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Thread: Variable speed
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24th December 2010, 06:38 AM #1Novice
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Variable speed
Hi all,
I'm new and uninformed about bench grinder.
I have a abbot&Asbey 8' bench Grinder. I use this for knife making.
I'm wondering if anyone knows if there is such a thing as variable speed controler which will slowdown my bench grinder?
I rang JCars and they said they have one which normally is used for power tool (such as router etc...) but I'm not sure if this will work on my bench grinder.
Any suggestions are much appreciated.
Thanks
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24th December 2010, 10:28 AM #2GOLD MEMBER
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Hi arda
Welcome
I assume you are talking about a 240V single phase grinder?
Did jaycar give you a model number? (If it would run a router I would thinkk it would run a grinder.)
Stuart
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24th December 2010, 11:01 AM #3Mechanical Butcher
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Routers have universal motors with brushes. A controller for it won't suit your induction motor grinder. It'd need a variable frequency drive, which might cost more than the grinder did. Carba-tec sell an 8" grinder with a slower 1475rpm for $219. A more complicated solution would be to make a special spindle for your grinder, belt-driven to get just the speed you want. Maybe replace the wheel on one end of your grinder with a pulley, driven from another motor?
Jordan
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24th December 2010, 11:06 AM #4Distracted Member
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If you put a 6" wheel on an 8" grinder, would that give a suitable reduction in peripheral speed? Pretty cheap solution.
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24th December 2010, 11:23 AM #5GOLD MEMBER
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24th December 2010, 05:42 PM #6Product designer retired
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Arda,
Buying a VFD is probably out of the economical question, since you would be up for around $200-$300 new. Second hand units are about half that price, if you can find one.
You would need 240V 1-phase to 240V 1-phase at about 0.5A
I presume the grinder was around the $140 mark.
You might be able to place a 4A diode in series with one of the leads. This would effectively cut the frequency in half. What the consequences are, I don't know.
I've done this with great success on small universal ac/dc motors. ie, motors that have brushes.
If one of our electrical guru's could respond, and it's ok, might be another cheap option.
Ken
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24th December 2010, 06:48 PM #7Mechanical Butcher
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That'll likely get one of admin's disclaimer messages!
A diode will rectify the AC and make it half-wave DC, which sounds like it'd slow down a universal motor. It's an interesting idea that I haven't tried. I've no idea what it'd do to an induction motor, but would do that with caution. You'd need to have a high current type diode, and safely house it. I'd be afraid of cooking the induction motor.
Jordan
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24th December 2010, 07:32 PM #8Product designer retired
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Maybe what I posted, wasn't very responsible, or feasible.
Perhaps you could buy a transformer, say 240V to 110V, and switch to 110V after the grinder was up to speed.
I'm not a sparky's bootlace, just trying to help.
Ken
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24th December 2010, 11:08 PM #9Member
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Hi
dc into induction motor would work as a brake,
( DC brake )
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25th December 2010, 05:39 AM #10Novice
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thanks for all the replies guys.
it is bit scary when it comes to modifying the electronic part of it as I don't have any knowledge of electronic component at all.
I will give the spindle idea a try - sound like a much safer option for me . I saw a variable speed motor from Hare and Forbes for around $500 (bit to expensive).
cheers for all the suggestions. Marry Christmas to Everyone and Happy new year!!!!!!!!!!
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25th December 2010, 01:28 PM #11
The unit is fitted with an induction motor, as stated by others.
The output speed is near synchronous with the input power supply frequency, there is a small allowance for slip. For a 2 pole motor to be synchronous on 50HZ mains they would run at 3000RPM (50 cycles per second x 60 secnds per minute =3000 rev per minute).With slip allowance the motor would be rated at 2800 to 2900 RPM.
Adjusting the input voltage to the motor would cause it to draw excessive current and overload, but it should attempt to maintain speed.
As a single phase motor, the unit is at least capacitor start, and possibly capacitor start/capacitor run. Operating via a variable speed drive is not an option for these motors, as the the values of the capacitors required for starting/running the motor would change with changes in frequency. The motor core is also optimised to run at 50Hz supply frequency, and excursions of 10Hz either way (40 - 60Hz) would significantly effect the optimization and system efficiency.
Finally, installing a half wave rectifier in series would mean that the motor would effectively be fed approximately 170VDC plus about 100V peak to peak of ripple, primarily at twice mains frequency, but extending to cover a broad spectrum of harmonics. The DC component would provide no worthwhile energy for the motor as AC is required to induce current in the motor rotor. It would however bring about considerable disfavour with the supply authorities who have a hatred for DC in the supply system as it creates currents that destroy the protective earth systems than ensure our safety.
In short there is no simple approach to slowing this class of grinder. The options are really limited to buying a three phase unit and VFD (effective range 50 -150% of nominal motor speed), buying/building something with a step belt drive and spindle, or building something arround a DC or universal motor.For the last option, the Jaycar controller and 9 inch angle grinder might supply a starting point, provided that you can locate appropriate abrasives, or build up something with a belt drive and spindle to take standard abrasives.
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25th December 2010, 02:57 PM #12Mechanical Butcher
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Thanks for the clarification Malb. Now I know why VFD isn't practical for single phase motors.
Jordan
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26th December 2010, 07:37 AM #13
Actually this indicates the pathetic state of Retail these days.
The units that Jaycar have to sell are kitsets that you need to assemble ftom scratch, which are based on published electronics magazine construction articles. Every one of these published articles for speed controllers advises that the units are not suitable for induction motors. It would not be unreasonable for the counter jockeys to be aware of this and pass this basic information on to the customer, even if they could not identify the unit that the customer wanted to use the device with. At least the customer would
have the basic info to use for further research.
But then our electronics supply stores have become so focussed on selling Chinese crap battery toys that anyone who can be trained to work a cash register and credit card machine can work there. As far as product knowledge is concerned, that went out the window 15 years ago.
This is a rant against the moneygrabbers, not the person who asked a question based on their needs and information provided.
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