Installing a dynamic brake on a VFD
- Thread starter davewo
- Start date
rsanter
Well-known member
Most VFDs have the option to program the time it takes to spin up,to full RPM and spin down to zero. Does yours not have that? Have you called the MFG to ask about programming that feature?
I just installed a Snyder (sp) VFD on my mill and that was one of the programming parameters. I think mine is set to 3sec for each function
On VFDs there is two types of dynamic braking. One adsorbs the energy into the resisters that are in the VFD and they dissipate it out as heat. The other way is that some VFDs back feed that energy into the motor that they are operating ( eddy current) and the motor dissipates it as heat
Bob
I just installed a Snyder (sp) VFD on my mill and that was one of the programming parameters. I think mine is set to 3sec for each function
On VFDs there is two types of dynamic braking. One adsorbs the energy into the resisters that are in the VFD and they dissipate it out as heat. The other way is that some VFDs back feed that energy into the motor that they are operating ( eddy current) and the motor dissipates it as heat
Bob
MBfreak
MEMBER EMERITUS
Hello.
The concept of braking resistors is using the resistor to dump the stored energy in the rotating mass and magneticc circuits by letting the drive motor be used as a generator which is switched onto the resistor (energy sink) thru some power semiconductor.
If you have a value from the manual, a way to configure the VFD for resistor braking and schematic that shows how to connect it up, use that.
The precise value of the resistor in ohms and the power rating in watts is not all that critical, at least not for a trial run. If you have access to electric heaters, you can use them for a trial. A 110 V 500 W heater has a nominal resistance of around 24 Ohms. Connect two in parallel with the third one in series and you have 36 Ohms ( close enough to 39) with more than enough Watt capacity.
Try that out.
Relying on dynamic resistor breaking to stop a tap before it bottoms out is AFAIK not a very reliable solution.
Best regards
The concept of braking resistors is using the resistor to dump the stored energy in the rotating mass and magneticc circuits by letting the drive motor be used as a generator which is switched onto the resistor (energy sink) thru some power semiconductor.
If you have a value from the manual, a way to configure the VFD for resistor braking and schematic that shows how to connect it up, use that.
The precise value of the resistor in ohms and the power rating in watts is not all that critical, at least not for a trial run. If you have access to electric heaters, you can use them for a trial. A 110 V 500 W heater has a nominal resistance of around 24 Ohms. Connect two in parallel with the third one in series and you have 36 Ohms ( close enough to 39) with more than enough Watt capacity.
Try that out.
Relying on dynamic resistor breaking to stop a tap before it bottoms out is AFAIK not a very reliable solution.
Best regards
I use a resistor on my mill with vfd. google "vfd's for newbies" a fellow named Forrest Addy has posted quite alot about vfd set- ups I don't remember what site he was n,maybe practical machinist, he would tell you that you can use a stove burner element(it is basically just a resistor) read and learn !
pete
pete
I can't find the one I bought -- no name on it , I got it from newark electronics - seems like it cost maybe 40 or so dollars at the time, but as long as the resistance is correct, you should be ok.
http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/vfd-braking-resistor-103374/
http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/vfd-braking-resistor-103374/
MBfreak
MEMBER EMERITUS
Hi.
There is no need to go so high in the Watt rating. The energy in the rotating and magnetic circuits needs to be converted to heat in the resistor mainly.
And it is a low duty cycle , so the resistor ( or whatever "heater stuff" ) you use will nit get very warm. For the machine size you have my belief is that 500 W is VERY sufficient.
When testing it out, brake as often as you think you will ever do from the highest speed you will ever run and just smell and watch the brake resistor.
If overly hot, increase W, if obviously not overloaded put it in a metallic grounded enclosure and wire it up permanently.
That is if it really breaks the machine the way you are satisfied with.
Ola
There is no need to go so high in the Watt rating. The energy in the rotating and magnetic circuits needs to be converted to heat in the resistor mainly.
And it is a low duty cycle , so the resistor ( or whatever "heater stuff" ) you use will nit get very warm. For the machine size you have my belief is that 500 W is VERY sufficient.
When testing it out, brake as often as you think you will ever do from the highest speed you will ever run and just smell and watch the brake resistor.
If overly hot, increase W, if obviously not overloaded put it in a metallic grounded enclosure and wire it up permanently.
That is if it really breaks the machine the way you are satisfied with.
Ola
laser3kw
Well-known member
saw dust + open heated element = problem. The do make enclosed models where the element is not exposed. The have an aluminum case to act as a heat sink. I really don't think you need dynamic braking on a table saw. Normally you turn them on use them, turn them off. The brake would offer no real benefit unless you had to stop suddenly and maybe reverse quickly..
Strouty
Well-known member
Silly question, but why don't you just reverse the motor when you get to the point you are worried about? Does the VFD not allow for this? I am used to a rotary phase converter, so I am not sure.
I never understood why a mill needs a braking resistor. If you insist on having one here is another source.
http://www.automationdirect.com/adc..._Braking_Options_(Braking_Units_-a-_Resistors)
http://www.automationdirect.com/adc..._Braking_Options_(Braking_Units_-a-_Resistors)
Strouty
Well-known member
I guess that is one good reason to use a rotary phase converter. I had no idea that you could not instantly reverse a VFD. I am going to stick with my rotary phase converter for my mill, lathe, and drill press. You just saved me a lot of money!
So VFD for saws, grinders, and other things that don't need quick reverse. I was going to run a VFD on my bridgeport but now I will stick with adjusting the belts for speed changes.
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Strouty
Well-known member
Here is some more info on reversing, it appears that you can get damn close to "instant" with a VFD.
http://www.garagejournal.com/forum/showthread.php?t=164002
http://www.garagejournal.com/forum/showthread.php?t=164002
Strouty
Well-known member
My bridgeport is the only thing that really would be nice to have the VFD, my lathe and drill presses are all variable speed, so that does not really help me much.
Strouty
Well-known member
Very inventive and seems to be cheaper than what you were looking at just for the resistor.
Just a side note for when you have your final setup picked out, most braking resistors are wired with high temp wire.
Also sometimes it is good practice to control the resistor with a contractor monitored by a temperature switch if you are going to be using this machine lights out or semi lights out. This prevents the resistor from overheating and possibly catching fire/melting down.
Also sometimes it is good practice to control the resistor with a contractor monitored by a temperature switch if you are going to be using this machine lights out or semi lights out. This prevents the resistor from overheating and possibly catching fire/melting down.
Falcon67
Well-known member
I have a 2 HP TECO on my 1 HP mill/drill (Griz G0519). I can instant reverse the spindle with the switch and have the E-Stop set for .5 seconds. I do not have a braking resistor on the unit. The E-Stop has saved me grief several times.
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Strouty
Well-known member
You could cook lunch while working, that is an added feature you never knew about.
