VFD operation on high inertia loads

[jakegordonII]

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.

 

[wired1]

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

 

[cbarn24050]

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.

 

[danielmurray]

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

 

[jakegordonII]

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.

 

[gcaudill]

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).

 

[jOmega]

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

 

[ischgl99]

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

http://www.dutchmenservices.com

 

[RadarRay]

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?

 

[ischgl99]

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

http://www.dutchmenservices.com

 

[wired1]

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

 

[jOmega]

Wired1,

How about personnel saftey as a second reason ?

jOmega

 

[wired1]

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

 

[ischgl99]

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

http://www.dutchmenservices.com

 

[jOmega]

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.

 

[ischgl99]

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.

http://www.danfoss.com

Mike Bensema

http://www.dutchmenservices.com

 

[Shortstub]

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?

 

[jOmega]

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?

 

[jOmega]

Mike Bensema

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

jOmega

 

[ischgl99]

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

http://www.dutchmenservices.com