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VFD operation on high inertia loads

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

does all the braking have to be done electrically, or can a mechanical brake be used as an assist?

RE: VFD operation on high inertia loads

Hi, this is a common problem, the inverter cannot return the regenerated power back to the supply, this causes the internal dc link voltage to rise. Some inverters have a provision for a large resistor to be fitted to absorb the energy however these are not usually large enough for this kind of application. You will need specialist help from a drive supplier to solve your problem.

RE: VFD operation on high inertia loads

Who is the drive manufacturer and what specific faults are you getting?

RE: VFD operation on high inertia loads

(OP)
The drive is a Square D, Altivar 66.  Internally, I am hearing things similar to what the three replies have already stated.  It has something to do with feedback that the VFD receives knowing where the rotating magnetic field is (stator) with respect to the rotor.  

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

jakegordonII ~

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

I used to work for one of the major centifuge manufacturers, most of the time, braking was not done at all, just coast to stop.  Is there a reason you need to brake the centrifuge?  Some of them come with brakes on the flywheel that can be used, but most are manual operated.  In additon to the braking resistor already mentioned, you can also feed water to the centrifuge during the decel, that will act as a brake, but you want to program the water addition so that it stops feeding before the centrifuge comes to a stop.  You can use a tach input if you have one.

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

Just out of curiosity I thought I could take a guess at what kind of load you have.  Assuming you have a 4 pole motor, 100HP and it takes 7 minutes (420 seconds) to accel.  Use the formula Tq =(wk2*delta S)/(308 *time) and solve for Wk2. Note Torque of 100HP/1800 rpm  motor is roughly 300Ft-#’s.

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

radarray, centrifuges typically use a fluid or friction clutch with the motor coming up to full speed right away or a wye-delta starting with a direct drive belt where it switches to delta after it is up to speed.   Not sure which machine he's got, but based on the motor size and that he is using a VFD, it sounds like it is a belt drive machine.  This is all assuming he is talking about a high speed centrifuge and not a decanter centrifuge.  For your torque calculations, you need to factor in the clutch or the wye delta starting.  The torque will be limited to the FLA of the motor, so you would have to calculate based on that.  The clutches are adjusted so that the amp draw does not exceed the FLA of the motor, either adding or subtracting clutch plates or adding or removing fluid, same with the VFD.  

Mike Bensema
www.dutchmenservices.com

RE: VFD operation on high inertia loads

mbensema:  the only reason I can think of is to reduce the downtime between batches.

RE: VFD operation on high inertia loads

Wired1,

How about personnel saftey as a second reason ?

jOmega

RE: VFD operation on high inertia loads

jOmega,  That's a worthy second reason - I stand corrected.

RE: VFD operation on high inertia loads

If the reason is to reduce time between batches, then use a brake resistor and call the manufacturer about the safe stopping time.  Personnel safety is often brought up as a reason to stop a centrifuge quickly, but it can actually be more dangerous if you don't do it correctly.  If you stop it too quickly from the motor or a brake, the bowl can come loose, pulleys can break, or you break the belts and have no means of controlling the stop.  This size machine probably has a 2000 pound or so bowl rotating at 4-5000 rpm, it's not going to stop on a dime without serious damage to the machine.  In an emergency, the best and safest way is to flood it with water, provided that doesn't cause a safety issue with the process.  

Mike Bensema
www.dutchmenservices.com

RE: VFD operation on high inertia loads

Mike,

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

Sorry, I should have included the line regen module, my oversight, and you're right, it does sound like the better option, but the duty cycle for a machine that size probably is low enough that a brake resistor would work provided it had adequate ventilation.
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

Just a reminder,

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

Shortstub,

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

Mike Bensema

If you need a copy of that Danfoss file, let me know.

jOmega

RE: VFD operation on high inertia loads

j0mega, I have it already, just couldn't find the link again to post it.

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

Mike,

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

Make that NFPA-79

Sorry for typo

jW

RE: VFD operation on high inertia loads

I've enjoyed the dialog also, it's been informative.  I would be interested in seeing if there is a braking requirement.  I still work with centrifuges, but not for the manufacturer anymore and do get asked about braking on occasion.  

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

Interesting thread. Too bad the original poster is not contributing any more.

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

jraef, how long does the centrifuge normally run when processing a batch of sugar?  Most of my experience is in the dairy and wastewater industries where the machines are only shut down when they need to be, many times weeks later.

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

mbensema
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

Thanks for the info jraef, you learn something new everyday.  14 batches a day is a lot of machine cleaning, glad i'm not a millwright at that plant.  

Mike Bensema
www.dutchmenservices.com

RE: VFD operation on high inertia loads

hi there about your centrifuge braking why dont you use a hydraulic drive very simple and no problem braking to come down to plough speed +-70rpm works like a dream i have done two latest one using load sensing with speed feed back
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

My first assignment (ASEA) back in 1964 was a centrifuge for sugar refineries. We had just switched from WL sets to thyristors. A DC motor and a four quadrant thyristor rectifier was ideal at that time with close to no-loss regeneration. And I start to think that it still could be ideal. DC motors are still made, you know.

RE: VFD operation on high inertia loads

It's not clear to me that the initial poster intended to brake his centrifuge at all.  He mentions braking issues with the drive but, it just might not be due to braking.

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

have you given any thought to using hydraulic drive.
closed or open loop system very simple systems and braking not a problem if you need further info let me know.

greye.

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