electric motor

[9fingers]

It is exceptionally unlikely that the commutator itself is damaged

The windings will have broken down somewhere causing a local short. They are NOT repairable.

You might find a cheaper source but the price is a market driven thing.

If you contact a motor rewinders, they still won't be able to repair it economically but they might have a tester called a 'growler' which should confirm if it is damaged or not.

A new 706 would set you back 500notes so worth spending £100 on it surely?

Bob

 

[jonny boy]

Thanks for everyone's help, I'll have to take it to my nearest Dewalt service centre to get a true dianosis of the problem. The saw stands me at £30, so any repairs costing over £70 just doesn't seem worth while. it's a shame really because this was a terrific saw with good cutting capacities.

jon.

 

[9fingers]

I would just like to point out though, the brushes are really soft, you could easily scrape a gouge in them with your nail. The consistency of hard chalk, is this right?

jon.

The brushes have been damaged by the heat of the sparking.

As I said in the beginning buy new ones with the new armature.
Also let them bed in before you work the saw hard.

Bob

 

[Digit]

A growler isn't needed if he has a meter Bob, just measure the resistance between each side by side copper segment. Should be about 0.3 ohms.

Roy.

 

[jonny boy]

Roy, I have a clamp meter, will this work?

jon.

 

[9fingers]

Roy,
In my attempts to test this sort of armature, i've not got anywhere conclusive with resistance measurements. I've put this down to shorted turns only making a difference with AC.

I've even tried an improvised AC bridge set up. In every case a replacement armature has fixed excessive sparking where I have not just tossed the tool in the bin.

Measuring deviations from 0.3 ohm when typical leads measure up around this figure is difficult. I must try a proper 4 terminal ohm meter sometime for this job.

Bob

 

[Digit]

I normally clean the com first Bob and have found from testing probably hundreds of brush motors that when arcing is bad the resistance suddenly jumps to the 20/30 ohm range.
I agree about the 0.3 ohm measurement difficulty but the change in resistance in a duff armature I have always found to be considerable and easily detectable with a digital VOM.

Roy.

 

[mister henderson]

When I worked for Black and Decker, many, many years ago back when B&D tools were made in England and got repaired when they went wrong, sparks going all the way round the commutator meant the rotor needed to be changed.

We didn't bother analysing further than that, we just changed them. At £100 it might be worth looking into the rewind option

 

[OLD]

On the armature the windings are soldered to the segments look for dry joints and resolder as required skiming the com. is to reduce wear on the brushes only.

 

[jonny boy]

Good afternoon, I've just taken out the armature and all the connections to the com. are in good order and the resin insulation around the windings looks sound. Before spending money on a new armature, can it be comfirmed that the armature is the problem and not the outer windings of the motor. Don't fancy the prospect of spending £80+ for no reason!

jonathan.

 

[9fingers]

Very unlikely to be a field coil fault. Measure the resistance of each coil. They should be same/very similar.

Use Roys method on the commutator. He has measured many more than me and seems to have good correlation of results.

If you dont have ohm meter then use a car battery and an car indicator/brake light bulb in series with each adjacent pair of commutator segments. look for equal brightness on each pair.

Same improvised test for the field coils. Look for equal brightness to each other but not as bright as the commutator measurements.

Then get your credit card out!

Bob

 

[OLD]

You need to test for continuity of the windings via the segments as others have mentioned, a simple aldi multimeter will do this or get a specialist to test it out for you.

 

[jonny boy]

Hi, the clamp meter i have say's it can check, AC voltage, AC current, resistance, continuity test and insulation test. No good?

jon.

 

[9fingers]

Use the resistance setting. Short the two probes together and note the reading (should be between zero and 1 ohm) subtract this from any readings you make to get the true resistance.

Bob

 

[jonny boy]

If anyone can take me through the steps, i'll fetch the armature in and clamp meter so i've got it in front of me.

jon.

 

[9fingers]

Read the thread Jon.
Between what Roy and I have written you have all you need to test the armature.

Bob

 

[jonny boy]

Hi, if anyone is interested, Roy is going to be along in a while to take me through the steps of checking to see if I have a duff armature!!

jon.

 

[jonny boy]

Sorry, i'm going through it via PM.

jon.

 

[9fingers]

What was the verdict Jon?

Bob

 

[jonny boy]

Hello, Roy took me through the steps of testing the armature for what it was worth. He suspected that the meter I have was unable to read sensitively enough for the process so we diddn't really find anything out. As I have told Roy though, the only real option I have is to take it to my nearest Dewalt service centre and see if they will test the armature to check if it is damaged before I spend the best part of £100 for a new one. I think when all said and done, the armature does seem to be the cause of the exessive spakrks.
Thanks for your help again Bob, much appreciated.

jonathan.