My chainsaw motor has burnt out Fortunately, it is not the armature but the field windings that have burnt.

How do you know that the armature did not suffer also?
All it takes is one shorted turn. !

 

Put the armature in a growler - OK. Then checked R between adacent comm segments, not the same.

 

"R" ?
You check the voltage!

 

R = Resistance. Checked with a multimeter.

 

When using a growler, you use the Voltage range. !!

 

I did not do it - rewind shop did and said growler test OK. Their charges were too high!

 

You need a really good meter to check the low resistance of motor winding.
I have a megger with a pretty decent Ohm meter in it but once you get down into the single digits its hard to tell if you are reading the leads or the motor.

Making a field coil is probably within the range of easy fixes provided you can get some magnet wire and epoxy ( boat epoxy in a pinch will work )
An armature or rotor on the other hand can be a problem as the connections gt complicated as the machines get bigger ( lap, wave frog leg simplex duplex triplex ect )
Balance is probably not that big of a deal on somethings small like a power tool.
Its a very big deal on things that get into the 5 and larger kw range where things reallyt start to shake and you need a dynamic balancing machine to get it smooth

I'm not sure its worth the trouble unless you are trying to prove a point.

 

Balance is probably not that big of a deal on somethings small like a power tool.

It is if it is a mains Universal (series field) motor they can reach 20,000 RPM.

 

Still not able to get a reply to my question. History:
Motor failed, opened, found field winding black, armature looked OK- rewound field, tested - ran but heavy sparking, dismantled, again field winding black, growler checked OK, armature looks OK.
What could be the cause

 

One of two, Shorted turn, wrong connection.
Verify field polarity using a battery and a compass.

 

Thanks. If field polarity was wrong, the motor would not run at all - but it did run albeit with much sparking. Does a growler not detect a shorted turn

 

Does a growler not detect a shorted turn

Yes, it should.

 

So, the question still remains - if growler tests OK and field polarity is OK, what could be wrong

 

Doesn't leave much!
Possible error of some kind in the fields still? Rewinding error?

 

If the field is backward then the direction will be reversed. If the field is in parallel with the armature then way too much current will flow in each, and the field will burn up .

 

Checked, they are in series, not parallel. Strangely, the first failure the field windings were black and showed short to ground and armature looked OK. After rewinding the field, when it was sparking a lot, the field windings were similarly black and armature looked OK.

 

It is if it is a mains Universal (series field) motor they can reach 20,000 RPM.

No its really not that big of a deal on a small machine.

Something small has a relatively low surface speed and not much mass.
Most of the time if any balancing is done at the factory then its done with some epoxy putty at one end or the other ( or both Ive seen ) and its a simple static balance.

Big large diameter machines require complicated dynamic balance.
Diameter is a huge factor here.
When you change the speed on rotating electrical machines you need to consider if it can even turn that fast without damaging itself.
This is why as high speed machines turn faster they get longer and skinnier ( until no longer practical )

 

ran but heavy sparking, dismantled, again field winding black, growler checked OK, armature looks OK.

Are you sure the brush rigging is set in the right position.
If you off it will spark like hell once you out of the neutral plane.

Are both field windings black or just the one you repaired.
Where did you get he magnet wire?
Did you insulate the winding from steel shell and pole shoe? ( fiberglass tape or something ?)

Can you post some pictures that might help.

 

Did you insulate the winding from steel shell and pole shoe? ( fiberglass tape or something ?)

Covered from post #3 on, e.g. Presspahn paper etc.
.

 

Covered from post #3 on, e.g. Presspahn paper etc.
.

I can't recall off the top of my head what the coating on a bucket of magnet wire is rated it.
But its not much, and if you do not epoxy or varnish it then it does not take much to damage that and you can get a failure.

Cringe-worthy to watch, but another foreman of mine would turn off the shop lights and turn the hi pot all the way to see where a winding was leaking to ground.
I'm just glad I am not the guy who paid for the repair because heaven knows how much life is left in the insulation after that.