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Three Phase Motor Brake

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solis365

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I have a woodworking shop and am looking to put a brake on a very large radial arm saw (16" blades), as they spin for a very long time after power has been cut. The motor is something like 1 to 1.5 horsepower, three phase. The idea is to send a large (~12A peak) "DC" current (only needs to be half wave rectified, no smoothing) into the motor to act as a brake. The blade cannot come to a stop immediately as this will loosen the bolt that holds it on. The best way to stop it is slowly, over a period of 8 seconds (the blade coasts for far longer than that normally).

I am inspired by the PDF linked below:
http://www.electro-tech-online.com/custompdfs/2009/08/Radial_Arm_Saw_BrakePDF.pdf

However the issue with this project is that it requires the user to: hit the off switch of the machine, then press and hold a momentary switch for 8 seconds until the blade stops. It would be safer if the brake were applied every time the power was turned off, for an automatic 8 seconds.

I would like to design such a device. My current thought is to have the kill switch on the machine also trigger a smaller electronic circuit. This circuit will turn a relay on, allowing the 12A DC into the motor winding. Meanwhile some sort of timing circuit (RC network most likely) will be set for 8 seconds, and when the capacitor charges the relay will be shut off. However after the relay shuts off power must be cut to the saw entirely. At this point the only way the saw could be "more off" (i.e., guaranteed off) is by pulling the plug.


This is probably more easily accomplished with a small microcontroller accepting interrupts from the "off" switch of the saw.

Also, this circuit will need to be powered from the same AC line as the saw. Though it can be rectified, the saw will probably draw upwards of 20A from the line at a moments notice. Especially if something binds while cutting, so the control circuit will need to be immune to these sorts of power draws on the line. If a small "wall wart" type circuit will provide the necessary isolation...
 

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marcbarker

New Member
I do quite like the elegant simplicity of the scheme in the PDF file, you switch off then hold a button in for a few seconds. Better than slowing it down by pressing a lump of wood in! Similar application I think to bench grinder or angle grinder, if I want to stop either of them quicky I shove a piece of metal in. Dangerous I suppose!
 
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user_88

Member
alternate braking method ... eddy current braking

If you were to place an aluminum plate on either side of the circular saw blade, as in the form of spacer plates, you might be able to utilize the method described here:
Eddy current brake - Wikipedia, the free encyclopedia
You would have to rig an electromagnet so that its flux vector is perpendicular to the blade.

... and then set up a timer on the electromagnet circuit .....

Overall, it sounds like a relatively simple scheme. You might have to experiment with the magnetic field strength .... and respective level of braking achieved.

... just a suggestion.

It may be that you would actually get a braking effect while only using the metal saw blade as the rotor ....However, if you could install the aluminum spacer plates ... in a manner so as to interact with the magnetic field, you would certainly have more braking efficiency.
 
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mneary

New Member
I worked on a product that used DC braking of an AC induction motor, and we learned a few things that might help you.

The resistance of the motor is much less than its AC impedance, thus the DC voltage that you need for braking is quite low. Your 12A can come from a DC supply as low as 12 volts!

You can use a capacitor to initially supply the 12V. If it's a capacitor discharge system you don't have to disconnect it once you've stopped. (No explicit timer.)

You can adjust the DC voltage to limit the initial braking, with less dependence on cutting up heating elements.

Be sure to protect the DC supply from the energy in the motor! Use a properly rated forward diode in series with the supply and a reverse diode across the supply!

The capacitor should be doing all the work.
 

Boncuk

New Member
Why don't you brake mechanically?

connect a two position lever to a spring loaded brake shoe normally used in cars and have it press against the saw blade?
 

solis365

New Member
Why don't you brake mechanically?

connect a two position lever to a spring loaded brake shoe normally used in cars and have it press against the saw blade?
mechanically braking the saw blade could damage it unless done very precisely. Also, as it is a radial arm saw, the actual blade slides forward and back, meaning the brake assembly would have to move with it, all while remaining mechanically stable enough to never interfere with the blade while in operation... I think it would be easier to design an adequately safe electronic brake rather than a safe mechanical one.

mneary said:
I worked on a product that used DC braking of an AC induction motor, and we learned a few things that might help you.

The resistance of the motor is much less than its AC impedance, thus the DC voltage that you need for braking is quite low. Your 12A can come from a DC supply as low as 12 volts!

You can use a capacitor to initially supply the 12V. If it's a capacitor discharge system you don't have to disconnect it once you've stopped. (No explicit timer.)

You can adjust the DC voltage to limit the initial braking, with less dependence on cutting up heating elements.

Be sure to protect the DC supply from the energy in the motor! Use a properly rated forward diode in series with the supply and a reverse diode across the supply!

The capacitor should be doing all the work.
so just have the "off" switch trigger some circuitry that passes a charged capacitor to the motor windings? then to get the timing right just adjust something like a series resistor to change the time constant?

this sounds pretty simple but the problem is the capacitor will apply braking force exponentially - lots of stopping force initially, and exponentially less. so even though it may take 8 seconds, most the the braking will be done in the first 1 or 2 seconds. This may still cause the blade to loosen. With just the heater wire (i.e., resistor), the braking is constant through the 8 seconds. Hence the need for a timer and control circuitry. do you have a way to use the capacitor method, but keep the braking force a little more constant over the 8 seconds?

the main concern is safety, which is why I want an automatic brake to be applied, as usually the operator is concerned with holding the wood and making sure it doesnt drop to the floor, is cut properly, etc. keep in mind there is a 16" steel blade rotating at a few thousand RPMs


thanks for the suggestions!
 
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