Help please: adding dynamic braking to an induction SCMS?

[Eric The Viking]

(changed the title to reflect the content better)

I need a spot of info for a conversion project...

This is about the Elektra Bekum/Metabo KGS3xx series, or the Axminster QMS254I (which is almost identical). These are they:

Mine is old but good, and I want to add an electric brake to it. Later 240V ones and the Axminster one have a brake, but there are several possible designs and I'm not sure which type has been fitted. The simplest approach is a high-power, low-resistance shunt resistor, across the stator winding when the saw is OFF, but this requires a double-pole changeover switch for the trigger, not a make-break one such as mine has. I'm guessing that's what the later saws have, rather than a complicated circuit, in which case it's a relatively easy mod to do to mine.

If the later versions have this sort of brake, it's easy to get the switch as a spare part and build the rest.

So if you've dismantled your saw, for a good clean, perhaps, does the trigger for yours have six wires going to it (to separate terminals), or just four?

Thanks to anyone who knows and can tell me!

Regards,

E.

PS: To avoid any argument breaking out, this is NOT a safety mod at all. It simply stops the blade a bit more quickly, to speed up setup time when making multiple cuts. Stopping an SCMS too fast is inherently dangerous if designed like mine - I'm aware of the issues!

PPS: If I can do this, I'll post pictures in due course.

 

[dickm]

Interesting - I was wondering about this sort of thing for my old Elu flipover saw, but had assumed it was completely out of the question. Anyone got any comments?

 

[Eric The Viking]

Interesting - I was wondering about this sort of thing for my old Elu flipover saw, but had assumed it was completely out of the question. Anyone got any comments?

There is a power tool repairer in town here (Infix). I have to get my Makita SDS drill serviced soon, so one possibility is to ask if they've ever worked on these low-power induction motors. They're quite friendly and may know.

Incidentally, if your Elu is a brushed motor, it's a much more complex problem, I think.

I hope Bob M. spots this thread, as I'd welcome his thoughts especially.

Cheers,

E.

 

[dickm]

There is a power tool repairer in town here (Infix). I have to get my Makita SDS drill serviced soon, so one possibility is to ask if they've ever worked on these low-power induction motors. They're quite friendly and may know.

Incidentally, if your Elu is a brushed motor, it's a much more complex problem, I think.

I hope Bob M. spots this thread, as I'd welcome his thoughts especially.

Certainly be interested to hear what they might have to say - ditto Professor Bob!
The Elu is an induction motor (110v); one of the reasons why I like it.

 

[Eric The Viking]

Here's what I have in mind (favourite option, subject to being told I've misunderstood something important!):

Quick calculation: my saw is 1100W at 110V, meaning the stator impedance is roughly 11 Ohms (10A current at speed). So, at a guess, the shunt resistor ought to be somewhere between 10 and 50 Ohms with a steady-state dissipation of about 55W (1/20 of the peak load).

The questions are, what resistance to use, and can I find a suitable switch?

If the resistance is too low, it may work too well, and decelerate the motor too fast. It's a relatively big blade, and the danger is that it may slip and undo the arbour bolt (left-hand thread). That's a far worse problem than having to wait for a minute or so for it to stop naturally.

If I can get the right switch, I can test-rig it. Ceramic resistors are relatively cheap (pennies, or they should be!), and I can start relatively high and work down to a value that slows the blade in, say <10 secs (still a long time, but shouldn't loosen the blade, one would hope). Then I can substitute a robust resistor for the cheaper one, for a final 'production' version.

If I can't get a replacement trigger that's designed for the saw, I can still achieve the result. with a 'stop' switch upstream. You'd flick the 'stop' switch (probably mounted on the chassis somewhere, then hold the trigger in until the blade stopped rotating.

I'm waiting for someone to tell me my sums are wrong or that you can't do it for some reason...

Cheers,

E.

 

[Racers]

Hi, Eric

I think you need to use a relay, then you could use the existing switch.

Pete

 

[Eric The Viking]

Hi, Eric

I think you need to use a relay, then you could use the existing switch.

Pete

Good point! I assume you mean like this:

I'll have to put a small box on the outside of the case anyway (for the resistor+heatsink), so if I can find a suitable relay (20A changeover with a 110V coil should do it), then it ought to work well. The existing switch is DP, but that's probably not important really - it doesn't need to isolate, and it's inherently failsafe in any case.

 

[Racers]

Hi, Eric

Yep that should do it, how about puting the other existing sw in the netural?

You could use one of those nice die-cast ally boxes as a box and heatsink.

Pete

 

[John Brown]

I'm interested to know how you get on with this - I have an Elektra Beckum flip-over saw that I'd like to modify.
Slightly off-topic, but can one do a similar thing with capacitor start motors?

John

 

[Eric The Viking]

Hi, Eric

Yep that should do it, how about puting the other existing sw in the netural?

Indeed I could, but it's Xfmr-isolated anyway, so small advantage. More to the point, having looked at Rapid's site, I think I'll struggle to find a small 110V relay (if at all). Plan B I guess is a potential divider for the relay coil (run it at 12V or 24V probably). I've got several ex-washing machine ones somewhere that would do.

I like the idea of box=heatsink. I really ought to weigh the blade, and work out how much energy will be dumped into the resistor in 3 secs or so. It should be fine, but it might get hot in repeated use. Elektra Beckum gave a 20% duty-cycle for the saw, but I'd struggle to approach that in real life, so I may be worrying unnecessarily. Experimentation is the key, I suspect.

The other thing I ought to do, though, is see if the later versions of the saw (Metabo) have a pinned arbour. There's the usual 'chunky slotted washers' arrangement, and the shaft end has flats, so I guess they didn't change that part of the design. I'm not going for a dead stop in 1/2 second, and I'll settle for improved spin-down rather than 'stop on a sixpence'.

Again it would be good to know about the Metabo 240V version...

Cheers,

E.

 

[Racers]

Hi, Eric

Lots of 110V relays on Ebay.

http://cgi.ebay.co.uk/Finder-4052-Serie ... 588befafff

Pete

 

[dickm]

Interesting stuff. Problem with my Elu flipover is that it has a separate NVR switch. I'm guessing this complicates things?

Related question - where is the best place to get 2pole 2way switches that will carry about 20A? I'm having occasional problems with my Lurem combination, which seem to be down to the c/o switch. It doesn't have to handle starting current, because I only switch over when everything is stationary (for obvious reasons!). But the Lurem has 2kW motors of very agricultural dimensions - indeed, one of them came off a bale elevator .

 

[Mcluma]

I bought 4 years ago just from EB the electronic brake - from germany -as the uk people didn't want to do it

I cannot for the life understand you guys using this saw without an electronic brake - you guys must be mad (hammer) (hammer)

You buy this saw on ebay for peanuts, buy the brake and you have the best saw in the world, that is if you can live with only a 50mm capacity - hence me upgrading to a 12 inch saw

 

[Eric The Viking]

Partial update:

I've been doing some experiments:

Thanks to Pete Maddex (who found them) I bought some 110V relays from eBay.

I've re-jigged the saw's internal wiring, and it now starts/stops via a relay rather than the trigger switch, per my earlier diagram (Pete: haven't gone down the switched neutral approach for reasons of space at the moment).

The other relay contacts go out to two flying leads to which I can connect a load. I'm proud to say I blew a 12V 12W bulb on the first attempt, and can make a 40W 240V bulb glow yellow for a moment. Since the latter bulb has a cold resistance of 110 Ohms (approx.), it's way too high for practicality, but even so, it is sinking quite a bit of power (optimistically, initially about 20W).

The other encouraging observation is this: during the experiment, the saw arbour bolt was finger-tight on the arbour bushes/plates. On braking (when the 40W bulb glowed), it unwound itself several turns! This is its own angular momentum versus the braking force applied from the current through the bulb. There's no other explanation I can think of, and it doesn't do it without the bulb 'brake' in place (and the arbour plates are keyed to the motor shaft so don't otherwise spin WRT the bolt at all).

All the above points to the idea that a 'self-excited' motor coil will produce sufficient current to brake itself effectively.

Obviously, all testing has been *without* a blade fitted, and in that situation, un-braked stopping time is only 5s. There is a reduction, but it's hard to measure accurately as the time is so short.

Issues:

1. I'm hamstrung because a necessary terminal adaptor for my storage 'scope hasn't arrived, so I still can't capture the back-EMF spike on switch-off. I need to do this to get a measure of how much energy will be dumped in the first 0.5sec of braking, as this determines the power rating and value of the necessary resistor.

2. The switch off causes substantial arcing across the relay. This was predictable, but is worse than anticipated. To avoid premature relay failure, I'll have to add a snubber/spark suppressor circuit. This will have to be carefully chosen to avoid 'quenching' the motor back EMF entirely, and it's presence will reduce the brake effectiveness too, probably.

3. It's a rats' nest of untidy wiring at the moment. It will tidy up to something compact and effective though (I think), but I can't do this until experiments are complete. The braking resistor will probably mount on one of the fins of the motor heatsink - the power dissipated won't cause any issues, and it's unlikely to be any sort of safety hazard (well insulated connections and isolated from the motor case).

Total component cost will be easily <£15, although the time taken has been rather a bit

I've taken a few WIP pictures, but they're not wonderful and I'm rather short of time at the moment. Will post then and more detail as soon as I can.

On the whole it's encouraging.

E.

PS: I have a spreadsheet for calculating the kinetic energy stored in the blade (the calculations are reasonably simple). I'll upload it if anyone's curious to work it out for their own saw. It's interesting to do what-ifs on RPM, blade weight, diameter, etc. It shows that stopping saws suddenly with blade diameters of >10 inches requires a LOT of braking power and involves relatively enormous forces - possibly why the American emergency-stop systems seem to only go up to that diameter! You also need an infinite supply of sausages, it seems. :roll:

 

[OldWood]

Following this with interest and a wee bit knowledge.

The wood working club I belong to has a Sheppach bandsaw driven with a 2kw single phase motor. Because of H & S concerns, a electromechanical brake was bought 2 years ago to fit to the then 4 year old motor. 3 weeks or so ago, the coil operating this brake (bit like a clutch), lost it's smell and the whole lot had to be taken out, and in due course refitted by me! . The club ended up buying a new motor as the repair shop condemned the bearings as well.

Interestingly one of the club members worked in a meat processing factory and his comment was that the brakes on the bandsaws there were always failing, so perhaps this dynamic one may well be worth our consideration too.

Rob

 

[Eric The Viking]

Hi Rob,

How does the electromechanical brake work - like a disc brake on a car, or some other way?

I'd guess there's a huge amount of kinetic energy in a running bandsaw. You could calculate it (I can't upload my excel spreadsheet, sadly, as the forum won't accept Excel files). The issue is the energy you need to dissipate in order to brake within a specified time (say 10 secs).

If you PM me, I'll see if I can send you the spreadsheet that way...

E.

 

[OldWood]

Hi Eric
PM sent. I'll answer the brake query here for all to see !!

The back end of the motor has a coil mounted on the body together with two steel plates mounted on 3 stand-offs, the inner plate being spring loaded. The end of the shaft has a sliding plate with brake material on both sides running between the two static plates - when the motor is energised the coil pulls the inner plate away from the brake disc and the brake is released ..... the spring compression does the braking action when the motor and coil are powered off.

Rob

 

[Eric The Viking]

It's a sort-of inverse clutch arrangement really then!

You have mail, by the way.

Cheers,

E.

 

[OldWood]

An addition to my explanation of the brake on the Sheppach bandsaw.

A major problem with this configuration is that the brake is default on, and there is no way of releasing it without powering the motor. OK the default status is fine as a safety feature, but it does mean that it is near impossible to turn the wheels and blade when setting it up.

It was a comment from Simon in a PM about measuring the run down speed that brought this back to mind. I think the other bandsaw in the club is a small Record - it has a rotary power switch which has a position where the brake is released for maintenance purposes.

Rob

 

[NetBlindPaul]

EM brakes are common on newer CE marked machinery and the arrangement you describe is pretty much standard.
Often these brakes are powered from 2 phases of the 3 ph supply to the motor or the L&N, thus they have no override possible.
Value engineering it is called!
More expensive machines have a brake release, however, to work on a machine spindle/tooling with the main electrical power to a machine not locked off is a bit of a no no under "business" H&S regs!