VFD operation on high inertia loads
VFD operation on high inertia loads
(OP)
We are attempting to drive a solid bowl centrifuge (very high wk-2 with normal 7 to 10 minute wind up time when starting accross the ine). Motor is 100 HP. any experience and learnings. We are having trouble with the inverter faulting out due to braking issues.





RE: VFD operation on high inertia loads
RE: VFD operation on high inertia loads
RE: VFD operation on high inertia loads
RE: VFD operation on high inertia loads
RE: VFD operation on high inertia loads
The rise in DC bus voltage is due to the back EMF generated when the motor is driven by the load, instead of the other way around.
Knowing the kinetic energy of the load and the time in which you want to remove that energy (decel time) you can determine how much power (HP/kW) will have to be absorbed via the motor and drive. Only then can you determine if a large enough braking chopper/resistor is available to handle the kW over the period of time which you need.
The kW is converted to heat in the braking resistor. This limits the power handling and the duty cycle of the resistor.
P.S. If you extended the drive decel time beyond the normal decel time of the load with no braking the problem should go away (if this is acceptable).
RE: VFD operation on high inertia loads
Contact Bonitron
http://www.bonitron.com/line_regen.htm
They have a Stand-Alone Line-Regeneration module that can be used with most VFDs available today. This is not dynamic braking. It is true regenerative braking in that it takes excess energy from the dc bus and passes it back to the AC supply.
It can be added to your Square D Altivar 66 drive if you have access to the DC bus connections, usually right across the DC bus caps.
HTH
jOmega
RE: VFD operation on high inertia loads
Most VFDs can handle some braking without using the braking resistor, how fast do you need to stop it? I think the rule of thumb is something like 10-15% of the drive capacity. Keep in mind that if you use the motor to stop the centrifuge, you have to keep the decel time within the limits of drive components, ie belts, gears, etc. If you stop too fast, you could cause damage to the driveline.
Mike Bensema
www.dutchmenservices.com
RE: VFD operation on high inertia loads
Therefore Wk2= (Tq * 308 * t)/ delta S. So the Wk2 at the motor is (300 * 1.5 * 308 * 420)/1800 = 32340 #ft^2. Which is a WK2 equivalent to about 80 inch roll of paper 12 Ft wide, considerable indeed. And I cheated a little using a 1.5 torque multiplier, because across the line the torque starting should even be greater, although I am unfamiliar how you can start across the line for 7 minutes without tripping the motor protection.
That is a lot of energy to absorb in a resistor bank, so an Active front end could be the way to go. Both Eurotherm drives and US drives have active front ends, although I don’t know if Eurotherm uses them in stand-alone apps.
Is starting time an issue?
RE: VFD operation on high inertia loads
Mike Bensema
www.dutchmenservices.com
RE: VFD operation on high inertia loads
RE: VFD operation on high inertia loads
How about personnel saftey as a second reason ?
jOmega
RE: VFD operation on high inertia loads
RE: VFD operation on high inertia loads
Mike Bensema
www.dutchmenservices.com
RE: VFD operation on high inertia loads
One limitation of the dynamic brake resistor solution is the duty-cycle of braking events. Have to have sufficient time between stopping events to:
- allow the resistor time to cool (most are not force ventillated)
- allow sufficient time to get rid of the heat from the VFD heatsink that was introduced by the braking transistor. Most do not operate linear mode but in switch mode...
As a cost savings, some VFD mfgr's size the braking transistor smaller than the output transistors. They're not intended for continuous operation and so can be derated. Another cost savings is to not increase the size of the VFD heatsink on which the brake transistor is mounted.... so heat build-up is a consideration. Ergo, the duty-cycle rating.... so many seconds on... with so many minutes off between events.
While the Line Regen Module is costlier.... it offers a more long term reliable solution without having to be concerned about duty-cycle and heating as stated above.
RE: VFD operation on high inertia loads
For anyone interested, Danfoss has .pdf file on their website about sizing braking resistors and uses the centrifuge as one example. I can't find the file anymore, but it was in there somewhere for the VLT 5000. www.danfoss.com
Mike Bensema
www.dutchmenservices.com
RE: VFD operation on high inertia loads
Regen works only if supply bus is active! What is done upon power failure to drive! Or is that not a safety issue?
RE: VFD operation on high inertia loads
You raise a very valid point.
If the ac line (mains) disappears, it raises several interesting considerations:
- Commutation failure of SCR's in Regen Module.
(Hopefully, module is fused.)
- If mains branch circuit fusing to Regen Module opens, then current flow path from DC bus to mains is interrupted and no damage should occur (depending upon which phase of Regen Module is in conduction at the time, and which phase fuses open.
- And of course, braking ability is lost.
Losing the 'mains' ac supply, is an argument for choosing dynamic braking as the method. The regenerative energy will keep the DC bus up ...and assuming that the power supplies of the VFD are sourced from the DC bus (as opposed to a conventional supply tied to the incoming AC power).... braking will continue until the regen energy is no longer able to keep the VFD DC bus 'pumped' up.
If the Line Regen module is chosen, and if personnel safety is a concern, one would expect that the OEM would provide some means of mechanical braking as a redundant backup (don't hold your breath on this one)...
I guess it comes down to :
~ What do you need ? and
~ what are you willing to pay ? and
~ Can you live with the compromise?
RE: VFD operation on high inertia loads
If you need a copy of that Danfoss file, let me know.
jOmega
RE: VFD operation on high inertia loads
Centrifuges usually don't have an automatic brake unless the customer is willing to pay more for it and most machines cannot be outfitted with one, ie belt drive machines. The reason is even with a brake it will take 5-10 minutes best case to stop the bigger machines, not much of a gain for personnel safety.
If the power fails, it will just coast to stop, not a safety issue. An e-stop usually does the same thing, cuts all power to the machine, so it will also coast to a stop. If it needs to stop faster during a power failure, then a UPS needs to be supplied that will enable flushing the machine with water to slow the stop. If he had a desludging centrifuge instead of a solid bowl, then that could be made to stop faster, but you're still talking several minutes.
The main issue is a centrifuge cannot stop fast enough to provide safety to personnel. Think of it as a freight train, if something happens, you can make some efforts, but you have to ride it out.
Mike Bensema
www.dutchmenservices.com
RE: VFD operation on high inertia loads
Thank you for the addition insight into the world of centrifuges.
If OSHA codes are adhered to, personnel safety should be a non-issue... but we both know that's not the real world. And so, there will be a time when someone will get entangled with the machine. And 5 minutes to stop is much too long. When it comes to personnel safety, the machine is usually sacraficed as necessary to get the thing stopped.
Some methods I've encountered over the years, include an eddy current clutch-brake with the brake very much oversized to produce the necessary braking torque.
On DC drives, I've seen the regen capacity doubled over the motoring capacity (bridge sized for twice as much current.
And on a few occassions, I've used Plug reversing under current limit control (VFD). This requires adding an encoder or zero speed switch on the centrifuge to sense when zero speed is achieved. No line regen occurs, and dynamic braking not required. But the motor-- now that's a different story. Since the inertial energy is dissipated as heat in the motor, it must be sized larger (over framed) or force-ventillated to get rid of the heat.
I don't have my copy of NFMA-79 here with me but I'll look on Monday to see if there isn't a breaking requirement. Seems I've seen a spec for centrifuges somewhere along the way.
Enjoyed the dialog.
jOmega
RE: VFD operation on high inertia loads
Sorry for typo
jW
RE: VFD operation on high inertia loads
I agree the machine should get sacrificed to save the person, but that is not easy to do with present designs. To stop the machine quickly, the brake would have to act directly on the bowl, otherwise the bowl can come loose due to the fast stop. The main issue is the manufacturers will not supply it until the customer demands it and is willing to pay for it. There are limits to what can reasonably be done, and you can't completely design out stupidity. I know of mechanics that pride themselves on being able to start dismantling a machine before it has stopped and also know of a plant where 2 men were killed doing just that. Going back to the train analogy, if you jump in front of it, no safety feature is going to save you.
I look forward to what you find in NFPA.
Mike Bensema
www.dutchmenservices.com
RE: VFD operation on high inertia loads
For what it's worth, the sugar industry stops centrifuges all the time because it is a batch process. 5 minutes stop braking time as opposed to 45 mintues stop time coasting adds up to increased productivity.
The standard for that process has been water injection, but now they are adding line regen drives to make it quicker. US Drives has done quite a few, probably as a result of targeted marketing to that industry. ABB, Baldor, Siemens, GE, Toshiba and Robicon are all manufacturers that I know of who offer line regen options. Schneider is probably one of the few big suppliers that does not.
I've also seen them used on Gold Concentrators, a type of centrifuge which is also a batch process operation.
Quando Omni Flunkus Moritati
RE: VFD operation on high inertia loads
http://www.ab.com/manuals/ms/193-2.16.pdf
etc. for more info
RE: VFD operation on high inertia loads
With these regen drives, is the decel time fixed or does it vary based on the load? With a VFD, if it is progammed to stop with water injection and for some reason the water is shut off, it will decel too fast and possibly trip. Is this the same problem when using a line regeneration?
Mike Bensema
www.dutchmenservices.com
RE: VFD operation on high inertia loads
Batch time varies from process to process acording to the production stage (there are several). Generally speaking though, the highest batch rate I have seen is 10 - 14 batches per day on the first stage. Without braking they might get 9 or 10. With braking they get 14 or even more. Subsequent concentration stages run longer and so get fewer batches per day.
With regenerative braking the VFD rarely ever tripps off because any excess energy is fed back into the AC line. Another new technology being used is having several VFDs on a common DC bus so that the regen energy from one is being used to spin up the others. I have no direct experience with that, I just know it is being done.
The water not only speeds the braking, it is part of the process as well so the batch is ruined if it fails.
Quando Omni Flunkus Moritati
RE: VFD operation on high inertia loads
http://www.vfds.com/teco-m.htm
for: Boost Fixed and Accel For high starting torque or to accelerate high inertia loads
http://www.csemag.com/index.asp?layout=story&webzine=cse&publication=cse&articleid=CA230289
http://www.hq.usace.army.mil/cemp/e/Et/mitergat.pdf
etc. for more info
RE: VFD operation on high inertia loads
Mike Bensema
www.dutchmenservices.com
RE: VFD operation on high inertia loads
for spot on speed control at start ramp to charge speed ramp to dehydration speed and braking to discharge speed.
charge speed is 520rpm dehydration speed is 900rpm and discharge speed is 70rpm with latest drive i have used proportional control for the pump programme from the dcs.
greye hunter
RE: VFD operation on high inertia loads
RE: VFD operation on high inertia loads
One characteristic about Schneider/Square D drives is that they have a dead-slow processor. This results in very poor motor torque response especially when operating in sensorless vector mode.
The unfortunate result of this slowness is that, even on coast-to-stop applications, the motor torque continues PAST the desired speed setpoint and then the drive has to correct by braking the load back down to setpoint. A need-less DC Overvoltage Fault results.
If this is the type of problem that is presenting itself here, then the answer is a faster processor drive like an ABB ACS800 or similar units from others. These units will process motor torque in a more timely fashion and pull off as the speed setpoint is approached. No regen is required and no trip occurs even in the total absence of braking capacity.
Come to think of it, the Reliance GV3000 is another drive in the dead-slow catagory and displays the same troublesome behavior on high inertia loads.
RE: VFD operation on high inertia loads
closed or open loop system very simple systems and braking not a problem if you need further info let me know.
greye.