Calculating Decel Time with VFD

[LionelHutz]

Hello;

In many VFD applications there is a braking resistor used to allow the VFD to delecerate the load in a reasonable time. I was wondering if anyone has references to a document showing how to calculate the shortest possible deceleration time given a certain motor, load, VFD and braking resistor.

I see this taking two calculations.

The first calculation is to find out how quickly the VFD can change the rotational energy in the load into heat in the resistor. I know this calculation would not include things such as frictional losses but it should be a reasonable safe approach.

The next calculation would be to get the average decelerating torque needed for the first calculations deceleration time to check that the current required to produce this torque does not overload the VFD. If it does, then find out the torque possible and re-calculate a longer time.

This approach seems reasonable but I'd sure like to see some one elses version if possible. I have yet to find any sources showing how to calculate this so I feel I might be missing something obvious.

 

[DickDV]

Your drive has a short-term current rating (usually one minute) which is a good value to use for maximum braking current. The braking torque would be approximately the same magnitude as the motoring torque at that current.

Use that torque to evaluate shortest possible stopping time given the load inertia.

To size the resistor, your braking kw will be about the same as the motor kw times the percentage of short-term overload torque available. That will be the heat kw in the resistor.

If the stop is infrequent and always into a cold resistor, you can size the resistor kw at 1/10 or the calculated heat kw. If your stop will be repetitive or near continuous, your resistor wattage will go up with duty cycle. At 100% duty cycle, the brake resistor wattage would be equal to the calculated heat kw with no derate.

Using this method will result in slightly conservative resistor sizing. This is due to some braking occurring from friction lossess and some braking energy being used to magnetize the motor. Better to be a little conservative than the other way around, in my opinion.

 

[Marke]

If you calculate the kinetic energy of the motor and driven load running at full speed, and if you ignore frictional losses etc, then that is the energy that you have to dissipate in the braking resistor. The rate of deceleration will determine the energy flow.
The maximum rate of energy flow is determined by the rating of the drive and the resistance of the braking resistor.
The short term dissipation rating of the resistor must be high enough to dissipate the kinetic energy.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[LionelHutz]

Well, this is good information.

DickDV;

I calculated an example using the current capacity of the drive and got 2.4 sec. as the decel time but when I calculated the quickest the drive can dump the energy into the given resistor I got 24 sec. This is using a built-in VFD resistor and calculated assuming a linear deceleration ramp. I know it won't be 24 sec because of losses in the system but that is still a big difference.

The VFD has a minimum resistance limit which limits the rate at which it can dump energy. Also, it's possible to connect a higher ohmic value resistor that limits the rate of energy dumping even further. Then, some VFD's come with a built-in resistor that is usually a higher resistance. So, to me, using the current rating only seems to only tell half the story.

Marke;

I do know the rate of deceleration will determine the energy flow....do you have any practical examples showing how to calculate the rate of deceleration given a set of motor and vfd parameters?

Peter

 

[DickDV]

LionelHutz, I think you will find that using the minimum resistance value will usually produce braking current very close to the current limit. The challenge is to pick the proper kw rating for the resistor.