Radial arm saw problems

[NozSpark]

I've got a customer who has a Dewalt 1251 radial arm saw which is faulty!

What happens is that when you operate the saw it starts up then the thermal trip in the switch operates and turns it off. After a minute or so you can have another go with the same result.

It's a single phase machine rated 1.1kW, 6.2A...

Motor plate markings 1.45kW, P2 1.1kW, 220-240V, 50Hz, 2800rpm, cos 0.98

Switch - Webber unimat WTN22-555 with a rated current of 5A

The start winding has a resistance of 7.36ohms and the main winding 3.61ohms

On start the clamped current was approx 32A

Now to me the winding resistances (especially the main) is low, 3.61ohms equates to 63A?

Does my "logic" sound right, or am I missing the point?

 

[ProDave]

Shorted turns on one of the windings?

The DC current would be 63A, but the ac current will be a lot less due to reactance.

If you can, try uncoupling the mechanical load from the motor and try again. It could be something seized causing too much load.

 

[NozSpark]

There is no mechanical load on the motor, the blade runs freeley when turned by hand (supply disconnected)

I realised that reactance would have a part to play, however I don't have a LCR meter to make proper measurements; and I don't know what readings I should be expecting:|

 

[ProDave]

Take the blade off, so there's little inertia and the motor should reach running speed really quickly and try again. What's the running current?

I still bet it's shorted turns, causing the high current and rapid overheating.

 

[NozSpark]

It says that it's rated at 6.2A Dave, I expect that's the running current?

 

[Sidewinder]

Is it a brush motor or an induction?

 

[NozSpark]

Unfortunately I don't normally do this kind of thing so I won't be buying a winding tester

I'm trying to help out this client who subs me their electrical work that they get under the local care and repair scheme, they're a joinery company!

---------- Post Auto-Merged at 20:11 ---------- Previous post was made at 20:06 ----------

Yes it is an induction motor, but canoey does not deserve a scoob (yet) as he could have looked that up given the info I've posted.

I've looked online for a stator (

 

[NozSpark]

Are we talking about the same machine here???, there is no brake on this saw.

These seem to stock it

DeWalt DW1251----B Type 1 France Radial Arm Saw Spare Parts | Tools and Parts Direct

 

[Sidewinder]

Will DeWalt not supply a new stator?

 

[NozSpark]

I've just found this, it seems like the winding resistances I took are correct

ProductDetail

 

[Sidewinder]

If it's one of these:

Dewalt Radial Arm Saw DW1251 | eBay

Don't bother, I can get them a better machine probably for less than Canoe will do the re-wind for!

This is not a competition thing, they are just CARP machines TBH!

 

[OnOff]

I've had one for over 20 years now. Good bit of kit. Had to replace capacitor in that time and been through a few NVR switches.

 

[Sidewinder]

I have a manual if you want it Noz, however, I would suggest they dump it for a good machine!

 

[Sidewinder]

Onoff,

Sorry but for serious business use they are carp mate end of!

They are like the current range of DeWalt merely DIY machines or at best suitable for one man outfits, they are just not serious enough for a proper joinery shop!

 

[NozSpark]

Yes, they have the newer version (1986 model)

Surely if the windings were faulty then I wouldn't have got the readings that I did?

Like I said before, I do not have a LCR tester, so I cannot test the capacitor. It looks healthy enough, no bulges or leaking electrolyte, although there is a slight smoke mark at the terminal end (when I say slight I mean that it was difficult to spot)

TBH it isn't getting chance to get up to speed, it only runs for maybe a second and sounds fine. I know that doesn't really mean anything though, It seems like I'll be going back to have a better look check the bearings for play etc.

If I disconnect the capacitor and spin the motor before operating the switch, it should keep running??

But then again if the capacitor was faulty then it wouldn't even start?

 

[NozSpark]

Just popped back in there today, disconnected and put my meter on the capacitor terminals and it showed 0.18nF,,,, should have been 20uF

To confim that it is the start capacitor I reassembled the trigger, plugged it in and span the shaft up with my drill (can be accessed on rear of motor),,, it then worked perfectly.

So I'm now after a 20uF capacitor..

I guess one of these will do?

 

[Sidewinder]

Don't see why not, last time I went to a trade outlet as I could not get the value I needed.

 

[OnOff]

Just popped back in there today, disconnected and put my meter on the capacitor terminals and it showed 0.18nF,,,, should have been 20uFTo confim that it is the start capacitor I reassembled the trigger, plugged it in and span the shaft up with my drill (can be accessed on rear of motor),,, it then worked perfectly.

So I'm now after a 20uF capacitor..

I guess one of these will do?

Though I got mine from RS that Maplin one will be fine. A tad expensive maybe but of course there's a Maplin on EVERY retail estate now! I also use Cricklewood as well:

Cricklewood Electronics - CCTV. CCTV Equipment. CCTV Systems. Digital CCTV Cameras

 

[NozSpark]

Cheers guys,,