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-   -   PF525 VFD Braking time (http://www.plctalk.net/qanda/showthread.php?t=114470)

alexbeatle March 1st, 2018 02:07 PM

PF525 VFD Braking time
 
Hello!

What would be the proper value for the DC Brake time on VFDs for the DC injection to the motor? Is there a way to calculate?

Having it currently set to 2s, but this creates inconvenience (or overloads when starting while in brake) when manually jogging the motor little-by-little.

Thank you.

DickDV March 8th, 2018 09:12 PM

In a perfect world, the DC braking current should stop flowing as soon as the motor stops spinning. As a practical matter, the timer is usually set maybe a second longer than actual brake time to cover for variations in actual stop time.

Using DC braking on a bump-bump-bump type jog application is a poor choice for the reason you mention. Switch to snubber resistor braking (often called dynamic braking) and set motor speed to zero between bumps without stopping the motor. You will be amazed at the improvement in precision and predictability you get.

GaryS March 8th, 2018 11:22 PM

From what I read from the post you are using the VFD for positioning
If that is the case then you do not want to use DC injection braking at all.
Typically DC Injection braking is used to stop a freewheeling los like a fan or pump .
The setup would be on a stop command, coast to stop with DC braking or if the load is in motion before the start command then the DC braking will stop the freewheeling load before the VFD starts outputting power. DC Injection braking will give you inconsonant stop times because the time the you set is the injection time not the actual stop time. The actual stop time could be much shorter fir the load allows it or it could actually be longer if the you don’t have enough time to stop load and Dc cut off before it stops.
A Much better choice would be Dynamic Braking ( Buss loader ) sat the VFD up to ramp to stop. The stopping time will be consistent and if you restart before the load stops, there’s no problem at all the VFD is still in control of the load. You can set the Acc and DCC time very short I have seen 0.5 sec on both. Remember the shorter the stop time the more energy that is dumped in the DC Buss.
For that matter the faster you want to stop a load the more energy you must dump someplace. In the motor, the DC buss Loader or Regen back into the line.

I use the term Buss Loader over the Dynamic Braking because with a VFD you don’t really have Dynamic braking. You dump the excess power in the DC Buss through a transistor to a Load Resistor. In the Old brush type DC motors you switched a power resistor across the armature, the current flow through the armature and the Resistor until the motor stops. That’s where I believe the term Dynamic Braking originated.
The time it takes to stop the motor is dependent on the load you are stopping and the amount of current you pass through the armature . The higher the current flow the shorter the stopping time.

You may want to check out Yaskawa,s web site they have a very good explanation of the different types of motor braking and how to determine what type is best for you.

jraef March 9th, 2018 05:42 PM

Dynamic braking is dynamic because it converts the kinetic energy of the spinning load into electrical energy in the drive, then dumps that off as thermal energy into the resistor. But it's dynamic in that the faster the load is spinning, the more energy it contains for braking, but the slower it gets, the LESS energy it has for braking. So Dynamic Braking can never actually FINISH the job, it suffers from the law of diminishing returns. often times the load friction is sufficient to stop it anyway, but sometimes it isn't. So at some point if that happens, you turn on the DC Injection Braking to finish the task. A lot of VFDs have a default setting of 10% speed as the turn-on point of the DCIB, but it varies a lot from application to application.

The down side to DC Injection Braking is that the kinetic energy in that spinning mass it still converted to heat, but that is heat in the MOTOR, not an external resistor. That's why by using ONLY the DCIB, you are causing overload trips; most good VFDs will track the motor heating effects of DCIB and bias the trip curve so that you don't damage the motor.

So as others have said, it's better to use DB as much as possible to keep the kinetic energy out of the motor, then use DCIB sparingly just to finish the task if necessary.

kalabdel March 9th, 2018 06:01 PM

Quote:

Originally Posted by jraef (Post 771414)
Dynamic braking is dynamic because it converts the kinetic energy of the spinning load into electrical energy in the drive, then dumps that off as thermal energy into the resistor. But it's dynamic in that the faster the load is spinning, the more energy it contains for braking, but the slower it gets, the LESS energy it has for braking. So Dynamic Braking can never actually FINISH the job, it suffers from the law of diminishing returns. often times the load friction is sufficient to stop it anyway, but sometimes it isn't. So at some point if that happens, you turn on the DC Injection Braking to finish the task. A lot of VFDs have a default setting of 10% speed as the turn-on point of the DCIB, but it varies a lot from application to application.

The down side to DC Injection Braking is that the kinetic energy in that spinning mass it still converted to heat, but that is heat in the MOTOR, not an external resistor. That's why by using ONLY the DCIB, you are causing overload trips; most good VFDs will track the motor heating effects of DCIB and bias the trip curve so that you don't damage the motor.

So as others have said, it's better to use DB as much as possible to keep the kinetic energy out of the motor, then use DCIB sparingly just to finish the task if necessary.


***Off Topic***
Hi jraef,

Have you done or doing any motor control teaching/training? If not, please do and let me know where/how (do you have a youtube channel?) . I've bumped into quite a few of your replies on this forum and others going back many years and you have quite a talent for easy to understand yet information rich replies.
I'm always pleased to read your replies.

Cheers
Kal

jraef March 9th, 2018 06:35 PM

Quote:

Originally Posted by kalabdel (Post 771420)
***Off Topic***
Hi jraef,

Have you done or doing any motor control teaching/training? If not, please do and let me know where/how (do you have a youtube channel?) . I've bumped into quite a few of your replies on this forum and others going back many years and you have quite a talent for easy to understand yet information rich replies.
I'm always pleased to read your replies.

Cheers
Kal

I do a lot locally, what I call "VFD-101" classes. But I'm getting more and more pressure to put them up on Youtube. The thing is, I have a face for radio... so I'll have to use one of those programs that lets me basically walk through a presentation on my screen while recording audio, because if there is a camera, I'll break it. If I have time to do it, I'll let you know.

Sydney March 9th, 2018 09:50 PM

I Second Kal
 
Jraef

I spend several hours a week going over threads that interest me. I have also read many of your posts and agree with Kal on your ability to put into words explanations of complicated issues that are easy for readers to understand. As for the radio face comment LOL, keep up the educational posts and I hope to
SEE you on youtube.

Regards
Sydney

rupej March 9th, 2018 10:03 PM

+1


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