Troubleshooting Variable Speed Drives

Fast, effective troubleshooting combines intuitive and logical thinking with special training, the right test instruments, and "SMARTS."Troubleshooting electrical equipment is always a challenge. As electrical equipment incorporate more complex electronics, the challenge becomes increasingly formidable. More and more time is required to troubleshoot this equipment; certainly, increased knowledge is

Fast, effective troubleshooting combines intuitive and logical thinking with special training, the right test instruments, and "SMARTS."

Troubleshooting electrical equipment is always a challenge. As electrical equipment incorporate more complex electronics, the challenge becomes increasingly formidable. More and more time is required to troubleshoot this equipment; certainly, increased knowledge is needed to do this task correctly and efficiently.

This certainly is the case with variable frequency drives (VFDs), which consist of a complicated combination of electric power components and sensitive electronic circuits. Furthermore, VFDs are directed by precision, low-voltage, low-current digital or analog circuitry that can be quite sensitive to outside disturbances. Most importantly, proper VFD operation is greatly dependent on many exterior factors such as the load itself, input voltages, surges, dirt, heat, moisture, and so on.

Thus, if you want to be an effective troubleshooter of VFDs, you must apply systematic common-sense thinking and troubleshooting techniques. Another essential factor is training.

Applying SMARTS

The secret to troubleshooting VFDs is using SMARTS, an acronym for a proven method used in troubleshooting numerous VFD installations over the last 20 years. SMARTS stands for Safety, Manuals, Application, Readings, Talk, and Symptoms.

This technique can be applied to all kinds of VFDs and other electrical equipment as well. It's practical, economical, effective, and fast.

Safety. This is the first step in all troubleshooting; safety for yourself, your fellow employees, and the equipment. Take as many test readings as possible with the power off. However, many measurements and test procedures will have to be done on energized equipment. Be sure to install warning signs, tape off the test location, and follow lock-out procedures.

It's important to be aware that both AC and DC voltages are present within the VFD. DC voltage levels will be equal to the peak voltages of the AC input. For example, a 480V drive will have a DC voltage of 676V (480 x 1.41). The DC voltage used in a VFD is stored in capacitors that are capable of holding a lethal charge for several minutes. Before attempting any internal hands-on work on the VFD control enclosure, you should first take a voltage reading of its capacitors with a verified working meter. Remember, your life could depend on this simple test.

With today's modern microprocessor designs on VFDs, it's very easy to program in a set of operation parameters that could inadvertently prove to be fatal. A motor designed to operate at 3600 rpm at 60 Hz can be made to operate at 7200 rpm when the VFD is programmed to supply a 120 Hz output. Imagine what would happen to boxes moving on a conveyor at twice the conveyor's rated speed. Make sure you understand the results of any changes or adjustments made.

The best advice is to read and follow all safety cautions on the drive and in the supplied manual.

Manuals. It's impossible to troubleshoot and repair a VFD without first studying the manufacturer's manual. With on-board diagnostics available on most units, you need the manual to decode these fault messages as well as to identify the probable causes for the respective faults.

The manual also provides you with test procedures and troubleshooting guidelines that will save many hours of unnecessary downtime. A good practice is to request additional manuals or copy the ones you already have. Many hours of lost downtime can be attributed to looking for the equipment manual.

Application. What is "application?" It's the VFD, the motor, and the load connected to the motor. Don't forget the external controls that control the VFD itself, such as interlocks, push buttons, digital controllers, etc.

The VFD will operate only when and how it's told to operate. At the same time, it will respond to any dramatic changes, such as incorrect or faulty load and power demands, by shutting down in an effort to protect itself.

The majority of problems with VFD applications (fans, pumps, conveyors, etc.) have nothing to do with the VFD control console itself. As such, you should troubleshoot the VFD controller last and troubleshoot the application first.

Readings. As part of your troubleshooting process, you'll need to collect readings of the drive. The most common readings needed are as follows.

* AC input line voltage.

* AC output load voltage.

* Line current.

* Load current.

* Frequency output to the motor.

* DC bus voltage.

Today's VFDs use operator interface panels and digital displays. Most of the readings needed for troubleshooting can be read on these displays. In some units, you can even choose one of four different languages.

When using test equipment to collect readings, be sure to verify the meter's capability for the intended reading. The supplied manual will usually indicate what readings are needed as well as the type of meter to use.

Solid-state VFD components are easily damaged by overvoltages and fault currents. Therefore, they may be checked early on prior to sequential, logic procedure. Also, be sure that insulation resistance readings are not taken on a motor with the drive connected and power removed.

It's good practice to log the readings on paper as you troubleshoot. This will help you from having to take the same readings over again.

Talk. That's right, talk. Talk to the operator of the equipment; talk to other mechanics working in the area; talk to anyone that might know anything about the equipment or an unusual activity near it. As a detective (troubleshooter), your job is to get the facts and get the equipment back up and running.

Here are some questions that you should ask. When did it stop working? Has anything else also stopped? Have there been any recent changes or additions to the load? Is this a random failure or has this been happening for some time?

The better your understanding of the events surrounding a failure, the more accurate and faster your troubleshooting will be. Ask yourself, "Am I troubleshooting symptoms or the actual problem itself?"

Symptoms. This last step is especially critical for effective troubleshooting of VFDs. It's vital that you learn to separate symptoms from the actual problem.

For example, a fuse blew; is this the problem or a symptom? Actually, the blown fuse is a symptom of the true problem. A fault of some type has caused the fuse to blow. Replacing a blown fuse without troubleshooting the cause will not correct the problem and, in many cases, will create even greater problems.

Another example: the drive trips in an overload condition. Is this the problem or a symptom of an undiscovered problem?

Collect all your symptoms and carefully review them. Let the symptoms as well as your readings and interviews with operators point you in the right direction.

Actual case history

Location: Baltimore Museum of Art, Baltimore, Md.

Application: Numerous VFDs for an HVAC system in new building addition.

Type of loads: Fans, pumps.

Problem 1: A 60-hp chilled-water pump VFD shutdown.

Problem 2: Only one standby chilled water pump remaining. The museum needs to maintain precise control of temperature and humidity to preselected constant levels to protect art work.

Getting down to the business of troubleshooting, we instinctively began thinking in terms of SMARTS.

Safety. Even with many years of experience, we always start troubleshooting with these simple questions. What are the dangers in troubleshooting this equipment? Am I safe in what I am doing?

Manuals. Guess what? The pocket for the manual was on the VFD controller door but the manual was gone. As such, a twin unit VFD next to the inoperative VFD had to be shutdown and its manual removed. (With the digital displays and error codes available for diagnostics, it's essential to have the VFD manual.) The fault code on the display indicated that control power had been lost and recommended checking control inputs, fuses, and the control transformer. But before taking readings, we reviewed the application.

Application. Sure we're electrical people, but you can't fix the piece of equipment if you don't know what it's doing or supposed to do. If you're not sure of the application, ask. Get someone to help you. Try to think of the things you'll need to know. For example, how is the drive controlled: auto, manual, local, or remote? What and where are exterior sensors and limit controls? Anything special about the load?

Readings. At the VFD control panel, the manual was reviewed and the suggested readings, as outlined, were taken.

As the readings progressed, we found a blown control fuse on the secondary side of the control transformer. When you see a blown fuse, a little voice in your head always seems to say, "Go ahead, change it; it probably got weak and needed to be changed. Your problems will be over." Don't do it. A fuse is a protective device, and when it blows, it means a problem exists and the fuse cleared a potential over-current or fault-current condition.

Using a meter in the resistance test mode, we checked for shorts in the control circuit, but did not see any dead faults or unusual resistance values. As such, the fuse was replaced and the VFD was powered up. Boom! It blew again.

Now, it made sense to start tracing wires from the transformer. In doing this, we started asking some questions.

Talk. "So, when did this unit actually go down? Did you lose any other equipment? Anybody do any work nearby?" We were told that some painters had just finished painting the mechanical room just prior to the drive shutdown. Although there appeared to be no relationship, we asked where they were painting. The painting included areas right over the VFD itself. Looking at the top of the VFD, we saw where pieces of cinder-block from the painting prep work had fallen down onto the enclosure. Behind the VFD a piece had fallen into the cooling fan and jammed the fan blade, causing the fan motor to short out. This fan motor operated at 120V and was directly wired to the control transformer. End of the story? Not yet.

Symptoms. Everything pointed to the fan motor as being the culprit. But after disconnecting it from the transformer and further testing, a fault still showed up.

We continued troubleshooting and discovered that someone had made repeated fuse changes and faults had damaged an additional gate-firing transformer.

Solomon S. Turkel is Senior Instructor with ATMS, Corp. Baltimore, Md.

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