Allen Bradley Flex 70 VFD Overvoltage of DC Bus

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seawillie

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Hello,
Has anyone had an A/B VFD go bad by burning out internal dc bus components? Size of our motor is 50Hp. Application is a large volume pressure washer unit. Our ramp time is 3 seconds and we have NO DC braking resistor. We have lost 3 VFD's. We had a visit from an A/B tech rep and followed his suggestion's. Jumpers were checked for internal configuration on an ungrounded 460 V Delta system.......A line reactor was installed. Power in was monitored for quality. Still we have lost another one.
My question is.....Is it possible to have the ramp up and down so fast that it would cause an excessive DC Buss Voltage big enough to burn out the internals of the VFD. We once again have NO Braking Resistor.
We will be doing one or more of the following:
Slowing the ramp time......Oversizing the VFD.......Installing the Braking Resistor........Isolation Transformer......
But for now frozen in time before we do those things I am wondering if the design guys would have had this happen in the test lab and confirm when a braking resistor would be needed?
We have 2 of the units with the A/B forensics (research) department evaluating them.
 
seawillie said:
Hello,
Has anyone had an A/B VFD go bad by burning out internal dc bus components? Size of our motor is 50Hp. Application is a large volume pressure washer unit. Our ramp time is 3 seconds and we have NO DC braking resistor. We have lost 3 VFD's. We had a visit from an A/B tech rep and followed his suggestion's. Jumpers were checked for internal configuration on an ungrounded 460 V Delta system.......A line reactor was installed. Power in was monitored for quality. Still we have lost another one.
My question is.....Is it possible to have the ramp up and down so fast that it would cause an excessive DC Buss Voltage big enough to burn out the internals of the VFD. We once again have NO Braking Resistor.
We will be doing one or more of the following:
Slowing the ramp time......Oversizing the VFD.......Installing the Braking Resistor........Isolation Transformer......
But for now frozen in time before we do those things I am wondering if the design guys would have had this happen in the test lab and confirm when a braking resistor would be needed?
We have 2 of the units with the A/B forensics (research) department evaluating them.
Click to expand...

how would a brake resistor help you?

it only deals with stored energy being converted back to electrical energy through motor regeneration and a pump really does not store enough energy to worry about.

most any electrical fault on modern drives will trip the drive long before it harms the drive any.

what component(s) are damaged?

do you get a dc bus overvoltage fault when this happens? is that why you suggested a brake resistor?
 
Braking Resistor

Braking Resistor

Hello,
We are getting fault 24 (Decel Inhibit)-Drive is not following commanded acceleration or deceleration because it is attempting to limit bus voltage. <= by the manual. In it's list of corrective actions it refers to the braking resistor. The motor is belt driven to the pump. Inertia would be the stored energy. I am wondering if the command voltage to slow the motor with a fast ramp speed can be SO far away from the lagging DC buss voltage that it would cause components to fail.??
 
seawillie said:
Hello,
We are getting fault 24 (Decel Inhibit)-Drive is not following commanded acceleration or deceleration because it is attempting to limit bus voltage. <= by the manual. In it's list of corrective actions it refers to the braking resistor. The motor is belt driven to the pump. Inertia would be the stored energy. I am wondering if the command voltage to slow the motor with a fast ramp speed can be SO far away from the lagging DC buss voltage that it would cause components to fail.??
Click to expand...
If repeatedly ignored, it could. It would have nothing to do with your accel rate, only decel. If attempting to decel too fast, the motor regens back to the DC bus and if there is nowhere for the energy to go, it has to dissipate through the pre-charge resistor circuit. If you do this over and over, the resistor can burn out and the next time you power the drive, the capacitor charging current spike can do further damage.

So first off, why are you attempting to decelerate a pump really fast? Do you need to decelerate it at all? Why not just let it coast to a stop? if it's a PD pump, as I might expect on a pressure washer, it will stop fast all by itself.
 
Great Thank You

Great Thank You

I have put in the request to change the ramps times. And no we do not have any reason to ramp down.....coasting would be fine....Thank You for the reply
 
I have melted many a drive by decelerating too quickly. Also have melted braking resistors!

Decelerate more slowly, and install a braking resistor. Make sure it is properly sized - your AB rep can help you calculate this. Make sure the drive and the resistor have proper ventilation and are not subjected to high temperatures.
 
bward said:
I have melted many a drive by decelerating too quickly. Also have melted braking resistors!

Decelerate more slowly, and install a braking resistor. Make sure it is properly sized - your AB rep can help you calculate this. Make sure the drive and the resistor have proper ventilation and are not subjected to high temperatures.
Click to expand...
But again, I fail to see the value in using braking on a pump at all. Just turn it off (set to Coast to Stop).
 
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