It's the 'value engineering' part that is the driver as it is easy enough to arrange switch gear that activates the brake OFF and yet inhibits power getting to the motor, or come to that any other part of the machine.
Rob
Rob
Help please: adding dynamic braking to an induction SCMS?
- Thread starter Eric The Viking
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Eric The Viking
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For anyone following this thread hoping for a low-cost DIY HSE compliant brake, I thought I ought to restate the original intent of my project. I've only ever been trying to reduce the setup time when using the saw, NOT to slow it to stationary within the time (10 sec?) of HSE regs.
In the transport industry, the principle I'm trying to use is commonplace. It's known as dynamic or regenerative braking (the latter is more descriptive really), and it either dumps kinetic energy into a resistor mat on top of, say, the tram or trolley bus or commuter train, feeds power back into the supply circuits, or uses the "regenerated" electricity to spin up a flywheel that's then employed in helping the vehicle start off again (like the KERS system on F1 cars, but less complex).
The problem is that the braking efficiency drops off dramatically as the speed of the vehicle reduces. On vehicles, this is highly undesirable! Thus they have additional mechanical brakes that are applied when the vehicle actually wants to stop, or, in emergencies, if the regenerative brake fails.
If I can get my experimental system to work, it will inevitably be the same. Regenerative systems cannot stop a motor completely on their own.
Another difference between what I'm trying out and commercial regenerative braking systems is the motor design itself: The big transportation motors are also usually DC-powered, as it's relatively easy to speed-control them (obviously you need to do that too!). DC makes regenerative braking simpler to design and more efficient in use. Mine, used with an AC motor is a bodge in comparison.
In AC motors, usual for industrial applications, the magnetic field rotates slightly ahead of the rotor when the motor is running at normal speed. The electric brakes for these motors are similar no matter how many phases they have: they feed DC (usually rectified single phase AC) into the motor windings. It's done with 'contactors' (basically heavy-duty relays) or triacs/thyristors (solid-state alternatives), and it's relatively complex and expensive. But it will actually stop the motor...
... Injecting DC (instead of AC) creates a non-rotating magnetic field in the motor. As the motor rotates in this field, a force is generated to oppose the movement. This starts off as a big braking force, but once it's stationary, any slight movement still generates opposing forces. It will behave a bit as though it's clamped in position. It's in essence clamped by an electromagnet!
So a 'DC-injection' brake will stop a motor completely, unlike my intended system. Mine is only intended to make the machine slow down more quickly. It has no ability to actually stop the motor. Air resistance and bearing friction do that, as they do on the unmodified machine. I'm hoping, though, that what it will do is to make the motor stop in a shorter, more practical time.
I've calculated that the present stopping time is the result of about 6 watts (simple average) of friction in the system. My simple regenerative braking system will work best when the blade is spinning fastest, so can theoretically dump most of the energy in the system at the beginning of braking. Once its efficiency drops off (as the blade slows), friction takes over, eventually causing the blade to stop moving completely.
I'm frustratingly held up in the experimentation waiting for time and parts. As soon as I can, I should be able to complete the testing and have something that either works, or fails for some not-thought-about practical reason!
Cheers,
E.
In the transport industry, the principle I'm trying to use is commonplace. It's known as dynamic or regenerative braking (the latter is more descriptive really), and it either dumps kinetic energy into a resistor mat on top of, say, the tram or trolley bus or commuter train, feeds power back into the supply circuits, or uses the "regenerated" electricity to spin up a flywheel that's then employed in helping the vehicle start off again (like the KERS system on F1 cars, but less complex).
The problem is that the braking efficiency drops off dramatically as the speed of the vehicle reduces. On vehicles, this is highly undesirable! Thus they have additional mechanical brakes that are applied when the vehicle actually wants to stop, or, in emergencies, if the regenerative brake fails.
If I can get my experimental system to work, it will inevitably be the same. Regenerative systems cannot stop a motor completely on their own.
Another difference between what I'm trying out and commercial regenerative braking systems is the motor design itself: The big transportation motors are also usually DC-powered, as it's relatively easy to speed-control them (obviously you need to do that too!). DC makes regenerative braking simpler to design and more efficient in use. Mine, used with an AC motor is a bodge in comparison.
In AC motors, usual for industrial applications, the magnetic field rotates slightly ahead of the rotor when the motor is running at normal speed. The electric brakes for these motors are similar no matter how many phases they have: they feed DC (usually rectified single phase AC) into the motor windings. It's done with 'contactors' (basically heavy-duty relays) or triacs/thyristors (solid-state alternatives), and it's relatively complex and expensive. But it will actually stop the motor...
... Injecting DC (instead of AC) creates a non-rotating magnetic field in the motor. As the motor rotates in this field, a force is generated to oppose the movement. This starts off as a big braking force, but once it's stationary, any slight movement still generates opposing forces. It will behave a bit as though it's clamped in position. It's in essence clamped by an electromagnet!
So a 'DC-injection' brake will stop a motor completely, unlike my intended system. Mine is only intended to make the machine slow down more quickly. It has no ability to actually stop the motor. Air resistance and bearing friction do that, as they do on the unmodified machine. I'm hoping, though, that what it will do is to make the motor stop in a shorter, more practical time.
I've calculated that the present stopping time is the result of about 6 watts (simple average) of friction in the system. My simple regenerative braking system will work best when the blade is spinning fastest, so can theoretically dump most of the energy in the system at the beginning of braking. Once its efficiency drops off (as the blade slows), friction takes over, eventually causing the blade to stop moving completely.
I'm frustratingly held up in the experimentation waiting for time and parts. As soon as I can, I should be able to complete the testing and have something that either works, or fails for some not-thought-about practical reason!
Cheers,
E.
NetBlindPaul
Established Member
The electro mechanical (EM) brake works by the magnetic field repulsing the brake disc when powered allowing an air gap and free rotation.
When the power is lost the springs close the gap and the friction between the disk & friction material stops the machine.
BTW, I can stop a 48" resaw in 10 secs with DC braking over and over IF needed, though, we normally go for 20-30 on these big machines as this still complies with PUWER98.
In response to the comments by Eric The Viking about regenerative braking, the only way this could be done with repeatability to comply with statutory legislation is if you utilise an inverter or PWM servo drive type unit with an active front end where by the motor is turned into a generator, then the active front end pumps that back into the “grid” you also need filters between the drive & the mains for this
We used to stop 5 off 60" dia. grinding wheels about 1" wide on a spindle of about 1000kg's in just over 10 seconds with this technique. Problem was any mounting discrepancies with the grinding wheels or defects within the grinding wheels, and they would either spin on the arbour or there would be a big bang the first time the system was tested!
When the power is lost the springs close the gap and the friction between the disk & friction material stops the machine.
BTW, I can stop a 48" resaw in 10 secs with DC braking over and over IF needed, though, we normally go for 20-30 on these big machines as this still complies with PUWER98.
In response to the comments by Eric The Viking about regenerative braking, the only way this could be done with repeatability to comply with statutory legislation is if you utilise an inverter or PWM servo drive type unit with an active front end where by the motor is turned into a generator, then the active front end pumps that back into the “grid” you also need filters between the drive & the mains for this
We used to stop 5 off 60" dia. grinding wheels about 1" wide on a spindle of about 1000kg's in just over 10 seconds with this technique. Problem was any mounting discrepancies with the grinding wheels or defects within the grinding wheels, and they would either spin on the arbour or there would be a big bang the first time the system was tested!
Eric The Viking
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Update:
Setback: A quick trip to Maplin's yesterday resulted in two spark suppressors (£1.60ea) to go across the relay contacts. Fitted them this morning. Result: dismal failure. If anything the sparking is worse than before, and there is now additional sparking when the contacts close (wot???) as well as when they open. So if left unchecked, that relay is not long for this world...
Discussed this with pater mine (81 year-old semi-retired electronics designer, who doesn't suffer fools gladly): realised I was being "an *****" to use a relay*. Having a re-think involving two triacs controlled by a relay. This effectively eliminates the switching sparking, but the relay's inclusion is a quick+dirty failsafe to ensure I can't short the incoming 110V supply with a hefty load resistor by accident. The braking triac can't switch on until significantly later* than the power-supplying triac switches off.
All this might take a while, as the circuit board is a total rebuild (but it looked pretty untidy anyway).
Slight smugness: Took advantage of the hiatus (will have to obtain some suitable Triacs next week) to wire a second bulbholder in parallel for the brake. I've written off the present relay anyway (have spares), so a bit of chunky sparking won't matter (unless of course I actually weld the contacts together :twisted: ).
Yup, can now get two bulbs glowing for about 1/2 sec (55 Ohms cold), and no appreciable decrease in stopping time yet. I can obviously go lower - I think the braking force increases in inverse proportion to the drop in load resistance (i.e. the current increasing). Triac switching will mean I can aim for a really low resistance, I hope.
This is getting FUN.
More news as it happens (so don't hold yer breath!).
E.
*he didn;t quite put it that way, but I understood the full force of the argument
**say >10ms (hopefully). There is a marginal risk that the power triac switching off will clamp on for an extra 1/2 cycle of the mains (i.e. 10ms), so, if the timing's off, there will be a momentary massive short. All part of the fun...
Setback: A quick trip to Maplin's yesterday resulted in two spark suppressors (£1.60ea) to go across the relay contacts. Fitted them this morning. Result: dismal failure. If anything the sparking is worse than before, and there is now additional sparking when the contacts close (wot???) as well as when they open. So if left unchecked, that relay is not long for this world...
Discussed this with pater mine (81 year-old semi-retired electronics designer, who doesn't suffer fools gladly): realised I was being "an *****" to use a relay*. Having a re-think involving two triacs controlled by a relay. This effectively eliminates the switching sparking, but the relay's inclusion is a quick+dirty failsafe to ensure I can't short the incoming 110V supply with a hefty load resistor by accident. The braking triac can't switch on until significantly later* than the power-supplying triac switches off.
All this might take a while, as the circuit board is a total rebuild (but it looked pretty untidy anyway).
Slight smugness: Took advantage of the hiatus (will have to obtain some suitable Triacs next week) to wire a second bulbholder in parallel for the brake. I've written off the present relay anyway (have spares), so a bit of chunky sparking won't matter (unless of course I actually weld the contacts together :twisted: ).
Yup, can now get two bulbs glowing for about 1/2 sec (55 Ohms cold), and no appreciable decrease in stopping time yet. I can obviously go lower - I think the braking force increases in inverse proportion to the drop in load resistance (i.e. the current increasing). Triac switching will mean I can aim for a really low resistance, I hope.
This is getting FUN.
More news as it happens (so don't hold yer breath!).
E.
*he didn;t quite put it that way, but I understood the full force of the argument
**say >10ms (hopefully). There is a marginal risk that the power triac switching off will clamp on for an extra 1/2 cycle of the mains (i.e. 10ms), so, if the timing's off, there will be a momentary massive short. All part of the fun...
Hi Eric
I know this thread is a bit old now, but I have just acquired a used KGS301 and would like to add some braking to it as it does take a very long time to spin down compared to my old brush motored saw. Did you ever come up with a solution to this, and if so could you possibly share some details?
Many thanks
Fergal
I know this thread is a bit old now, but I have just acquired a used KGS301 and would like to add some braking to it as it does take a very long time to spin down compared to my old brush motored saw. Did you ever come up with a solution to this, and if so could you possibly share some details?
Many thanks
Fergal
Mcluma
Established Member
Fergal":1kc9v3n0 said:
Do like I did call Metabo (i called of course Elektra Beckum) and buy the module new no messing about - comes with clear instructions etc
The law in germany was that they needed to be braked, hence they sold the aftermarket upgrade
Eric The Viking
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Mcluma":2hg94igd said:
I would have done, but my saw is 110V. And the 240V module (as a spare part) is/was stupidly expensive, so I can only speculate as to the cost of the 110V version.
I wasn't ignoring Fergal - we have corresponded about this by PM - but I haven't got any further with the project recently.
E.
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