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Variable Frequency Drive (VFD) Troubleshooting and Maintenance


In the United States 50% of the total electrical energy generated is consumed by rotating equipment. 65% of this total is consumed by centrifugal or flow related applications such as fans, blowers, compressors, and pumps according to current estimates. Variable speed drive technology offers a cost-effective method to match driver speed to load demands and represents a state-of-the-art opportunity to reduce operating costs and improve overall productivity.

You can divide the world of electronic motor drives into two categories: AC Drives and DC Drives , a motor drive controls the speed, torque, direction and resulting horsepower of a motor. AC drives control AC induction motors. DC drives typically controls a shunt wound DC motor, which has separate armature and field circuits.

Motor drives are also known as DC Drives,  AC Drives, SCR Drives (Silicon Controlled Rectifier Drive), Variable Frequency Drives, Variable Speed Drives, Adjustable Speed Drives, Electric Motor Drives, Electronic Motor Drives, AC Motor Controllers, AC Inverters, Pulse Width Modulation (PWM) Drives and a few other names. We will be limiting our discussion to Variable Frequency Drives in this article.

What’s a Variable Frequency Drive (VFD)?

A VFD controls the speed, torque and direction of an AC Induction motor. It takes fixed voltage and frequency AC input and converts it to a variable voltage and frequency AC output.

Well involved control circuitry coordinates the switching of power devices, typically through a control board that dictates the firing of power components in the proper sequence. A microprocessor or Digital Signal Processor (DSP) meets all the internal logic and decision requirements. This description shows that a VFD is basically a computer and power supply.

Trouble Shooting a VFD

A typical block diagram of a Pulse Width Modulated (PWM) VFD is shown below. PWM drives are the most prevalent type of AC drives being marketed today. You’ll see that the AC line is converted to DC (within the converter Section of the drive) and then synthesized back into a variable frequency, variable voltage output. By doing this, motor speed is varied by changing the frequency, and motor torque is maintained by keeping the volts to frequency ratio constant (for most applications).

At the user end, you are going to do very little electronic circuit board repair on a modern, microprocessor based motor drive. Fortunately, most failures are not on the circuit boards, but rather within the power sections and that isn’t really difficult to troubleshoot. We , at our end, can repair these boards, but a typical plant maintenance person simply isn’t likely to develop any expertise in repairing them.  Effective troubleshooting, on a VFD requires a methodical approach. There are two techniques, one is the classic divide and conquer, and the later as we term it is stochastic. The classic method is particularly effective when knowledge of the equipment is only rudimentary. It is best illustrated in the diagram below. Assume a signal is present at A, but is not at B. How many steps a good troubleshooter would use to isolate the box that isn’t passing the signal?

The answer is three! A typical troubleshooter would divide the circuit in half, by checking between boxes 4 and 5. If the signal were absent, then the next check would be between blocks 2 and 3. If the signal were present here, then a final check between blocks 3 and 4 would isolate the problem to either block 3 or block 4. This is, divide and conquer method.  Stochastic troubleshooting takes advantage or in-depth knowledge and experience, and attempts shortcuts. Stochastic refers to an educated guess, based upon random tests and observations. Typically, the more familiar a repair technician is with any particular piece of equipment, the better he can deduce the problem from random tests, without a time consuming disassembly in order to make measurements, as would be required with the classic method of troubleshooting.

Steps to Troubleshoot A Dead VFD.

1.      First be careful. The capacitors within the power section can maintain a lethal charge even after the power is removed. Don’t put your hands into the power section before determining that the capacitor voltage has been discharged.

2.      With the power off, check the power sections. Here’s how. Place your digital Multimeter in the Diode check mode. Find the positive DC Bus (Sometimes this brought out to terminal, sometimes it isn’t). Place your negative lead on the + Bus, and then check in turn with your positive digital Multimeter lead each incoming phase. You should read a diode drop from each phase. If it reads open, then the charge resistor (see diagram 2) is open. This is a common failure.

3.      Next place the positive digital Multimeter lead on the – Bus, and place the negative digital Multimeter lead on each incoming phase again. You should read a diode drop, not a short or an open. Place both digital Multimeter leads on the Bus, one on the + and the other on the -. You should read the capacitors charging; you should not read a short.

4.      To check the inverter section, place the positive digital Multimeter lead on the – Bus, and reach output phase by placing the negative digital Multimeter lead on each one. You should not read a short, and in fact should read a diode (there are diodes connected across each output transistor). Check the remainder of the inverter section by placing the negative digital Multimeter lead on the + Bus, and checking each output phase again with the positive digital Multimeter lead. You should not read a short, but rather a diode drop again. If you read open from either of these checks, then the Bus fuse is open -see Diagram 2. (The charge resistor and the Bus fuses may be in the + or – Bus, depending upon the manufacturer).

5.      If no problems are present within the power section, and the unit will still not go, you have it either connected improperly, programmed wrong (the most common problem we find), or you have a bad circuit board.

6.      If you do have a shorted transistor ….and here’s a word of caution. If you have an older generation PWM drive which uses Darlington transistors in the Inverter section, be careful if you find one shorted. When they short the almost always fail the base driver circuit as well. Latest PWM drives use IGBTs in the output section, and they are much less likely to fail the driver sections.

7.      Nervous about all this? The cost of VFDs has dropped dramatically within the past few years, along with an increase in features and reliability (just like computers). So, buy a spare and sent the broken AC Drive to us for repairs! We earn our bread & butter repairing these for you! In addition, we also do all industrial electronic controls repairs, servo motor repair, servo drive repairs, encoder repairs, resolver repairs, industrial power supply repairs, chart recorder repairs, industrial monitor repairs, human interface repairs, teach pendant repairs, timer/ counter/ temperature controls repairs. You name it and we have the capabilities.

Recommended Maintenance Requirements for VFD.

  • Keep it Clean. Dust on VFD hardware can cause a lack of airflow, resulting in diminished performance from heat sinks and circulating fans. Dust on an electronic device can cause malfunction or even failure. Dust absorbs moisture, which also contributes to failure. Periodically spraying air through the heat sink fan is a good preventive maintenance measure. Discharging compressed air into a VFD is a viable option in some environments, To use compressed air for cooling, you must use air that is oil-free and dry or you are likely to do more harm than good. That requires a specialized, dedicated, and expensive air supply. Plus you still run the risk of generating electrostatic charges (ESD).
  • Keep it Dry. Water and moisture produce circuit board corrosion. Moisture is enemy number two after dust. So try & keep the drive area moisture free.
  • Check Mechanical Parts. Check circulating fans for signs of bearing failure or foreign objects – usually indicated by unusual noise or shafts that appear wobbly.
  • Check Capacitors. Inspect DC bus capacitors for bulging and leakage. Either could be a sign of component stress or electrical misuse.
  • Spare VFDs. Store them in a clean, dry environment, with no condensation allowed.
  • Monitor Sink Temperature. Most VFD manufacturers make this task easy by including a direct temperature readout on the Keypad or display.