Today's troubleshooters use many tools, from simple handheld meters that measure resistance and voltage to sophisticated recording oscilloscopes that capture fast transients.
Ever watch veteran electrical troubleshooters in action? If not, you should: It'ssomething to see. Years of experience coupled with intelligent "hunches" seem to guide their investigation. To help you in your troubleshooting efforts, we've compiled the following tips to solving motor control, power distribution, and adjustable speed drive problems.
Testing contact quality on energized motor starters. Typically, you must get a "failed" motor to run again as soon as possible. Obviously, you're under pressure and must use all the troubleshooting techniques you've acquired over the years. One tried-and-true technique involves testing contact quality on energized starters.
To understand this method, consider a typical manual or magnetic motor starter having three sets of power contacts and three overload (OL) relays. When a starter energizes a motor, equal currents (theoretically) flow through each contact and OL relay. So, you can measure the voltage drops across the contact or OL relay poles and compare them with one another. How do you do it? First, connect your multimeter's lead set to the correct input terminals and set it to read AC millivolts (mV). With the starter energized, begin with the lefthand pole and place one probe's tip on the line-side terminal. Then, carefully position the other tip on the corresponding load-side terminal, but electrically upstream of the OL relay. Then, note the reading. Repeat this process for the other terminals and compare the results. For each reading, try to place the test lead probes in the same relative position at each contact and OL relay.
You can apply this online millivolt-drop test to contactors serving other loads too. With a little experience, you can measure the voltage drop on a cable termination. Where there's a small amount of exposed bare conductor on the cable, place one probe on a strand and the other on the corresponding lug or bus bar. You shouldn't use this procedure to establish a precise benchmark, substitute as a preventive maintenance program, or replace current-injection OL relay testing or thermographic surveying. It only provides a basic check for use in breakdown troubleshooting situations.
Suppose you do the testing as indicated above on a magnetic starter and get the readings shown in the Table (in the original article). Reviewing these readings, you would see those of the OL relays are nearly the same. However, the starter contact reading for Phase C is significantly higher than the others. This may be the result of an internal problem such as excessive wear, loose hardware, or poor contact pressure.
By taking phase current readings and using Ohm's Law, you can calculate and compare the respective contact resistance values (technically, impedance). With the resultant high-resistance value for Phase C, this condition (if left as is) can cause high heating and eventual starter failure. The result will be unannounced downtime. Remember, even though you're making a fraction-of-a-volt measurement, you're connecting your meter to points energized by line voltage. Therefore, follow these safety guidelines:
• Use only a test instrument having internal protection circuitry that prevents damage, should you inadvertently apply input terminals to line voltages while set at lower voltages.
• Use the utmost caution and exercise all required safety practices.
• Wear appropriate protective equipment such as safety glasses, rubber gloves, etc.
• Make sure you have good work-area lighting.
• Be well aware of the circuit voltage, available short-circuit current, immediate surroundings, working clearances, and the capabilities and limitations of your test instrument. Special thanks to Scott Falke, former High-Voltage Electrician at the Lawrence Livermore National Laboratory, for this troubleshooting tip.
Troubleshooting control power transformers. Getting a process line back into operation can be stressful, considering the electrical components tied into the control system. Sometimes it's not the exotic sensor or mysterious black box (known as the PLC) that's the problem. The source can be as mundane as the ubiquitous control power transformer (CPT) in a motor control center.
Typical CPT problems include open circuits, partial shorts, complete shorts, and grounded windings. First, you should check the primary of the CPT to verify you have power.
Partial shorts. Sometimes, a partial short occurs in the CPT's secondary, causing a voltage drop. Overheating of the CPT is the usual symptom because large circulating currents are flowing through the shorted windings.
Troubleshooting here involves a sequence of steps. First, verify line power is available by taking a voltage reading at the CPT's primary. Then take a reading at the secondary. If the readings are somewhat lower than normal, suspect a partial short.
Another method is to use a sensitive ohmmeter with the CPT's leads disconnected and the system de-energized. Here, a lower-than-normal resistance reading indicates a partial short. However, the difference in resistance from normal will be very slight.
Complete shorts. Sometimes a CPT's winding shorts out, activating a circuit breaker or fuse to protect the circuit by de-energizing it. However, there may be instances where the CPT continues to operate, resulting in excessive overheating (due to very large circulating current) and melting of CPT winding insulation. The most apparent symptom is a strong odor. Another is no voltage output across the shorted winding. But be careful: The short may be in the external secondary circuit and not the CPT's winding.
The best way to find the location is to disconnect the external secondary circuit from the CPT and take a voltage reading at the CPT's secondary. If the voltage is normal, the problem is in the external secondary circuit. If the voltage is zero across the secondary leads, the CPT is shorted and needs replaced.
Grounded windings. In older transformers, especially overloaded units, insulation breakdown is common. Here, the insulation physically breaks down or deteriorates to the point where the winding's bare wire exposes. If the exposed wire comes in contact with a grounded surface (such as the CPT housing), the CPT shorts to ground.
Should the above condition develop, and a point in the external secondary circuit also becomes grounded, part of the CPT winding will be shorted out. Here, the symptoms are overheating, which you can detect by touch or smell. You can also detect this by a low-voltage reading at the CPT's secondary. The only alternative here is to replace the damaged CPT.
The best method for detecting this condition is to use a megohmmeter. First, disconnect the leads from the CPT's primary and secondary windings. Then, connect the megohmmeter's negative test lead to an associated ground and its positive test lead to the winding you're testing. Record the reading. Finally, take an insulation resistance reading between the windings by connecting one test lead to the primary and the other to the secondary.
Diagnosing ASD "trip" problems. Repeated "tripping out" (shutdown) of an installed adjustable speed drive (ASD) is not only an annoying and elusive problem, but an expensive one when manufacturing or production processes stop.
While altering the trip levels of the ASD is tempting (and in some cases economically justifiable), it's not a long-term solution. Repetitive tripping of a drive may be a warning sign complete failure is not far off.
Most modern ASDs monitor many different fault conditions that can trip the drive. Use these parameters as a guide: DC bus overvoltage; DC bus undervoltage; current overload; and ground fault protection relay tripping.
DC bus overvoltage faults. Line transients from the AC source are a common cause of an overvoltage fault. (See Fig. 1, in the original article) The best way to discover the cause of this is to connect a voltage recorder to the AC line inputs and time-stamp the transient event. If the transient events are greater than 2 p.u. (per unit) and less than 0.5 cycle duration, it's likely they're caused by lightning, utility switching (transformer taps, kVAR capacitors), load switching within the building, or some power line fault clearing event.
If you can correlate the transients to a regular utility switching event, contact the utility. It's possible the utility is unaware of the effect it's having on customers. It may even be willing to modify practices or install equipment to minimize such effects. If you can't determine the source of the transient event, then you should install a surge protection device at the service entrance to the building.
For shorter magnitude, longer duration transients, which are classified as swells (1.3 p.u. to 2.0 p.u. and 0.5 cycles to 30 cycles), it's possible an isolation transformer or series line reactor will take care of the instantaneous overvoltage. An isolation transformer will also reduce common mode noise from the ASD. Swells longer than one or two cycles may require some additional voltage regulation like an uninterruptable power supply (UPS). Be careful though: Not all UPS systems can regulate overvoltages.
A longer duration overvoltage lasting greater than 30 cycles is a less common fault affecting ASDs. It occurs when the local utility is slow to respond and compensate for large loads from industrial and commercial customers being switched off, such as at the end of a work shift. The problem is fairly easy to detect with a recording power line monitor. If you can't resolve the problem with the local utility, then a UPS designed to compensate for overvoltages may be the answer.
If line voltage monitoring doesn't record a transient or swell associated with the overvoltage fault, it's possible the increased energy at the DC bus is coming from the load side of the inverter, not the line side. We call this motor regeneration. One problem with elevators and large centrifugal loads, regeneration occurs when the motor is "coasting" and changes from being a motor to a generator.
You can detect regeneration by looking at a change in direction of the DC current level, as measured at the output side of the DC bus. A simultaneous measurement of the DC voltage will confirm current flowing back into the DC bus is causing the overvoltage. Installing a dynamic brake is the most common way to solve this problem. If you've already done this, check the resistance measurements according to the manufacturer's specifications. If it's a solid-state type, check the transistors for proper conduction, using the diode test function on your multimeter. It's also possible the dynamic braking is too excessive, stopping the motor too suddenly. If your drive permits, lengthen the deceleration time to minimize the regenerative effect.
One final word about overvoltage transients: Inadequate building grounds can cause transients to propagate through a distribution system at magnitudes greater than normal. This might explain why one building is having more problems than adjacent ones.
DC bus undervoltage faults. Your best bet in detecting ASD undervoltage faults is to use a recording line voltage monitor. Because most ASDs have enough "ride through" to handle short duration (less than 0.5 cycle) sags or dropouts, you don't need to detect fast undervoltage events. While this makes the problem easier to detect, fixing it is more difficult due to large current surges caused by large loads either inside or outside your building. (See Fig. 2, in the original article.) In either case, you'll probably need voltage regulation like that from a UPS.
Another possible cause for undervoltage is when the line voltage supply has "flat-topped" peaks. (See Fig. 3, in the original article.) This happens when you have other large electronic loads in the building. Because all electronic loads convert from AC to DC, the AC-to-DC conversion is responsible for almost all nonlinear (harmonic generating) currents that cause flat-topping. Basically, all electronic loads draw a short burst of current at the voltage peaks. (See Figs. 4 and 5, in the original article.) If the AC source is weak (either the transformer kVA is too low, or the conductors are too small or too long), then the peak currents flatten the peak of the voltage waveform. This reduces the charge on the DC bus capacitor, which presents a problem when someone switches another large load on in the building. As a result, the voltage peak drops further, the capacitor charge becomes even lower than it was due to flat-topping, and the DC bus voltage drops below the trip point. About the only way to recognize this problem is with an oscilloscope or power monitor that can display the voltage waveform.
Current overload faults. While there are several different causes of overload tripping with ASDs, some are easier to find than others. First, make sure there's a proper match between the motor and drive. While this may seem obvious, the actual motor is sometimes different than the original specification due to cost and/or size concerns. Also, make sure the motor is sized properly for the load. Measure the motor current on all three phases and check against the nameplate rating to be sure.
Another overload consideration is whether the load has changed. Sometimes an equipment operator increases the demand of a load without knowing the consequences. What's more, the ASD may not be programmed properly. If the acceleration time decreases, or trip points change, this could account for overload tripping.
You should also make sure the ambient air temperature at the ASD installation is within specified operating temperatures. An increase in environmental temperatures can cause component failure and/or higher-than-normal conduction of components.
An open phase connection can also cause an overload condition known as single phasing. Basically, the other two phase windings in the motor have significantly increased current, and the motor keeps running. If this is the case, the drive will probably trip when you restart it. Open circuits are sometimes difficult to find because wiring is not always easily accessible. The best way to check for single phasing is to measure the in-rush (acceleration) current on all three phases. The one with no in-rush current is the culprit.
Voltage imbalance between phases at the motor terminals can cause excessive current to flow in one or two motor windings. This, in turn, can cause the overload to trip. It could be due to a problem with one of the output transistors in the drive not conducting properly, or a bad connection between the drive and motor, resulting in a voltage drop on one of the phases.
Shorted windings can also cause an overload condition. However, because the ASD may trip and shutdown before you can take a visual reading, you should use a DMM or oscilloscope capable of measuring and recording the peak in-rush (acceleration) current. Make this measurement on all three phases to see if a shorted winding in one of the phases is causing more current to flow in it than the others. Be careful though: A voltage imbalance may be the cause of any current variations between phases. This means you may have to disconnect the drive and motor and test them separately.
If you exceed the overload current on each phase, and it's beyond the nameplate rating of the motor, you may need to investigate whether you're exceeding the mechanical load connected to the motor for the application.
Ground fault protection relay tripping. The fast edged pulses of a pulse width modulation (PWM) drive may cause leakage currents to flow between the motor windings and the grounded motor frame through the capacitor created by the metal windings, winding insulation, and metal frame. With normal 60 Hz sine wave operational, this isn't a problem. However, the high switching frequency of the PWM signal reduces the motor's internal capacitive reactance, causing greater amounts of leakage currents to flow. This can cause nuisance tripping of the drive, as many GFP relays trip at 300mA or less.
Measuring this phenomenon can be difficult, as it may require isolating the motor frame from ground and then reestablishing a connection to ground via a cable that allows you to take ground current measurements.
To reduce leakage currents, use a common-mode choke and dampening resistor or special cabling that employs either EMI suppression, low-pass filtering, or ferrite granule coating that absorbs the radio frequency (RF) energy and turns it into heat. Isolation transformers also minimize common-mode leakage currents, but may not be a cost-effective solution unless you're using them to solve other problems.
Special thanks to Mark Mays, Senior Product Specialist, Industrial Group Marketing, Fluke Corp. for these troubleshooting tips. You can order an application note titled "Measurement of Adjustable Speed Drives" by calling (800) 443-5853.
