G201X power switch

[crochambeau]

Hello,

Apologies if this has been covered before, it seems my question is built on pretty generic search terms..

My operations manual stipulates:

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

Does this refer to limit switch, or main power switch? If it means main power switch, why?

My situation is such that I will be running everything off of one AC circuit. I've a large linear power supply that I intend on using, but plan on switching it on first, before anything (especially the PC), because I expect huge inrush current as three caps the size of pint glasses charge up. After that stabilizes, I will power my PC, etc up and then apply power to the drives. I'm not fond of the idea of having my drivers active while the computer boots.

The power supply has a DC breaker (rated at 25 amps) that I was planning on using as my 24 volt "power switch". In my mind, that is a switch on the DC side of the power supply.

Does this configuration skirt potential problems with DC switching? The manual doesn't go into a lot of detail beyond "don't do it". I'm hoping this was aimed at switching while the machine is active, which I certainly do not plan on doing.

[H.O]

Don't switch the DC between the powersupply for the drives and the drives them selves- that's what it says ;-)

The reason is the same as with your powersupply and the inrush current you expect when turning IT on. If you have a fully charged capacitor bank in your powersupply and then "dumps" the G201X across it with a switch you'll get a huge (or at least large) inrush current into the G201X (because it too has capacitors inside) and that is not healthy for it.

That's one reason, the other is when turning that switch off you can get a large voltage "spike" due to the inductive nature of the motor, that spike can also kill the drive.

Why not simply use the Disable input of the drive to keep it "off" untill everything is up and running?

/Henrik.

[crochambeau]

Ah, I hadn't considered making "Disable" a default setting on power up. Does Disable prohibit power from reaching the motor entirely? My goal is to power up the PSU as unloaded as possible, I'm not too concerned about the additional load that the drivers themselves will provide at power up.

I still don't fully comprehend how instantaneous voltage at power on (assuming the voltage level is held within the 80 volt tolerance window) will do any harm. Is it safe to conclude that the primary area of concern is the powering off of the device?

This brings me think about the following:

I will have a fuse inline with the DC buss to each driver. What are my concerns if the fuse blows? This would obviously happen while the machine is under load, and back EMF is at peak levels. Would it be wise to integrate a post fuse pulldown resistor in the event the machine is overloaded?

The aforementioned breaker on the DC side. I will be powering additional things with the 24 volt feed (such as vacuum switch, cooling fans). While I fully plan on keeping the current well within the 25 amp window, I need to consider what will happen should this breaker throw.

Would putting a normally closed 24 volt relay at the disable input be a wise idea? This will prohibit the driver from being in active duty unless 24 volts is on and the relay is switched (allowing stationary motors while the PC boots) and would complete the disable to ground circuit should anything go wrong on the 24 volt line.

Am I playing with fire here? It's too late to buy the unkillable model, and I'd just as soon build my machine correctly in the first place (still in the design phase, though I have most of the important bits on hand).

I should add that my power off process would be at AC mains, not DC circuit. I just want to build this thing so it can withstand any mishaps, as I am inexperienced with CNC (I've been wrangling electrons for years).

[H.O]

Ah, I hadn't considered making "Disable" a default setting on power up. Does Disable prohibit power from reaching the motor entirely? My goal is to power up the PSU as unloaded as possible, I'm not too concerned about the additional load that the drivers themselves will provide at power up.

Yes, it turns off the "powerstage" of the G201X so that no power is allowed "out" to the motor, it will look like there's no power connected to the G201X.

I still don't fully comprehend how instantaneous voltage at power on (assuming the voltage level is held within the 80 volt tolerance window) will do any harm. Is it safe to conclude that the primary area of concern is the powering off of the device?

Because there are capacitors inside the drive and when powering up the dirve these capacitors are discharged. When you instantly apply <80V to the drive there will be a very large current surge into the drive in order to charge these capacitors to same voltage as your powersupply. This current surge is not healthy for them.

This brings me think about the following: I will have a fuse inline with the DC buss to each driver. What are my concerns if the fuse blows? This would obviously happen while the machine is under load, and back EMF is at peak levels. Would it be wise to integrate a post fuse pulldown resistor in the event the machine is overloaded?

Put a diode across the fuse, anode towards the drive, cathode towards the powersupply capacitor. That way, when the fuse blows, power is allowed to go from the drive back to the powersupply but not the other way around. For belts and braces place a MOV (varistor) at the power terminals of the drive.

The aforementioned breaker on the DC side. I will be powering additional things with the 24 volt feed (such as vacuum switch, cooling fans). While I fully plan on keeping the current well within the 25 amp window, I need to consider what will happen should this breaker throw.

This is the same powersupply as for the motors, yes?
Why not simply put a fuse on the primary side of this supply and then put individual fuses for the various loads on the secondary?

I must admit I probably don't see the whole picture here...Do you have one single 24V powersupply or do you have a 24V powersupply AND a higher voltage supply for the drives?

Don't worry, we'll get it sorted without blowing anything up.

/Henrik.

[crochambeau]

Originally Posted by H.O Yes, it turns off the "powerstage" of the G201X so that no power is allowed "out" to the motor, it will look like there's no power connected to the G201X.

Perfect, I am now comfortable with the idea of powering these up with the power supply, the DC side of which will be normally on.

Originally Posted by H.O Put a diode across the fuse, anode towards the drive, cathode towards the powersupply capacitor. That way, when the fuse blows, power is allowed to go from the drive back to the powersupply but not the other way around. For belts and braces place a MOV (varistor) at the power terminals of the drive.

Excellent suggestion regarding the diode across the fuse. Regarding the MOV, any reason I shouldn't go with a clamping voltage of 76 volts?

Originally Posted by H.O Do you have one single 24V powersupply or do you have a 24V powersupply AND a higher voltage supply for the drives?

I have one 25 amp 24 volt power supply, this will feed all three driver/motor combinations, and various other low draw accessories (which will also be fused now that you mention it). I have a readout for supply voltage and amperage and should be able to observe any potential problems before they run away. If I'm really going to be using only ~ 60% of the rated motor current, everything full on at once will be roughly 50% of the power supply capability.

Many thanks for your assistance! With safety valves in place the power up/down procedure will be simplified.

Curtis

[H.O]

Hi,

Regarding the MOV, any reason I shouldn't go with a clamping voltage of 76 volts?

No, not really, it should work nicely. On the other hand, if the powersupply voltage is 24V why such high voltage on the MOV?

Sounds like you have it under control! Good luck!

/Henrik.