Troubleshooting Your VFDs

Aug 1, 2000 12:00 PM, By Stan Turkel

Find more articles on: Variable Frequency Drives

Variable-frequency drives (VFDs), also known as adjustable-speed drives (ASDs) have become the preferred method of controlling speed to meet load requirements. The most common drives use a pulse width modulation (PWM) design, which is affordable, reliable, and cost effective for most applications.

While simple in their design, they can give you problems when it comes to taking operational and troubleshooting measurements. Knowing what measurements to take will save you time and money. You'll need this type of testing for troubleshooting and diagnosing a defective unit as well as when performing routine maintenance.

Where do you start taking measurements? The four main aspects for testing a drive system are the building's power supply, the drive unit itself, the motor, and the load. Each item in a drive system works together and becomes the entire drive application. This creates a potential problem: Any one item can shut down the drive. Knowing what measurements to take at each section is critical for the troubleshooting and maintenance of the drive.

The facility's power supply. In today's power-hungry society, it's getting hard to obtain a good source of clean non-distorted power. Over and under voltage conditions greater than plus-or-minus 10% will trip most drives. A voltage unbalance between phases of 3% to 5% can cause tripping of the drive's overload fault protection device.

With the drive in operation and carrying a load, measure the incoming line voltage at the input side of the drive itself. Using safety precautions, measure the incoming line voltage between A-B Phase, B-C Phase, C-A Phase. Sometimes, you may want to also measure A, B, and C, to ground. What you want to look for is over and under voltage conditions as well as unbalance between the phases. It's important to take the above readings during peak loads on the drive as well as during off-hours to see if you're experiencing voltage swings as a result of the dynamics of your facility. Then, take a current reading of each of the three phases on the line side of the drive. Again, look for any unbalance between phases.

The following formula will help you calculate the percentage of unbalance between phases.

Percent Voltage Unbalance = Maximum deviation from the average voltage x 100 / Average voltage

Taking readings in the VFD. There are several measurements you should take at the drive that involve getting into close quarters while the drive is in operation. Use caution when testing a powered drive unit. Remember to adhere to all safety procedures while taking these measurements.

PWM type drives take incoming AC line voltage and rectify this to a constant DC voltage that is then supplied to the switching, or inverter section, to create an adjustable alternating frequency and variable-voltage source to the motor. Measure DC bus voltage in the drive for over and under voltage conditions, which can be generated by line power changes or load regenerative conditions.

There are two important DC bus voltage measurements you should take. The first is the actual DC bus voltage, which should be equal to the line-side peak voltage (rms voltage x 1.41). Once the capacitors are charged, the reading should remain constant. On a 480V system, the DC bus voltage will be about 676VDC. You should take the second DC bus voltage measurement to determine the amount of AC ripple found on the DC bus. This reading helps pinpoint capacitor breakdown and reduced filtering of the DC bus, which can cause current trips.

DC bus voltage measurements. With the drive in operation and carrying a load, take a voltage reading at the DC bus. You take this measurement at the connections to the drive capacitor or capacitor bank. (See drive manual for exact location). Set the meter on DC volts and measure the positive and negative sides of the DC bus. This should be equal to the line voltage x 1.41.

Now, remove the meter from the circuit and set it on AC volts and take the same measurement. The meter should show very low AC voltage ripple, as this is a filtered DC source. You should discuss readings above 5VAC with the drive manufacturer, as this may indicate a possible breakdown of the capacitor filtering.

All drives maintain a constant volts-per-Hertz ratio to the motor. This ratio is kept constant, regardless at what speed the drive operates. Thus, as the frequency (Hz) changes the motor speed, so does the voltage. The only exception to this rule comes with flux vector drives. These types of drives may change the ratio, depending on special torque requirements. One of the most effective ways of troubleshooting a drive is to verify that the volts-per-Hertz ratio is being maintained at different speed settings.

Drive output volts-per-Hertz ratio measurement. Using safety precautions, set the analog meter for the maximum AC volts. With the drive running at full speed (60 Hz), measure the voltage to the motor at the drive motor terminals. For example, a 460V motor operating at 60 Hz should have a ratio of 7.6V applied to the motor for every Hertz applied. The voltage should be equal to the nameplate voltage for the motor.

Now, set the drive to 50% speed (30 Hz), and take the same motor terminal voltage reading. It should now be half of the last reading, or 230V for a 460V motor. Then, adjust the drive to 25% speed (15 Hz), and the motor voltage should now be 25% of the full voltage reading, or 115V.

What about leakage current? Drive problems can also appear if the leakage current from the drive's power transistors is excessive. A transistor does not actually open up like a mechanical switch when it turns off, it just reduces the amount of current it lets through. Sometimes a transistor that starts to become defective will show signs of excessive leakage current when turned off.

Transistor leakage current measurement. With the drive energized and a run command given, set the drive to zero speed (0 Hz) and measure the voltage to the motor between phases. In this state, the drive should not be firing any of the transistors, and there should be 40V or less leakage, depending on the manufacturer. You should discuss voltages above 60V with the manufacturer. This higher reading may indicate a pending transistor failure.

The motor. Even though you took the voltage and current readings at the motor terminals in the drive itself, you will also want to take the same measurements at the motor. Your meter should be the same as used at the drive. The analog meter gives you smoothed readings and should match the expected volts-per-Hertz ratio.

Essentially, the voltage and current values should be the same. A voltage drop or poor connections may be cause for concern. Also check for any unusual vibrations (vibration is usually a sign of excess bearing wear).

Taking temperature readings. We all know many electrical failures are the result of excessive heat, which breaks down insulation of conductors and windings. Temperature readings of motors, conductors, and heatsinks of electronic components are valuable diagnostic measurements, and you should consider them part of a yearly maintenance program. The key is to be consistent in the location of the readings, and to take them under similar loading conditions. There is much information available from the vendors of temperature probes and meters.

The load. If it turns, then it must be okay, right? Depending on your application, there are still a couple of things you may consider measuring. If you're concerned about speed regulation for a process, then a tachometer reading at full load conditions is in order. Always verify rotation direction. I have uncovered instances where equipment was simply operating backward; and unknown to those using the equipment.

The torque stresses the shaft and drive train components. Having started up hundreds of drive applications over the years, I still can't understand why folks need to ramp up these large loads in 10 sec or faster. It just makes sense to adjust the ramp speed of a load to as long as possible to reduce all kinds of stresses; both mechanical and electrical.

There are two important areas not included in this article: harmonics and overvoltage reflections at the motor windings. Although harmonics is a concern, it's not normally associated with the tripping of drives and shutdowns. You can usually control overvoltage reflections at the motor windings by keeping the motor leads as short as possible. Several motor manufacturers can provide special windings to withstand such overvoltage conditions.

Turkel is a senior instructor, ATMS Technical Training Co., Owings Mills, Md.

Sidebar: What Type of Testing Meter Should You Use?

You can use any clamp-on true digital multimeter (DMM) Cat. 3 rated for testing motor drives. The best recommendation is a clamp-on 1000V Cat. 3 unit. A true rms clamp-on DMM, or an AC-only clamp-on attachment for a DMM will work. If your meter is not rated Cat. 3, then don't use it.

Sidebar: Use an Analog Meter to Test the Load Side Output of the Drive

You would expect to use a true rms DMM to test the load side output of the drive. But for this measurement, the DMM might give you an incorrect reading. Why?

The output of a VFD is a series of very high transmittal oscillating positive and negative voltages. Using IGBT transistors, the oscillations approach 20 kHz, which vary in base frequency and duration (width of pulse, hence the name pulse width modulated drives). A digital multimeter takes many samples per second and converts this analog information into digital information for display. Using a digital meter for output readings causes a problem: It will attempt to follow the high-frequency switching of the IGBT transistors, giving false information. The analog meter has a smoothing effect and ends up reading a voltage equal to what the motor actually sees.