Demagnetizing dead-time between powering and regenerating
Demagnetizing dead-time between powering and regenerating
(OP)
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
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





RE: Demagnetizing dead-time between powering and regenerating
Curt Wilson
Delta Tau Data Systems
RE: Demagnetizing dead-time between powering and regenerating
RE: Demagnetizing dead-time between powering and regenerating
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?
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RE: Demagnetizing dead-time between powering and regenerating
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
RE: Demagnetizing dead-time between powering and regenerating
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
RE: Demagnetizing dead-time between powering and regenerating
Your assumption about the flux remaining and the speed it rotates changing is pretty much what happens.
RE: Demagnetizing dead-time between powering and regenerating
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Demagnetizing dead-time between powering and regenerating
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.
RE: Demagnetizing dead-time between powering and regenerating
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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