Miter saw Induction motor brake

[Dlyxover]

I know this kind of question has been asked before on the forum but

I bought a Elektra Beckum KGS 301 miter saw that has a 1600w induction motor.

http://uk.rs-online.com/web/p/corded-mi ... s/0470793/

but the electronic brake did not work, which I had come to terms with and just let it spin down after use altho that dose take quite some time.

I had a project that need a thin kerf cut so took off the 250mm, 3.2mm blade and replace it with a 160mm 1.6mm blade, gave it a quick blip to check all is as it should be and the blade stopped nearly dead when I released the switch.

The brake worked! fired it up to full speed and and released the switch; blade did not stop dead took about 20 seconds to come to a stop.

This intrigued me, I switched back to the 250mm blade and it did nothing, ran with no blade it stopped dead.

now to the question.

why is it doing this?
inside the control box of the saw is a relay/contactor and aboard, all looks in good shape nothing obvious.

Could be relay/contactor have failed and is unable to switch correctly to stop the 250mm blade?
(due to the momentum from the weight of the large heavy blade)

Any help is appreciated

 

[Eric The Viking]

I think you've answered your own question: the bigger blade has a lot of kinetic energy stored in it (it's a flywheel).

I have the earlier KGS300, which is getting close to the end of its useful life. But... it's a 110V version (like hen's teeth), and has no brake. If you go back through the forum, you'll see I experimented with adding one, using lightbulbs as a load.

The principle works well, but it's dashed tricky to do in practice. I blew quite a few 25W mains bulbs experimenting, and at the moment I've given up. I couldn't find a relay man enough to take the current necessary:

I was intending to use a changeover relay to the motor - one way applied mains to run the saw, but release the trigger and it would dump the energy from the motor into a bank of lightbulbs (or power resistors - couldn't decide and didn't get as far as making it).

I could eventually get an unloaded motor to come to a dead stop with a bit of a jolt, but calculations based on the mass of my normal 250mm blade indicated that there was a _lot_ of energy to dump. This involved an unhealthy back EMF, which wrecks contacts, etc.

Then there was all the practical stuff about where to mount the resistors/lightbulbs, how to do the wiring, etc.

So now I just apply a block of softwood very gently to the blade, and double check the blade guard is behaving itself before starting to use it. I have frightened myself a couple of times by passing my hand under a still-spinning blade because the guard didn't drop back correctly. I shortened the return spring, which has helped a lot (I think I also fettled the lever and cable), but I find I need to keep the area inside that mechanism as clean as possible, so it now gets brushed and hoovered-out pretty regularly, and lubricated, usually with something like silicone spray (so sawdust doesn't go sticky inside the guard).

To be honest, although I'm fond of the machine, it's also frustrating because it has quite a few design issues that make it awkward to use. The worst is the big, fat motor (I frequently get my hand stuck under it!), but I struggle to get a clean cut on both sides of the blade, and the distance stop for repeat work is on the wrong side really. On a good day, the finish off the saw is amazing, but often it's burned, or too ragged to use without re-finishing. I do like the hold-down clamp though.

I realise the above doesn't help much, but I do share your frustration

E.

PS: Axminster sold a close relative for a while - they might still carry spares. You have a far better chance than I do of finding a new relay or circuit board, because yours is 240V. Otherwise there are other spares suppliers - I was able to get a new return spring and even the main plastic moulding (the handle and blade guard shell - it's one piece).

 

[sunnybob]

I have never owned a braked motor and got curious about them. This is a good simple explanation, which if you click the links moves onto extremely complicated methods.
https://electrical-engineering-portal.c ... king-motor

 

[Dlyxover]

Thanks for the replay Eric The Viking.

I like the saw too I got it for £30 off ebay.
Been using a block of wood as you do to stop the blade, the induction motor is a all round win (and finger press, its catch me out all the time)

I've found the manufacture of the brake unit and the naughty contractor

https://www.klibo.com/contactor_kb_04.html?&L=1

I did also spot this 10a brake board that is the same as in my saw

https://www.klibo.com/brake_10a.html?&L=1

The contractor dose a bit of a burn clutch/brake smell to to it
Found a replacement on ebay for about £25 all in

https://www.ebay.co.uk/itm/Schaltschütz-K-B-KB-04-Uc-230V-3S-1Ö-731-Geräteschütz-Motor-Schütz/130498846292?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

I've got some spares for its already from

https://www.powertoolspares.com/metabo/ ... 510/spares

I did find a replacement brake unit assembly but was quoted £170 for it

Will report back the outcome