Rockwell PowerFlex700 with Heavy Load

Hi,i've got a big problem dealing with a 15KW vertical drop lift driven by a PF700.
Here's the thing,first,PF700 shows HW overcurrent alarm when run for a while.i connect a breaking unit with dynamic Resistor onto the convertor.After that,Continuous running seems OK,but when the time bewteen start & stop is very short(ex:trigger it then stop),PF700 still shows HW overcurrent.i think the breaking unit & resistor is OK with this load,so what's the problem now?,HELP!

i haven't worked with them, but you might want to check the settings in regards to the over / undervoltage settings.

out units fault out until i increase this value.

regards,
james

James Mcquade said:

i haven't worked with them, but you might want to check the settings in regards to the over / undervoltage settings.

out units fault out until i increase this value.

regards,
james

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Thx,I'm gonna check it tomorrow.

An overcurrent limit is typically an overload situation in the positive direction: you're either accelerating the drive against a physical brake or against a heavy load and the drive cannot supply sufficient current.

An overvoltage limit is typically found when you are lowering a load or decelerating the drive and the DC bus is raised by the back-EMF regeneration of the decelerating load. This is where a dynamic brake and resistor pack might be involved.

The PowerFlex 700 drives have a torque proving feature that makes them suitable for equipment hoist applications. Hoist applications can be complex and dangerous and I strongly recommend that you obtain qualified local assistance.

as Ken said, this sounds like you should be using a professional in this area. I certainly wouldn't be changing the drive current limits above what the motor is rated at.

Check your ramp times, very fast ramp times (especially when stopping a high inertia load or changing direction) can cause over-currents. I've got no idea what a suitable ramp time would be in a lift application, I luckily don't have to work with things carrying people!

Ken Roach said:

An overcurrent limit is typically an overload situation in the positive direction: you're either accelerating the drive against a physical brake or against a heavy load and the drive cannot supply sufficient current.

An overvoltage limit is typically found when you are lowering a load or decelerating the drive and the DC bus is raised by the back-EMF regeneration of the decelerating load. This is where a dynamic brake and resistor pack might be involved.

The PowerFlex 700 drives have a torque proving feature that makes them suitable for equipment hoist applications. Hoist applications can be complex and dangerous and I strongly recommend that you obtain qualified local assistance.

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Yeah i've seen the board,BUS DC increase upto 690VDC when decelerating from 50Hz to 5Hz.(My high speed is 50Hz,Low is 5Hz,Ramp Time is 1.5 sec).& I'm dealing with automotive body.STOP MODE is DC BRAKE.with external dynamic brake unit & external resistor.

Hardware overcurrent is the last line of defense for the drive before it melts down an IGBT. As errors go it tends to be much less forgiving of false trips but it is also a much higher current than most other overload errors. Generally speaking HW overcurrent will only occur if the drive is connected to a load type that the drive can't control the current of using it's normal current regulation means (it starts controlling to a current but the current rises so fast it can't regulate it). I have only seen a HW overcurrent twice in the last 10 years. One was on a PF70 that was ultimately deemed defective. The other was very recently on a PF700 that was connected to a motor HP larger than the drive rating.

Look for intermittent shorts and megger the motor. I suppose it could also be a supply power quality issue.

Keith

I see HW overcurrent occasionally (a few times a year in our washdown food plant) and every time it has ended up being caused by a wet disconnect or junction box, or shorted motor winding. I am not 100% sure why we see HW overcurrent instead of ground or phase faults but I suspect it has to do with having long motor leads with multiple ground points in most cases.

If you are getting HW overcurrent on a good, healthy motor circuit, then this tends to indicate that your drive is too small for the load, and/or your motor FLA setting in the drive is incorrect. If it is indeed a high current situation, you should have a Motor Overload fault first.

Since HW Overcurrent on newly installed Architecture Class PowerFlex VFDs has been our nightmare for the past five years or so (since starting replacing Legacy 1336s), one quick note...
Circuits (motor & field wiring) which have been running trouble free for years on 1336 VFDs started to fault on HW Overcurrent shortly after the installation of the PowerFlex drives.
Applications tweaks, grounding, isolation transformers, sometimes multiple VFD and motor replacements...You name it, we did it...To no avail...
What did solve the issue (and became SOP for every 1336 to PF migration implementation) was the replacement of the field (load side) wiring of every problematic application...
Apparently, the PWM (Carrier Frequency) has the tendency to deteriorate the insulation properties of the "standard" (THHN class)conductors since HF radiation generates ozone which will alter the Teflon's electrical properties, especially within sharp direction changes sections.
Evidently, replicating reflection, wave combining or resonance using common test instruments is almost impossible to achieve and most of the time the circuit is determined to be "good".
IMHO, if old, non-inverter duty , HF exposed wiring is present within HW Overcurrent issues applications, the replacement of the field ( VFD load side)runs might just be the needed solution.

dmargineau said:

What did solve the issue (and became SOP for every 1336 to PF migration implementation) was the replacement of the field (load side) wiring of every problematic application...

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Good info, thanks. So apparently the 1336 were not sensitive to degraded insulation, while the PF700 is?

I believe that the PF's default PWM is 7KHz (It is user adjustable, however, if modified, the VFD needs to be derated!), probably a lot higher than the 1336 since the PF motor control is significantly "tighter".
Evidently, the higher carrier frequency will cause (or accelerate) the conductors' deterioration.
The frequency of the HW Overcurrent occurrences within the replacement PF applications was also considerably higher when the load field wiring of multiple VFDs was routed within the same conduit run.
I think we are already "talking" EMI.

dmargineau said:

I believe that the PF's default PWM is 7KHz (It is user adjustable, however, if modified, the VFD needs to be derated!), probably a lot higher than the 1336 since the PF motor control is significantly "tighter".
Evidently, the higher carrier frequency will cause (or accelerate) the conductors' deterioration.
The frequency of the HW Overcurrent occurrences within the replacement PF applications was also considerably higher when the load field wiring of multiple VFDs was routed within the same conduit run.
I think we are already "talking" EMI.

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I've been using PF series for years,from 40 to 700.& I'm gonna using 755 in next GM project.HW Overcurrent is rare to see indeed.We always choose PF one level higher than the motor depending on the current,so when the motor is 15KW,we use 22KW PF to match it.But considering the high speed and the short decelerate distance,I'm afraid this is not enough......

i would strongly recommend that before you look at replacing your load side wiring, that you give it more time to decelerate. also, starting and stopping it fast might not be a good idea, sometimes its a good idea to have a minimum runtime timer to prevent it from stopping right after starting. if you do replace the field wiring, there are inverter rated motors and cables, that can handle those carrier frequencies.
I also recommend the practice of slightly oversizing VFDs, if possible.

Drive overcurrent faults can usually occur with excessive motoring current and with excessive braking currents. It sounds like this problem is on the braking side.

Since this only occurs during decel, I would not run out and replace all the wiring or replace the motor. Instead, a careful measurement of braking current needs to be made. If it exceeds the short-term rating of the drive, either the brake ramp needs to be lengthened a bit or you need a bigger drive.

This sounds like a case where the braking kw is higher than the motoring kw and the components were sized for motoring only. It's an easy mistake to make.

Reminds me of a job I did many years ago in a large tire manufacturing plant. The motoring hp was 15 and the braking hp was 82. The drive and motor looked to be way too big but everyone else's "motoring only" sizing did not work!

dmargineau said:

What did solve the issue (and became SOP for every 1336 to PF migration implementation) was the replacement of the field (load side) wiring of every problematic application...

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Also a thanks from me, as I will most likely be replaceing a few 1336s this summer.