Thread: variable speed conversions

That motor looks like a 2HP 3 phase motor with a couple of big capacitors on it so it will run on single phase and make 1HP.

I reckon it would be possible to ditch the capacitors, wire it in delta setup and use a VSD.

But I would rather buy a used "Brand name" 3Phase motor than that one.

Thanks for the reply Bob.

Its a single phase motor.

Am off to google, looking for the mugs guide to "delta setup" and "vsd" to continue my education.


Bill

So ............. you need to have a 3 phase motor, you hook it up to a magic black box called a vsd (variable speed drive ?), which in turn is hooked up to the shed's single phase supply.

The VSD takes the single phase input, does some jiggery-pokery with it and feeds it out as 3 phase.

The VSD is able to vary the frequency on the 3 phase supply which then varies the speed of the 3 phase motor in question.

Whilst dropping the voltage to a single phase motor might slow it down it would also do various damage to the motor so varying the voltage to a single phase motor is not an option.

So for my little tiny home made lathe (see attached photo) I need to buy a 3 phase motor and a magic black box, this will be expensive, so I might as well forget about it and just pay for a factory built lathe with variable speed control.

Is that a reasonable summary ?


Bill

Originally Posted by steamingbill

Thanks for the reply Bob.

Its a single phase motor.

I serious doubt it is a single phase motor . I have dozens of 3 Phase motors with those connectors and not one single phase motor with those connectors.

Originally Posted by steamingbill

So ............. you need to have a 3 phase motor, you hook it up to a magic black box called a vsd (variable speed drive ?), which in turn is hooked up to the shed's single phase supply. The VSD takes the single phase input, does some jiggery-pokery with it and feeds it out as 3 phase.

That is basically it.

The VSD is able to vary the frequency on the 3 phase supply which then varies the speed of the 3 phase motor in question.

Yep

Whilst dropping the voltage to a single phase motor might slow it down it would also do various damage to the motor so varying the voltage to a single phase motor is not an option.

There are black boxes especially made to change the speed of single phase motors but they are not cheap and very hard to find. The only one I have seen recently was a kit you had to build and that limited the size of motor that could use it. Nearly all so called single phase variable speed motors are 3 phase with a built in minimal VSD.

So for my little tiny home made lathe (see attached photo) I need to buy a 3 phase motor and a magic black box, this will be expensive, so I might as well forget about it and just pay for a factory built lathe with variable speed control. Is that a reasonable summary ?

No.

A used 3 phase motor for that lathe should only cost ~$35. But ideally you need to find one
A VSD will cost $125 delivered.
You may also need a couple of extra switches and a 10k potentiometer for about $15.
All up $175.

However there are 3 gotchas
A) VERY IMPORTANT - if you are not up for wiring AC I do not recommend doing it your self - then the cost of a sparky has to be added to the cost
B) The motor should be a 240V 3 phase that can be wired for Delta - not all 3 Phase motors can do this or do it easily.
c) You might find the motor but the shaft/pulley it comes with is not compatible with your other pulley. This can be a right PITA - you might have to buy new pulleys or get a bush made up. Unless you are metal work savy or know someone who is it can set you back a few pennies.

Thanks Bob,

Have got a better understanding of the important bits now.

Bill

Originally Posted by BobL

I serious doubt it is a single phase motor . I have dozens of 3 Phase motors with those connectors and not one single phase motor with those connectors.

Bob,

Am intrigued.

What is it about the connectors that screams 3 Phase ?

Is there any sort of hope yet that this is really a 3 phase in disguise and that I am already half way there in my quest for a variable speed drive ? The seller definitely describes it as single phase.

Bill

Originally Posted by steamingbill

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What is it about the connectors that screams 3 Phase ?
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I'm not 100% certain but that motor has 3 pairs of contacts - most three phase motors that can be configured between delta and star have that arrangement.
A single phase motor usually has two contacts - Active and Neutral.
But the 2 x 3 contact block could have something do do with the fact that it is a reversible single phase motor.

There are some guys on the metalwork forum that are more knowledgable than me - maybe post something there about it.

Hello,

If anyone has stumbled onto this thread and, like me, didnt know what star and delta meant .............

I found these pages useful

Should a 3 phase motor run on star or delta

and

The difference between star and delta wiring systems? - Yahoo! UK & Ireland Answers

and

How to check the Windings of a 3-Phase AC motor with an Ohmmeter

I found them by googling "3 phase star delta" lots of other pages out there if you do the search.

Bill

Bill if you are after variable speed for your lathe why don't you buy a couple of step-down pulleys for both the motor and the main shaft of your lathe unless you are after far more speeds than that setup would give you ?

Stewie

Originally Posted by Stewie D

Bill if you are after variable speed for your lathe why don't you buy a couple of step-down pulleys for both the motor and the main shaft of your lathe unless you are after far more speeds than that setup would give you ?

Stewie

Stewie,

Yes thats an option - does cost a few dollars by the time I've bought the pulleys and had them drilled out to fit my shafts.

Have been checking out local scrap merchants for pulleys as well.

Being fond of gadgets I was interested in the high tech solution and what the difference in costs might be.

Bill

Originally Posted by steamingbill

Stewie,

Yes thats an option - does cost a few dollars by the time I've bought the pulleys and had them drilled out to fit my shafts.

Have been checking out local scrap merchants for pulleys as well.

Being fond of gadgets I was interested in the high tech solution and what the difference in costs might be.

Bill

We have a great selection of variable speed drives to have a look at online

Single and three phase motors and variable speeds , An old Singer sewing machine has a variable speed control unit, it is single phase,is it because it has brushes? My reason for asking is I have just brought an old lathe(wood) with the pully system on it and intended to buy a variable speed control unit to use on it. can't be done? help CD.

Hi CD.
Mains motors fall into 2 broad categories.

Universal (brushed) motors as in portable drills, circ saws, routers, vacuum cleaners, etc and sewing machines. The speed is dependant on the effective applied voltage and independant of supply frequency. Speed can be varied by "chopping" the mains supply so a user determined portion is supplied, or by reducing the voltage with a variable transformer or high power variable resistance system. Old style sewing machine motors with foot or knee speed control had a chain of carbon buttons wired in series with the motor. The foot pedal or knee control compresses the buttons reducing the resistance of the chain, increasing the applied voltage and motor speed. In the fully released position, the buttons seperate, breaking the circuit and switching the motor off.

Induction motors, as found in many lathes, mills, drill presses and general industrial machinery. These are synchronous motors (speed is dependant on input frequency) and mostly independant of voltage. These are your 3 phase motors and the bulk of the 1HP+ single phase motors,the exception generally being high speed motors. 50Hz supply motors needing to operate above 3000RPM will be some form of universal motor as it is impossible to get a 50Hz supplied induction motor to run above 3000RPM, but 1500 and 1000 RPM variants are also possible by adding more magnetic poles in the windings. The motors will run slightly under these speeds,or slip. The degree of slip will vary with the load (torque) required of the motor. Typically motors operate at rated power with about 4-5% slip giving nominal speeds of around 2850, 1440, or 960 RPM.

Three phase motors will determine the direction of rotation from the sequence of the three phases in the supply and will self start. This makes it possible to employ a variable frequency drive which takes a single or 3 phase supply, converts it to a DC voltage, then converts it to a variable frequency three phase at the voltage similar to the supply voltage. With a well matched combination of drive and motor, it's practical to create a variable speed drive with a wide speed range extending to at least 22,000RPM, as used in CNC routing spindles.

A basic single phase induction motor cannot establish it's operating direction naturally and will rock a few degrees either way and hum on application of voltage but not rotate. To make them usefull, a second 'start' winding is added, usually with a speed sensitive switch a starting capacitor. This produces a temporary second pole in the motor giving it a kick start in the required direction and supplying additional starting torque. Once the motor gets to about 80% of rated speed, the speed sensitive switch switches the start winding and capacitor out of circuit and the motor continues to run in the same direction and gets to its nominal speed. If the motor is slowed below the switching speed, the switch will re-energise the starting winding and capacitor. However the start winding is designed for very short term use, and will overheat and burn out within seconds if kept energised. This is what limits the application of variable speed drive electronics to single phase induction motors, once the motor slows, the speed sensitive switch re-engages the start windings and burns them.

Your lathe upgrade is possible under certain conditions. You need to use a 3 phase motor, and unless you have 3 phase power available, the motor, needs to be capable of operating on 240V, needs to be modified to make it work on 240V (accessing internal winding connections, not always easy/possible), or you need to import a specific VFD unit from UK that takes 240V single phase input and outputs 415V three phase variable frequency. If you have a three phase supply available, then any 3 phase 415V motor and compatible 415V VFD will do the job.

Hope this helps helps you understand the factors involved.