Connecting AXBB with Gecko G201X step drives - charge pump?

[mschoonmaker]

Hi Folks,

My AXBB controller is arriving today, I'm retrofitting a Romaxx WD-1 CNC (currently using Mach3). It appears the current control system uses the Mach3 Charge Pump driving a relay which seems to deliver or cut the drives' power. I've read elsewhere that pulling power to drives is a good way to damage the drives. If this is the case, it seems that I should perhaps use the AXBB's CHP output (to drive a relay) to enable/disable the drives, instead of completely pulling power from the drives. It also appears that the E-stop is in series with the Mach3 Charge Pump circuit which again drops power to the drives.

I'm interested to hear other, more experienced users' perspectives on this, and maybe even a suggestion or three?

Many thanks!

[ger21]

You typically kill the AC power to the drives power supply. That's fine.
You NEVER cut the DC power to the drives.
Personally, I don't believe it's necessary to cut power to stepper drives, or bother using the Enable.

Steppers won't move unless the software is sending step signals to the drives, and I've never heard of steps being sent when they shouldn't be.

[cncdrive]

I don't see why is it a problem to kill the power to the drives.
If removing the power from the drives damages them then they could damage anytime you switching the power off. And I guess you will not power them up for infinate time, but sometime you will power them off.
So, I would be surprised if it is a problem to remove power from the drives.

What is dangerous for stepper drives is if you disconnecting the motor from the drive while it is running, that could really damage the power bridge in drives...

[mschoonmaker]

Regarding the removal of power from the drive - from page 4 of the G201X drive manual, it states:

TERMINAL 2 Power (+) Connect the power supply “+” to this terminal

The power supply voltage must be between 18 VDC and 80 VDC. The maximum power supply current required is 67% of the motor’s rated phase current. An unregulated power supply may be used as long as the voltage stays between the limits; keep the ripple voltage to 10% or less for best results. The drive has a 2 second power-on reset time before the motor is energized.

CAUTION! Power supply voltage in excess of 80 VDC will damage the G201X.

CAUTION! Never put a switch on the DC side of the power supply! This will damage, if not destroy, your drive!


Really not sure why, but this is what caused my post...

[GaryLa]

The DC supply side should decay, not be immediately removed.

There is a lot of capacitance involved in both the power supply as well as the drives.

Switching off the AC and letting the DC decay is the only option.

[cncdrive]

CAUTION! Never put a switch on the DC side of the power supply! This will damage, if not destroy, your drive!

Interesting. I do not have any Geckodrives and not in contact with the company, but as you are one of their customers can you ask them about why is this?
I would be interested, because I see no reason for that, however I have designed a few servo and stepper drives in my life.

[mschoonmaker]

I have submitted the question to their tech support. Will report back here when I get a response.

[GaryLa]

CAUTION! Never put a switch on the DC side of the power supply! This will damage, if not destroy, your drive!

Inductance. If the DC supply drops instantly, the inductance will skyrocket the circulating current's voltage. This burns out the drive.

[cncdrive]

Then the drive has too small bulk capacitor inside in my opinion.

[GaryLa]

Then the drive has too small bulk capacitor inside in my opinion.

It's a function of inductance and the rapidly collapsing DC. It causes a spike which can easily exceed the FET ratings. It has nothing to do with Geckodrive. This can adversely affect many circuit types.

There's a similar problem on the coil of a relay. Unlike motors, the DC in the coil runs one-way so we add a snubber diode across the leads to force the spike to circulate within the inductor (the spike flows in the reverse of the existing current).

This is exactly the same problem you get when you plug or unplug stepper motors while power is applied. The spike can destroy the FETs.

BTW, spikes can also damage capacitors by exceeding their voltage rating.