CNC, Dynamic Braking and RPC

Folks, have any of you had experience with a CNC connected to an RPC which uses dynamic braking? I just spoke with a tech guy at GWM about sizing up a unit for my Mazak Slant 15 (circa 81) and he unloaded that potential problem on me.

It seems that many of the Mazaks are/were notorious for dumping power back into the line when braking engages and there is nowhere for this excess power to go. He said that since the RPC is in close proximity to the machine, it will simply bounce the power back, resulting in adverse effects to the computer (error codes, shutting down, etc.).

I found limited info to this phenomena, or how to deal with it. Some mentioned a power resistor but that was with drive motors of 1-2hp. I'm dealing with 15hp driving my chuck. I also read somewhere about using a heating element, but I believe it was also referencing a small motor situation.

If any of you have info/insight into dealing with this I'd be elated if you could share it.

Best,
Mark

I'm not entirely convinced......

An "RPC" has two lines connected to the incoming power directly, so there is nothing there to "bounce it back".

The third line would, or should, accept power back and transform it, by motor action/back EMF, at least partially over to the direct power lines.

I'd suggest about a 66% to 80% equivalence to a direct 3 phase connection, if I had to estimate it.

That would mean that it might depend on how much power was dumped back, and also on what the RPC was. If oversized a bit, it might be better.

I definitely am not buying the "bounce back" deal. Maybe the person means the RPC is a bit higher impedance than the line, and that is true. But it can't "reject" power, it will send it back, just not as efficiently as a direct 3 phase line.

O. on re-reading your post, I should add that "close proximity" has , as far as I can determine, NO influence whatever. The wires go where the wires go, and "proximity" should have no effect.

perhaps he is trying to describe some other effect, but I can't determine just what.

if the machine is set up to "invert" the voltage back onto the line, it likely has no provision for an overvoltage resistor and switch. But it might.

I remain unconvinced until someone gives a better explanation of what is meant. Until then, I would suppose they mean line impedance vs the impedance through the RPC, and as I said, I expect about 66% at worst, and up to 80% effectiveness vs a direct connection.

Bottom line there is that it should work, but may not be as good at extreme situations.

Indeed. I contacted another company and brought up this phenomena - they confirmed it could be a problem but noted that it happens to 1-2 customers per year on average. So I guess it isn't as common as what I was lead to believe.

We'll see what happens...

I have a bit of experience with the Mitsubishi FR-SE spindle drive in the Mazak QT-10 & believe it is similar to the drive in the QT-15. This is a regenerating drive that converts the energy from the rotating spindle & held in the DC bus capacitors back into AC which is then sent back to the AC source.

When this drive decelerated the spindle it would often get an error. Turned out that the +5V on the control board was low even though it was in spec. Anyway, while going thru the documentation, I could not find any way to add a braking resistor using existing circuitry. ie., I didn't see a plug in braking resistor option. If you have a problem you might try contacting the drive manufacturer (Mitsubishi) & ask if it is possible. I also found that the decel time can be set by switch SW5-3,4,5 the Cushion Time (p.29 of BNP-A72737-35A). By lengthening the decel time, the amount of regeneration is reduced and may eliminate any problems caused by it.

Hope this helps should you have any problems,

Cheers,

Hitachi has external braking control modules and resistors you can add to the dc bus of another mfgr spindle if that is what you needing. pm me for details and specs if interested.

btw, they also have a couple different size stand alone dc bus regen to line modules that are priced what I consider cheap too; iirc like 3kw size for $2000 maybe? pm me if interested and I will look em up too.

as for off the wall guess on line regen causing rpc problem.... keep in mind the spindle motor wont generate much more than its rating in stopping I think and so if it starts ok on the rpc pulling 2-3-4x nameplate rating, I wonder why deceling at much lower current/power should cause problem...

Modern VFD's have detection circuits that will bleed any over-voltage on the DC bus
capacitors. This not only operates when braking but in normal operation when the
motor is regenerating. This is called a brake circuit. Some DSP's/Microcontrollers
designed for VFD operation have the brake signal output designed in. In simple terms,
there is a transistor which receives a modulation signal that shunts current from the DC
storage caps to ground through a resistor.

Not knowing any particular details regarding your RPC I assume that you have a
situation which is similar. Your RPC is more elaborate than a motor and a few run
capacitors. I would think that a RPC that has a dynamic brake designed in would
take care of the motor regeneration effects. Question? What RPC is being used?

mrogowski,

Is the control a Fanuc 6T? If Yes, both the servos and spindle drives are regenerative DC units. And the spindle drive is quite tempermental.

Bill