Before you rip that variable frequency drive (VFD) off the wall and send it in for a rebuild, stop.

Some simple checks diagnose a VFD in moments. Here’s how to make the necessary inspections with a multimeter.

By Daniel Schubert • Yaskawa Product Training Engineer

Variable frequency drives (VFDs) are a staple component of many industrial and commercial applications in which motors run to complete tasks. VFDs can control and protect motors and in some applications, even offer energy savings. But as with any system component, VFDs can fail. Here we explain some troubleshooting techniques that engineers and plant personnel can use to check and get VFDs running. The checklist for the major no-power checks we present here include:

• Safety — with the system at less than 10 Vdc
• An input check — as a diode check
• A dc bus check — as a visual check
• An output check — as a diode check
• A review of checks

Upfront warning: Safety when working with VFDs

Our number one concern is you the reader, so if you don’t feel as though you possess the experience to perform these tests, contact a professional to perform them for you. Life-threatening voltage and current are present in the VFD even after the incoming supply is removed. Before testing, follow the lockout/tagout procedures for the system at hand. After this is completed, follow arc-flash procedures for the given system and follow local codes.

Locate the +(positive) and –(negative) dc bus terminals on the drive. Refer to the manual for the unit or contact the manufacture of the VFD if you’re unsure of the location of these terminals.

The multimeter should have at least a 1,000-V CAT III rating and the ability to make diode checks.

Set the multimeter to Vdc. Connect the +(Red) and -(Black) leads from the meter to the dc bus terminals on the VFD. If the value is above 10 Vdc but reducing, wait until the excess dc bus voltage is below 10 Vdc. This time varies depending on the drive capacity. If voltage doesn’t fall below 10 Vdc, ensure input power to the drive is removed or contact the VFD manufacturer or installer.

VFD input check (at the rectifier)

In today’s VFDs, the input or rectifier section is comprised of input diodes that convert an incoming three-phase ac sine wave into a rectified dc supply. There are at least two diodes for each phase. They are positioned in opposite conducting orientation to allow full-wave rectification. To check the input section, we need to perform simple diode checks. These checks include testing the forward and reverse bias direction of both diodes in each phase. This process uses the input terminals R/L1, S/L2, T/L3 on the drive and the dc bus terminals.

If you are unsure about where the input terminals are, refer to the VFD’s manual.

With a multimeter set to diode check …

… put the +(Red) lead on the input terminal (R/L1) and the –(Black) lead on the (+) DC bus terminal. This isolates the positive R/L1 phased diode. A good diode should read around 0.5 Vdc in the forward-bias direction. Repeat this process for the S/L2 and T/L3 terminals while leaving the –(Black) lead on the (+) dc bus terminal.

Note: When making this measurement, look for consistency across the all three input terminals. The measurement of 0.5 Vdc is only an approximation and can change depending on the VFD and model size. If at any time the meter reads 0 V, then that diode is shorted.

The next step is to check the reverse bias direction of the diodes. Move the –(Black) multimeter lead to the R/L1 terminal, and move the +(Red) multimeter lead to the (+) dc bus terminal. Then check the remaining two inputs by moving the –(Black) multimeter lead to the S/L2 and T/L3 terminals. The multimeter should eventually display (OL) after charging the drive’s filter capacitors. OL occurs when the power supply in the meter is unable to force current through the diode in the set direction.

We are now done with the upper diodes and need to check both directions of the remaining diodes on the rectifier. We start by placing the +(Red) multimeter lead on the (-) bus terminal and the –(Black) multimeter lead on the R/L1 terminal. This again should read around 0.5 Vdc. From there, move the –(Black) multimeter lead to the remaining S/L2 and T/L3 terminals while looking for consistency between the three measurements. Some engineers consider a variance of more than 0.05 Vdc between them to be a bad sign, as this could mean that one or more diodes need replacement.

Finally, move the -(Black) multimeter lead to the (-) dc bus terminal and the +(Red) lead to the R/L1 input terminal, then check the S/L2 and T/L3 input terminals, again making sure that the meter reads OL after a short time charging the filter capacitors. When charging the filter capacitors, the time can vary and increases with drive size.

We have now checked all the diodes in both bias directions. If at any time the meter reads 0 V, then that diode is shorted.

DC bus check on VFDs

After the diodes rectify the incoming ac wave into dc, the dc bus or dc capacitors store the voltage and have a smoothing effect on the dc bus voltage ripple. To fully check the capacitors, an engineer or plant worker would need to pull the individual capacitors out of the system and use a tester supporting high-microfarad capacitors.

In lieu of this (for no-power checks) visual inspection for any signs of physical damage or electrolytic fluid leaking from the capacitor is sufficient. Sometimes it’s even possible to smell if the capacitor isn’t good anymore … and the smell will most likely be a strong scent. If the unit in question has a lot of hours on it and maintenance personnel are already replacing other components, it’s not a bad idea to go ahead and replace the dc bus capacitors.

VFD output check (at the inverter)

The third and final section is the output or inverter section. This usually consists of insulated gate bipolar transistors (IGBTs). The IGBTs take stored dc from the bus capacitors and work together to form a simulated ac output wave to the motor. The VFD uses pulse-width modulation (PWM) to control the voltage and frequency applied to the motor. The IGBT includes an emitter, collector, gate and free-wheeling diode. The VFD modulates the pulses applied to the motor by varying how long it applies a voltage between the gate-emitter junctions of the IGBT. This is called gating and it happens thousands of times per second.

The gating signals themselves can’t be checked without power and are usually checked once power is applied, and the drive is running with no load — in other words, with no motor. That check involves using an oscilloscope to ensure the IGBTs are gating properly.

The free-wheeling diode completes the output circuit and handles any regeneration from the motor going back into the drive. This regenerated energy is then introduced back into the dc bus capacitors.

Our last check is basically another set of diode checks. Fortunately, in most cases the IGBT fails because the free-wheeling diode has shorted. How do we check this? We check it the same way we did the VFD’s input. More specifically, check the free-wheeling diodes just as the rectifier’s diodes were checked … but this time, use the U/T1, V/T2, and W/T3 terminals instead of the R/L1, S/L2, and T/L3 terminals. If the measurements show a good diode, you’re done. If the measurements show a short circuit (less than 0.5 Vdc in both directions) then you have a shorted IGBT.

Review of checks out-of-range values

Note that these checks cover the major components of the drive’s main circuit. If you have readings that aren’t in this range, then it’s likely that you must remove the drive and either rebuild or replace it.