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Decelarating Ramp


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#1 bob

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Posted 17 July 2006 - 12:06 PM

Hi,

I have at my site of work one SVTL600 AsiRobicon drive driving a very high inertia load. The motor is coupled to the drive via an epicyclic gearbox. The motor run at fixed speed but the speed is manually adjusted to match the load rate. The decelerating ramp is set at 60 s and the accelerating ramp at 120 s. I do understand that a low decelerating ramp could cause the drive to trip on VDC max. Ive reduced the deceleration rate to 30 s but the drive did trip on VDC max when brought from zero to rated speed.
I do not understand how the decelarating ramp could cause the drive to trip on VDC max when accelerating.

Ive also another query.I am also thinking of coasting the drive to stop instantaneously if there is anything unusual on the production line. There is no brake chopper on the drive. Any problem, like overstressing the DC Caps, in doing this.

Thanks.

Bob

#2 mariomaggi

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Posted 17 July 2006 - 01:55 PM

Dear bob,
you are true, trip on VDC max is not normal.
Your info are not enough to give you a good suggestion. What kind of load? For example, a big drum washer could have items inside that during starting could have an unbalance, this make in fact a short shock in the regenerative direction, this could generate an overvoltage.
Please add more info, at least power and speed.

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Mario

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#3 kens

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Posted 17 July 2006 - 10:37 PM

Hi Bob, the VDC Max trip is unusual during the acceleration ramp. Has this happened only once or is it recurring? Is it possible that there was an overvoltage condition on the supply to the drive when it tripped? It may also pay to check on the control wiring to the drive, it may be that there is an intermittant fault on the input that caused the drive to begin to stop when it shouldnt have. Have you tried to stop the load in 30 seconds? Did it work?

When you say 'coasting the drive to stop instantaneously' do you mean that the drive will release the motor or do you want a controlled stop? If you release the motor from the drive the time to stop will be determined by the inertia of the load and also the frictions of the equipment. If you want a controlled stop then I believe that you will need to install a brake chopper and resistor to dissipate the energy. I am not familiar with the SVTL600 AsiRobicon drive, is the DC bus accessable?

Ken
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#4 jraef

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Posted 18 July 2006 - 01:15 AM

One possibility that can give you OVDC trips on acceleration is an eccentric load, as previously mentioned by Mario. I see this happen quite often in vibrating feeders for material handling and jack pumps in the oil well industry. What happens is that after the load moment center moves past its apex, it begins making the motor rotate faster in relation to the applied frequency and the power applied by the VFD serves as an excitation source. This means its relative synchronous speed has been exceeded and it becomes an induction generator for the remainder of that half cycle, until the eccentric load reaches its nadir and the motor must work again to lift it. The problem is that the caps can only store a limited amount of the regenerated energy and without a brake chopper, they have nowhere to put it until the motoring half of the cycle begins. During constant speed operation, this excess energy from the regen half cycle can often be utilized in the motoring half cycle if it hasn't built up to exceed the trip levels. But during acceleration, the relative synchronous speed in constantly changing as the VFD is attempting to compensate for the eccentricity of the attached load. So the regen energy comes in sudden bursts that often exceeds the storage capacity of the caps.

There are 3 ways of dealing with this that I have used. The easiest is to use a dynamic brake chopper on the DC bus to bleed that excess energy into resistors. Another similar method is to oversize the VFD in order to attain more bus capacity, the usual rule of thumb is to double the size of the VFD, although I have had successes with less oversize than that, depending on the load. The third is to utilize a custom macro available in many VFDs specifically designed for eccentric loads, which tracks the regen cycle and backs off on the output frequency as it approaches the load apex to make sure the VFD output is closer to synchronous of the overhauling load so as to avoid it going super-synchronous and allowing the motor to regenerate in the first place. You should contact ASI and see if that drive has this feature available.
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#5 bob

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Posted 18 July 2006 - 06:50 AM

Hi,

Thanks everybody for comments. The motor is a 400 k W 2 pole one and the load is a center-driven winder. I have got another type of drive on a similar application and there is no problem in reducing the decelerating ramp to 30 s. The software in the AsiRobicon is very cumbersome. I agree that a brake chopper and resistor will solve the problem but these are quite expensive components and are not available at hand. I also agree with jraef that the problem could hopefully be solved using soft features of the drive but support is almost inexistent .
My problem is that I need to stop the drive as fast as possible as soon as there is a tripping upstream on the line of production and 60 s is too long.
I am thinking of opening the main contactor on the drive which will obviously stop the drive.
Any comment.

Bob

#6 jraef

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Posted 18 July 2006 - 10:01 PM

Yes, unfortunately when Siemens bought Robicon here in the US, they did NOT buy the ASI portion of it as it was completely redundant to the Siemens products. I see that ASI is attempting to survive on their own as ASIAnsaldo once again however. Did you try contacting them directly?

http://www.asiansaldo.com/
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