grinder speed change

[formerfiddler]

Hi all,

has anyone changed the speed of their grinder down to 1425 rpm? Specifically an Optimum grinder (= Axminster) with the 200mm x 32mm wheels... It would seem to me that you could just replace the circuit board inside...

Ciao,

formerfiddler

 

[TEP]

Hi fiddler, I'm no 'spark' but I believe that electric motors are wound for what ever speed they are designed to run at. So I don't think that just changing any board inside would make a lot of difference to the speed.

Although I am probably wrong about this, but hopefully a electrically minded member will be along soon to put us right :roll:

PS - Welcome to the mad house 8)

 

[Taffy Turner]

Hi Fiddler,

Tam is right. The vast majority of grinders will be fitted with a synchronous motor. There is no easy (or cheap) way to vary the speed of one of these, as the rotational speed is fixed dependent on the windings of the motor and the frequency of the AC supply.

Whilst I am not familiar with your grinder, I would be very surprised if there was a circuit board of any kind.

Hopefully Tony will be along soon - he is the real expert around here on electric motors.

Regards

Gary

 

[CHJ]

For Standard Synchronous Induction Motors on a 50 hertz system, the formula is:
50 x 60 x 2 = 6000 no-load RPM divided by the number of poles.

A four-pole motor operating on the bench under no-load conditions runs at 1,500 RPM (6,000 divided by four poles).

But when an AC motor is loaded, the spinning magnetic field in the stator does not change speed. Instead, the rotor is held back by the load from “catching up” to the field speed.
The difference between the field speed of 1,500 RPM and the rotor speed of approximately 1,425 RPM is called the “slip.” The slip varies with the load over a narrow operating range for each motor design.

A two pole motor operates at 3,000 RPM (6,000 divided by two) unloaded and approximately 2,850 RPM under load.

There are specialist six or more pole motors, and variants with speed/pole switching capability but these would not normally be used for something as basic as a grinder.

 

[Paul.J]

CHJ wrote
For Standard Synchronous Induction Motors on a 50 hertz system, the formula is:
50 x 60 x 2 = 6000 no-load RPM divided by the number of poles.
A four-pole motor operating on the bench under no-load conditions runs at 1,500 RPM (6,000 divided by four poles).

But when an AC motor is loaded, the spinning magnetic field in the stator does not change speed. Instead, the rotor is held back by the load from “catching up” to the field speed.
The difference between the field speed of 1,500 RPM and the rotor speed of approximately 1,425 RPM is called the “slip.” The slip varies with the load over a narrow operating range for each motor design.

A two pole motor operates at 3,000 RPM (6,000 divided by two) unloaded and approximately 2,850 RPM under load.

There are specialist six or more pole motors, and variants with speed/pole switching capability but these would not normally be used for something as basic as a grinder.

Couldn't have put it better meself

Paul.J.

 

[formerfiddler]

Wow, thanks CHJ! Even I'm an electrical noob, I do get the jist of your explanation.

You're right, there is no circuit board inside . just a black cylinder and the switch... .

I have an external speed reducer with two settings on it but I don't understand how it is supposed to work - on some motors (angle grinder) it reduces the speed but will not take any load. I haven't tried it on my stationary grinder yet. Will do tomorrow.

The grinder works well but I have to be very careful with the point of the skew chisels...

I changed the direction of rotation from down to up by turning the machine around and mounting the switch, cable and guards on the opposite side. An easy fix and I get a better edge that way...

Thanks for your input!

Ciao,

formerfiddler

 

[CHJ]

..... just a black cylinder and the switch... .

The 'cylinder' is most likely a capacitor to aid start-up.

..... I have an external speed reducer with two settings on it but I don't understand how it is supposed to work - on some motors (angle grinder) it reduces the speed but will not take any load. I haven't tried it on my stationary grinder yet. Will do tomorrow.

Don't try your speed reducer, it will not work, or if it does it will be for the wrong reasons and may be heading for smoke, your angle grinder is a different type of motor, basically an AC/DC (commutator & brushes) and these work on a different principle.

The fact that your reducer does not support the load is probably because it has very basic circuitry and is only supplying part of the wave form to your angle grinder and not compensating for the feedback from the motor when it slows that more expensive controllers do.

As a matter of Caution, I do not advise the change in rotation direction that you have done with the bench grinder, they are designed the way that are with the cutting forces driven into the tool rest for very good safety reasons, a tool grabbed by or caught by the wheel in a moments inattention can have very serious result such as the shattering of the wheel as it jambs in the spark guard, possibly with fingers still in close pursuit.

 

[formerfiddler]

Good morning!

CHJ said

As a matter of Caution, I do not advise the change in rotation direction that you have done with the bench grinder, they are designed the way that are with the cutting forces driven into the tool rest for very good safety reasons, a tool grabbed by or caught by the wheel in a moments inattention can have very serious result such as the shattering of the wheel as it jambs in the spark guard, possibly with fingers still in close pursuit.

I do have a grinder with a 120mm wheel made by Elu (the one with the leather strop / belt sander) rotating up which I consider inherently less dangerous than rotating down.

Consider that with downward rotation (as in the lathe itself) the wheel is always biting into the tool at an acute angle! With upward rotation the tool cannot "grab".

I did replace the original tool rest on the Elu with the Veritas unit but I considered the wheel diameter too small for sharpening turning tools which cut bevel down as opposed to wood chisels...

I can see that removing the tool rest on the Optimum / Axminster grinder to accommodate the Woodcut jig could be potentially dangerous but this grinder is only used with the jig. I always stand off to one side because of flying grit etc.

That being said, a woodworker's workshop is a dangerous place and all of the tools can cause serious injury or death (as it says in the disclaimers). I tremble with fear when I enter the shop and breathe a sigh of relief when I leave with all my fingers etc. intact. As my pseudonym indicates, I have played the violin professionally for untill retirement.

 

[TEP]

Mornin' fiddler, I have a grinder turned on its base so effectively it revolves 'up', but I HAVE NEVER used it for turning tools. It is only used for sharpening carving chisels. Only because it has a rubber/carborundum impregnated wheel on one side, and a leather hone on t'other. As you can imaging they would not work very well if the wheels were turning conventionally.

In my experience there is no need to sharpen turning tools they way you are suggesting. Also if you find the tool tip digging in when sharpening with the normal rotation I suggest you may be pressing too hard onto the stone/belt.

 

[tnimble]

The speed setting can be changed without rewinding the coils for a different number of poles.

A AC/AC convertor is used. It is a deivce than converts the 50Hz frequency to for instance in this case a 25hZ alternating current. These convertors are used alot in industry to control the speed of large motors. They are however not cheap and most models are only for 3 phase motors.

A low speed grinder would be way less expensive than an AC/AC frequency convertor unit. The above unit costs aprox £ 300 for the left unit and £ 400 for the one on the right.

 

[CHJ]

Agreed but slightly misleading quoted against the original topic talking about a basic single phase motor, not one designed to run at lower speeds for noise control or power train reasons.

I use a similar electronic frequency convertor on my lathe (as do many others) to control it's speed between 90-3500rpm but it is feeding a 3phase motor.

A normal 50hz (domestic/workshop) single phase motor run at 25hz would have very little torque available and would probably run quite warm into the bargain due to lack of iron content. (the higher the frequency the less iron needed, hence why 400-1200-1600hz are used on aircraft)

Even running a 60hz (american) single phase motor on 50hz can be problematical as to whether you will get away with it dependant on loading.

 

[tnimble]

Indeed its far from ideal running a cheap lightweight motor that way. However the above unit is for 1 phase motors.

 

[Taffy Turner]

The above unit costs aprox £ 300 for the left unit and £ 400 for the one on the right.

Hence my comment that there is no easy (or cheap) way to vary the speed.

It would be cheaper to buy a Creusen low speed grinder than one of those, and would be a much better solution for the reasons that Chas has laid out.

Axminster sell the Creusen - see here: -

http://www.axminster.co.uk/product-Creusen-Creusen-HP7500TS-Powerline-Grinder-(Slow-Speed)-22331.htm

Gary

Edit - Strange - why isn't that URL coming up as a clickable link - it normally works OK for me???

 

[CHJ]

Edit - Strange - why isn't that URL coming up as a clickable link - it normally works OK for me???

Gary, I think it is because the PHP software does not like some of the symbols used in the link, PHP is not fully html complient.

http://www.axminster.co.uk/product-Creu ... Slow-Speed)-22331.htm

 

[Taffy Turner]

Thanks Chas,

Interestingly although your link is now showing as clickable, it doesn't work! :? It just takes me to an error page?

Gary

 

[CHJ]

Thanks Chas,

Interestingly although your link is now showing as clickable, it doesn't work! :? It just takes me to an error page?

Gary

Yes that' because the (speed**** bit is missing off the end of the link.

If you type in (or copy paste) the last section onto the end of the explorer link it will work.

PHP does not recognise the brackets so ignores them.