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AC Drive Failures
multible drive failures

Have had three AB power flex 70 frame 8 drives fail losing bus caps and transistors. with no trip alarms over/under bus voltage, over current. Have monitored line voltage for six months finding no problems. Has anyone else had similar problems

By Brian Beal on 27 March, 2014 - 5:54 am

Are you cycling line power frequently? Each time you apply line power, the DC bus is charged through a circuit that typically has a thermistor. Frequent power cycling heats up this circuit.

if you are blowing caps the voltage rating of the drive circuit is being exceeded.

you have to check the voltage on the caps and the incoming ac voltage againts the respective ratings,

with a lot of equipment being manufactured who knows where, you run into nominal voltage ratings that may not be compatable with your area,

case in point we had a system rated at 3ph 440 vac blowing caps left and right,

that particular unit was made to EU 400 vac ratings, we and were assured by the factory that 440 vac was okay, but the power company had the transformer taps set at 500 vac!

Only the high dollar model could handle that, our units could not, the factory seemed to be living on another planet, they had no clue, but replaced all units as they had performed the installation and checkout!

How long are the leads between the drive and the motor?
Do you have any line reactors or filters in place?

1 out of 2 members thought this post was helpful...

Gray Wolf... What has AB said?


It is clear that the DC-Link is being overloaded (overvoltage). Does the application work in regenerative mode? In other words, does the motor brake the load often? If yes, you can try extend the deceleration ramp or install a brake resistor to make the DC-Link voltage lower when stopping the application.


There is no "Frame 8" in a PowerFlex 70 drive, although there is a Frame B.

Generally, this sort of issue is the result of one of two issues:

1) Extreme line transients, sometimes too fast for the drive Over Voltage protection to respond, in which case it might be too fast for an inexpensive meter to detect as well. You would need a feast sampling recording meter, like a Fluke Scope meter or a Dranetz meter to see it.

2) You have a line contactor ahead of the drive and are opening the contactor every time you shut down. All drives have a pre-charge circuit, in this drive is is a resistor that is shorted out by a contact after the bus charges. By constantly cycling power to the drive, you burn out that resistor, after which the caps and transistors fail in short order.