Electical Eng Question!

Black Bug

Black Bug

Well-Known Forum User
Hey folks,

Well here's a seriously off topic question for you!

As part of my plans for world domination I've recently aquired a lathe/mill combo, linky, which is all very nice and maybe one day I'll be able to make something on it in one go without copious amounts of cursing - yeah right!

Anyway the thing is powered by a single phase 550kw/0.75hp motor. Downside is that this drives the lathe spindle via belt and pulleys. So to change the spindle speed you have to arse around swapping the belts to different pulleys, which means unclamping the motor and secondary pulleys to allow for a bit of give etc. All in all a pain in the behind.

So I was thinking, would it be possible to make a little black box with which I could control either the voltage or current supplying the motor and therefore vary the actual speed of the motor to give me much more flexibility and ease of use. Basically a dimmer switch.

Is this possible? Conceivable? Safe?!! Would it be best to control the voltage supply or the current supply? If I've got my head around this right then putting a potentiometer in series with the motor would control the voltage, putting it in parallel would control the current, right?

Does such a black box already exist that I can just buy off the shelf, and if so what's it called?

Thanks for any thoughts!

Cheers,
Rob
 
Bug,

Confused here

Are you sure that it is 550KW and not 550W --- 550Kw is equal to 737 Hp :eek: which would be a motor the size of a small car (the motors we used underground to mine coal were only 330Kw)


The main route today for varying an AC motors speed is via variable frequency (VFD) and pulse width modulation (PWM).

Frequency drives are cool and very accurate (motor speed is a product of frequency and number of poles or winding nodes) vary the speed of the rotating magnetic field, factor in the slip and hey presto new motor speed
If I've got my head around this right then putting a potentiometer in series with the motor would control the voltage, putting it in parallel would control the current, right?
Click to expand...
Don’t even think about putting a pot or resistor in series or parallel that only works on low voltage/current dc stuff.

There are books and books available on AC and DC motor control theory, if you want I can lend you a couple for good bedtime reading.

 
Thanks Steve, or should that be curses, foiled again?!! Variable frequency makes sense now I think of it, bit of the old Faraday stuff eh? Takes me back to my days messing around with guitar pick ups as well!

Sounds a bit too tricky to knock up in the workshop though, are such things available off the shelf? Can't find anything under 'variable frequency drive' on google other than this suspect book, http://www.pearsoned.co.uk/Bookshop/detail.asp?item=100000000056992
I don't think I could cope with reading that at any time of the day ;)

Cheers,
Rob
 
I'm not much help but my friend has a lathe which had the same problem as you and he definately retrofitted a speed controller/direction selector on the front without too much drama. Just so you know that it's definately feasible.
 
Well I'm half way there Russ as the lathe comes with a built in reverse switch - otherwise the mill part would just turn the wrong way, nice design Warco! If you get a chance to ask your friend what they used for the speed control that would be great.

Haven't had any luck finding variable frequency drives in the UK but there seems to be a few US sites, this one in particular seems like it will fit the bill if all else fails:
http://www.driveswarehouse.com/product_info.php/products_capacities_id/5/products_id/107

cheers,
Rob
 
Loads of drives available in the UK, industry uses them all of the time as speed control is nothing new

You could use some of the aftermarket drill speed controllers that are available that fit directly in line and were available when drills just had a single speed (ahh the good old days) 750watt unit should do.
http://www.quasarelectronics.com/motor_controllers_drivers.htm

This kit is for a motor with brushes, you will need to check which type you have.
Triac circuits such as this are cheap but work well.

Remember safety first, fuse, breaker, earth, cable size ---- its all very important stuff to keep black bug alive
 
Thanks Steve, is there an easy way to determine if the motor is brushed or brushless. I had a look at it last night to see if there was any useful info on it - typically not.

All the variable frequency units (RS, Farnell and that US site) output to 3 phase voltage which is a bit of a drag. I even found a guy on Ebay who sells inverters and motors and his suggestion was to just buy one of his inverter and motor kits - joy!

Oh, what became of my simple dimmer switch idea?!! ;)

Cheers,
Rob
 
If the motor uses brushes then there may be an inspection plate, my guess is that it is of a brush design if correct the unit I posted -- (£20 all in) would suffice, but I must reiterate that triac type speed controllers are only good for brush motors.

Most household/domestic units are brush type

Washing machines

Tumble dryers

Electric drills

Vacuum cleaners

Its only when you get to a full blown induction motor that you need the more exotic controllers such as inverters (which essentially are pulse width modulation/frequency modifiers

 
D'oh!

Well I went looking for an inspection plate but found this instead. Looks like a life of pulleys and belts ahead of me or the semi ludicrous swapping in of a 3-phase motor running through an inverter from single phase.

Anymore ideas folks to save me from certain insanity?!!

Cheers,
Rob
 

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Mr Bug

Ok so its an induction motor which pushes you down the route of a more complex controller.

Leave it with me for a couple of days and I will have a ponder.
 
Thanks Steve, good to have your input on this. In the meantime I'll hook a push bike up to the spindle and get my girlfriend pedalling ;)

Cheers,
Rob
 
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