DC? oops, I must 'ave been thinking of real motors. Didn't DC die with old Tom Edison, or was that the incandescent blow torch? I can't remember, too far back.
Maybe old 'mega will put us out of our misery and spill the beans.
I still hope he's gone to 3ph, I'd hate to have to look for DC motors for a project.
Which brings me to a story that a little bit of knowledge is a dangerous thing.
Some years ago, about '97 last century, I bought a Bridgeport mill from the firm I worked for when they downsized the toolroom.
The first problem was it was 3 phase and I only had single phase.
So it was, do I get a 2HP single phase motor and attempt to fit it, or investigate the phase change method I'd read and heard about?
The 2HP was out, due to the arm and a leg syndrome.
Having read about using capacitors and an inductor, whatever that was, to effect a miraculous appearance of two more phases I decided to give it a go.
Off came the motor and it was quite plain that there were more wires than you could shake a stick at.
In the box at the top on the motor casing, there is a reversing switch, and this had the three supply wires going into it with three more wires coming out going into the motor.
There are also three more wires held together in a skruit connector.
Having learned to be methodical years ago, I made a sketch of the various connections and then pulled everything apart.
It soon became apparent that the three wires tied together and the three wires going into the motor were definately related, a meter across the wires gave me three pairs that had about 8 ohms between each.
I deduced that the three pairs tied together were the star point and the other three were the other ends of the stator coils.
Now according to someone or other you connected the single phase mains supply to two of the wires and attached a capacitor of about 100 Uf 400 volts across the last wire and the live input wire.
You also had an inductor in the circuit to smooth things down a bit and add a bit of balance to the phases.
I thought I'd better try this out on another motor I had spare to see what would happen.
Just to be on the safe side I added a 3 amp fuse in the input line.
Beauty, it started and ran like a dog on heat.
So now we're in business.
All was assembled and neatly put together with proper crimped connections and spade terminals.
It was noticed that after running for about ten minutes the motor got a bit warm, not actually smell the insulation cooking warm, but sausage on the barbie warm.
I shut down and waited for 1/2 an hour. The same thing happened.
Maybe a bit more capacitance to ush things along a bit, so another 20 Uf was added. It helped, for about 15 mins.
I ran it like this for almost 8 years, doing short runs and shutting down as soon as poss to keep the motor cool.
I even added a small fan to the top of the casing to force draught the motor, and this prolonged the melt down time to twenty minutes continuous running, which was OK, as I rarerly ran it for long periods.
Now it can be told, having come across this forum and the discusion on phase converters, it was apparent that I should have used another 3 PH motor to generat the extra two phases etc etc.
Someone up there loves me.
I've just obtained a 3HP motor for $50 on Ebay,and an electrical box, $10, to put the capacitors and related switch gear in, and a pair of 10amp 3PH switched sockets for $20, so now it'll be a proper set-up.
I think you can call that a learning curve.
In the meantime the Bridgeport has found a new home at my mate's factory. He always drooled at the thought of having a Bridgeport so when I bid on and got another turret mill, an Ajax, on Ebay, I let him have it.
The Ajax is a british made mill, almost identical to the 'port, a bit longer table at 56", and having a built in feed motor and fast traverse for all directions, with built in coolant pump.
The spindle taper is INT 40, so I let all the R8 tooling, from the Bridgeport, go to my mate.
Now I'm back on Ebay to get an INT 40 milling chuck with ER 32 collets.
I don't want one of the collet sets, that my local tool dealer advised me to get, only the metric sizes of 6, 8, 10, 12, 16, 20 and two other metric sizes of 7mm and 13 mm to cover the imperial range.
The ER series of collets will collapse down 1mm so the range shown will cover all the milling cutters from 1/4" shank up to 3/4" shank and the equivalent metric sizes up to 20mm.
The Bridgeport weighs about .8 tons, and the Ajax weighs about 1.3 tons, with most of the weight going into the knee casting that houses the feed motor and gearbox.
Ian.  |