As an industrial electrician, you'll encounter many complex electrical circuits and drives that will break down as they age. Chances are, you won't be an expert in repairing these devices, but you can take some practical steps to solve problems and help get your systems operational again in a timely manner. Let's start with the motor drive.
Solid-state electronic AC motor drives are becoming more common within industrial plants. They control a wide variety of devices like pumps, conveyors, air handlers, chillers, machine tools, mixers, and a host of other devices once designed to run at constant speed or be powered by DC. Since failure in these devices can be often attributed to the rectifier section, you'll need a fundamental understanding of transistors, diodes, silicone controlled rectifiers (SCRs), and insulated gate bipolar transistors (IGBTs).
Pulse width modulated (PWM) inverter drives are the most prevalent type of AC drives (Fig. 1 here). The AC line voltage is converted to DC (in the converter section) and then reconstructed back into a variable frequency and a variable voltage output. Changing the frequency varies motor speed, and motor torque is maintained by keeping the ratio of volts-to-frequency at a constant.
Troubleshooting motor drives. Because most failures occur within the power sections instead of the circuit boards, they aren't very difficult to troubleshoot. The typical plant maintenance technician will rarely see enough failures to build up any proficiency in repairing circuit boards.
Effective troubleshooting on a variable frequency drive (VFD) requires a methodical approach. The classic divide-and-conquer method, taught by most technical schools, is effective when knowledge of the equipment is limited. A good troubleshooter will first isolate the box or section that isn't passing the signal and then work on it.
So how can you quickly and efficiently troubleshoot a dead VFD? Remember to always put safety first. The capacitors within the power section can maintain a dangerous charge even after the power is removed.
First make sure that the capacitors are discharged before putting your hands into the power section. With the power off, begin checking the power sections of the drive. Then, place your digital multimeter (DMM) in the diode check mode. Find the positive DC bus (sometimes this may be brought out to a terminal), place the negative (black) lead from your DMM on it, and then check each incoming phase in turn with the positive (red) lead. You should read a diode drop of about 0.6V on each phase. If it reads open, then the charge resistor is open and needs to be replaced. This is a common source of many problems.
Next, place the DMM's positive lead on the negative bus and the negative lead on each incoming phase in turn as you did before. You should read a diode drop, not a short or an open. Place one DMM input lead on the positive bus and the other on the negative bus. On this measurement you should read the capacitor charging rather than a short.
To check the inverter section, place the positive DMM lead on the negative bus and the negative lead on each output phase. You should read a diode drop because diodes are connected across each output transistor. Again, you shouldn't read a short. Check the remainder of the inverter section by placing the negative lead of the DMM on the positive bus, checking each output phase again with the positive lead of the DMM. You should read a diode drop again and not a short. If you read OPEN from either of these checks then the bus fuse is most likely open. If no problems are present within the power section and the unit still won't function, it's either improperly connected or programmed or has a bad circuit board.
Newer PWM drives use IGBTs in the driver sections of the output, and are much less likely to fail. These devices perform like a metal-oxide semiconductor field effect transistor (MOSFET). When the voltage at the gate exceeds the threshold voltage, the device turns on. If the voltage applied to the gate contact is less than the threshold voltage Vth, then the device is turned off (Fig. 2).
Visual PLC troubleshooting techniques. Most PLCs incorporate light emitting diodes (LEDs) in their design, which offer a good source of diagnostics. They can provide valuable information about the wiring, and input/output (I/O) modules within the unit. Typically, I/O modules have at least one LED indicator; input modules normally have a power indicator, while output modules usually have a logic indicator.
A lit power LED on an input module indicates that the input device is operating and its signal is present at the module. However, this indicator by itself can't isolate malfunctions to the module. Consequently, some manufacturers provide an additional diagnostic indicator known as a logic indicator. If a logic LED is lit, the logic section of the input circuit has recognized the presence of the input signal. If the logic and power indicators don't match, then the module is unable to correctly transfer the incoming signals to the processor. This indicates a module malfunction and most likely points to the problem area.
The output module's indicator functions in a similar fashion to the input module's indicators. When on, the logic LED indicates that the module's circuitry has acknowledged a command from the processor to turn on. In addition to the logic indicator, some output modules incorporate either a fuse indicator or a power indicator, or sometimes both. A blown fuse indicator displays the status of the protective fuse in the output circuit. The power indicator displays that power is being applied to the load. Similar to the power and logic indicators in the input module, if both LEDs aren't on simultaneously, the output module is malfunctioning — again pointing to the probable problem area.
As you can see, LED indicators greatly assist the troubleshooting process. With power and logic indicators, you can immediately pinpoint a malfunctioning module or circuit. Although they can't diagnose all problems, they serve as a good first round indicator of a system malfunction.
Troubleshooting the PLC inputs. If the field device connected to an input module doesn't seem to turn on, a problem may exist somewhere between the line connection and the terminal connection to the module.
First place the PLC in standby mode so the output isn't activated. This will permit you to manually activate the field device. A limit switch can usually be manually closed to achieve this result. When the field device is manually activated, the module's power status indicator should turn on, indicating the power link is working properly. If this occurs, then the wiring most likely isn't the root of the problem.
Next, analyze the reading of the PLC's input module. Place the PLC in its test mode. The device should read its inputs and execute its program, but not turn on its outputs. If the PLC reads the device correctly, then you know the problem isn't located in the input module. If it doesn't read the device correctly, then the module could be defective. However, several causes are possible. First, the logic side of the module may not be operating correctly. Second, its optical isolator may be blown. Third, one of the module's interfacing channels could be damaged. In any case, you'll need to replace the module.
If the module doesn't read the field device's signal, then further tests are required. Bad wiring, a faulty field device, a faulty module, or an improper voltage between the field device and the module could be causing the problem. First, measure the voltage to the AC input module. Your DMM should be in voltage measuring mode and should display the voltage that powers the module. If the voltage is present and at the correct level, you know you have a bad input module because it's not recognizing the applied voltage. If the measured voltage is 10% to 15% below the specified signal voltage, then the problem most likely is in the source voltage to the field device. If no voltage is present, then the wiring is broken or shorted or the field device is dragging it down. Check the wiring connection to the module to ensure that the wire is properly secured at the terminal or terminal blocks. You can also perform an insulation check on the wiring to look for shorts and/or damaged insulation. Be sure that the system is de-energized first before conducting this test.
To further locate the problem, confirm that voltage is present at the field unit. With the device turned on, measure the voltage across it using your DMM. If no voltage is present on the load side of the unit (the side that connects to the module), then the input device is probably defective. If there is power, then the problem is in the wiring from the input device to the module. In this case, the wiring must be inspected and tested to find the problem.
Troubleshooting PLC outputs. The first step in troubleshooting the outputs is to isolate the problem to either the module, the field device, or the wiring.
First check that the source of power to the output module is at the specified level. This value should be within 10% of the rated value. In a 120VAC system, for example, it should be between 108VAC and 132VAC. Examine the output module. If the fuse is blown, check its rated value to be sure the correct fuse was installed in the first place. Also, check the output device's current specifications to determine if the device is pulling too much current.
If the module's output status indicator fails to turn on despite receiving the instruction to turn on from the central processing unit, it's faulty. If the indicator does turn on and the field device doesn't activate, then check for voltage at the output terminal to be sure that the switching device is, in fact, operational. If no voltage is present, then you should replace the module. If voltage is present, then the problem lies in the wiring or the field device. At this point, make sure the field wiring to the module's terminal or to the terminal block has a good connection and that no wires are broken. This can be accomplished in the same fashion as described earlier.
When you finish checking the output module, check to see that the field device is functioning correctly. Check the voltage coming to the field device while the output module is on. If voltage is present, but the device doesn't respond, then the field device is probably defective or damaged.
One trick for checking the field device is to test it without using the output module. Remove the output wiring and connect the field device directly to the power source. If the field device doesn't respond, then it's faulty. If the field device responds, then the problem lies in the wiring between the device and the output module. Check the wiring, as noted earlier, looking for broken wires, shorts, worn insulation, and oil or grease on the connection points and along the wiring route.
Troubleshooting the CPU. PLCs also provide diagnostic indicators that show the status of the central processing unit (CPU). These indicators include such display messages as POWER OK, MEMORY OK, and COMMUNICATIONS OK.
You should first check that the PLC is receiving enough power from the transformer to supply all the loads. If the power received is in accordance with specifications and the PLC still isn't working, check for a voltage drop in the control circuit or for blown fuses. If these conditions are all proper, then the problem lies in the CPU. Most likely, the diagnostic indicators on the front of the CPU will display a fault in either memory or communications mode. Should one of these indicators be lit, it's highly likely that the CPU needs to be replaced.
Troubleshooting the input and output sections of a motor drive can be easy when approached logically and addressed section by section. All you need to do is measure volts and ohms with a DMM. As with motor drives, the most practical method you can use to diagnose PLC input/output malfunctions is to isolate the problem to either the wiring, the module, or the field device. When these systems have both power and logic indicators for you to view and interpret, then module failures become very easy to recognize and isolate.
Olobri is a product development manager for AEMC Instruments, Foxborough, Mass.
