AC motor drive question

i need to calculate decelerate rate of ac drive for safely but fast stop of the process in steel plant. Could anyone give me any suggestion or documents how to calculate decelerate rate of ac drive ?

I think you would need to know
1. total mass of the load you wish to stop
2. Total inertia of the rotating elements
3. Time frame you want it from full speed to zero

From the above you should be able to calculate braking energy and use that number to size the braking resistor.

Dan Bentler

This is a good article on dynamic braking:
http://www.mrplc.com/kb/index.php?print=87
Be prepared to get your brake resistor and say your prayers if your inertia and braking time formulas are bigger than what your drive can stand. Otherwise your drive is going to trip when braking.

First and most important thing whether motoring or braking: YOU MUST UNDERSTAND THE BEHAVIOR OF THE LOAD MACHINE. Assuming that you do, then the stop time limit is the shortest stop the machine will tolerate. That covers things like broken gears, broken drive shafts, bent rollers, and how the process material acts when suddenly stopped. For example, on this last one, if the process material is a sheet of steel, will it fold up and jam if stopped too fast. Or, in the case of liquids, if a fast stop launches all the liquid out of the machine and into the basement, that clearly would be too fast.

Again, assuming that you understand where all the limits are, then you need to calculate or at least estimate braking hp. Sometimes, you can get a good estimate of this by watching the acceleration time. For example, if it takes 25hp to accelerate the machine up to max speed in 10 seconds, then you can figure it will take about 50hp to stop in 5 seconds and about 100hp to stop in 2.5 seconds. These will be a bit conservative due to friction losses helping you stop. Lacking any of this, as mentioned above, you will need to take rotating inertia and stop time and figure braking hp.

Don't be supprised if braking hp comes out higher than motoring hp. (I've got one application where the motoring hp is 15 and the braking hp is 75!) If that is your case, you must size the motor and drive for the larger of the two. You can use the short-term rating of the drive which will reduce it's size by about 1/3. You can figure peak short term hp in the motor which will reduce it's size by 1/2.

As for the braking resistor, if you are sure that the only time braking is used is when the resistors are completely cooled off, then you can figure the resistor wattage at 1/10 of the braking wattage. If the braking is repetitive, you may have to up the wattage to the near continuous rating.

Hope this helps.

Thank you for reply.Widelto could you please tell me equation of peak braking power. In the documents it does not shown.

Braking hp is the same formula as motoring hp---- hp=torque x rpm/5250. Torque would be in lb-ft. For braking, the torque is negative.