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Help wiring a DC Injection brake

bbarenz

Plastic
Joined
Jan 18, 2021
I bought a used Square-D 8922 EMB 20/480 DC injection brake and I'm trying to wire it in to an old Okuma LS lathe with a 3 phase (240V) motor. It didn't come with instructions but I was able to find a manual however they don't define the terms used and it's unclear to me how to complete the wiring. The diagram from the manual is below as well as the wiring diagram for the Okuma lathe. I'm pretty sure the main power wiring is correct (L1,L2,T1,T2) shown in the diagram. I connected I5,I8 in series with the Start/Stop circuit however I'm not sure where 25,28 and X0,X0 are supposed to go. I originally thought X0 was a capacitor however it shows the same M1 symbol as show in series with 25,28 so I'm not sure what M1 is mean to represent.


DC injection Brake Circuit
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Okuma LS Lathe circuit
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Those diagrams are too tiny to read.

'X' is generally used for the secondary windings of a transformer or the load side of some other power source. In this case it looks like a signal source from the module for the connection of a "dry" contact.
'M' refers to the contacts and coil in a motor starter.
'OL' for the contacts and heaters in overload relays.
'L' is the line side of a self-contained starter, switch or other device... 'T' is the load side.

This is a diagrammatic view using NEMA symbology. The symbol for a normally open contact in this context closely resembles that of a non-polarized capacitor seen in board-level schematics. They are not to be confused.

I can't hardly see what's going on in the first diagram because of the watermark and how small everything is, but X0 seems like an electrical interlock using an auxiliary contact on the M1 starter to control operation of the brake module. M2 appears to be a customer supplied contactor for the brake module. I5, I8, 25 and 28 seem like they might be control elements within the brake module which are to be inserted in series with the usual three wire start-stop arrangement, etc.

My guess is the module runs on a timer that starts when the M1 auxiliary between X0 and X0 is closed. I5 and I8 probably open when 25 and 28 close - preventing M1 (power) from closing, whilst closing M2 (brake). Note that M1 and M2 are electrically interlocked to prevent Bad Things™.
 
I built a few brakes to stop a planner heads (without brakes the heads would coast 20 mins or so )It takes a interlocked starter like a forward rev type or a 2 speed motor contractor, I would build a dc supply around 50 volt dc and for a 50 hp it would be up in the 100 amp range. At stop the dc would be applied to 2 of the 3 phase windings with a timer, this very important as the dc would heat the hell out of the windings and rotor of the motor and only would be good for a few stops a hour. I think you are short the dc supply and the right starter....Phil
 
I tried that many years ago on a Gidding and Lewis production lathe.... never going past the proto stage.

For a test only PS, I used a automotive 12V battery charger, and was stopping a 3HP 3p 208V, 1700 rpm motor by injecting the 12Vdc across two of the three winding's.

In practice I would have needed current limiting, the stop was way too abrupt. And yes you need a timer!
 
I'm no help on the wiring but I would think you might get some clues looking at diagrams from Versibrake in Germany or Powerdrive in UK. They may be similar. I also question the size of the brake. I'm not familiar with the Sq D brakes but it looks to be a fairly small one. I have a Versibrake 20 and it is rated to stop a shaper running much smaller tooling than what your lathe will handle with a chuck. I believe my brake is only rated for 20-30 stops per hour at less weight. Dave
 
You might be better off to go to a pugging style brake, still needs a f/r starter...It just applies rev 3 phase at a low voltage (50/75 volts) to the coasting motor...Phil
 
Does this lathe have a cam-lock spindle nose? If not, a brake might be a very bad idea. Wouldn't want the spindle to stop and the chuck to keep going... and going... and going...
 
the dc would heat the hell out of the windings and rotor of the motor and only would be good for a few stops a hour.

Is this true for all dc injection brakes, or something about your application?

I am also planning on installing a dc injenction brake on a lathe, and cycle it frequently. Is there a way to do this without risking the motor?

Is it typical to wire the brake so that the braking current flows through the heaters in the starter? -- to add some protection.
 
Just calibrate the braking current to match the motor's nameplate FLA and set the timer to cut out after however long it takes for the motor to come to a stop. You won't overheat the motor or trip the overload relay at that current. Heaters are usually set between 110-125% of FLA depending on the motor's temperature rise and service factor to prevent nuisance tripping. Most motors are marked with a 1.15 SF - meaning they've been designed and tested to handle a 15% overload just fine for short periods of time during normal use. Hell, putting 150% FLA through most motors for 10 seconds every 10 minutes probably won't hurt them. Just remember that electrical heating is a function of I^2R. Heating scales exponentially with current through a fixed resistance.

K.I.S.S.
 
I've worked on jet brakes on radial arm drills and they stop the spindle very,very suddenly. In an emergency that's a really good thing but I'm not sure I'd want to be stopping any other machine as sharply as that on a regular basis.

Regards Tyrone.
 
Good point and yes it has 6 cam locks on the chuck. It's a 12" diameter 100lb chuck which would be a really bad day if it came off :)
 
Thank you this gave me some good insight and I'll report back with findings. I'm used to IEEE symbols and the NEMA symbols were definitely throwing me off. I found a symbol guide on line that answers a lot of questions.
 
Every dc inj brake I worked on would heat the motor badly, and everyone would exceed the fla by 3 to 5 times to work, the problem is 2 fold, 1 the impedance of a ac winding is high, where on dc it is low so it takes reduced voltage and more amps to excite it ,makes HEAT... then the rotor is a big short so it makes HEAT... so there is more heat being made that just the conversion of motion to heat, 2nd the braking force is high when the rpm is high but falls off quickly at slower speeds , so it not a real good brake but it works. I think the best brake is a vfd puged into rev with braking resistors, at least the heat is removed somewhat from the motor windings. None of the braking meth will stop on a dime and all are tough on the motor windings. If you need to stop on a dime install a brake on the motor shaft, or pug in rev with full voltage on the winding (ouch)...Phil
 
That's a valid point. They have a chart in the manual for selecting the HP size of the motor (see below) but the "Type" designations which seems to correlate HP to model numbers don't match any numbers found on the unit so I'm at a loss as to what the actual rating is. capture3.jpg

My other thought is to install a mechanical brake but there isn't really a good place to put it. There is plenty of room on the back side of the motor however it doesn't have a rear shaft. I considered moving the motor back and extending the shaft off the front to accept a brake rotor and caliper but that would require a shaft coupler and shaft extension which would put more force on the motor bearings.
 
What I done is machine a new shaft for the motor and build a new end-bell to fit a sterns brake and a bigger bearing, (not for the faint of heart) but is the best fix , a lot of bucks or work...you can find deals on ebay for the brake...Phil
 








 
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