Hi all, one of the problems with single phase motors is the HP to physical size factor.
The duty cycle of the motor will give you the run time for it.
For instance, if you check on the electric specs plate on the motor and it states continuous run, you can be safe in thinking that it can be used at it's stated HP capacity to run a machine all day long, but if the plate states int. or intermittent run, then you have a whole different ball game.
Here we have a motor, that rated at 3/4 Hp, will for a short time, usually 1/4 hour or less, act like a 3/4 HP motor, but soon gets too hot to use.
You can pump in the amps to get the wattage out of a small motor package but in the end the current drawn has got to go somewhere and that is in the form of heat.
I use to see 1/3 HP motors from washing machines driving small lathes on the home workshop circuit.
They work fine up to a point, as most of the work is cut and try, with the lathe being switched off for longer than it runs as the operator battles with the mysteries of turning etc.
Which brings me to another point, not mentioned before by anyone, and that is the number of start cycles per hour for single phase motors, usually about 4 or 5, after that the start windings just add to the miseries as the whole motor gets hotter and hotter.
You just can't get the heat away quick enough, and as it follows an exponential curve, with every switch on cycle, you get that familiar smell of hot insulation.
I was told once that the "modern" single phase motors have insulation that is designed to run hot to improve the insulation properties, crap!
Incidently if you do come across a 1 ph motor that won't start but only gives a loud humming sound, then 10 to 1 it's the start windings or centrifugal switch that has gone west.
It's also vital to test for a short to the frame of the motor.
NEVER touch a motor frame when it's running, unless you want a nasty surprise with a shorted out winding.
Also NEVER run a motor without an earth connection.
Last but not least, before running a motor test it with a meter to see if you have a high resistance between the live input wire and the frame.
If the resistance is low then ten to one you will blow something at start up.
If you do get a start problem, it doesn't mean the motor is dead.
If you isolate the start windings by disconnecting them from the switch plate, then the motor can be used by wrapping a cord round the shaft and giving it a pull.
The old familiar phrase "Don't try this at home" would probably fit in here.
As soon as the cord is free the motor is switched on and if you get enough revs with the " cord starter" it will run, and quite happily too, in any direction, as it is the start windings that determines the direction the motor will run in.
Some 1 ph motors can't be reversed, due to inaccesibility of the wiring connections etc, but for the majority it's only necessary to swap over the start winding connections to effect reversal.
I once bought a practically new 3/4 hp motor with the centrifugal switch burnt to a crisp.
It was a 2,800 rpm job from a circular saw, and I paid about 1 pound back in 1975 from a scrap yard in UK.
I cleaned out the innards of the motor and reinsulated the wiring connections.
The centrifugal switch was on a Tufnol board that had burnt out and was totally unrepairable, so I just brought the two start winding wires out and connected them to a momentary contact switch that was used to get the motor going. One quick press for a split second and the motor was up and going.
Later I rewired the input connections and added a relay with a time delay, to effect the start up requirements.
By adding a relay to the start circuit in place of the start switch meant that if the motor was overloaded it could even stall, and as there was no centrifugal switch present to switch the start windings back in with speed drop, no catastrophic burn down can happen.
This also works to reclaim a motor if the start windings are actually burned out.
I use a 1 HP motor on my Colchester Bantam lathe, but to avoid switching the thing on and off all the time, I added an electric clutch from a car's airconditioner.
Now the motor runs continuously, with the electric clutch being switched on and off for as many times as you want, and never gets hot.
The bonus is that the clutch gives a softer start, as opposed to the jerk when a motor is switched on, even without a load.
The clutch is designed to be switched on and off continuously, and only consumes 2 amps at 12 volts DC, so a battery charger of 4 amps output will power it quite well.
It is activated with an ordinary car type toggle switch, mounted in a conveniant position on the headstock, rated at 12 volt 4 amps, and fused accordingly.
I'm adding an electric brake as well, so that when the clutch is deactivated the brake will come on for a second or two to stop the lathe quicker, very necessary when your'e using a turret lathe on short run cycles, and beats having a foot type mechanical switch.
I think I'd be right in saying that all of the Chinese and other foreign import lathes have motors that are connected directly to the drive train and so get switched on and off all the time as required.
They're designed to do this, but I dislike the thump in the drive train when the motor starts up.
An electric clutch and brake here would not add too much to the cost, but would extend the usefullness a hundredfold.
Ian.  |