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Thread: Mars Lathe DC motor conversion.
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28th September 2012, 04:05 PM #31
Hi Ewan,
I calculate you'd need a 650 Ohm 50 watt resistor for a 0.2 duty cycle.
Regards
Ray
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28th September 2012 04:05 PM # ADSGoogle Adsense Advertisement
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28th September 2012, 06:16 PM #32
Thanks Ray....but could you be as kind as too show me how you get those numbers?
I couldn't resist and went outside this arvo for a play, Here are some results (from the 4 amp motor, not the 6)
The motor is generating about 130v at 2000rpm.....oh, and a small RE magnet cannot hold on to a 4" flywheel at 2k....lucky i was not in the way otherwise i'd be back in the hospital for sure.....or dead.
From 1000rpm-stop
No braking. 11 sec
1w 100k resister. No real change
1w 10K 8sec
From 2000rpm-stop
No Braking 20sec
1w 10k let the blue smoke out.....
5w 30ohm 6 sec.....the blue smoke left us on the second try.....
2x5w 30ohm in series (so 5w 60ohm) 8sec, repeatable.
I think i might get some 100-500ohm 5w just to see what the difference is. It will clearly be exponential, the higher we go in resistance the less difference in time.
I have vids of the testing (for timing) if anyone is interested.
A resister like Ray is talking i think i would be better winding my own.....then again you never know what you'll find on ebay! 100 Ohm 100R 100W Watt Power Metal Shell Case Wirewound Resistor | eBay1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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28th September 2012, 07:00 PM #33
Hi Ewan,
I used the formula from the page you linked to. Dynamic Braking Resistors Calculations - Engineers Edge
Ires = MotorPower * 0.9 / 800 For 1100 watts and 90% efficiency that makes Ires=1.24 amps
Resistor Value = 800 / Ires = 646 Ohms
Power Rating (240V) = 400 * 400 * duty / 646 = 49.5 Watts for 0.2 Duty Cycle
Braking resistors don't need to be rated for full duty cycle. That article recommened 0.2 as a reasonable value.
Regards
Ray
I've got a bank of big power resistors, I'll have a look and see if there are any suitable looking values.
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28th September 2012, 07:11 PM #34
Thanks Ray, that makes sense, i based my math on the Pmotor being in amps not watts. I thought it sounded a bit small. Based on the fact i will be only using the motor to aprox. 150 volts (2500rpm) I think the resister could be a bit smaller then 650 ohm, especially with my findings today. Add all the extra momentum of a lathe to the mix and the braking time is going to be considerably longer that just with the flywheel.
1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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28th September 2012, 07:17 PM #35
Hi Ewan,
I checked that resistor bank, big ceramic wirewound resistors, about 14inches x 2inch, so not much use for your job I'm afraid, but here is an ebay supplier...
2 pcs 50W Power 650 Ohm 5% Aluminum Casing Wirewound Resistor Green | eBay
Regards
Ray
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28th September 2012, 07:42 PM #36SENIOR MEMBER
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I don't think so Ewan.
There's a lot of friction/drag in the drive system, and I think you will find that the whole shebang will grind to a halt very quickly, with or without braking.
There's no braking with the AC system, yet the spindle stops in less than two seconds regardless of the drive ratios in use.
I can't see the DC system being any different - but I have been wrong before
Just my two cents worth.
Rob
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28th September 2012, 07:50 PM #37Senior Member
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G' Day Ewan,
With my setup (1.5 HP motor & KBMM controller) I didn't bother with braking resistors, as my lathe has a threaded spindle, I merely set the deceleration time to 1 second (default is 2 seconds), in practice with most of my work in the 200- 500 RPM range my lathe comes to a stop within 2-3 revs, when screw cutting at around 35 RPM it pretty well stops dead.
Regards,
Martin
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28th September 2012, 07:56 PM #38
Hi Rob,
Unlike your needle roller bearing lathe mine is a plain bearing. AT 1000rpm with an 8" chuck it will spin for a good 20 seconds with the belt slackened(and the tumbler in neutral, so no lead or feed shafts running). I generally don't turn the motor on and off at the moment, i use the belt as a clutch, like so many old flat belted machines did. I would have to test it with just turning the motor off but it would be at least 10 seconds i reckon.
Thanks for looking Ray, ebay turns up plenty of results for sure! I also have some old northbridge PC heatsinks that are the perfect size for those resistors.1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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28th September 2012, 07:59 PM #39
Thanks Martin,
I will test for sure with the DC motor and no braking, at threading speed i'm sure it will be a fast stop but at higher speeds the problems i mentioned in the last post start to creep in. I would just by a KBPB...if i were a rich man! My bearings also need some work but i need the LeBlond up and running for that.......so many things to do......1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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28th September 2012, 09:06 PM #40SENIOR MEMBER
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Interesting subject, which I know nothing about.
I did read in the KBE notes on one of their regenerative controllers that they don't recommend it if braking is not required, as it will make the DC motor run much hotter.
Just thought I would throw that into the pot.
Cheers
Rob
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28th September 2012, 10:55 PM #41GOLD MEMBER
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So is DC motor braking just something to consider because you can or is there a reason for needing it on a lathe that has been converted from an AC induction motor. I'm with Rob on this, I know absolutely nothing about it other than obviously when the DC motor gets powered off it generates it's own power and this can be used for regenerative braking. But surely a DC motor would not coast for as long as the AC induction motor it replaced? DC motors have brushes which create slightly more drag or friction than AC induction motors which have nothing but their bearings.
I'm thinking it is just something to consider when using DC motors in a conversion since it's an added improvement in the overall machine?
Simon
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28th September 2012, 11:23 PM #42SENIOR MEMBER
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I can see where it would be very useful on an industrial machine which has a quick/repetative forward and reverse cyclic function - such as on a production line.
I will wait until I get the DC motor conversion running before going down this path.
However, it is a good discussion item and worth exploring.
Cheers
Rob
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29th September 2012, 04:44 PM #43
Can any of the other electrical guys give me you opinion on the "auto inhibit". From what i gather it needs to be reset for everytime the motor is stopped and re-started without the ac being cut. I think they have mixed the braking and armature switching picture into one, hence my confusion. As i will be cutting the AC line everytime the motor is stopped i'm thinking the auto inhibit relay is not needed. Here is more from the manual: 9.1 AC Line Switching:
The control can be turned "on" and "off"
using the AC line (no waiting time is
required). Auto-Inhibit® circuitry
automatically resets critical components
each time the AC line is interrupted.
This, along with Acceleration Start and
CL, provides a smooth start each time
the AC line is applied.
9.2 Armature Switching – If the armature is
to be disconnected and reconnected with
AC power applied, the Inhibit Circuit™
must be simultaneously activated and
deactivated. Connect I1 and I2 together
to activate the Inhibit Circuit™. See
Figure 9.
I will need a relay on the braking resister though (i think) that is normally open and closes when the AC is cut. Otherwise i will be grounding some 35w (with the 650 ohm resistor) all the time (turning it into heat anyway).
My trip to the shed today was to time the slow down speed of the lathe. From 1000rpm-0, 15sec with the belt slackened, 8sec turning the motor off, from 500rpm-0, 8sec and 4sec.1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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29th September 2012, 09:42 PM #44Novice
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Ewan,
If you are after power resistors a great source is www.rockby.com.au they sell surplus electronic components at very resonable prices.
An example is at the moment in their clearance page 7 they have 22R 20W power resistor(qty 20) for $3
Although I usually visit the shop for my orders they will express post for $11 which is better than the 3+ week wait for ebay postage/
PS if you do get an order I would recommend getting Anti-Seize & Extreme Lubricating C/Pound stock#37719 included in your order, it is a great product @$4 a tin. I liked it so much I bought a 12 tin box of it when it was $6,I use it on gears, bolts and even for tapping.
Cheers Andy
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2nd October 2012, 07:23 PM #45
Thanks Andy,
I was amazed at the amount of high power resisters on ebay, so finding one is not a problem.
I had a rummage yesterday and today and found nearly everything i need, relays, switches, switched pot for the speed control, tranny for 12v DC supply etc. I just have to put it all together and make up a diode bridge to get 12v DC for the controls and Fan. I'm keeping as much as i can low voltage and using several relays to switch the AC and braking resistor.
Now for a quick game of "what is it", this tranny looking thing was in one of the treadmills. But, it only has 1 coil (only one pair of wires) It measures 1 ohm resistance, my guess is a big-### resistor? No markings anywhere. Its about a 4" cube.1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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