phase convertor

[Andrew Wildman]

Dave, if you don't mind me asking, how many quid was the transformer?

[DavidA]

Andrew,

No problem.  It was £225 + £15 carriage.

Quite expensive for a transformer, But now I have it it should be very versatile.

I did notice something about it that I didn't expect. But I'll come back on that when I have done a few measurements.

Dave.

[DavidA]

Thomas,

Andrew wrote,

.. Also will require 500v caps which you may find hgarder to get hold of...

This is true.  But you can always put two or more capacitors in series and this will increase the working voltage. But it will reduce the capacitance.

So two 250 volt 10 MicroFarad capacitors in series will give you a working voltage of 500 Volt. But an overall capacitance of only 5 Microfarad.

It means more capacitors are needed, but it does work.

Dave.

[John Rudd]

And.....dont forget to fit bleeder resistors across them all.....

Id like to think I can sell you something else of use to you.......

More seriously, caps exploding isnt funny, Ive had it happen to me, apart from the mess ( of the cap that is.... ) my nerves arent up to it anymore..... ( runs and hides ....)

Id sure like to hear when its finished and how it performs....

What rating are you aiming for Dave?

[DavidA]

Joun,

Don't worry, I wont forget the resisters on my set up.

The transformer is a 4 KWatt model.

I'll probably aim at a rating of 1.5 KWatt (to match your motor if I use it as an idler), otherwise it will be rated to the largest motor I have. That would be my Denford lathe at 1.2 KWatt.

With the 240 to 415 Volt transformer I could just use the Steinmetz capacitor set up to derive the artificial phase and leave it at that. But using an idler (Ok, I know it really should be about 2.5 KWatt, but it is only on a low duty) I hope to get a better start up. The chuck is quite heavy.

Anyway we will see.

The motor on my Emco milling attachment is less than 1 KWatt. I have a big drilling machine but the motor on that is only 0,5 KWatt.

So it should all work.

I need to find my AVO and take measurements. Then do some calculating.

Maybe need to build a switched bank of capacitors.

Dave.

[DavidA]

A progress report; sort of.

I have been waiting for the arrival of a capacitance meter to find out what the value of the capacitors I have actually is as against what it says on the can.

The calculated value required is 4.16 MFD.

My nominal 7.5 MFD unit measured at 6.7 MFD.  Near enough for a test. I'm expecting a high voltage because of this.

So, being a believer in checking the system in it's constituent parts as it is being assembled, I decided to find out what Voltage I was getting form the new auto transformer.

A kettle lead was to hand so the kettle end was cut off and the wires connected to the transformer 220 Volt primary.  Switched on .

And the lights went out.

The RCD in the house had tripped.

Further examination revealed there was a leak between the live and the earth inside the molded plug on the lead.

A new plug was fitted.  Power on.

A gentle hum from the transformer.

Measured 220 Volt at the primary and 415 at the secondary terminal.

So far so good.

I then connected up capacitor and the three phase leads to the Milling attachment.

Now, I'm not sure if I should get the full artificial phase voltage when the motor isn't running. But I measure only 163 Volt across it on the input side of the stop/start switch.

Pressed the start button.  Nothing happened. No comforting 'clack' of the contactor slamming in.

I have to state that I have never seen this attachment running. So it is quite possible that there is a problem with it or the switch.

Called it off for the night. Further investigations tomorrow or Tuesday.
I think I will use the new motor I bought as I know it should work. Then come back to the mill.

Dave.

[John Rudd]

A progress report; sort of.

I think I will use the new motor I bought as I know it should work.
Dave.

Of course it will work, think positive...( no pun intended.... )

[DavidA]

John,

I have every confidence in the motor.

But there again, I had every confidence in the kettle lead.

Dave.

[vintageandclassicrepairs]

Hi,

Now, I'm not sure if I should get the full artificial phase voltage when the motor isn't running. But I measure only 163 Volt across it on the input side of the stop/start switch.

Pressed the start button.  Nothing happened. No comforting 'clack' of the contactor slamming in.

You need to use the "real" phases for the control circuit, ie. the output phases from the transformer if the controls are 415v

HTH
John

[DavidA]

John,

Thanks. I wondered about that but didn't want to get involved with it tonight. It does make sense.
I'll check tomorrow to see which phase is feeding the solenoid. It is a 415 Volt control.

Dave

[DMIOM]

A progress report; sort of.

I have been waiting for the arrival of a capacitance meter to find out what the value of the capacitors I have actually is as against what it says on the can.

The calculated value required is 4.16 MFD.

My nominal 7.5 MFD unit measured at 6.7 MFD.  Near enough for a test. I'm expecting a high voltage because of this.

...........

Not surprised if your caps are Mega Farads.... 

[Bluechip]

Not surprised if your caps are Mega Farads.... 

    

Dave

[DavidA]

DMIOM,

...Not surprised if your caps are Mega Farads....

If you go to the following site..

https://en.wikipedia.org/wiki/MFD

and look at the third item in the section on science, technology and medicine,  you will find that MFD is the old abbreviation for MicroFarad; which was the way I intended it to be understood. Old habits die hard.

I understand that humble pie is best served with a chilli sauce.

I didn't use the Mu character as I don't know how to get it.

Anyway, onwards.

It dawned on me last night that the best way to go would be to first connect the motor directly to the three phase supply, by-passing the contacter.

Upon doing this the milling attachment whirred into life. A success.

The artificial phase measured 459 Volt. I did predict it would be high.

So, just the contacter to sort out and a new capacitor to get (from RS) and then I can get on to trying the lathe electrics.

If I could only find my clamp meter I could also check a few currents.

Dave.

[Bluechip]

DavidA:

I have some Ducati motor start caps. marked eg. 12 MFD and they aren't all that old. So, the terminology is not quite obsolete .

I also had some ancient LEMCO & Dubilier silvered mica and waxed paper caps. marked uuF for picofarads and mmF for microfarads.

Slung 'em out some years ago together with anything with 'Hunts' on it ... 

Dave

[DavidA]

I posted on another thread that today I had a go at getting the Denford to run.

As I have mentioned, I bought this lathe from the company I worked for; but hadn't seen it run for a while.

I first got the EMCO milling attachment running using my capacitor type three phase system. So far so good.

When I connected the lathe to the same system it knocked out the RCD. We are now up to date.

Today I had a look inside the control panel. Lots of unexpected electrics.
It has a DC injection brake system fitted. And I don't know anything about them.

But I don't think I really need to use it. So I will by pass it ; at least for now.

I connected the three phase supply direct to the motor. (after removing the original connections) and tried it. A loud(ish) hum and signs of the motor trying to turn.
Turned it off again and reasoned that this thing hadn't been run for a while and maybe best to disconnect the variable speed belt and just run the motor on it's own; no load.

Try again.  Motor runs slowly with a distinct two - phase growl.

Not a problem as I know that the calculated capacitance for the 2.2 Amp motor is around 16 MFD; and I only had 7 MFD in circuit.

Time to order up those capacitors I referred to earlier.

Then also time to set up the idler motor. That should help it start easier.

Dave.

[DavidA]

Just adding a bit.

The capacitors arrived today (RS overnight post, free)

It was too cold to bother much in the shed today, but I had to just have one 'play'.

I fitted the 10 MFD capacitor.
The motor hesitated briefly then spun up to speed.

The voltages at the motor connection block were

U1 - V1  399 Volt
V1 - W1  445 Volt
W1 - U1  419 Volt

And the currents

U1  0.85 Amp
V1  1.35 Amp
W1 2.42 Amp

Lot's more test to be done yet; both on and off load.

By the way, its a Star connected motor.

Dave.

[DavidA]

Another update.

I now have the motor I acquired connected up as an idler and find it starts easily with 30 MFD attached.  This now feeds my lathe via an isolator switch.

As mentioned before, I did decide that the DC injection braking was only a complication, so spent some time in the shed bypassing all that and have finished up with just the basic control panel, I.E forward, backward and stop.
At the moment I am having a bit of a problem with the contactor controls. I have a fairly good idea of where the problem lay.
Basically the lathe won't start up when I press the go buttons. But if I push in either contactor manually then it runs, but stops as soon as I release. So my contactor solenoids are not powering up.  Tomorrow will be a 'having fun with my test meter' kind of day.

This raises a point for anyone else who may go the same route and use the book.

It gives you very good circuit diagrams , but nothing on connecting up the interlocking switches when you have two contactors. One for forward and one for reverse.  Your on your own with that one.

I suppose that if you were really desperate to get running you could just use one contactor (with overloads) and place a manual change over switch between the overloads and the motor to swap two phases. Thereby getting your reverse.

Dave.

[DavidA]

'It lives, Igor. It lives,'

I took the desperation route and set up the machine using just one contactor and no overloads.

But now I know it works (including the speed control) I can make everything neat and tidy. At the moment the pilot motor is held down to a table with G clamps and the lathe control panel is laying on a wooden chair with all it's innards exposed, Nothing has any covers on. (( need to get at the wiring to measure things). All rather dangerous  . Re connecting the other contactor will give me  a lathe that actually runs in reverse as well as forward.

I do believe my original problem was down to a disconnection in the contactor relay circuitry. Things quickly start to get complicated when the interlock switches are included.

But the pilot motor / capacitor idea works well. with the lathe running I am getting around 406 Volt average. A small tweak on the capacitance will probably improve that.

As the lathe is the biggest load I am likely to put on the system (and that draws less than 2 AMP), I'm quite satisfied. The only fly in the ointment is that as the transformer is fed from a 16 Amp trip in the house, sometimes the trip drops out on starting the pilot. No big deal as everything is fine once the pilot motor is running. Maybe an additional few MFDs just for starting will improve things.

I'll include some proper reading next time.

Dave

[Andrew Wildman]

Dave, i changed my MCB to a c curve type rather than the standard b type as I had a similar problem with the transformer inrush current.

[DavidA]

Andrew,

I'll probably do the same. It seems to happen only when it is cold.


An update.


I burnt a lot of midnight oil and managed to get the control panel wired up properly.  So it now works as it should. Backward, forward and stop. Only the sud pump to sort out.

So, having got everything running I decided to do a quick test on the threading capabilities.

12 TPI would do.

Chucked up a bit of 1" steel bar. faced it off and took a light skim off the length.  This is the first time the lathe has cut metal for three years.
Next, set up the gearbox.  The 100/127 wheel set and it's banjo are still in a drawer, so the machine is still set up for Imperial.
I followed the instructions. Got to be careful with the Synchro as some operations are done with the machine running, and some are not.

So, spindle, is turning, leadscrew is turning. Close the half-nuts.

Nothing much seems to be happening. But wait, the saddle is moving toward the headstock, but very slowly.

Re check the gearbox settings. all ok.

A look at the reduction gears showed that someone has got then on all wrong. Instead of being a 1:1 gear train fron the spindle to the gear box it is reducing the ratio to 10:1 so I would be cutting something like 120 TPI.
And that is what it looks like.

Ho Hum. Never a dull moment.

Dave.

[DavidA]

Back to the phase convertor theme; well, nearly.

In the book on single to three phase conversion the author also briefly mentions using a three phase squirrel cage motor as a generator. His example has the motor driven by another motor, but a single phase one.

As there seems to be no electrical connection between the two, I assume that the three phase motor (with it's added capacitors) could be driven by, say, a small Diesel engine: or even a steam engine.
You would, of course,  have to set up governor to keep the engine a the required speed as the load changed.

But it has the advantage of having true 120 degree phase shift between all phases.  And with a bit of additional plumbing it could keep your shed warm.

Has anyone tried this ?

Dave.

[awemawson]

Three phase electric motors are frequently used in small hydro-electric systems in the third world. I seem to remember that the only modification is capacitors across the three windings

http://www.redrok.com/cimtext.pdf

[DavidA]

Andrew,

Thanks for the links.

Yet something else to add to the 'future project' list.

Dave.