Powerflex 750 Brake Chopper Duty Cycle 4

[123MB]

Hi All

I have a very specific product question about Rockwell drives. I figure since you're all in the US and love your Rockwell drives you should be able to field this one for me easily.

The question is, what is the design specification duty cycle for the brake IGBT in a Powerflex 750 drive (20F1ANC140JA0NNNNN). I have asked Rockwell tech support but have not been able to get the answers. All they seem to know is that the IGBT is rated for 100% of motor torque as braking torque.

I'll even settle for what these internal chopper transistors are typically rated for, i.e. what's the most realistic brake duty rating - 10%?

Our application is a linear rod pump which will use dynamic braking at a duty cycle of around 50% and the cycles are very frequent - i.e. once every 10 seconds. I.e. the drive lifts the rod and the drill string to the top and then controls the speed (overhauling braking)as the rod drops under gravity. I expect braking torque will be less than 100% motor torque, but duty cycle is expected to be high, hence I am trying to get an idea for the chance that the brake IGBT is going to die.

This is all contributing to the decision to provide line side isolation of the drive to cater for the event that the brake IGBT fails (shorts closed) and the brake resistor heats uncontrollably. The process guys think that providing the ability to open the line side supply to the drive if the resistor is overheating is not neccesary and that wiring the thermal switch to a drive fault input is enough. I am trying to make the point that this is an arduous application and hence more care needs to be taken.

Thanks

 

[waross]

So the IGBT is rated for 100% motor torque and your application is less than 100% of motor torque and your duty cycle is about 50%.
The duty cycle of the IGBT will depend on the current through the IGBT and is meaningless without a stated current.
I don't see the duty cycle being an issue until the % current squared times the application duty cycle exceeds 100%.
Your current is less than 100% of rated current and the application duty cycle is less than 100%.
You may want to evaluate the Wattage rating of the braking resistor compared with rated motor current and a 50% duty factor.
A properly selected resistor should not overheat.
This may be a more productive avenue to address your concerns.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[controlsdude]

The question is, what is the design specification duty cycle for the brake IGBT in a Powerflex 750 drive (20F1ANC140JA0NNNNN)
PowerFlex 753 AC Drive, with Embedded I/O, Air Cooled, AC Input with Precharge, no DC Terminals, Open Type, 140 Amps, 75kW ND, 55kW HD, 400 VAC, 3 PH, Frame 6, Filtered, CM Jumper Installed, DB Transistor, Blank (No HIM)
<< p/n gives you this, assuming your using this for heavy duty application.

Here is a pdf that would calculate it from rockwell. Maybe this will answer your questions.

http://literature.rockwellautomation.com/idc/groups/literature/documents/at/pflex-at001_-en-p.pdf

Page 69 "appendix a" gives you the size based on above.

Its all engineered for you, sounds like their standing behind something thats been predetermined.

 

[123MB]

Thanks guys

Well what can I say, if it's a moot question fair enough.

For external brake choppers, for example, the duty of the chopper itself (aswell as the resistor) is selected.

But it sounds like Rockwell have got it all sorted. I still don't understand exactly how hard the internal brake chopper can be pushed in so far as duty cycle is concerned, but it seems like something I shouldnt have to worry about

Thanks again.

 

[waross]

All they seem to know is that the IGBT is rated for 100% of motor torque as braking torque.

Don't exceed motor rated torque when braking.
If you must de-celerate faster than you accelerate, you may be in trouble.
However, on a 10 second load cycle, if you are exceeding rated motor torque and by implication, rated motor current when braking, then your motor may be in more danger than your IGBT.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

You can also use the A-B Motion Analyzer on-line tool to figure it out for you.

https://motionanalyzer.rockwellautomation.com/

The tool looks as though it is for Servos, but the PowerFlex drives are in there as well.

If you must de-celerate faster than you accelerate, you may be in trouble.

As a generalized rule, you can never electrically or electronically de-celerate a motor faster than it can accelerate. So your "trouble" will look exactly like failure...
The only way to stop a load faster than it can accelerate is via mechanical brakes.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[BrianPetersen]

If there is a gearbox or coupling or some such thing attached to that motor, and that drive mechanism was sized based on motor rated torque (which would normally be the case), and you slam it to a stop with any sort of brake much faster than it would accelerate under motor power ... you may break stuff!

 

[123MB]

Thanks for the link jraef, I had a look at the tool but unfortunately I have not yet had a chance to set it all up with my data etc but I hope to very soon. Looks like a very handy tool.

In general this load will require > 100% FL motor torque for a short amount of time both when starting and stopping.

However the drive is oversized hence when AB say that the braking IGBT is rated for '100% motor torque' it is probably closer to 120%-130% when the oversizing is considered.

But getting back to the braking IGBT - AB have said that the '100% torque' rating of the IGBT 'is a continuous rating' - in their words.

So does this mean that this IGBT is able to conduct the full load current continuously, i.e. without any off time? seems a bit too much to ask, but is that typically how these integrated brake choppers are sized?

For external choppers I have used in the past, their rating is also tied to the braking duty cycle, hence they have to be specified accordingly.

 

[123MB]

For information all I have confirmed that the brake IGBT on these drives is actually capable of continuous operation at full load for an indefinite amount of time. Thanks all.

 

[jraef]

Actually that depends on the drive in question. On the “Architecture” Class drives like the PowerFlex 70, 700 and now the 750, that’s true. For the “Component” Class drives like the 4, 40 and new 520s, the standard is 10% duty cycle like it is on most small low cost drives. Braking duty cycle of a transistor is about heat dissipation. When small drives are made with IPMs that contain all of the diodes and transistors in one potted unit, the 7th transistor for braking is rated the same as the other six so although rated for the same current, that motor current is no longer divided across 6 units in terms of the heating effects, hence the low duty cycle. On larger drives where they use individual IIGBTs, the brake chopper transistor is larger and often has its own stirring fan on the heat sink. On stand-alone retrofit brake choppers, different duty cycles are based on economics; you don’t have to buy extra duty capability that you don’t use.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[123MB]

Jraef

Thankyou, you are an absolute legend. Exactly the information I was after.

Interesting to know about the 520 drives, we use them alot also here