PowerFlex 755 - Dynamic Braking (Check my math!)

[phuz]

I have a customer who has a centrifuge application. The plant engineer did all the calculations on the centrifuge to arrive at a WK^2 of 6458 ft-lbs^2.
Using the simple formula provided by Eaton, and a starting speed of 650RPM and a final speed of 135RPM over a period of 90 seconds, I come up with a torque value of about 120 ft-lbs. The PowerFlex 755 says that 100% of the motor torque can be used for braking, which in this case is 233 ft-lbs. This tells me that the drive, itself, is more than capable to slow the centrifuge within the time requirement without needing a braking resistor. Does this seem accurate?

 

[jraef]

Sort of...
What the PF755 is telling you is that if you use Flux Braking or DC Injection Braking, you can get 100% of the motor torque as braking torque. But in BOTH of those methods, the kinetic energy of the spinning mass gets converted to HEAT IN THE MOTOR. On something like a centrifuge, you need to consider that in discussion with the centrifuge motor supplier, it's unlikely that you can get away with that.

Using Dynamic Braking where you use the braking resistors will move that kinetic energy OUT of the motor into the resistors. That is much safer for the motor, and so long as you don't need to do it very often, should be fine for the resistors and chopper transistor. Download a manual on how to calculate braking capacity by searching for
PFLEX-AT001K-EN-P

Best practice on a centrifuge is to use a Line Regenerative VFD (like the 755TR) that can pump the energy back into the line source, because you can do that as often as necessary.

 

[PLCMentor]

I had a bad experience with a centrifuge a long time ago. We actually had regenerative braking electronics on it but the drive and the braking electronics did not play well together. I dont think I would ever install a centrifuge without some sort of braking assistance - resistors or regen. What about estop? Does it not have any emergency requirements for faster stop?

 

[phuz]

I had a bad experience with a centrifuge a long time ago. We actually had regenerative braking electronics on it but the drive and the braking electronics did not play well together. I dont think I would ever install a centrifuge without some sort of braking assistance - resistors or regen. What about estop? Does it not have any emergency requirements for faster stop?

There are no requirements on this, but with this mass, I think we'd be limited to a friction brake anyway.

 

[phuz]

Sort of...
What the PF755 is telling you is that if you use Flux Braking or DC Injection Braking, you can get 100% of the motor torque as braking torque. But in BOTH of those methods, the kinetic energy of the spinning mass gets converted to HEAT IN THE MOTOR. On something like a centrifuge, you need to consider that in discussion with the centrifuge motor supplier, it's unlikely that you can get away with that.

Using Dynamic Braking where you use the braking resistors will move that kinetic energy OUT of the motor into the resistors. That is much safer for the motor, and so long as you don't need to do it very often, should be fine for the resistors and chopper transistor. Download a manual on how to calculate braking capacity by searching for
PFLEX-AT001K-EN-P

Best practice on a centrifuge is to use a Line Regenerative VFD (like the 755TR) that can pump the energy back into the line source, because you can do that as often as necessary.

Thanks. We've already calculated all that.
I'm not familiar with the 755TRs though, and it looks like the price and lead time is double (of an already June delivery for a regular 755), so not really an option in this case. Good information nonetheless!

 

[kamenges]

I came up with the same 120 lb-ft for decel torque at the load. Is the motor shaft direct coupled to the load or is there some reduction there?

I have gone this direction in the past successfully:
https://www.bonitron.com/overvoltage/line-regen/m3645/

Well, technically it was the previous version but the same idea applies. This device is connected to the brake resistor connections and sends the regen power back to the AC line. This also probably the kind of set-up PLCMentor[/b} was referring to.

The next step up is this:
https://www.bonitron.com/common-bus/m3645p/

The output of this thing gets connected to the DC bus terminals of the PF755 and handles all bus functions. More expensive but also more elegant.

Overall I'm betting Bonitron lead times will be far less than Rockwell lead times. I'm a big Bonitron fan. I have never had any issues with their stuff.

Keith

 

[phuz]

I came up with the same 120 lb-ft for decel torque at the load. Is the motor shaft direct coupled to the load or is there some reduction there?

I have gone this direction in the past successfully:
https://www.bonitron.com/overvoltage/line-regen/m3645/

Well, technically it was the previous version but the same idea applies. This device is connected to the brake resistor connections and sends the regen power back to the AC line. This also probably the kind of set-up PLCMentor[/b} was referring to.

The next step up is this:
https://www.bonitron.com/common-bus/m3645p/

The output of this thing gets connected to the DC bus terminals of the PF755 and handles all bus functions. More expensive but also more elegant.

Overall I'm betting Bonitron lead times will be far less than Rockwell lead times. I'm a big Bonitron fan. I have never had any issues with their stuff.

Keith

Direct coupled, yes.

Educate me. Why is line regen *better* than a braking resistor, which is a fraction of the cost?

 

[kamenges]

"Better" is always a funny expression. It's not necessarily better or worse. That evaluation is application specific. A big part of this evaluation is the cycle rate of the event. The centrifuge will store 9750 joules at 650 RPM, which really isn't all that much. You would probably have to cycle the thing pretty frequently to get your money back out of a line regen unit. And if you intend to install the centrifuge in Tuktoyaktuk you will never get your money back out of it since you are looking to constantly heat the environment anyway.

Keith

 

[phuz]

"Better" is always a funny expression. It's not necessarily better or worse. That evaluation is application specific. A big part of this evaluation is the cycle rate of the event. The centrifuge will store 9750 joules at 650 RPM, which really isn't all that much. You would probably have to cycle the thing pretty frequently to get your money back out of a line regen unit. And if you intend to install the centrifuge in Tuktoyaktuk you will never get your money back out of it since you are looking to constantly heat the environment anyway.

Keith

That was what I figured. For a 90 second decel that occurs AT MOST once every hour, I think it'd be a long time to recoup that. Was just quoted several thousand for a 10 ohm (7000W) braking resistor from PowerOhm. I didn't expect a braking resistor to be almost half the cost of the VFD itself. I'm thinking the quote of 7000W is a bit overkill for this application.

 

[kamenges]

Yep, those bad boys can get pricey. I would hope that gets you an enclosure and thermal protection as well.

7000 watts does seem a bit hefty. But it will be more than the 100 -200 watt average you get for the decel profile. The resistor assembly will have to have enough thermal capacity to survive the 11000 watt initial power hit it takes at the beginning of decel.

I assume you told PowerOhm that this happens for 90 seconds once an hour?

Keith

 

[phuz]

Yep, those bad boys can get pricey. I would hope that gets you an enclosure and thermal protection as well.

7000 watts does seem a bit hefty. But it will be more than the 100 -200 watt average you get for the decel profile. The resistor assembly will have to have enough thermal capacity to survive the 11000 watt initial power hit it takes at the beginning of decel.

I assume you told PowerOhm that this happens for 90 seconds once an hour?

Keith

This was our local AB vendor that carries PowerOhm, but I'm not sure he fully understood, so I may have a chat with PowerOhm directly.

 

[GaryS]

Motor Breaking

A few things to keep in mind when doing breaking
I see here some people are talking about DC injection braking. DC injection breaking at best will only provide 30% of motor torque at motor rated speed and it drops rapidly as the motor speed goes down it will provide 0 torque before the motor actually stops.
Dynamic Braking or a better description foe this would be Buss Loader. A vfd with an internal chopper transistor buss loader can only provide 10% breaking torque. That would be 100% break torque 10% of the time or 100% torque 10% of the time. It will do this very well, but you must look at your application carefully. A vfd with an external buss loader / DB can give you 100% motor torque 100% of the time. The external buss loader will dump the energy as heat through the external mounted resistor.

In your application I would forget the Powerflex drive ( There old technology ) and use the new Yaskawa Matrix Drive
And set it up as a Flux Vector drive with an encoder. This will give you absolute motor control on all 4 quadrants of the motor operation 100% breaking torque down to 0 speed the energy is pushed back into the ac supply line. I have set up Flux Vector drives to actually hold the load at zero speed no problem.

 

[kamenges]

Originally quoted by GaryS:

In your application I would forget the Powerflex drive ( There old technology ) and use the new Yaskawa Matrix Drive

Have those finally come around a little bit? The last time I looked into them it would have cost me my house for a 10HP equivalent and I could have built a new house in the time it took me the get the U1000. But I would agree that for a single point free-standing regenerative axis this would be the way to go from a technology standpoint.

Keith

 

[phuz]

Sounds really cool, but overkill for this application.
Also, they have no interest in deviating from an AB standard.

 

[jraef]

Line regen is not about recouping energy costs, in fact unless you have somewhere else to use the energy, within your facility or a Net Electric metering (NEM) contract* with your utility, you don't "spin the meter backward".

it's all application specific. Line regen braking is about NOT burning up your resistors or the brake chopper transistor when the braking has to dissipate a lot of energy. Resistors have a finite life and heat x time = failure. So it depends on what you are trying to stop and how often. I did a gold concentrator once with braking resistors, it's basically a giant bowl spinning like a centrifuge with the lighter material moving up the sides and out, leaving the heavier gold behind. It was a batch process, so it started and stopped dozens of times per day. The braking resistors lasted about 6 months, the braking transistor about a year. Changed to regen braking and never replaced anything in the drive again (as long as I was involved). But on the other hand I did a 900HP whole log chipper and used DC injection braking, which as I said, traps the heat in the motor. But they only braked it to a stop 2 times per day, so it didn't need line regen braking.