VFD Help needed - what braking options?
I have just installed a 1/4 HP VFD driving a 1/6 HP motor on the table of a surface grinder. The table runs back and forth with the VFD cycling the motor forward and reverse. It reverses kind of slow with the default settings - 6 secs & coast to stop. How quickly can I have it stop and with how much braking before I start to tax the limits of the VFD or shorten it's life considering the toll which the repetitive back and forth might have on it.
Brake starting frequency - 60 cps or less
Braking level 0% to 100%
Braking time 0.01 to 30 secs
There is also an option for automatic deceleration control (regeneration prevention).
The manual also says that a braking resistor will not work on this drive.
Normally you cannot damage the VFD by shortening the decel time as they have built in safeguards, the most common if you go down too low is overvoltage, due to the regeneration exceeding the supply.
As soon as it sees this, the motor is turned off and will coast.
A bit of detail on the drive might help.
If the drive does not have provisions for an external resistor the parameters you posted won't apply, or are for DC injection braking.
The minimum decel time depends on how much inertia is in the table drive system.
If you just unplug the drive when it's running the table, how long does it take to coast to a stop? The decel time can be set to a value somewhat less than the coast time as a starting point.
You can also just try setting the decel time lower and lower until the drive trips, then add a bit of time and you have your setting. You won't hurt the drive...
Any experiments like this should be tried with the heaviest mag chuck you plan to run plus some weight to simulate a heavier than average workpiece.
Six seconds is a lot of time, a lot of drives come out of the box with very high accel/decel settings. Wild ass guess is that you could run a second or less on the decel.
It's unlikely you would shorten the life of the VFD by the repetitive reversing, that's what they are made for.
A 1/4 hp VFD is very small and won't have much internal ability to bleed off energy however, so if your table inertia is high and the drive friction low you might not be able to get quick reversing.
If you can't get the decel under ~1.5 sec you might want to switch the drive for one that has an external resistor or simply a larger hp drive.
The external resistor is the best bet, with a brake resistor the drive essentially gets turned into a generator on decel and the table inertia gets dumped into the resistor and turned into heat.
With a brake resistor you could easily set the decel to 1/2 sec or less.
Let's put it this way... you're running a 1/6 HP motor. Resistors and whatnot won't increase the power of that. All you've got is 1/6 HP. Put your drive to its lowest setting. Since you're overrated already, ain't nothing gonna break.