# VFD's and regenitive voltage.



## sparky105 (Sep 29, 2009)

line reactor


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## miller_elex (Jan 25, 2008)

MOV

Metal-oxide varistor


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## varmit (Apr 19, 2009)

You need to install a regenerative braking resistor. The drive probably has terminals to connect the resistor to. The resistor "burns off" the power generated by the motor when slowing. A parameter change, on the drive, will be required to tell the drive that the resistor is there. A line reactor will not fix this problem. Sizing the resistor can be a bit of educated guessing. Unless the cycling is extremely rapid, I would suggest a 50% resistor: The resistor wattage would equal about 50% of the max motor current. Example: a 1 HP motor would have a load of about 740 watts, so a 50% resistor for a 1 HP motor would be 370 watts. 

Where in KY are you located? I usually get my braking resistors from Post-Glover ( the mfr. ) in Florence.

Good luck.


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## Glen Walker (Dec 30, 2009)

We deal with a lot of Decantur Machines in Western Ky at the Coal Mines. What size machine is it that your working on, Hp? Most of the machines that we deal with range from 200Hp up to 500Hp 4160V. If your regening power back to the drive it must be directly coupled and on machines like that we usually have a reactor on both the line and load sides.


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## Glen Walker (Dec 30, 2009)

If this Screenbowl is very large he may need to look at installing a contactor to break the load leads to the motor. These machines are basically a big centrifuge, and can take up to 20 minutes for them to coast down.


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## Introyble (Jul 10, 2010)

varmit said:


> You need to install a regenerative braking resistor. The drive probably has terminals to connect the resistor to. The resistor "burns off" the power generated by the motor when slowing. A parameter change, on the drive, will be required to tell the drive that the resistor is there. A line reactor will not fix this problem. Sizing the resistor can be a bit of educated guessing. Unless the cycling is extremely rapid, I would suggest a 50% resistor: The resistor wattage would equal about 50% of the max motor current. Example: a 1 HP motor would have a load of about 740 watts, so a 50% resistor for a 1 HP motor would be 370 watts.
> 
> Where in KY are you located? I usually get my braking resistors from Post-Glover ( the mfr. ) in Florence.
> 
> Good luck.


Newer VFD's recover the regenerated energy. I agree with this post (above)


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## John Valdes (May 17, 2007)

Glen Walker said:


> If this Screenbowl is very large he may need to look at installing a contactor to break the load leads to the motor. These machines are basically a big centrifuge, and can take up to 20 minutes for them to coast down.


Contactors are never placed on the load side of VFD's unless you want it (VFD) to trip out every time the contactor opens or closes. 
With a VFD, the correct resistor bank and switching transistor, this long stopping time can be greatly reduced. Greatly reduced. 



Introyble said:


> Newer VFD's recover the regenerated energy. I agree with this post (above)


I agree also. But correct me if I'm wrong. The standard VFD does no more than bleed excess buss voltage to the resistor bank and is dissipated as heat.
If you mean a regenerative class VFD then your statement makes much more sense to me. Or are all VFD's today regen? :thumbsup:

I also concur that a line or load reactor would be a waste of money as they would accomplish nothing in regard to the problem. However, they do have their place. I use line reactors on every application. I only use load reactors when distance is an issue or noise is an issue. But not to address bus over voltage.


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## Hunter1151 (Nov 4, 2010)

John Valdes said:


> Contactors are never placed on the load side of VFD's unless you want it (VFD) to trip out every time the contactor opens or closes.
> With a VFD, the correct resistor bank and switching transistor, this long stopping time can be greatly reduced. Greatly reduced.
> 
> I agree also. But correct me if I'm wrong. The standard VFD does no more than bleed excess buss voltage to the resistor bank and is dissipated as heat.
> ...


Be sure drive has chopper, even though dc bus might be fine on longer decal time if you need fast decal and have a hard load, like a fan, then the dc bus will go high, this needs to be burned off with resistors


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## etb (Sep 8, 2010)

jtashaffer said:


> What is the best thing to do on the LINE side to stop this from happing. ( Resistor bank, *Diode*, ?,? )


on the LINE side of AC equipment?!




Introyble said:


> Newer VFD's recover the regenerated energy.


Some of the earliest drives ever made were also regenerative. True that there are more available on the market now than in times past, but by no means to all VFDs do this. In fact,



Hunter1151 said:


> Be sure drive has chopper


is a good point. Some VFDs aren't even able to control a bus/brake resistor, so be sure yours can and that it's configured correctly. The more complicated drive setup gets and the more $ that is involved, the more I like to take my time on things like that and check them.

E.g., if you can, manually ramp the load down and watch the bus voltage. Ramp it harder and harder until the bus reaches the cut-in setting and verify the chopper starts. Checking things has saved my butt on several occasions. For example, once we found a firmware bug in the drive that cleared all the parameters we set (including brake resistor) each time we changed modes.


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## John Valdes (May 17, 2007)

Do we have two trolls working in unison?


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## rick279 (Nov 9, 2010)

jtashaffer said:


> I am working on a decanter centerfuge which it ramps up and down the whole time it runs. It is sending regenitive voltage back to the VFD and shorting it out. What is the best thing to do on the LINE side to stop this from happing. ( Resistor bank, Diode, ?,? )


You need Dynamic Braking ( DB ) resistors and maybe a DB module depending on the VFD, or a Regen AC drive. The other answers I saw were wrong. Reactor, SCR, etc... There is nothing you can do on the line side, the issue is the motor regen.


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## John Valdes (May 17, 2007)

rick279 said:


> You need Dynamic Braking ( DB ) resistors and maybe a DB module depending on the VFD, or a Regen AC drive. The other answers I saw were wrong. Reactor, SCR, etc... There is nothing you can do on the line side, the issue is the motor regen.


Not all the answers were wrong. And you are correct. Resistor bank, with switching or regen drive.


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## rick279 (Nov 9, 2010)

John Valdes said:


> Contactors are never placed on the load side of VFD's unless you want it (VFD) to trip out every time the contactor opens or closes.
> With a VFD, the correct resistor bank and switching transistor, this long stopping time can be greatly reduced. Greatly reduced.
> 
> 
> ...


John Line reactors are sometimes used for high bus voltage when line voltage exceeds a certain level, usaully around 507V+-?. This is not only an issue with the transformer but can also vary at different times of the day depending on your location. By adding a 5% line reactor, we lower the input voltage.


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## John Valdes (May 17, 2007)

rick279 said:


> John Line reactors are sometimes used for high bus voltage when line voltage exceeds a certain level, usaully around 507V+-?. This is not only an issue with the transformer but can also vary at different times of the day depending on your location. By adding a 5% line reactor, we lower the input voltage.


Rick,
The average line reactor is nothing more than a 1:-1: ratio isolation transformer. 480 in, 480 out for example. Pure and simple. Some reactors have a few more bells and whistles, but basically they all do the same thing. 
Of course reactors do prevent voltage spikes. A choke effect.
The line reactor does not limit voltage, it only affects impedance. 3% is the industry standard.

I have had several occasions where drives were tripping at certain times of the day. Afternoons in the summer and mornings in the winter. Power correction caps on POCO's side are used for these fluctuations. They turn on and off as needed. This is what trips drives. Some plants have there own Power Correction Cap's.

A line reactor has no correction for a steady over voltage. It allows it to pass and the drive trips if it reaches the threshold.


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