will allow AC and DC voltage readings up to 300 volts, resistance, and continuity checking, as well as current and milliamp readings up to 10 amps. Some of the higher end units offer auto scaling and even source milliamps, voltage, and RTD for simulations. It is important for the technician to recognize the selection of lead plug-ins. This is necessary to change for reading amps or transmitter signal milliamps (ma). Since amperage is read in series of a circuit and voltage is read in parallel, the leads must be plugged into the proper ports for operation. If leads are left in the amperage ports while testing for voltage, you will most likely blow the internal fuse. Make sure you keep spares because this is a common mistake. When testing voltage or current, always start at the highest setting on the meter if signal values are unknown. You can then select lower ranges to improve the reading resolution. When measuring resistance, always verify that power is off and locked out. It is usually a good idea to lift field wires from terminals when measuring resistance to isolate your components and avoid reading paralleled resistance elsewhere in the circuit. When reading fuses for continuity, pull them out first. You could read a low resistance through motor windings or a transformer and think the fuse is good! When troubleshooting power circuits such as 480vac or 600vac motor circuits, it is often necessary to own a clamp current meter. This will allow for independent phase current testing without lifting any wires on terminals.
Motor circuit troubleshooting
Recognizing issues in the motor control circuit should be a straightforward task. Motor issues typically present themselves as blown fuses, tripped feeder breakers, open windings in a phase, bad contactor, defective overload, seized motor, or mechanical jam. First, check the fuses or feeder breaker above the starter contactor. With power off, each of the three-phase fuses can be pulled to check for continuity. Analyzing the motor circuit requires a clamp-on amp meter. Watch the motor control circuit while an operator engages the run command. If the contactor energizes and trips a breaker and immediately blows a fuse, this is a sign of a short in either the motor windings or the wiring. With power off, read resistance between each phase on the starter load side outgoing terminals. The larger the motor, the lower the resistance, but the resistance should be the same across each phase. If a phase is shorted in the motor or wiring, you would read 0 resistance. Also measure resistance between each phase and ground. This resistance should be in the millions of ohms. If a short exists in the motor or wiring to ground, you will read very low resistance. Now, isolate the motor at the motor terminal junction box and see if the short is in the wiring back to the starter or in the motor windings. On dredges, old corroded wiring or water in field terminal boxes is often the culprit. If a short is not discovered, the issue may be an open wire or winding on the motor. A bad contact in the starter contactor will cause the same open effect. When one of the three phases is open, the motor will hum loudly while vibrating, and the two phases with connectivity to the motor will read high locked rotor current with a clamp-on amp meter. The phase that reads no current is the open circuit. With power off and locked out, you can test the starter contactor contacts and overloads with a continuity test across the individual phase of the contactor. You should read open circuit until you physically press in the contactor. While pressed in, you would read 0 ohms if the contactor and overloads are good. If this proves okay, the next step is to check your wiring and isolate the motor windings with resistance checks along the way.
PLC I/O troubleshooting
It is common for laymen to blame PLC code or hardware for problems. The fact is, if your dredge has been operational for several months, the code is probably solid, and it will not change. Most cases of a system not starting or display reading not working involve a faulty field device or connection. PLC input field devices are used as system interlocks, as well as operator display annotations for alarms and status. If the PLC code is well written, these interlocks not only ensure safe operation of the system, but should annunciate alarm failures if the interlock is not valid. Being familiar with the PLC code or interlock schedule will be very helpful in troubleshooting control issues. When an operator comments that he can’t start his hydraulic pump or has a bad reading on his display for the discharge pressure, knowing which transmitter and field device is the interlock is necessary. This can be found using the electrical diagrams, as Figure 1 shows, along with the PLC ladder logic printout. Go to the field device in question and look for physical damage. Next, open any junction boxes that the wiring would pass through to get from the PLC cabinet to the field device. Check for water, corrosion, or frayed wiring, which is a common issue on older dredges. If the device and all wiring appear intact, it is necessary to troubleshoot with the meter. You need to determine if the problem is the field device or an actual input module failure. If you are troubleshooting a transmitter, there is a dedicated heading for this task. For the common field switch (pressure switch, push button, flow switch, level switch, etc.), use your multi-meter set for voltage and test voltage between the input terminal at the I/O cabinet and the neutral common while the field device is exercised. If the device and wiring is good, you should see the associated voltage at the input. Now, verify if the proper PLC input light illuminates on the input module window. You may have a bad input module. If input power is not at the input terminal when testing, go to the device and open the connection box while testing for voltage there. One side of the switch should always have input power applied. If not, you probably have a bad isolated fuse, which could simply feed that field device or a bank of devices. Usually, when a complaint is that several instruments suddenly stopped working, it is narrowed down to a fuse. Now, you need to find what blew the fuse. With the fuse pulled, check which field power wire is shorted. You may have to lift them from the terminal blocks individually to find the offensive short. It’s probably a pinched wire in a junction box or water in the field box somewhere.
Typical transmitters found on dredges include pressure sensors, magnetic flow meters, encoders, density meters, ladder angle sensors (inclinometer), and RTDs. Most of these transmitters are 4-20ma signal instruments. Each of these transmitters can come in two-wire, three-wire, or four-wire configuration. It’s important to know which you have for troubleshooting purposes, and they typically have the wiring termination connection and unit measure range stamped on them. When replacing a transmitter, make sure to purchase the same range and wiring configuration as the original unit. This is necessary for scaling in the PLC. When testing 4-20ma signals, the meter is required to be in line (series) with the ma current signal. Fluke sells a 773 meter that is a nice addition to any technician’s toolbox. This meter has a small retractable clamp designed to clamp around the small signal wires found in a 4-20ma circuit. This avoids the requirement to break a connection somewhere to read the signal.
Production is usually read in tons per hour or cubic meters per hour. In some cases, an on-board computer calculates the production rate with signals from density meter and velocity (flow meter). Some dredges still prefer the yield meters, which have independent flow and density needles and an analog scale to infer the production via the cross needles. In either case, they rely on the accuracy of the flow meter and density meter. As magnetic flow meters are exposed and wear, the effect can change the accuracy. It is important to calibrate flow meters upon installation and during annual service. Nuclear density meters rely on variable material amounts to interrupt the scintillator’s crystal pickup of the nuclear source. When material interrupts the path from the source to the crystal, oscillation is affected, which, in turn, becomes a weaker electrical signal. This can be calculated into density based on the dry specific gravity of the material being read. For accurate computation, an operator must know the material count (specific gravity offset) of the media (pond, lake, channel, river). These setup values are entered as fixed parameters.
Each dredging operator or dredge owner should have a good downtime database or method to validate his individual downtime occurrences and reasons. You can’t control what you don’t measure. This would certainly shed light on the amount of downtime and the cost of issues due to control or electronic equipment failures. I’m certain if this downtime was measured, management would recognize the need for skilled technicians with the ability to react to and resolve issues associated with the evolving automation and controls found on today’s dredges.
Jay Wise is the owner of Baltimore-based Kruse Integration. He has 35 years of experience in industrial automation, with 20 years in dredge automation.