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Thread: Wire wheel safety
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4th Jul 2020, 08:26 PM #1Most Valued Member
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Wire wheel safety
I’m by no means a safety freak that wants to live in a fluffy bubble, but I ALWAYS wear a full face shield when issuing a wire wheel on bench or angle grinder.
Picked these out of my jacket tonight after about 15mins using one on the angle grinder.
10mm bolt for size reference.
The thought of one in someone’s eye makes me go cold.
Take care guys.
Steve
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4th Jul 2020, 08:35 PM #2Member: Blue and white apron brigade
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4th Jul 2020, 09:17 PM #3Most Valued Member
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I know what you blokes mean, I found a wire stub, embedded 5 mm in my ankle. It had gone through my loose fitting jeans and sock. I didn't feel it till I had my shower, and wondered what the pain was.
The amazing thing was, I used the wire buff at waist high level.
KrynTo grow old is mandatory, growing up is optional.
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4th Jul 2020, 09:43 PM #4Most Valued Member
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Thanks Bob for reminding me how forgetful I am
Steve
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4th Jul 2020, 09:52 PM #5Member: Blue and white apron brigade
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4th Jul 2020, 10:16 PM #6
Speed
When a wire wheel is mounted on the average bench grinder, It would be handy to control the speed of the grinder to some degree for some jobs , slow it down , hence less hazardous . Thinking of trying one of these . I think these use a SCR device ? https://www.ebay.com.au/itm/4000W-AC...wAAOSwFohexhi3
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4th Jul 2020, 10:38 PM #7Member: Blue and white apron brigade
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I've had mixed success with those things. I used one to control the speed of a DC motor by varying the AC input to a rectifier - it sort of worked but the power loss was significant. The second one was set up to control the speed of small hand held cake mixer - that one worked OK. A third one was setup to control the speed of an aquarium pump but they didn't like each other and the SCR blew up after about 30s of use.
Either way on a grinder the problem is going to be power loss, it's not like grinders have a lot of spare power to begin with.
My wire wheel was on one of 2 3P Grinders I have that run via one VFD but I had more important uses for that than a wire wheel so I now have they wore wheel on a SP grinder.
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4th Jul 2020, 11:30 PM #8
Aldi grinder
I have a ALDI 1200 watt angle grinder with variable speed . Using the grinder, I've noticed the power output still seems to be holding up OK as you slow the rpm down with a little thumb wheel, don't know how it works but its been a handy tool. . For $30 it was a good deal . A note of caution: I did manage to destroy two of these ALDI grinders, after a extended period of heavy duty use, they overheated. The latest example of the same ALDI grinder I have has lasted very well with light to medium duty use
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4th Jul 2020, 11:46 PM #9Most Valued Member
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4th Jul 2020, 11:54 PM #10
Hi Guys,
You cannot control the speed of an induction motor with a thyristor or triac dimmer. A universal motor yes !
You can however use a triac to switch one on and off.
I do believe that there are now available single phase input/output VFD's that will control the speed of a single phase induction motor.Best Regards:
Baron J.
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5th Jul 2020, 09:24 AM #11Member: Blue and white apron brigade
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Good point. And is the reason it works on the angle grinder.
Another problem at low speeds is both the lower speed and insert power losses means reduced grinding so there is a tendency to press harder which can lead to overloading/heating the motor.
Would be OK on a universal motor powered wire wheel provided you weren't pushing too hard
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5th Jul 2020, 06:58 PM #12
Hi Guys,
I dug this drawing out and cleaned it up a little. Its been a while...
VFD Block Diagram.png
The drawing shows a block diagram of how a VFD works. This is what is needed to build one. The electronics are very similar to an AM transmitter. The carrier frequency is in this case modulated by a low frequency sine wave from say, 35 Hz to 70 Hz to give the speed variations. I've modified the drawing to show only a single phase output, which would be needed for a grinder with an induction motor.Best Regards:
Baron J.
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5th Jul 2020, 07:44 PM #13Senior Member
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5th Jul 2020, 10:44 PM #14Member: Blue and white apron brigade
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This week I'm going to be giving a general live video presentation about VFDs to WA mens Sheds Association so if its OK with you I would like to sue that diagram in my presentation. My talks is more about applications but I thought I would start of with a few minutes of intro of a "handwaving this is how they work" .
Cheers Bob
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5th Jul 2020, 11:29 PM #15
Baron, your single phase in/out VFD may work with a synchronous motor, but I suspect would be dubious with either a cap start or cap run motor.
For a cap start, the start winding centrifugal switch would generally engage if the input frequency dropped below about 40Hz for a nominal 50Hz motor, ultimately baking the start windings which would be energised for much longer periods than they are designed for, even allowing for the lower currents because of the higher impedance of the start cap at the lower frequencies. Available starting torque would also be significantly reduced, to the extent that motors may fail to start in some circumstances.
With a cap run motor, the aux winding current would be reduced because of increased impedance in the run cap, significantly reducing the available torque from the motor.
While the induction motor is quite common, it has a drawback of needing at least two sets of windings with a phase delay between them, and with a single phase motor the delay is created by a capacitor with a frequency dependent impedance, and the system is optimised to operate at the local mains frequency which maintain tight tolerances.I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.
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