Demagnetizing dead-time between powering and regenerating 2

[rulmismo]

Hi,

I have a traction application with a VFD-Motor supplier.

It gives me information about response times to change from 100% powering to 100%regenerative braking. (about 4s with jerk limiting 0.8m/s^3)

What it puzzles me is that before passing to braking it waits for 1s dead time without applying any effort. It says that is for "motor demagnetizing time".

In my understanding to change from powering to regenerative braking at speed, the supply frecuency should be reduced (< "rotor" frequency) and I don´t get why this dead time (what physical limitation there is).

I assume flux is always rotating in the same direction, simply faster or slower than rotor flux (to change from powering to regenerative brake), so the winding inductance don´t have so much effect and it should not need really to demagnetize totally and wait the dead time, simply to reduce frequency.
(i upload a diagram to clearify my idea)

What do you think? probably some of my reasoning is wrong but don´t know what..

Regards
rUL

 

[cswilson]

I'm with you on this one. We frequently use induction motors -- vector controlled -- as servo motors, and go instantaneously between motoring and generating. I don't think the limitation is in the motor on this one.

Curt Wilson
Delta Tau Data Systems

 

[LionelHutz]

Isn't the jerk limiting simply a self imposed limit on how quickly the whole machine can accelerate and decelerate? It is propably based on a "people" or "cargo" limit which does not allowing a sudden change from accelerating in one direction to accelerating in the other direction.

 

[jraef]

I always considered "jerk limiting" an issue for preventing damage to gears from backlash issues, at least that's they way it is referred to in some manuals.

What I am familiar with is that the "dead time" for allowing the magnetic field to collapse is an issue necessary when going from motoring to DC Injection Braking, because otherwise the regen happens immediately and can damage the semiconductors. But in the case of regenerative braking, I don't get it, you would WANT the regen to happen immediately and you are keeping the circuit active to ALLOW the regen energy to reverse flow (as opposed to a DCIB trying to block it). Is maybe someone mixing things up here?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[rulmismo]

Hi,

Jraef,I think don´t understand what has to do "DC injection braking" with "regenerative braking appearing inmediatly" and what relation has to damaging semiconductors.
Maybe you mean some transient interaction between remaining magnetic flux and DC injection....?

It is not a jerk problem. The dead time, they call it explicitily "motor demagnetizing time" as if they need to demagnetize totally before entering regenerative zone.

I will ask my supplier but they are japanese and is not very easy to get understandable information from them.

Regards

 

[waross]

When a VFD driven motor goes into regeneration there is no demagnetization. With a regenaration capable VFD, whenever the frequency of the VFD is reduced below the frequency of the back EMF the motor should regenerate. This depends on the motor staying magnetized. A common method of braking with VFDs that are not regenerative capable is to limit the voltage on the DC bus with resistors. This may be similar as far as the motor and the back end of the VFD are concerned and also depends on back EMF which means that the motor must stay energized. In both cases the motor is braked by extracting energy from the back EMF. The back EMF depends on the motor staying energized. This extracted energy drives up the voltage of the DC bus. A regenerative VFD will return this energy to the grid. A dynamic braking capable VFD will waste the energy in a resistor.
If the VFD is neither regenerative capable nor dynamic braking capable, then DC injection braking may be used. In this case the energy extracted from the load is wasted in the resistance of the motor windings. If the DC is injected before the magnetism and back EMF decay, the back EMF voltage may damage the supply of the DC injection. If the supply of DC is not able to withstand the application of the AC back EMF, then a delay time must be allowed.
If there is a time limit between acceleration and de-celeration, that may have more to do with passenger comfort than electrical issues.
Is it possible that the time interval is not dead time, coasting, but the minimum time for a smooth transition from full acceleration to full de-celleration?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Well, there is no demagnetizing when moving from motoring to regenerating so there is either a translation problem or they are simply wrong.

Your assumption about the flux remaining and the speed it rotates changing is pretty much what happens.

 

[waross]

I always considered "jerk limiting" an issue for preventing damage to gears from backlash issues, at least that's they way it is referred to in some manuals.

"Jerk limiting" Remindes me of more than one job where the crew tried to keep the boss's son away from the production areas as much as possible. Grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ozmosis]

I would say this would only be an issue if you are removing and re-applying power to the motor from the VFD during this transition period. In this case, you would need some 'dead band' to demagnetize and magnetize when reconnecting the motor. If the VFD is running the motor in all quadrants and is capable of this, then I would say there is no need for any thought on transition time. This would be carried out internally in the VFD within the control function and would be micro-secs than secs.
1 second is a long time if your VFD/Motor is going from motoring to regenerating. During this period, if it is regenerating, I would say in 1 second the DC link voltage would be shooting up through the trip level if nothing is happening on the brake chopper or (if using) some form of active front end.

 

[jraef]

My reason for bringing up the DCIB issues was that I think maybe whomever made that statement about needing the dead time was confusing the issues of transitioning from motoring to braking with DCIB as being pertinent to regenerative braking. They are not related, I'm just speculating on where the error was made...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376