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VFD Troubleshooting

Modern Variable Frequency Drives are complex. By the mid 80’s, almost all VFDs had adopted microprocessors as their primary design architecture. By the early 90’s, Vector Drive technology was commonplace, and AC Drives began to appear in torque sensitive applications previously thought to be the exclusive domain of DC Drives.

Modern drives are remarkably reliable, and with almost all adjustments being digital, they rarely need tuning once started and commissioned properly. In the early days of service companies, including here at EMA, it was commonplace for field service engineers to carry a supply of transistors, resistors, and integrated circuits along with a soldering iron on most service calls. Many long nights and days were spent troubleshooting down to a discrete component level, and even changing circuit gains to stabilize a drive by swapping resistors in and out of an operational amplifier circuit. Thankfully, for the most part, those days are over.

Here are a few generic tips for troubleshooting a VFD .

1. Check the input bridge.

This is done with power removed, and waiting a bit until there is no voltage on the DC bus. Switch your digital VOM to Diode Check mode. Place the negative VOM lead on the positive DC bus. This is often labeled as “+” on or near the power terminals. Then place the positive VOM lead on each of the incoming three phases. You should read a forward diode drop (.3 to .6 volts) on each leg. If this checks out, then place the positive VOM lead on the negative bus. This is usually labeled “-” on or near the power terminals. Then place the negative VOM lead on all three incoming phases and again, look for a forward diode drop. If you read a short, then the input bridge is shorted, if an open, then likely the charge resistor is open.

2. Check the output devices.

Keeping the positive VOM lead on the negative bus “-“, place the negative VOM lead on each of the three motor output terminals. You should read again, a forward diode drop. Place the negative VOM lead on the positive bus “+”, and the positive VOM lead on each of the motor output terminals. Look for a forward diode drop. If you read a short, then the output device is shorted. If you read an open, then either the output device is damaged, or the DC Bus fuse is open.

Physically inspect the output devices. (They can fail in a manner where they can be physically exploded, but you’ll still read the diode drop.)

3. Check the bus capacitors.

Physically inspect the capacitors for signs of damage. This can include a cracked or deformed casing, or the pressure plug popped out of the top. Then measure from the positive to negative DC bus with your VOM set to OHMS. You should not see a short.

If your drive has several caps, it is possible that one could be open. The only way to be certain of this is to put an oscilloscope on the bus and look for ripple. Caution, this is a power on check, and should only be done by technicians familiar with doing such tests on a high energy circuit.

4. Check the internal power supplies

Assuming, that all of your power off checks pass, then apply power to the drive. In most VFDs, the power supplies are obtained from the DC bus via one or more switching power supplies. A quick check of this is if the keypad lights up. If it does not, then your switching power supply is dead. (assuming that your bus charge light indicates bus voltage).

5. Check the program

Many of EMA’s service calls on reportedly faulty drives turn out to be programming errors. It is possible, although it should be used as a last resort, to reset all programming to factory defaults. This is done differently by different manufacturers, and is usually detailed in the VFD manual. This puts the VFD back to the default programming that it came with when new.

A very common error:

The drive will be programmed to get its reference or its stop and start from the keypad instead of remotely. Many drives now have indications on the keypad if the drive is set for Remote or Local operation. (Local is keypad control).

Know what the most common component failure is in many VFDs?cooling fans! The drive manufacturers are well aware of this, and have made great strides in correcting this problem. But, check the cooling fans. A fan failure can lead to nuisance tripping, but even worse, excessive heat damages and reduces the life of drive components.

EMA regularly offers classes on AC Drives at various locations across the country; click the Training tab for further information.

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