Motor: Is it possible? Is it likely?

One way to test would be to connect two of those three windings to single phase power through a momentary switch. Then wrap a rope around the motor shaft and pull it to spin the motor up. as soon as the rope comes off the shaft hit the switch. If the motor continues to spin up and then runs with some vibration then it is probably a three phase motor. I have spun up three phase motors this way to get them running on single phase power. You need to get the shaft spinning at a pretty good clip but it does work. Eric

Bob, My first thought would be that the motor was converted by a motor shop and as such they would have properly put a name plate on it to give it correct specification.

As for testing and not knowing the original voltage I would first consider trying to read the ohms of each windings and compare to an existing 3 phase motor.

The other choice for this free motor would be take to a local motor shop (if it exist) and get them to convert it back.

Good luck and have fun with your new (among many) adventures.

Les

Thanks Les. I had not considered that the motor shop at DeWalt might have done that. It just threw me that it looks like a 3 phase motor, but was labeled single phase. I may just see if I can find a place to strap the motor down and see if she will rope start.

Not necessarily. It is quite possible to run a three phase motor off single phase... by using a capacitor. The fact that when it was given to me I was told it had been converted to single phase from 3 phase and that it has totally the wrong number of wires for single phase or single phase cap start sort of throws a monkey wrench in the works too.

You're absolutely correct Bob, a three phase motor can be started and run with a capacitor. Single phase motors with a cap are actually two phase motors and the cap is used for phase shifting. You knew this already I'm sure but others may not. In fact, I can't think of any single phase motor that isn't actually a two phase motor. Typically the second shifted phase is disconnected once the motor comes up to about 85% oiperating speed but not always. Eric

Try this old forum listing:

That sure sounds like your saw or very similar...

[ ... ]

Is disconnected once it reaches the threshold speed, unless it is a "run" cap (oil filled AC cap) instead of a starting cap (Non-Polar electrolytic, and not capable of surviving very long with AC) Also, the run cap is typically lower in capacity than the starting cap.

Also ... "universal" motors (AC/DC motors, with commutators and brushes) are single phase only, but *induction* motors (AC only) are typically either three-phase or two-phase used as cap start or cap run.

Also, there are "Hysteresis Synchronous" motors (also AC only) with permanent magnet rotors which are run cap equipped and two phase unless wound as three phase. They are normally used only when totally stable motor speed is needed -- no slip like in induction motors. These were commonly used for capstan motors in high quality tape recorders and turntables where audio pitch shifts with speed.

An induction motor normally experiences slip, and a 2-pole one on 60 Hz will run at about 3500 RPM or so, depending on load, while a H-S will run at a fixed 3600 RPM.

And -- what makes an induction motor what is is is that the rotor has a number of poles which are each surrounded by a cast metal to make a shorted turn. Often, all of the parts between the poles are joined to two rings of the same metal at the ends. When there is an AC field, it inductively couples to the poles and creates a current in the loops which makes temporary magnetic fields on the rotor poles, which are dragged towards the stator poles. These fields decay and new ones form on adjacent poles so the rotor "slips" behind the rotating fields in the motor stator.

Enjoy, DoN.

I would not call a cap start motor a two phase at all.

I would call it a dual winding single phase. The starter winding - large diameter short winding with a cap - dumps the AC power through the winding as XL is canceled by XC for the tech guys.

The run winding is used once up to speed and an internal switch throws at speed - switching run in and start out.

When the power company runs low voltage - starter winding is used longer, meaning more power used and then the run is enabled.

Two phase is 2 of 3 phase in a Delta wiring of a 3 phase motor. It is called an "Inverted V" or lost leg.

This is a basis of very high rel 3 phase. Input and output are 3 phase. If a phase is lost on the input the output has 3 phase at 66% power IIRC.

One could in theory loose two one inside and outside. Dropping more, but still running just fine. Hospitals and Police/Mil run these like this in case of attack.

Mart> >

Call it a "temporary two phase", as that is present only as the motor is starting, and is generated by the start cap.

Yes, two windings, but one gets standard phase, and the other gets shifted phase, by the XL and XC causing a shift. This provides a second magnetic field at an angle to start the motor spinning. You

*can* start the single phase motor with just one widing, if you have a way to spin it just before you switch on power -- such as a rope wound on the shaft.

Not quite. The inertial (also internal) switch switches the start winding out, but the run winding is connected full time -- as long as there is power to the motor.

Look at the inertial switch when you have a motor open. It only has the contacts to open the start winding -- not any more contacts to close the run winding.

Or -- just apply an ohmmeter to the motor from outside (and with AC power disconnected, too, to keep from frying the ohmmeter). It will draw current and show a fairly low resistance. If the run winding were open, you would only see a swing of the needle (if a meter) or a short display of low resistance moving slowly to infinite resistance, as the start cap charges. (Well ... some leakage in the cap, so you won't really get all the way to infinite, as a perfect capacitor would.)

If you apply power *only* to the start winding, you still lack the shifted phase relative to the run winding, so the motor still won't start, without something external to spin it up. But, because it is sitting there drawing lots of current, it *will* blow up the start cap in short order. :-)

And then the start winding is switched out, leaving just the run winding which was already enabled.

Now -- I grant that this is not a precise phase relationship between the two windings -- dependent on the value of the cap and the inductance of the winding -- as well as the frequency of the AC applied to it.

This is when the phases are 120 degrees apart. There are also two phase power lines (uncommon these days) which have a 90 degree phase relationship -- and a tricky transformer circuit can convert between true two phase and true three phase -- either direction. I forget the full name of it, but 'T' is part of the name.

Understood -- but this is still talking about phases shifted 120 degrees relative to each other. So you could call them "partial three phase". The two phase for motor starting is different, with 90 degrees being ideal, but usually not hit precisely.

Enjoy, DoN.

You can get this on a common / ground lead and a power lead that is hooked to the internal switch. The switch is in the start winding at dead stop. Once turning fast, the switch flips and the run winding is powered and the start is turned off. It is a Single Pole, double throw switch that is controlled my speed.

Having a phase shifted internal winding gives you on a scope a waveform and the non-shifted is only the power line. The run doesn't get anything.

The funny motor you are talking about is a milk station that has

A neutral/GND and two Legs of xxx volts that are really 2 legs of 3 phase.

I have 2 phase on my property. Two Highlines and I could have 3 phase if I wanted with three transformers. I created my own from Single phase.

Mart> >> I would not call a cap start motor a two phase at all.

My father's bench grinder had such a motor, probably assembled by my somewhat more technically savvy uncle from a bad appliance motor. To use it I had to plug it in and quickly yank on the vee belt, being careful to let go before my hand reached the front pulley. Once I wasn't quick enough but the belt was so loose I didn't get hurt.

-jsw

Not any which I have seen when I opened them up. They are single pole single throw NC (Normally closed), and in series with the start winding and the start cap.

For a two-voltage motor (120/240VAC), when wired for 120 VAC, there are two run windings connected in parallel, and the start winding cap and centrifugal switch are connected from line to neutral.

When wired for 240 VAC, the two run windings are in series, making the junction point a tap on an autotransformer which gives a voltage which is 120 VAC different from each end. The switch, cap and start winding are connected between the center tap formed by the two run windings and one end. Actually, to reverse the motor (only at start time), you can select whether the other end of the start winding is connected to one or the other of the 240 VAC line inputs. (I say "only at start time", because the relative phase of the run and start windings only determines the direction it is spun until the centrifugal switch opens. Afterwards, you can switch quickly between the two positions and the motor will keep spinning in the same direction. (You can also switch ends on the start winding, leaving it connected to the same end, but that requires a bit more complex a switching setup -- especially if you are using a normal drum switch and want the entirity of the motor to be disconnected from power in the "stop" position.

The run gets the normal phase from the power line, from 120 VAC or 240 VAC single phase. The cap and start winding between them generate a temporary phase in the start winding which differs from that in the run winding, and which gives the angular field to start the motor spinning.

A "milk station"? IIRC, the funny motor which I remember writing about is the Hysteresis Synchronous motor, with a permanent magnet rotor and speed locked to the power line frequency. Nothign to do with milk that I know.

Looking below, I see that I must have written that about the Hysteresis Synchronous motors in some other article, perhaps in a different newsgroup or mailing list.

Two highs and a neutral (if the two highs are fed from two phases of a three phase power line), yes. If you are calling the two lines of a 240 VAC power with neutral center tap, you can call those only either "in phase" or "180 degrees out of phase", depending on how you look at it, but you cannot generate three phase from that. You need a 120 degree phase difference to allow this to work.

Of course, you can approximate three phase with a three phase motor wired to 240 VAC, with a run capacitor shifting the phase to the third winding. What the actual phase angle is will depend in part on the value of the capacitor, and the inductance of the motor winding. This is a rotary converter, and it can be self starting if the run capacitance is connected only to one of the other windings, not equally to both.

Or, you can have a separate start capacitor which is automatically switched by the current in the main two windings.

Or -- you can start it with a pull rope just before switching on the power. Some people on this newsgroup do this. A bit of a problem if you walk out of the shop with a machine running, the power drops, and then returns while you were away. The rotary motor sits there not starting and getting quite hot.

If your near there, you could always go to a DeWalt factory site to ask and make sure. Here's one at: 7957 Central Ave; Capitol Heights, Maryland 20743 - (301) 333-0863

A medium saw mill was sitting 20 yards from this chair. The 3 phase runs down both sides of the highway. I am a tap, two high line fuses and 2000 feet of poles. (After a hurricane, it was four 100' boom trucks to fix my power lines. They shortened it by 200' but it is real

2 phase. This is in the country, the 2.5 miles away micro-plex town is moving in on my acreage.

I used to teach the stuff. And know what I have. Mart> >> You can get this on a common / ground lead and a power lead that

So you have 208V between phases?