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murdoch

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I've just bought and set up a dewalt dw721kn single phase saw. Unsure if this is normal but when I press the trigger, the saw make a loud buzzing and grinding sound while the blade turns slowly, sounds like it's struggling to get going. After 3-4 seconds it's picks up speed and sounds great and cuts well. When the trigger is released it then continues in a loud buzzing for around 7-8 seconds. Is this normal or can anyone help?
 

sunnybob

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Thats a big motor. Does it need a 20 amp supply?

What plug was supplied with it?

How good is the circuit youve connected it to?
Youre describing insufficient power to make the motor work properly.
 

murdoch

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Thanks, no plug was supplied so I just chucked on a 13 amp plug. I'm not great with electrics so I may call a chap I know to check it out, don't want to hurt myself!
 

sunnybob

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I know its against masculine nature, but have a look in the manual. It will tell you if it needs a 20 amp supply.

Big induction motors take a lot of current to get going.

Is it on the end of a 50 metre extension lead? Undersized or over long runs of cable will give the same symptoms.
 

Myfordman

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No motor should continue to make a buzzing noise after it has been switched off. This indicates a definite and unusual fault.

If what you wrote is correct, then this aspect is nothing to do with the power supply and needs solving first.
 

Distinterior

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Can you tell if the buzzing is still occurring whilst the motor is running...? I imagine it may be hard to hear over the noise of the motor running!
Also, without the motor running but the switch on the wall turned on ,ie, power going to the saw switch/trigger, can you still hear the buzzing?
If so, does the trigger area get slightly warm after a while?
The reason I ask these questions is, I bought an Elu 1751 a few years back and although a bit dusty, it was in really good condition. It hadn't been used for many years and had been stored in a shed somewhere. I had similar problems to what you describe and I ended up replacing the Capacitor and its been fine ever since. I don't think capacitors like a damp environment...coz when I took the old one out to replace it, there was quite a bit of condensation around the Capacitor and it looked like it had been sweating...
 

woodbrains

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Myfordman":vq58865v said:
No motor should continue to make a buzzing noise after it has been switched off. This indicates a definite and unusual fault.

If what you wrote is correct, then this aspect is nothing to do with the power supply and needs solving first.
Hello,

Agreed, if a motor requires a large start up current, it will blow a fuse or trip a MCB. Can't see how it will run on less current than it needs. It sounds as if the motor is bad, arcing across the windings where insulation has failed, or bad connections in the terminal box. It could be a bad start capacitor, though once the motor gets going it usually runs fine. I would question the seller about this, has he sold on a machine he knows that is about to fail completely?

Mike.
 

murdoch

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Thanks for the help, it was on a long extension lead but I've just managed to put it straight into a socket and it now starts fine, the more I've used it this afternoon to build a shed the better it seems to have got too. The buzzing seems to stop during use but returns when trigger is released, almost sounds as if the electronic brake is continuing after the blade has stopped.

The saw was bought as an ex demo machine and is in as new condition, 2016 model.
 

sunnybob

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Who am I to say I told you so.
The buzzing, if its an electrical type buzzing, disconnect it and check the wiring on the NVR switch.
If you follow the wire from the mains cable, identify which side of the nvr it is connected to. look carefully at the side of the NVR and there should be a wiring diagram on there showing which numbers for mains in and which numbers for load (the machine). I suspect it has been connected backwards, causing the energising solenoid to buzz when the machine is off.


I have found several machines wired backwards, including my brand new pillar drill.
 

Myfordman

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If your saw has a braking system, (mine does not) then I correct what I said earlier. It would be permissible for the motor to make a buzzing noise during braking.

A way to confirm is to operate the switch and when the motor is running , switch off at the wall keeping the switch pressed.
The motor should coast to a stop and make no buzzing noise.
Now that you have the motor starting up normally, there is probably nothing wrong with the machine.
 

sunnybob

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Just realised you mentioned a trigger.
The nvr switch i refer to is the green button red button kind of switch. I dont know your particular machine but if you dont have a two button switch ignore my directions. I still think its the same problem, but dont know how your trigger is wired.
You need a drawing to be safe.
 

acewoodturner

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I have a Elektra Beckum mitre saw with a brake on the induction motor. When I release the trigger there is a buzz as the brake kicks in. It lasts for about 4 seconds or so as it brings the blade to a stop. If I press the trigger during this 4 seconds the blade will not start rotating till the brake/buzz has stopped acting. Best saw I ever bought as its the quietest machine I own.
 

sunnybob

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With that information explaining the buzzing, I think youre good to go.
 

murdoch

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Thanks for everyone's help, all sorted and working well. Ditching the extention cable fixed startup and it seems the buzzing at the end is normal. Now to finish that shed!!
 
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