Understanding Variable Frequency Drives

Here are basic guidelines revealing how to troubleshoot variable frequency drives and get them up and running fast.The trick to troubleshooting variable frequency drives (VFDs) is to fix the problem and not the symptoms. When a fuse blows, is this the problem or a symptom of the problem? Of course we know it's only a symptom; something is causing the fuse to blow. We can replace all the fuses we want,

Here are basic guidelines revealing how to troubleshoot variable frequency drives and get them up and running fast.


This same idea holds true for fixing drive problems. The first and most important rule for troubleshooting VFDs is DO NOT TROUBLESHOOT THE VFD!! Troubleshoot the VFD application instead.

So what is the VFD application? It's everything from the incoming power to the load itself. Fig. 1 shows the individual subsets of the overall VFD application.

Incoming power. Like a car, a VFD will not operate properly with poor quality fuel. The incoming line power must be stable and adequate. Characteristics such as undervoltages, overvoltages, surges, etc. are usually the result of poor power quality.

VFD drive panel. The VFD controller panel is the heart of the application. The VFD panel takes in a fixed voltage and frequency and gives out a variable voltage and frequency to the motor. In doing so, drive gives off heat. The bigger the drive, the greater the heat. Without proper cooling a drive will shut down on overtemperature.

Overtemperature and overcurrent go hand-in-hand. Sometimes the faults will even interchange. An overcurrent fault could be the result of overtemperature. A clean well-ventilated drive always seems to work better and last longer. A couple of the best troubleshooting tools are a brush and vacuum cleaner.

AC Motor. In a VFD application, the motor reacts to the varying voltage and frequency from the drive and develops torque for the load. In doing so it draws current from the VFD. If the demands of the load are too great, the motor starts to draw excessive currents from the VFD. Eventually the current demands become too large for the VFD to handle, so it trips out on an overcurrent fault.

Overcurrent faults are the most common faults that will shut down a drive. There are at least 20 to 30 causes of over-current faults. Finding the actual cause of the fault in question is the challenge. When dealing with these type of faults, always look at the dynamics of the motor and the load.

Load. The load is the reason for the application. Without a load there is no need for the drive and motor. Troubleshooting drives always begins with a clear understanding of the load. This simple concept can save you much time in your troubleshooting.

The fast, effective troubleshooting

Expert troubleshooters always have a well defined approach to troubleshooting. This is not a set of troubleshooting instructions (which varies from equipment to equipment) but a way of thinking. The thinking process can be refined to the application of "SMARTS."

SMARTS is a system that will help keep your thinking straight when troubleshooting.

Safety is always your first concern. There are three questions you should always ask yourself before you do anything. 1. Is what I am about to do safe for me? 2. Is what I am about to do safe for those people around me? 3. Is what I am about to do safe for the equipment? These simple questions can help in preventing a deadly accident.

Manuals are required to troubleshoot today's equipment. Microprocessor-based controls have smart displays and powerful diagnostics. However, these wonderful capabilities are useless to us if we don't have the manual to interpret the information being provided.

Applications are what we troubleshoot - not individual pieces of equipment. A drive shutdown is usually the result of external conditions. The last item to troubleshoot should be the drive control panel.

Readings from the drive display panel indicate the cause of a shutdown. With the fault information in hand, a review of the manual will give you probable causes for the fault.

A log of past faults is helpful in tracking recurring problems that have yet to be fixed.

Other readings, such as line voltage and motor currents, give you an indication as to the overall health of the application. Recording readings over the life of the equipment is an excellent method of preventive maintenance. Don't forget temperature readings; a rise in heatsink temperature is a strong indication of future problems.

Talking and communicating with machine operators or users of the equipment is one of your best troubleshooting tools. You should ask questions like when and what else was going on when the application failed can save you many hours of troubleshooting. Not all of your talking will be with people. Sometimes a review of the log of past events in the building automation system is all you need to solve the problem.

Simple solutions are usually all it takes to put a system back into operation. The vast majority of problems seen by the author over the last 20 years have been simple in nature. Some simple problems have been made complicated by over-reactive troubleshooting methods, thereby becoming very difficult.

Always investigate the protective interlocks and safety relays first. Why bring in an expert to reset peripheral devices such as a smoke 'stat or freeze 'stat in a duct. Remember a drive needs two signals to operate: a start command and a speed-reference signal.

Symptomatic troubleshooting is how experts focus in on core problems quickly and accurately. Look at it this way. If the receptacle in your office was not working right now, would you drive down to the main substation in the city to check the incoming feeders?

Of course not, it wouldn't be practical. But think, how were you able to make the decision not to make the trip? You probably saw other lights on and knew in a split second that power was available to your part of the city. This split second decision saved you a 3-hr trip.

As you observe symptoms, ask yourself what could cause this symptom. Some symptoms can have multiple causes, but by negating or throwing out the ones we know not to be the problem, we can determine the true problem with greater speed and accuracy.

Variable frequency drive problems will fall under one of three classifications as shown in Fig. 2. This chart will help in troubleshooting a drive application. Note we are troubleshooting the application and not the drive.

Troubleshooting case history

A recent troubleshooting experience is a good example of how easy it is to be misled in separating symptoms from real problems. This customer has numerous large 125-hp VFD units mounted in roof top air-handling units that are used to supply conditioned air to a large manufacturing facility.

Having provided start-up services for the installation, we were asked to assist in troubleshooting a problem the customer was experiencing. One of the 125-hp units was making strange sounds and acting unstable. It was as if the unit was hunting all the time and experiencing many current surges.

The customer was sure that the drive was misfiring somehow, creating very high current surges. In fact, the door-mounted current meter would swing from a normal 130A peak up to 180A and then swing down to 90A or so. The problem seemed so serious that the customer was ready to take the unit out of service and run it in its bypass mode.

As we walked into the penthouse, we heard the sound coming from the unit. Our first reaction was to get this "troubled" unit shutdown in a hurry. As we were getting ready to shut down the unit, we noticed the volt and frequency meters on the door were locked-in and stable. At 50-Hz operation, the unit was operating at around 385V. The volt/hertz ratio was right on the money. We immediately realized that there was nothing wrong with this drive unit.

We then explained to the operating personnel what a constant volt-hertz ratio is and how the drive was obviously reacting to a dramatic swing in load demand. The real question was how a fan could show such swing.

Tracing out the duct work and taking sample static pressure, we obtained readings that got us. to the real problem. An internal damper that was blocked open when the drive was installed had come free. It would drop down, closing off air flow. This damper action caused unloading of the fan motor. Eventually the static pressure would get high enough to blow the damper open, allowing the fan to load down. It was like a leaf blowing in the wind in the middle of the duct.

The drive was doing its job, responding to the load changes to the fan motor. Once the damper was blocked open again the drive settled right down and operated correctly.

In retrospect, the troubleshooting process began as soon as we looked at the door meters. We asked how a drive could hunt when the volt/hertz ratio was stable. What would cause current shifts like this, especially on a fan motor?

While this was the fastest troubleshooting we had ever done (less than 2 min), it was the most unusual example of how a drive can be affected by the load.

Remember, troubleshoot the application first and the drive last. Good Luck... in your troubleshooting efforts.

Solomon S. Turkel is Senior Instructor and Course Author for ATMS, Inc., Baltimore, Md.

Hide comments



Plain text

  • No HTML tags allowed.
  • Lines and paragraphs break automatically.