Universal Motor Question(s) - Need advice

[rfdes]

I'm not a 'motor guy' so I'm confused regarding the following:

I have a 15 amp  motor from a table saw that runs off 120 vac.  There are no labels/markings on the motor so I have no additional information.  This motor does have brushes so I am making the decision that this is a Universal AC/DC motor.  My understanding (correct me if I'm wrong) is that a Universal motor does not need a start cap when run from AC.  My confusion lies in that there are 2 wire pairs present with one wire pair a low resistance (0.5 ohm)winding  and the other pair a higher resistance (4 ohms) winding.  This made me believe that one winding is the field/main winding and the other is the start/aux winding.  Is this correct?  If the Universal motor does not require a start cap then why the additional winding?  I am trying to understand this so that I can wire it up for testing and I don't want to ruin the motor.

If someone would help me understand this issue I would appreciate it.

Thanks
Jim

[CaptDon]

A universal motor doesn't need a cap when run from any voltage A.C. or D.C. The polarity of the field coil vs. the armature determines direction of rotation. Are you saying there are 4 wires on the field coil?? The extra pair of wires may have had something to do with the brake switch if the saw had a quick stop feature when switched off, sort of a safety brake.

[Kleinstein]

With brushes it likely is a universal motor. There is also an old version of induction motor with brushes, but that is rare and quite large for the power rating.

Having 2 windings (for the stator part) is not so rare. It is common to have just 2 identical halfs just from manufacturing. 2 unequal parts could be used for 2 speeds. With both parts in series one would get a lower speed and usually less power. I have not seen this with a table saw, just with drills.
2 speeds could also be used for a soft start - though I have not seen it made this way. A soft start could make sense for >15 A nominal rating.

An auxiliary winding could be used for a break feature (new machines). Either just shortening that extra winding when off. Some Makita handheld circular saws do it this way.

[BeBuLamar]

I think the 2 pair of wires are 1 for the armature and 1 for the field. That allows you to wire it for either direction.

[rfdes]

Some really helpful information here.  Thanks to all for their input. 
I guess I will experiment with the windings and see if anything smokes.

[themadhippy]

I think the 2 pair of wires are 1 for the armature and 1 for the field.

quick way to tell is if one pair of wires goes to the brushes,normally thats the armature winding

[Kleinstein]

Some really helpful information here.  Thanks to all for their input. 
I guess I will experiment with the windings and see if anything smokes.

For a first test one can use a lower voltage, also DC. With 120 V motor, it should start running with 12 V DC.

[richard.cs]

It could have both a shunt field and a series field making it a compound wound motor. Many small universal motors are series field only, and rely on loading from their cooling fans to prevent overspeed, but that's not inherent in a universal motor, they can have shunt fields instead, or be compound wound (both).

A series field gives a constant-power characteristic (and overspeed at low load), a shunt field motor attempts constant speed but with some speed drop on load due to the armature impedance, a compound wound can balance the two such that the series field compensates the on-load speed drop of a shunt wound motor, or just a little bit of shunt field can be used to limit overspeed on a motor that otherwise acts as series wound.

[rfdes]

I managed to draw up a diagram showing the configuration of this universal motor.  The field coils are wired in series with the
commutator, just as expected.  AC power applied between the Blue and Grey (Both blacks disconnected) rotates the motor in
the correction direction.  Running the motor for several minutes showed no sign of overheating.

The question is what is the purpose of the black wires?  Is this a 'brake' mechanism and if so, how are they wired?
Any more suggestions as to the black wires?

Thanks

[Doctorandus_P]

Grey and Blue seems to be the normal way to connect the motor, because those windings have the same resistance.

I am guessing that if you short circuit the two black wires, the motor will run at a slower or higher speed. Can you find the manual for the originial equipment of this motor? It probably has a three position switch for two speed settings.

[rfdes]

No, the manual only shows and ON/OFF push button switch.
I may do some more experimenting.
Thanks for your input.