Thread: Variable speed bench grinder
10th Mar 2008, 03:07 PM #1
Variable speed bench grinder
I am after a small bench grinder with a variable speed from zero upwards. There seems to be nothing in Australia and I found one in the USA but they don't ship internationally and besides that the voltage is wrong for us. Does anyone know if such grinder is available for purchase in Australia?If you don't know where you are going you will finish up somewhere else.
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10th Mar 2008, 08:09 PM #2
Ring GMC on 1300 880 001 and ask them whether the BG150VS has been released yet (will only go down to 1600rpm though, not zero).
10th Mar 2008, 08:22 PM #3
Thanks Batpig. I did look at most bench grinders and the lowest I see on the Australian Ebay is something like 240 RPM but I prefer zero.If you don't know where you are going you will finish up somewhere else.
10th Mar 2008, 08:37 PM #4
You'll have to build it yourself if you want such low speeds. A Variable frequency drive, good quality mandrel and 3 phase motor are the main components. I'd be very surprised if you can find a single phase motor that could do under 1600 rpm without pullies to reduce the rpms.
10th Mar 2008, 08:42 PM #5
10th Mar 2008, 08:55 PM #6
I think you would have to be careful as most motors that size are cooled by a fan on the motor and the slower you go the less the cooling. Another option to the AC motor would be a DC motor and DC drive system. An alternate could be wrecking a tread mill for the variable speed drive and motor.
11th Mar 2008, 04:15 PM #7
11th Mar 2008, 06:35 PM #8
Variable speed bench grinder options
Bench grinder wheels on very low rpm are mostly used for sharpening of knives, chisels, planer blades and such. Since wheels for that purpose have very fine gauge aluminium oxide grit, they tend to get soiled and logged and are used with water, to get rid of debris and to cool the workpiece. Such wheels mostly run in water baths and are induction motor driven by means of worm drive reduction gears. That means there is just one speed, which is dependant on mains current frequency (about 3000 rpm at 50 Hertz) and motor poles (two poles for smaller bench grinders and 4 poles -resulting in about 1500 rpm- for very large 3-phase industrial types).
The efficiency of induction motors is best between maximum no load rpm and a load rpm of 95-85% of full rpm. With higher load and more rpm loss the equation between output Watts and useless heat gets poor very quickly. People regularly working with stationary circular saws powered with an induction motor, know from experience that there is a certain "torque curve". Up to a certain point, the rotor manages to follow the rotating stator field and a good torque is to be expected. When more load pressure is applied, this torque and the rpm collapse quite rapidly, whereby much more of the power intake is converted to heat. This is a result of the induced magnetic rotor field running too far behind the mains generated magnetic stator field.
Therefore, it is a bad idea to run a small induction motor on some sort of power regulator, since even with a true frequency converter (the right way of doing this, contrary to an ordinary triac light dimmer) the efficiency of the motor will be very poor. With small motors it already is, below 500 Watts input soms 60% can be expected, only upward of a few kiloWatts, it gets better than 70%. The only exception goes for modern electric locomotives which use, contrary to what you would believe from the story above, frequency regulated induction traction motors. This is done because of low motor maintanance costs (no brushes and vulnerable armature copper windings), but these motors are almost 99% efficient and have a special rotor bar layout meant for variable frequencies. So these motors are specially built for huge starting torque from zero rpm at standstill, whereas a bench grinder motor has low starting torque and is rather meant to run on a fixed 50 Hertz. To illustrate this, watch closely how a bench grinder revs up from standstill. It starts slowly, but when almost reaching full no load rpm, it accelerates more quickly. This indicates the most efficient point of the torque curve, the range in which the rpm should be kept under load.
As you will know, series wound brush motors react more favourably to power regulation, this needn't be freqequency controlled but needs only be voltage controlled. But these motors have poor efficiency at low rpm too, again moreso when they are small. So, provided there are brush motor driven bench grinders (and i know a few from Metabo and AEG, but they are long since out of production since the 70's), that would be an -less than ideal- option.
Have you considered a strong electronically controlled drill? Modern models have input powers in excess of 1000 Watts and often feature tacho speed control and coil temperature measurement. So they are built to generate good torque at low speeds (especially with a gear box in 1st gear) and for quite some time too, with a proper overheating protection as an extra. Such a drill could be coupled to a grinding wheel accessory, which were very popular for hom DIY-use in the 70's, and these can be found regularly on eBay. Metabo has good versions and offers good matching drills to power them. Should your grinding power requirements be larger than a few hundred Watts of motor power, a custom built layout with an industrial 3-phase induction motors and drive pulleys chose for the rpm-count needed (as mentioned a few times already), would be the only alternative i could come up with.
Lots of success and greetings from Holland!
11th Mar 2008, 07:25 PM #9
16th Mar 2008, 05:41 PM #10
Variable speed bench grinder
What I can't work is out how these things work then. They are on Ebay USA and are advertised at o-10000 rpm at no load. Maybe I buy one of those and buy a voltage converter.If you don't know where you are going you will finish up somewhere else.
16th Mar 2008, 06:19 PM #11
That one in the photo will just have a Universal (as in brushes, like a drill motor as Gerhard is talking about in his last paragraph) as opposed to a brushless Induction motor. Will therefore be noisier, and probably wouldn't last as long as conventional Bench Grinder either.
Cracker of a reply Gerhard!
16th Mar 2008, 06:33 PM #12
Inkspot, why do you want a variable speed grinder?Regards, Bob Thomas
16th Mar 2008, 07:01 PM #13
To grind some metal frames and to set one side up as a coil winder since I am not able to get a small table top coil winder anywhere.If you don't know where you are going you will finish up somewhere else.
16th Mar 2008, 07:27 PM #14
16th Mar 2008, 09:31 PM #15
Thanks Bob. I bought that book already, received it a week ago, and don't particularly like the design but after all the searching and opinions received it looks like the best option to build this.If you don't know where you are going you will finish up somewhere else.
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