Using magnetic contractors with dc

[Hukshawn]

I changed my lathe to a dc motor and drive. The drive doesn’t have a reverse option, so I have my original barrel switch between the drive and the motor doing my fwd/stop/rev control and a small control box beside it doing main power and speed.
I’d like to change everything to one box with push buttons. I have some big contractors I gathered from a factory I worked in years ago. Definitely big enough to handle the amps (the motor will draw 20-25a max) but disconnecting dc at high amps can damage contacts as it tends to arc. Dc doesn’t want to let go cause it’s constant. Ac doesn’t have that problem because the sine wave passes 0 volts as it oscillates. I’m wondering if anyone has any experience with dc voltage and contractors. Should I expect burnt out contacts at some point? Or is it an unlikely problem?

 

[markba633csi]

Don't worry about burnt contacts because you don't want to be disconnecting dc at high amps anyway- controller will blow up. You could set up a latching system to do the reversing but you would need a time delay to bring up the controller after the direction contacts have closed, and you would need your stop button to kill power to the controller but leave the last directional contactor closed. It gets kinda hairy as far as complexity.
Mark

 

[Hukshawn]

Hmmm...

The controller has those enable/inhibit terminals. I could use those to stop. But yeah, gets kinda silly trying to coordinate stop and direction without messing up.

Unless, I use the stop button to inhibit/stop and have either direction start button to basically reset the contacts to the desired direction and re-enable the driver at the same time. So the contactors don't actually let go until the drive is beginning it's restart ramp up procedure. This can be done with the parts I have.

It's probably good I mentioned this, cause I have been using the barrel switch to start/stop the motor leaving the drive engaged not realizing it was a problem. That is probably a lot of start up amps hitting the driver.

 

[Z2V]

Just curious what you are using for a motor/ drive

 

[Hukshawn]

Kbcc 125. Not reversible. The 125R is.

 

[whitmore]

I changed my lathe to a dc motor and drive. The drive doesn’t have a reverse option,... I have some big contractors I gathered from a factory I worked in years ago. Definitely big enough to handle the amps (the motor will draw 20-25a max) but disconnecting dc at high amps can damage contacts as it tends to arc.

Well, everyone with a DC-capable welder can tell you that motor-drive voltages
and DC current can take a bite out of metal. Switches rated for 125VAC are usually
limited to 30VDC, but a DC motor might work on 90 to 180VDC.

Hopefully, the motor's DC controller can help; there's usually an 'inhibit' pin
used for thing like limit switches, so you can use a 'reverse' sequencing switch that first inhibits,
then swaps polarity, then lifts the inhibition. In the old days, there were wafer switch
options for break-before-make or make-before-break sequencing.

 

[Hukshawn]

All the old pre-PLC components I have from oldachinery control panels and boards are all sequencing style components. The only issue with setting up a sequence is there would need to be a delay added. The controller is designed to ramp the speed down at a preset time before it stops. A delay would have to be added in the sequence in order to break after all stop to avoid the break arc.
Which was what brought me to leaving the contactor engaged until restart when current/voltage would be zero.
I ordered a push button enclosure,
when that comes in I'll start playing around with a wiring circuit and post my findings. I'm sure I've drummed up some interest.

 

[Hukshawn]

Here, I did a video to show my set up. It’s eligant in its complexity but keeps my operating procedures down to a 3 button operation, like I wanted.

 

[Bi11Hudson]

There are some finer points to the previous replys that I didn't pick up on, it is well past midnight for me....; But a couple of comments from working on overhead bridge cranes with DC hot rails:

The first is to use a "double break" relay (contactor). The double break system isolates the two contacts so there is less chance of arcing over when they break under load.

The other is the use of "plugging". Although outlawed(?) today it was a common practice way back when. With the bridge running 5 points south (wide open), the controller was pulled to 1 point north. This for braking action. The same using the rack.(trolley east & west) There was a little burning at the contact tips, but the contacts lasted for months, before needing replacements. That on a 30 ton (small) crane in continuous use for 2+ shifts.

On the big cranes(200 T & up), a NEMA Size 5 contactor had contact tips the size of a half dollar. I guess that is the key to what I am trying to say. Big enough contacts in a double break configuration would handle your problem. Just make sure the DC rating of your relays is up to the size motor used.

 

[Hukshawn]

Yeah and these contactors don't have dc ratings on them. So another reason to zero the load before switching.
So now, when the contactors actually switch over, there is no load, zero. There will be no arcing. And I can't mess up and do something stupid like press both buttons because of that little white spacer between the contactors. That's a switching lock. I don't know what they're formally called but it switches back and forth with the contactors. It will not allow both contactors to fire at the same time, it physically locks out the other out.