Spindle Brake and VFD?

My 13” is outfitted with the optional spindle brake. In time the machine will hopefully be powered in conjunction with a VFD.

I’ve read that VFD’s don’t like sudden stops.

Does the brake present any significant problems as far as wiring or motor functions go, or can it be disconnected at the motor without much difficulty?

Anyone else have the brake with a VFD?

Thanks

Could you use the DC braking function of the VFD instead? Maybe wired to the spindle brake switch?

Thanks for the reply.

I don't know. I was hoping I might somehow be able to utilize the switch, but I don't know if in order to do so I would have to (or even can) remove the brake "pad" from the mechanism so that it doesn't mechanically stop the spindle and cause havoc with the VFD.

I should say right here that it's ALL new to me.

A good VFD will have a couple of solutions to your question.

1. You can program the VFD to either ramp down to zero speed, or coast to stop. If you have a switch on your spindle brake, you can easily wire that into the VFD.

2. A good VFD will have an electronic overload (clutch) built into it. When you hit the spindle brake, the VFD will sense the overload in as quick as 0.1 seconds and immediately cut power.

I just want to clarify WJ Birmingham's post and say that a nice VFD will actually have three stop options, coast to stop, ramp down, and DC braking.

Thank you for the excellent insight, guys.

Now that I am aware of the Coast, Ramp and DC brake function possibilities, I suppose the questions become which option is best suited for general lathe use, which option is most practical in terms of wiring (Or not) with the motor and spindle brake on my machine, and finally which model of VFD to look into when the time comes?

Thank you

Some VFDs have a limited braking effect built in. These can handle the rotational inertia of small spindles like a drill press or a small milling machine that isn't spinning a big face mill at high speed.

Real VFD braking requires a high wattage braking resistor, usually in the range of 20-100 ohms but with huge current/heat dissipation capacity. All that energy has to go somewhere, so it turns to heat whether braking mechanically or braking magnetically on the motor.

For a lathe with a heavy spinning chuck, you're asking a lot of any braking system to slow it quickly. Add in the factor of most old SB lathes having threaded spindles that can unscrew on you in a sudden stop and you can have a very bad day indeed.

If you're fortunate to have a Cam-Loc or L-type spindle nose, you're in good shape to consider aggressive VFD braking. Just dig into your VFD manual and read up on the built-in capability. If it's not enough, you'll need to buy a braking resistor specified by your VFD. They come up on eBay frequently from machine teardowns, but finding just the right one can requires patience. Ohms too high and it won't give you fast braking. Ohms too low and you'll overload the VFD current limit.

In all cases, most VFDs can be programmed to ramp down at a specified rate, so take advantage of that setting to minimize the stop time without overloading. Your mechanical spindle brake switch should be easily adapted to tripping the VFD's stop (or e-stop) function after setting the VFD to coast to a stop.

Good luck!

What size motor does your lathe have? Some of the smaller VFDs don't have DC braking. Make sure whatever VFD has the features you want before pulling the trigger.

Assuming your VFD has the feature, and you've considered the excellent threaded spindle point made above, I think you've got maybe two options to preserve or emulate the spindle braking feature of your lathe:

1) remove or disable the mechanical brake, but install a micro switch in the mechanism to serve as a digital input to the VFD for a brake-to-stop. I don't know, however if it would be acceptable to hold the brake in this manner indefinitely. On my VFD, you may program the ramp down and hold times individually, but it then lets go and the spindle is then available to spin by hand. Don't know if that matters to you, but probably worth considering.

2) retain the mechanical brake, and use the micro switch to instead instruct the VFD to go to "base block", which is just e-stop. I'm not an electrical engineer, an electrician, or even someone who knows that much about this stuff, but my understanding is that in base block, the SCRs are completely isolated from the output of the VFD, so you should theoretically be able to brake the spindle mechanically without hurting your VFD. I would love for someone smarter than me to come in and confirm this, however.

I rigged this with the VFD... using the DC break on a 1HP motor. It is not instant, but stops it in about 1 second. There are trimmers, and you can turn it down... but as mentioned, there is an overload, that trips if you are too aggressive. I used a momentary switch for start, and a resetting DPDT kill switch which triggers the brake/stop function.