Thank you, Mr. Strantor. Your concerns are warranted and very real.

To avoid that issue, the typical DC-injection braking circuit has a power resistor of some 50 to 100 ohms in series with the braking capacitor(s) to limit the initial peak current through the motor windings during capacitor discharge to something less than the normal current draw of the motor. The object is to stop the motor over a time period that would be, say, three to four times as long as it takes to bring the saw up to speed.

to answer the original question,
the overhead on capacitors voltage rating is zero.
outside the spec, anything could happen,
you might have a good one, and it works fine at say 25% over voltage,
you could have a bad one, that pops just a bit over voltage
and it will probably get worse over time,

 

I would steer clear of an electrical braking system, certainly one put together by an owner irrespective of their skill level. Instead we should ask why does it "coast" for 30 seconds or so? where is the inertial component here? the motor shaft? some kind of flywheel?

Why not add some mechanical brake or retardation, one that the user can easily and reliably press or activate and have the thing come to a stop in a second or two?

 

Could you mechanical break the motor ,
electrical breaking is good if your trying to recycle power,
or need small size, but do you ?

 

Thank you, Mr. Strantor. Your concerns are warranted and very real.

To avoid that issue, the typical DC-injection braking circuit has a power resistor of some 50 to 100 ohms in series with the braking capacitor(s) to limit the initial peak current through the motor windings during capacitor discharge to something less than the normal current draw of the motor. The object is to stop the motor over a time period that would be, say, three to four times as long as it takes to bring the saw up to speed.

I can tell you first hand that dynamic braking can loosen the attaching nut!! But what I saw was with a grinding wheel. Years ago my dad added dynamic braking to a home made bench grinder. On the third stop the nut came loose unthreaded itself, and after a few seconds the wheel, still spinning rapidly, slid off, rolled across the work bench, and flew off, onto the floor, where a large chunk broke off and hit the far wall. Fortunately we were all able to back away quickly and not injured.
The caution is that drive braking is not a wise choice for threaded attachments that lack secondary locking.

As for the complaints about every new thing: Television programming as seen in the USA is most certainly a propaganda vehicle almost entirely. The advertisements are the obvious portion, the programs are more subtle. But certainly TV pprogramming pushes sets of values with almost every image.

 

Thank you, Mr. Strantor. Your concerns are warranted and very real.

To avoid that issue, the typical DC-injection braking circuit has a power resistor of some 50 to 100 ohms in series with the braking capacitor(s) to limit the initial peak current through the motor windings during capacitor discharge to something less than the normal current draw of the motor. The object is to stop the motor over a time period that would be, say, three to four times as long as it takes to bring the saw up to speed.

Back in the old days, I heard a story from one of the auto tech instructors at our local vocational school. The students took apart and reassembled a V8 engine, and had it running on a test stand in the shop. As the instructor held the throttle wide-open, the harmonic balancer, a 8-lb chunk of steel on the end of the crankshaft, came off the engine at 6500 rpm and scooted across the shop floor where it bumped up against a wall and sat there spinning for quite a bit of time. Lessons were learned.

Just asking

is this the sort of thing your talking about

 

Basically dump the tool and invest in one that has the desired braking capability, that's what I'd do.

 

Just asking
is this the sort of thing your talking about

That is a very inefficient way of DC braking, the DC will rapidly diminish rather than continue at the desired strength throughout the braking cycle,!

 

That is a very inefficient way of DC braking, the DC will rapidly diminish rather than continue at the desired strength throughout the braking cycle,!

thanks @MaxHeadRoom ,motors are not my thing, so was researching and wanted to ensure I was on the right track,

 

The scheme in post #45 might be adequate, and it will slow the motr quite rapidly, but not hold it stopped at all.AND, the capacitors will indeed charge to more than 150 volts, butit would work to split them and put two in series. If the leakages are similar the voltages should be the same, or close.

 

Just asking

is this the sort of thing you're talking about

Yes, but it is missing the 30-100 ohm power resistor in series with the capacitor that limits the peak current through the motor windings. In the circuit you have provided, without a current-limiting resistor, if the capacitor is large enough, the inrush of peak current could fry the motor windings, or create mechanical failure issues by locking-up the motor rotor. The goal is to brake the motor within some acceptable time with capacitor(s) of a size large enough to overcome whatever inertial loads are present. In my case, it is for my brother's woodshop-quality large bandsaw, which has a significant rotational inertia, resulting in a coasting-down time approaching some 20 seconds or more, during which it is silent and quite dangerous.

 

Basically dump the tool and invest in one that has the desired braking capability, that's what I'd do.

Well, certainly that is one option, but this is a $1,900 industrial band saw, and for reasons unknown, they don't come with built-in braking mechanisms, either electrical or mechanical.

 

That is a very inefficient way of DC braking, the DC will rapidly diminish rather than continue at the desired strength throughout the braking cycle,!

Yes, that is a known characteristic of the circuit.
The capacitor(s) are sized to match the inertial load of the device such that they become discharged to 0v at around the time that the motor halts.
Whether inefficient or not, the method is widely used as possibly the best passive solution, in contrast to having an external DC supply available to provide a constant DC voltage source for the injection braking.

 

Well, certainly that is one option, but this is a $1,900 industrial band saw, and for reasons unknown, they don't come with built-in braking mechanisms, either electrical or mechanical.

https://www.grizzly.com/products/grizzly-21-5-hp-industrial-bandsaw-with-brake/g0531b

Really, this is the solution, not a customer modification to the electrical system of your current tool. Sell the current saw, buy a better one and spend your time more productively.

I can certainly understand the desire to attempt this, but know from personal experience that these kind of cost saving hacks are often a source of their own problems.

 

Whether inefficient or not, the method is widely used as possibly the best passive solution,

Although not 'Widely' within my perview.

 

https://www.grizzly.com/products/grizzly-21-5-hp-industrial-bandsaw-with-brake/g0531b

Really, this is the solution, not a customer modification to the electrical system of your current tool. Sell the current saw, buy a better one and spend your time more productively.

I can certainly understand the desire to attempt this, but know from personal experience that these kind of cost saving hacks are often a source of their own problems.

Please consider that there are many engineers who participate in this forum and have many decades of related experience among them. So if this suggestion was poor advice there would be many posts to that effect describing why it was a poor choice. So just because some scheme is not utilized by the OEM for this tool is not an adequate reason to back away in fear. Certainly there will need to be a bit of analysis done and certainly the installation must be done correctly. But it is an entirely reasonable method of braking an induction motor.

 

Please consider that there are many engineers who participate in this forum and have many decades of related experience among them. So if this suggestion was poor advice there would be many posts to that effect describing why it was a poor choice. So just because some scheme is not utilized by the OEM for this tool is not an adequate reason to back away in fear. Certainly there will need to be a bit of analysis done and certainly the installation must be done correctly. But it is an entirely reasonable method of braking an induction motor.

Yes I acknowledge that, and I was not implying anything about anyone's skills or capabilities or the quality of advice, there are many experts here with experience far beyond mine, I just hope there's a safe and truly robust outcome for this.

 

https://www.grizzly.com/products/grizzly-21-5-hp-industrial-bandsaw-with-brake/g0531b

Really, this is the solution, not a customer modification to the electrical system of your current tool.

I am assuming, deducing from the add that the brake is mechanical.? !

 

Yes I acknowledge that, and I was not implying anything about anyone's skills or capabilities or the quality of advice, there are many experts here with experience far beyond mine, I just hope there's a safe and truly robust outcome for this.

If you were wondering, I have a Master's degree in Electrical Engineering and a Bachelor's in general engineering.

How does that compare to your skills, education and experience?

 

I have a Master's degree in Electrical Engineering and a Bachelor's in general engineering.
How does that compare to your skills, education and experience?

Although possessing # 2 does not guarantee or indicate the level of #1 & #3 at all.

 

That bandsaw may have been $1900 two tears ago, now it would be a lot more. And a used bandsaw might go for $1000 if the seller was really lucky.
The advances during that time will generally be made towards reducing the quality to increase the profit. That is what I have seen. Also adding features and calling that quality. Features seldom equate to quality, even in industrial tools.
So selling it and buying a new one would be poorly advised at best,