Universal Motor wiring advice request.

[rfdes]

Hi -
I found a Craftsman model CMXETAX69434502 table saw in a dumpster recently. By it's condition it appeared unused. However, the unit's electromagnetic switch had been removed and I assumed that the saw failed shortly after it's first use. With the switch missing, I am trying to understand how this unit was wired. The motor is a universal motor and I was successful (or so I think) in figuring out the wiring(see pictorial). Applying AC to the Blue/Grey wires gives me the proper rotation and the motor runs fine. Applying AC to the Black wires rotates the rotor in the opposite direction. The question what is the alternate wiring (Black wires) used for? My guess is a brake system. After some research, this is the model magnetic switch that was used in this saw.

I could use some guidance as to how this motor was wired with this magnetic switch, particularly with the black wires.

Thanks for any help you can offer.
Jim

 

[Diver300]

I think that you should connect power to 3 and 7 of the switch, and connect blue and grey to 4 and 8. That will run the motor when the switch is turned on.

You should also connect the black wires to 1 and 2. That will brake the motor when turned off.

(Usual disclaimers, these are best guesses only. If it goes wrong, all you get is my apologies. Keep your fingers away from live wires and saw blades.)

 

[rfdes]

Thank you for the response. I don't understand how shorting the black wires when off causes the motor to brake'

 

[Diver300]

Thank you for the response. I don't understand how shorting the black wires when off causes the motor to brake'

It's a technique used all the time with DC motors. Car wipers and windows usually short the motors out when turned off to make them stop quickly.

Motors are also generators. When they spin, they generate a voltage. Shorting that voltage causes current to flow that opposes the motion. You mentioned that putting power on the black wires makes the motor go backwards. Therefore current flowing in the 2.5 Ohm winding and the armature is producing torque that is in reverse. Allowing that current to flow when the motor is turned off will produce torque in reverse, which is braking torque. That obviously stops when the motor stops.

You can try it out. Short the black windings and spin the motor. It will most likely be possible to feel the difference in how fast the motor slows down. It might not, because there might be no residual magnetic field, so no voltage generated, so no torque. When braking from running, there will be the field present from when the motor was running.

Be aware that there might be a big voltage on the windings when spinning the motor by hand, and the saw blade will be just as sharp without electric power.

 

[rfdes]

Thank you - that makes sense.
Take care.