# variable speed control induction motor



## prezio (9 Dec 2015)

Hi
I have bench grinder Sealey BG200XL - it has induction motor 560W
I would like to use it also for another jobs and I need to reduce speed.
1 way is put small pulley on the shaft on grinder with reduction on another shaft with 2-5step pulley.
and another is - get variable speed controll to install on it.

Is anybody from you who knows./ has experience with any opf these ways ?
The second one is more interesting for e but the problem is that the motor is induction and 560W. I found speed control for induction motor but only to 300W max

Is anybody from you who can help me - please?


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## Random Orbital Bob (9 Dec 2015)

send a pm to myfordman Prezio. He's a whizz with all things electronic.


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## prezio (9 Dec 2015)

I've sent - will see. thx


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## CHJ (9 Dec 2015)

It is not possible to alter the speed of *a standard single phase induction *motor and maintain it's power output.

An induction motor speed is controlled by the amount of magnetic poles within its design and the A/C supply frequency.

The motor speed will slow off load if you supply it with a single phase supply of lower frequency but the motor will very early in the speed reduction fail to perform and start to overheat, on load this will happen more rapidly.

The nature of the construction relies on the several things to keep it running at designed speed and power consumption.
1. The motor magnetic poles have to maintain synchronisation and partially rely on inertia to keep this.
2. The motor running at correct speed generates an internal power which offsets the supplied power and limits drawn current, even at correct frequency there will be some slip as will be seen by the difference in actual speed under load and theoretical speed.
3. Slowing the motor down means the poles don't have enough magnetic strength and inertia to maintain synchronisation and greater slip occurs and mechanical power output drops, at the same time the internally generated power reduces and the supply current rapidly increases because it has no internally generated power to oppose it**.
4. This high increase in supply current draw will generate excessive heat in the motor windings.

** The reason starting a motor takes several times its running current when first switched on.

Older design small universal A.C./D.C. (commutated) motors can be controlled by reducing the power input within limits but mechanical power output will vary dependant upon how sophisticated the controller is and self cooling may be a problem if run continuously. (why bigger electric drills have more than on mechanical speed selection to maintain torque)


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## prezio (9 Dec 2015)

Follow this information I have only one choice to use pulleys with reduction size 3:1 or 4:1 ;(
Is anybody has exeperience with pulleys reduction on bench grinder?
Any problems with that ? if I use it for for e.g. disc sander or belt sander even drum sander


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## Myfordman (9 Dec 2015)

Prezio, I'll reply to your PM here if that is OK.

Chas is absolutely correct; single phase motors can only be run at or very near the design frequency

Nominal speed = (supply frequency (Hz) x 60)/(number of poles/2)

The speed controller you have found is either designed for universal(brush type) motors or might be a bodge giving small speed variations with no consideration of the motor or the resultant power output.

If you would like to post a link to the controller you have found I'd be intrigued to see it.

Regards

Bob


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## prezio (9 Dec 2015)

I can but it is in Polish language
https://sklep.avt.pl/avt1613.html
and
https://sklep.avt.pl/avt5360.html


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## Wildman (9 Dec 2015)

so what use is a controller like the one below please
http://www.ebay.co.uk/itm/2000W-50-220V ... SwxYxUz0vW

I bought something similar advertised as a motor speed control not had the nerve to try it out yet. Was going to use it to reduce the speed of a small clarke 3 wheel bandsaw for cutting mild steel.


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## Monkey Mark (9 Dec 2015)

Wildman":6vbhjz76 said:


> so what use is a controller like the one below please
> http://www.ebay.co.uk/itm/2000W-50-220V ... SwxYxUz0vW
> 
> I bought something similar advertised as a motor speed control not had the nerve to try it out yet. Was going to use it to reduce the speed of a small clarke 3 wheel bandsaw for cutting mild steel.


As mentioned above, those speed controllers are designed for Brushed Universal motors, not for use with induction motors.
If your bandsaw has a brushed motor, it will most likely be fine (dependent on specifications).


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## CHJ (9 Dec 2015)

prezio":28wtbzkx said:


> I can but it is in Polish language
> https://sklep.avt.pl/avt1613.html
> and
> https://sklep.avt.pl/avt5360.html


Those controllers are supplying a digitally switched (possibly just square wave) variable voltage and or frequency output that will only work with light duty fans etc. (says not suitable for universal motors so does not have feedback circuits)

If you connected it to a normal incandescent light bulb you would probably see the bulb flickering as it is dimmed.

The mechanical power output of the motor is not so critical as the fact that if falls off just means the rotor slips more and less air moved.

The controller is more than likely (hopefully is) controlling the total current output as well to stop the motor winding overheating.


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## prezio (9 Dec 2015)

OK - thanks
I will use pulley reduction - I need more space for pulleys and belt ;( but If I don't have what I like , I like what I can to do


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## SlowSteve (10 Dec 2015)

Hi,

What you see above isn't STRICTLY correct - there *IS* a way of doing speed control on an induction motor and retaining it's torque and power consumption constraints - it's just not especially practical for a lot of people at the moment - although will be in a year or so.

For a lot of AC motors, you would use a could of transitor-like devices called TRIACS which makes for a small neat circuit you can build youself for a few pounds or buy a controller "off the shelf" from Maplin for perhaps £20- but this doesn't work for Induction motors - for all the reasons MyfordMan listed. For DC motors there is a different way called PWM where, very roughly, you ping big transistor with different pulses of current. To do PWM it's a small circuit controller chip which costs about 90p and a handful of lines of code - again very DIY-able and you can buy "off the shelf" controller from Maplin for about £10 - but that doesn't work for AC motors.

However - there IS a new way of doing speed control for Induction motors called Space Vector PWM - it's essentially an exceptionally messy mix of the two methods above- the problem is that right at the moment there is no off-the-shelf chip you can buy for a few dollars to do it - and so no off-the-shelf plug in controller - so you need a £50 microcontroller and to write a good few hundred lines of fairly tricky code. So, if you know your way around an Parralax board can make your own libraries, then it's do-able. If you're not in that group - which most people aren't - then there are a couple of companies building dedicated chips to do this right now which are getting released in 2016 - so by 2017 you should be able to get a cheap off-the-shelf controller to do this for you.

I know it's not a huge help for your project right now - I just wanted to flag up to everybody that it IS being worked on, and within a year or so, it will be easy to buy something to do this job.


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## prezio (10 Dec 2015)

Thx I will be looking on it in the future
For now 50 quid is to much - in 80 I can buy electrical motor (brushed) and 5 quid controller for it
and/or - use pulleys on the grinder to reduce speed like in lathes


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