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Craftsman 18V Ni/Cd Circular Saw with DC Motor Problem - HELP !

whiteoutage

Jul 20, 2017
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Video hopefully explains issue better than I can.
Once in awhile it starts normally at full speed. Video shows motor speed is very slow and has thumping sound. 18V is present at Idle, DC power supply is strong, all connections are good, but volt drops to less than 1V on start up. Thinking armature maybe? From what I could tell the brushes look good. When I took motor assembly out of gear box, all looked fine - plenty of lub, no unusual gear wear, blade spins freely. Is she gone or have I got a chance?
UPDATE: I took motor out of gearbox and bench tested motor independently of gearbox. Motor now runs at full speed, but for the first couple of seconds at start up still has same issue of low voltage and thumping . Also, I found motor specs (last one).
http://www.robotstorehk.com/rs_755vcwc.pdf
 
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Minder

Apr 24, 2015
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You may have a bad, (short in the armature) winding, the only/best way to confirm this is with an armature tester called a Growler.
 

Tha fios agaibh

Aug 11, 2014
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I think it's a power supply problem.
See if the motor briefly runs in gearbox with a 12v automotive battery. Be careful, it could get hot.
 

whiteoutage

Jul 20, 2017
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I think it's a power supply problem.
See if the motor briefly runs in gearbox with a 12v automotive battery. Be careful, it could get hot.

I just swapped power supplies (from a Gateway 19.5V laptop to a universal laptop PS with variable voltage. Without gearbox it runs strong without said issue at both 14.4 and 18.5 volt settings. Will now optimistically test with gear box. Wanted to use the gateway ps cause it has double the power and have used it with DC drills before without issue - maybe I hosed the gateway PS, or 19.5V is too much for motor with 20V max rating -no clue on this either?
 

whiteoutage

Jul 20, 2017
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Not so good with gearbox. First run on 14.4V was OK - ran strong at outset but voltage gradually settled at 16V and I possibly heard some slight extraneous motor noise at full rpm. Second test at 16-18V setting reverted back to the original slow speed/low volt situation. Back to square one - maybe short in armature - anyway to test in circuit since it appears this motor's armature is pressed in place?
UPDATE: Bit the bullet to weather sub zero temps to test on bat while car was running and it ran fine as far as I could tell. Possibly seems to work better with less voltage, if so, why?
 
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Martaine2005

May 12, 2015
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A laptop Power supply is not a sufficient power source. What is it? 3.5A or so?.
The motor start up current or stall current could be 5 > 10 times that of the PSU. That is why it runs ok on the car battery which can deliver 100s of Amps.

Martin
 

whiteoutage

Jul 20, 2017
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A laptop Power supply is not a sufficient power source. What is it? 3.5A or so?.
The motor start up current or stall current could be 5 > 10 times that of the PSU. That is why it runs ok on the car battery which can deliver 100s of Amps.

Martin
You may be right, but:
The Gateway is 9A @ 19.5V, and other variable laptop PS is rated 5.5A @ 18V. Yes, the gear box adds some resistance, but I would certainly think both power supplies would at least start the motor outside of the gear box when there is very little resistance (that's 180 and 120 Watts respectively). Though I agree that both PS power supplies will not be enough to properly cut wood; however, according to above the spec'd .PDF attachment, the no load amperage is only 2A( both PS's are far above that, and the motor's rated peak efficiency is only 16A). I have measured the Gateway output 10A+ peak power or 200W with my drill driver and the smaller PS can output 120W continuous regardless of voltage selected. Do you think a shorted armature could be limiting my power supplies?
 
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Martaine2005

May 12, 2015
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A cheap cordless tool can have a 300W motor.
Most good tools have 500W + motors. These can draw 30A peak.
Anyway, 18V x 16A = 288W peak. Well above your biggest supply of 160W (peak).
Your other supplies may be ok after they cool down!.
Also, no load speed is after it’s started rotating. It might draw 12A just to get going.

Martin
 

whiteoutage

Jul 20, 2017
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Yea, suppose starting load is quite heavy. You are probably are also correct on cooling of PSs as I just ran it for about a minute then it had same issue again on restart. Nothing got hot or even warm so bearings and mechanical should be good. Multimeter measured starting load at a little over 10A @19.5V - maybe peak max output of the larger PS. After starting and while running, the no load Amps dropped to about 4A. Not sure if internal DC magnets cause resistance when spinning the blade by hand while unplugged, but I can definitely feel resistance that seems to approximate the short distance between the commutator contacts? Sure do appreciate your spot on advice! In hindsight I should have asked all you wizards here first then would not have had to take it apart, etc. Though it was a dead simple tear down with very good construction. When the weather gets half way descent, I will test cut some lumber and report back. Thinking I will get 14.4V running on alternator/bat, and not expecting it to cut thick hardwood. Will be more than satisfied if it can cut 3/4' plywood and a 2x4. The blade on this thing is amazing thin so will have to take is easy. If good results I may MOD my old computer server PS rated at 750W.
Thank you Martain2005, Fios... & Minder!

BTW - what's everyone's thoughts on powering this saw with a medium size laptop battery? I happen to have a newish unused one lying around.

Addendum: also wondering if motor will automatically take more amperage when using less volts - accepts 18V battery, so if I power off a car will the saw use same Power or Watts as a fully charged 18V battery?
 
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whiteoutage

Jul 20, 2017
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Generally high voltage = higher current, also voltage determines RPM.
Wow, not good to hear, but great to know. So powering from a car's alt/bat @ 14.4V may limit power to say only 75% of motor's 18V rated of capacity?
Just realized the laptop battery I have (11V 4ah) is even wimpier than my laptop chargers.
Cost of Ni/Cd battery packs at least for older tool models is astronomical. How would you all suggest powering these relics to get max power - there is a graveyard of Ni/Cd's cordless devices that are still very nice tools? Portability is not really an issue with me although converting to Lithium if not too pricey may be interesting (I just don't like that the battery packs are rarely interchangeable between tools).
 
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Tha fios agaibh

Aug 11, 2014
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So powering from a car's alt/bat @ 14.4V may limit power to say only 75% of motor's 18V rated of capacity?
At a lower voltage it will run slower but power is Volts times the Amperage. So the 12v car battery has more power than your Gateway ps because it can deliver more Amperage.

18v is the maximum voltage it is rated for. It will run on much less voltage provided it has adequate current.

If it runs without getting hot on the car battery you could try converting the NiCad to a LiPo battery. Sometimes they sell adapters so you don't have to modify it.

Honestly, this tools best days are probably behind it and not worth the time and money to restore it.

Investing in a new brushless motor saw is a better place to throw your money.
 

Minder

Apr 24, 2015
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It looks like a very common motor that is available from a few places, Amazon included.
Advertised as small power tool motors.
 

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whiteoutage

Jul 20, 2017
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It looks like a very common motor that is available from a few places, Amazon included.
Advertised as small power tool motors.
Motor? Wondering if you are implying it may be damaged - 4 amps @ 19.5v does seem like a lot of power just for no load, no?

Did not know new models have brushless motors, is their an advantage besides brushes which I have never come close to wearing out? Again, I think an old Ni/Cd tool like this is pretty great especially considering replacement cost, hence this thread.
Could not find a Craftsman adapter form Ni/cd to lithium
 
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