So, I have a 1 HP Bridgeport on a VFD. The time has come to install a braking resistor. I had it for a while, but wanted to install the DRO first. With the DRO, and wiring better organized, installing brake resistor is more pleasant.
The question is about stopping time.
Right now (without the external brake resistor), I have the mill set to stop in 1 second. That works at most speeds, except at highest RPM it trips on overvoltage and then coasts to stop.
Just what can I realistically expect with a brake resistor? Would you say that I could expect to stop it in 1/2 second at all speeds?
Yes. I tap with my Gorton Mastermill, which has a HEAVY motor (5hp) and a big spindle.
I can do an "invisible reverse" at speeds up to 5000 rpm. Changes direction so fast you never see it.
I do have a 5hp VFD and a big breaking resistor. A tubular ceramic btw.
A braking resistor is essential if you are doing braking, or fast reverses.
I specialize in OmniTurn CNC lathes, which use a 5hp motor and can do an invisible reverse at 4000 rpm, and they too have a big braking resistor.
I have a 3/4hp motor on my Big Delta drill press, with an older Hitachi VFD..no braking resistor..and it trips if I try to tap with it at just about any speed.
Some VFDs come with a small internal resister, some with none, some with no provisions for an external resistor
Gunner,
"I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. In my youth I travelled much, and I observed in different countries, that the more public provisions were made for the poor the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer." -- Benjamin Franklin, /The Encouragement of Idleness/, 1766
This is very encouraging. I think that what I will do after installing the resistor, is try to take the stopping time as low as possible, at highest RPM. Then I will back out just a bit. Almost instant stop, is a very convenient thing.
Practically, the minimum braking time for the motor-VFD combination (with suitable braking resistor) will be approximately twice the minimum starting time that the VFD can drive the motor at. For the simple reason that it's either going to be limited by motor torque or VFD capacity on the way down in exactly the same way that it is on the way up.
I've just installed the brake resistor on a 5 hp Hitachi VFD going to a four horse spindle.
Hitachi allows two accel/decel/max speed profiles based on an input. So, I set up #2 to have a lower max speed and very high decel. This is for CNC tapping, it stops and reverses on a dime.
At more or less the highest RPM, the mill can be stopped in 0.9 seconds. As I found out, I originally wired it for 3 seconds stopping time. So this is an improvement (3.3 times less).
I am very happy.
I wish that the VFD reduced braking efforts, if it detects overvoltage, instead of tripping on overvoltage. This is really not too smart on the part of the VFD, unless I missed something. If it could do that, then the mill would stop almost instantly at low RPM, and longer at higher RPM.
Some do. Instead of a fixed ramp time from 0 - set speed, some also have a ramp rate, in Hz/sec. That would take less time at the lower speeds. Most of these drives have SCADS of hidden options that only become obvious on the 10th reading of the manual.
It should do that now. What vfd do you have and how much DC braking have you programmed it for?
Gunner
"I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. In my youth I travelled much, and I observed in different countries, that the more public provisions were made for the poor the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer." -- Benjamin Franklin, /The Encouragement of Idleness/, 1766
Would you happen to know about the specific case of Hitachi VFDs? My CNC mill would only stop from top speed in 1.4 secs or trip (9 secs. before brake) . Takes the same 1.4 secs. from low speed.
I found one "back door", there's a second profile allowed (switched based on an input) so I set max Hz to 30 and decel time to 0.2; I'm still working a way to to switch for tapping.
I have not yet used DC braking, but I believe that it is for something else (like fans in a windy tunnel or something). Maybe I am wrong, the drive has it etc.
i
Brakes are great when there is a problem afoot. Stopping a machine eating itself up or the like is important.
I really enjoy the function in my 1/2" hammer drill. If it catches on something - I let go of the trigger and it stops. I don't get wrapped up in the cord and drill bit.
Martin
Mart> >> >>
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