DW745 bench saw suddenly stopped working during cutting

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Timbertel

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Hi All Having replaced the brushes on this hardworking bench saw I’m suspicious of the motor simply because if after unplugging and leaving it for a while re plugging it in it starts working again.
Advice required from someone technically minded as to whether I should replace the armature(?)/ motor rather than buying a new saw.
Many thanks in advance for you responses
 

okeydokey

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How was it before you changed the brushes? what prompted you to change them in the first place?
Is there a thermal cut off near the motor that triggers it to stop after a period of use and after cooling off allows you to start again? Might you have disconnected this during the brush swap?
 

Timbertel

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Thanks for your response. The brushes were changed sometime ago because of intermittent stopping and not starting. I did check the reset button when it happened but it didn’t feel as if it clicked and it wouldn’t start again after pressing it. I don’t recall disconnecting it during the brush change but I will check. Perhaps there has to be a cooling period. Never had any problems like this previously I just wondering whether it is just wearing out perhaps?
 

Timbertel

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Spoke to PowerTool Centre they suggest replacing the armature as a starter and then possibly the field coil, £189 for both with no guarantees!
 

DBC

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Sorry. Ignore the above I see this has already been suggested.
 

Scruples

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why would such a motor have a capacitor?
Electromagnetic noise suppression. Brush motors cause sparking around the armature where the carbon brushes contact with the commutator. The sparks cause radio frequency noise which interferes with TV and Radio. The capacitor absorbs the excess current that produces the sparking and suppresses the interference.
 

Chris Hawkins

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Ok this happened to me - exactly the same issue. Even the bit about sometimes restarting after a period of time. I know this makes no sense, but I cut off the plug and replaced it. Never had another problem. Regardless of logic maybe worth a go.

Good luck.
 

imageel

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I don't know that model personally, however I would have thought an intermittent connection in either the field coils or the armature are unlikely - usually either will fail and fault to chassis == earth and trip a breaker/fuse or more likely go open circuit - permanently!.
I'd check there isn't a thermal fuse fitted and if so my advice would be to dismantle the motor as far as one can and thoroughly clean any dust from it and as others have suggested do the same to any switches, connectors since over time and in dusty conditions ingress is quite likely and may cause this type of problem. As Scruples mentioned this type of motor does not have a large start/run capacitor but may have a smaller one across the armature windings and these sometimes go faulty but more often open cct than short.
At that price quoted for new coils and armature I wouldn't risk taking a punt on it being them if it was me...
 

TomTheToolMan

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I had the same problem. Thought it was dust in switch. Took it apart cleaned it and it started working. Next time I used it, same again. Did some more investigation, this time with a meter, and discovered the "clamps" on the fuse holder in the plug were too wide giving intermittent connection to fuse. Not sure how it happens bur sounds like others might have had same problem. Anyway, bent fuse holder clamps tighter and problem has gone. Worth a look and cheaper than £189
 

okeydokey

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Just thinking to myself so you have the same problem as before you changed the brushes? Try the easy fix idea from Tom.
Maybe Ive missed something but which bench saw do you have and are you able to run a lead direct from the motor to a normal plug/wall socket and use the switch on the wall to switch it on and off. That would eliminate everything external to the actual motor and help diagnose if the motor is or not ok.
If the motor is faulty then perhaps see if a local repairer could run it on his bench and sort it out.
This firm is in Essex
Electrical Motor Repairs | Leigh-On-Sea - N J Electric Motorwinders
 

Chris70

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There’s even a remote possibility that you have a break in the saw’s mains cable. These breaks can be infuriatingly difficult to find and can occur when the cable(s) are strained frequently. Just a thought. A volt stick might help you here?
 

Sandyn

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Electromagnetic noise suppression. Brush motors cause sparking around the armature where the carbon brushes contact with the commutator. The sparks cause radio frequency noise which interferes with TV and Radio. The capacitor absorbs the excess current that produces the sparking and suppresses the interference.
Filter caps are also critical safety components. Y caps are designed to fail open circuit, X caps are designed to fail short circuit. Their performance is tested to ensure predictable behaviour. When they fail, they will remain in a failed condition. As long as no one has changed parts, There is a low probability that failure of these devices would cause an intermittent fault. Failure of an X cap should blow the fuse or trip the circuit breaker. Failure of a Y cap possibly wouldn't be noticed by the user.
Whenever I have had problems like this, it's been due to mains cable/ plug as others have suggested. On a hardworking saw bench, could be a motor problem.
 

Timbertel

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Hi All prompted by various comments (thanks) I have carried out further investigation. I opened up the motor. I’ve cleaned the contact point of the brushes on the Armature. I noted a groove or indents on the armature see pic I don’t think it’s wear but can’t be certain, might be how it was made? The brushes seem to be worn again although only replaced about 18 months ago, they measure 20mm, should they be longer I wonder to get better pressure at the contact interfaces? I opened the plug and found the neutral terminal to be loose so tightened that, fuse carrier and live ok. I shall give it a run to see how it goes. Thanks to all for your suggestions I’ll keep you all posted!
 

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Myfordman

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Noise suppression
I should have posed my question differently. What makes the author of that suggestion think that the failure of a (suppression ) capacitor would stop the motor from working.
The suggestion sounded like it was from someone who has no understanding how a brush motor works but just repeating something that is applicable to an induction motor.
 

DBC

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Hi All prompted by various comments (thanks) I have carried out further investigation. I opened up the motor. I’ve cleaned the contact point of the brushes on the Armature. I noted a groove or indents on the armature see pic I don’t think it’s wear but can’t be certain, might be how it was made? The brushes seem to be worn again although only replaced about 18 months ago, they measure 20mm, should they be longer I wonder to get better pressure at the contact interfaces? I opened the plug and found the neutral terminal to be loose so tightened that, fuse carrier and live ok. I shall give it a run to see how it goes. Thanks to all for your suggestions I’ll keep you all posted!
Is the width of that groove in the armature about the same as the width of the brushes? I only ask as that looks like the condition of the armature in my dropsaw when I was having the same problem as you? I replaced it a few years back. No problems since.
 
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