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DICKEYBIRD
02-12-2013, 02:19 PM
I want to add a motor reversing relay to my CNC lathe equipped with a 90v 3/4 hp Baldor DC motor. Would this relay handle the power? If memory serves the motor is rated @ 7.5A.

http://www.ebay.com/itm/NTE-R55-11D20-12F-relay-12vdc-2p2t-10A-/290725669169?pt=LH_DefaultDomain_0&hash=item43b099a931

Alan Douglas
02-12-2013, 02:37 PM
I believe you'll need a relay with DC-rated contacts. The one referenced is rated for AC only. However if the relay only operates when DC is not flowing, it should work.

MaxHeadRoom
02-12-2013, 02:44 PM
If you intend reversing when running then you need a relay with magnetic arc-blow out, If you stop or inhibit a drive first you can use an ordinary relay.
P&B PRD model is one with Arc Blow out.
The one you show is on the light side for DC motor switching.
Max.

Mike Amick
02-12-2013, 02:46 PM
I think it would probably work fine .. the starting current for the motor is pretty close to
relay limits .. but .. I wouldn't be afraid to use it ..

are ya sure you wouldn't want to spend just a little more and be sure

http://www.ebay.com/itm/NTE-R55-11D20-12F-12vdc-DPDT-20A-Ship-2-00-in-USA-tomorrow-/261157004446?pt=LH_DefaultDomain_0&hash=item3cce2ba49e

edit: Max was replying the same time as me .. I would give his advice more consideration.

J. R. Williams
02-12-2013, 02:46 PM
Be careful when setting up a relay system to reverse a DC motor as many controllers do not like the load connections to be rapidly reversed without going thru a dedicated OFF time. Check the wiring specs for the controller for reversing procedure. No problem with the motor.

MaxHeadRoom
02-12-2013, 02:47 PM
I have used the typical cube relay before for DC switching, the contacts just do not stand up.
Max.

DICKEYBIRD
02-12-2013, 02:54 PM
Thanks, definitely not my area of expertise. Further review of the manuf. spec. shows 13A@28VDC so it's too small.

Probably work for my use 'cuz I usually cut small stuff at way less than full power but I need to look around some more.

DICKEYBIRD
02-12-2013, 02:59 PM
Oops, I missed ya'lls other learned responses, thanks!

Definitely will be making sure Mach is programmed to make the motor come to full whoa before reversing. My B.O.B. has 2 relays, 1 already switches the Inhibit terminals on the KB controller and the other one will switch the reverse relay...when I get a good one.

DICKEYBIRD
02-12-2013, 03:08 PM
OK, how 'bout this one? http://www.ebay.com/itm/Siemens-Furnas-Industrial-DC-20A-Control-Relay-DPDT-46PG25D24C-24VDC-Coil-/271140892567?pt=LH_DefaultDomain_0&hash=item3f2141af97

MaxHeadRoom
02-12-2013, 03:12 PM
That looks like the P&B I referred to, it has the Arc blow out magnet between the contacts.
Max.

DICKEYBIRD
02-12-2013, 03:18 PM
OK thanks Max. That puppy looks manly enough for the job and then some.

Hopefuldave
02-15-2013, 01:24 PM
HI Milton,
Only thing I'd worry about is that it's rated 20A for *non-inductive* loads - and a DC motor's very much inductive... No problem if Mach does it's job and removes motor power and waits for it to stop before switching, but if anything glitches it could produce some pretty arcs! In which case:

It may do the job, as you'll be running at around 1/3rd the non-inductive rating - If I were using it (and overly worried that anything that can go wrong, will go wrong) I'd either get one rated for inductive loads, a higher non-inductive rating or put a MOV surge arrester across the motor terminals to deal with any inductive spikes if/when the relay opened with the motor spinning.

Just my ha'pennorth,
Dave H. (the other one)

MaxHeadRoom
02-15-2013, 01:50 PM
HI Milton,
Only thing I'd worry about is that it's rated 20A for *non-inductive* loads - and a DC motor's very much inductive... No problem if Mach does it's job and removes motor power and waits for it to stop before switching, but if anything glitches it could produce some pretty arcs! In which case:

The arc blow out does a pretty good job, I have used these on large inductive loads OK.
Max.

DICKEYBIRD
02-15-2013, 04:10 PM
Thanks gentlemen; as always your knowledge and input is greatly appreciated.:)

The relay just arrived and it's definitely a manly bit of kit and bigger than I thought it'd be. I hope I can find room for it!

Ebay sellers (and FedEx) amaze me. I ordered it Tuesday around 3:30 pm and it was on my desk when I got back from lunch today at 12:00 PM. It was $24.99 with free shipping and arrived from Roanoke, VA via FedEx ground. Can't beat that! Now I've got something else to play with this weekend.

As to the arcing, hopefully my memory (and coding skills) will allow me to program in a 1 or 2 sec. delay after every M5 and before each reversal. M5's instantly open up the I1/I2 inhibit circuit on the KB controller too so that should help, no?

MaxHeadRoom
02-15-2013, 04:42 PM
You mean close the I1 & I2 to inhibit.
This resets the drive.
Also, you should have a couple of spare N.C. contacts on each of the PRD's, if you are so inclined, you could put a dynamic braking resistor across these, when both are open the resistor is placed across the motor.
This helps if you have high rpm and/or a high overhauling load.
Max.

DICKEYBIRD
02-15-2013, 05:33 PM
You mean close the I1 & I2 to inhibit.
This resets the drive.
Also, you should have a couple of spare N.C. contacts on each of the PRD's, if you are so inclined, you could put a dynamic braking resistor across these, when both are open the resistor is placed across the motor.
This helps if you have high rpm and/or a high overhauling load.
Max."Relay question for the non-electrically challenged amongst us." At least I freely admit my limitations.:o

Actually, I knew the correct usage of the inhibit function but typed it wrong. One side of the brain didn't want to recognize closing a switch & shutting off something.:rolleyes:

I thought about dynamic braking but I don't think it's worth the trouble on this little thing especially with the lightweight ER32 chuck. It slows to a stop real quick. I'll program a conservative pause since I'm not working on a tight production schedule or anything.

MaxHeadRoom
02-15-2013, 05:38 PM
Well if you did want to include it later it would just mean a ~100ohm W.W., say 10-20w resistor across the two contacts.
Max.

DICKEYBIRD
02-16-2013, 12:50 PM
Well if you did want to include it later it would just mean a ~100ohm W.W., say 10-20w resistor across the two contacts.
Max.If it's that simple then I maybe should do it. I think I have some chunky resistors in my stash. Which 2 contacts would it go across?

The way I plan to wire it is terminals 1 & 3 go to the output of the KB, 7 & 9 go to the motor with jumpers from 1 to 6 and 3 to 4 to reverse the polarity and A & B coil terminals go across the B.O.B. relay with 24V supply. The other B.O.B relay handles the KB inhibit switching.

The other question I have is about the power supply for the 24vdc coil. The lathe already has a modified PC power supply powering up several other 5v & 12v items. If i use the blue wire (-12vdc) along with one of its many red wires, (+12vdc) that would make 24vdc and would be OK to use, yes? The coil draws about 90 mah.

http://i57.photobucket.com/albums/g227/DBAviation/ReverseRelay_zps43ee5191.jpg

MaxHeadRoom
02-16-2013, 01:45 PM
The only thing with single relay is there is no OFF position, for that you would need two relays.
One FWD one REV.
This would leave a pair of unused N.C. contacts on each relay, one pair could be used for brake resistor, the other for inhibit.
I thought you had it mapped out otherwise I would have mentioned it.
If using two relays it also advisable to use a spare contact on the M3/4 relays in circuit with the coil as an interlock to avoid dual pick-up.
You can use the +-12v for the coil, but remember to include a reverse biased rectifier across the coil.
Max.

DICKEYBIRD
02-16-2013, 02:28 PM
OK, thanks Max. I'm pretty strapped for room in my cabinet so I'll probably stay with the single relay for now and see how it works out in use.

Wow, glad I asked about the +-12v thing as I didn't have a clue it needed a rectifier to work! My lack of electrical knowledge is embarassing at times but fortunately it's not what you know but who you know.:)

MaxHeadRoom
02-16-2013, 02:43 PM
Wow, glad I asked about the +-12v thing as I didn't have a clue it needed a rectifier to work! My lack of electrical knowledge is embarassing at times but fortunately it's not what you know but who you know.:)

The relay would work OK, but the rest of your system may not like the back EMF spike without it.
Max.

DICKEYBIRD
02-16-2013, 03:03 PM
Ahh OK that makes sense. I should do the same with the main power contactor as well.

MaxHeadRoom
02-16-2013, 03:17 PM
DC coils need a BEMF diode, positive stripe to +ve, and on AC coils a R/C snubber should be used, this consists of a typically 220ohm and a .5f capacitor in series. You can buy these ready made.
This is connected across any AC coil or solenoid.
Max.

DICKEYBIRD
02-16-2013, 03:22 PM
I did a little googling on the subject to educate m'self and try to decide what part to pick up at Radio Shack on the way home from work but I'm getting more confused than ever. I ran into typical forum arguments about what's best and long theoretical diatribes.

I hate to keep bugging you but do you have a recommendation on a type/value rectifier to use?

MaxHeadRoom
02-16-2013, 03:41 PM
Any of the 1N400x series, I keep a raft of 1N4007's on hand, they are only $3/100!!
You may see some negative comments about their speed, but these have been used for decades for this purpose and so far appear to be doing the job.
I have used nothing else for relay/solenoids.
Max.

DICKEYBIRD
02-16-2013, 03:51 PM
Awesome, thanks Max! I'm pretty sure my local RS carries a bunch of those in varying numbers.

Too bad RS is shutting down over a hundred stores. No telling how many budding young (or old) electronics wizards-in-training soon won't have a place to go to feverishly paw through those drawers looking for a final part to finish their latest brilliant creation.:(

MaxHeadRoom
02-16-2013, 03:58 PM
I generally use DigiKey, overnight delivery to Canada, no duty.
$8 freight, or if I am in no hurry I wait until the order is $100, then its free, their prices are pretty good also.
Max.

DICKEYBIRD
02-20-2013, 10:53 AM
Update:

I got worried about me screwing up and coding a spindle reverse with it still spinning and letting the smoke out so I investigated what it would take to build a safety circuit (hardware, not software) to prevent it. I found it would be way over my head to design such a thing so I did a little searching and found that good ol' KB has the solution in their KBCC-125R control. It has forward-brake-reverse capability or forward direct to reverse with simple SPDT switching...AWESOME! I got real lucky & found one NIB on ebay for $65 + shipping.

Question: I'm planning to wire up the NO contacts of the 2 relays on the C11 B.O.B. in series with the S1, S2 & S3 terminals for the forward-brake-reverse option. In the page below the KB manual mentions isolation for those terminals when activating them with an analog signal. Isolation shouldn't be an issue when activating them with the relay contacts should it? My history with isolation issues is pretty sordid so I thought I should ask.

I do have the KBSI-240D signal isolator installed for the voltage following input as recommended.

The thing is twice as big as the KBIC-120 it's replacing so it'll be a struggle to find room in the existing enclosure but I will find a way. It also has the big ol' KB heatsink so heating won't be a problem.

http://i57.photobucket.com/albums/g227/DBAviation/KB-RIsol_zps3bc5a832.jpg

DICKEYBIRD
02-20-2013, 11:23 AM
Oops, here's the correct schematic:

http://i57.photobucket.com/albums/g227/DBAviation/KB-RIsol2_zps5350eaa5.jpg

EVguru
02-20-2013, 11:23 AM
Just a note;

If you have a relay/contactor that is designed to switch off under load, then a back EMF diode across the coil can damage the contacts. It allows the coil current to 'freewheel' and slows down how fast the contacts open, making it more likely to arc.

If you need back EMF protection, use a zenner diode rated at the coil voltage.

DICKEYBIRD
02-20-2013, 11:32 AM
Thanks Paul. That advice applies to my comment earlier in the thread about adding a diode to my mains power contactor?

I haven't added anything to it yet. I'm going to look up the datasheet and see if it's equipped with one already.

MaxHeadRoom
02-20-2013, 12:19 PM
Update:

' KB has the solution in their KBCC-125R control. It has forward-brake-reverse capability or forward direct to reverse with simple SPDT switching...AWESOME! I got real lucky & found one NIB on ebay for $65 + shipping.
[/IMG]

That was one of the versions I mentioned in an earlier post, this has 4 quadrant control, so you can reverse without power relays.
Also has Dynamic braking.
Another feature is usually on these types you can set the pot for centre position zero, with one end +rpm the other -rpm.
Max.

DICKEYBIRD
02-20-2013, 12:43 PM
That was one of the versions I mentioned in an earlier post...I'm thinking the best way for me to approach any electronics issues from now on is to send you a check for an agreed amount, get your advice and bank the money I'd save from not burning up stuff and not buying stuff I don't need.;)

mayfieldtm
02-20-2013, 05:40 PM
Aw Heck!
I'd be tempted to get a 3phase motor and VFD Drive.
At 3/4 hp, it shouldn't be to crazy expensive.
Be really nice though.
Relays (switch contacts) and DC don't always mix very well and can be a real pain in the A$$ sometimes.

On the other hand that relay might work just fine.

Tom M.

MaxHeadRoom
02-20-2013, 06:02 PM
Relays (switch contacts) and DC don't always mix very well and can be a real pain in the A$$ sometimes.

On the other hand that relay might work just fine.

Tom M.

With the 4 quadrant drive it should not be needed, the reversing is done electronically with low level switching.;)
Max.

DICKEYBIRD
02-20-2013, 06:36 PM
Yep Tom, I dumped all the agony of home grown relay stuff and got the KB control with the reversing/dynamic braking built in. I think it has an onboard relay but I'll betcha' it's engineered to last a long, long time under heavy commercial use like all KB products. It oughta' last forever with the light use I'll put it to.

Oh yeah, the manual says 10 ma switching.