a different motor question

Can anyone help me I need to slow down a 120v radial arm saw. It is an older model with 6 brushes. The saw runs at 5500rpm and I need to go under 5000rpm. There is no name plate on the motor itself. The unit is just plugged into a 120v circuit which is protected by a 20amp breaker. I did see a controller on e-bay for 16.95 thay looked like it would fit the bill. It mentioned a 20amp capacity as well. Since this is a radial arm saw the motor is a intregal part of the saw and can't be replaced. We are attempting to use an abrasive wheel on the saw after our 10" saw died today. Found a 12" old Dewalt saw and thought it would be perfect but wasn't thinking the max rpm for a 12" abrasive wheel is lower then for the 10" we had. Also the new saw has a 1" arbor whereas the old 10" had a 5/8" so we can't just put a 10" wheel on the 12" saw.

If it is a brush type motor, it is most likely a "Universal Motor" and you can control the speed by changing the applied voltage, just like a variable speed hand drill motor. Because you say it's old, the only other possibility is that it is an old "Repulsion Start-Induction Run" motor, in which case that will not work and there will be no method of reducing the speed. The way to tell the difference is that if it is a RS-IR motor, there is a speed switch that shorts the brushes once it gets to full speed and you can hear it when it does. If you still see sparks from the brushes at full speed, it is a Universal motor. Also, if it is direct drive, meaning there is no pulley / belt system to get the blade up to 5500RPM, that is another indicator that t is a Universal motor. I don't think an RS-IR motor can spin higher than 3600RPM.

No idea what your $16.95 device could have been, but it's not likely to be the right thing. It was probably an old DC drive. You could theoretically use it if your motor really is a Universal type, but it would take some decent electrical knowledge to do it safely because it would involve re-wiring the saw and controls.

If it is a Universal motor, all you need is a device called a Variac, or Variable Transformer. It's like a giant dimmer switch; Ebay has some available. But you need to know the motor amps. The size of the breaker feeding it does not tell you enough information to be safe. Look for a nameplate for the entire saw, or look for a HP rating on the motor. From a HP rating you can derive the current required: 1HP = 746W, motor Watts = A x V x PF (assume .7) x eff (assume 80%). So if you know the HP and voltage, you can use your math skills to get Amps.

Thaks Jraef. I don't hear anything dropping out when it hits speed. I will check and see if it is still sparking at speed on Friday. The unit is a direct drive.

Repulsion start induction run

Sounds to me like a RI motor that needs its centrifugal mechanism repaired. A RI motor will run at excessive speed if the centrfugal flyweights are not working. RI motors are usally of 2 types "Brush lifting" and "Brush riding" Most brush riding motors are small ie. 1/3 HP and smaller Larger RI are usally brush lifting ie. the brushes are lifted from the commutator by action of the centrifugal flyweights. Most that I have seen or serviced have radial commutators. When the centrifugal mechanism lifts the brushes it also slides a shorting necklace of little copper jumper bars to short all the commutator bars together, this converts the rotor to a induction configuration. If the flyweights and /or related mechanics are not functioning the motor remains in the start or repulsion mode, In repulsion mode the motor acts much like a series wound motor with poor speed regulation. When the motor operates properly it will exhibit well regulated speed properties like an induction motor. A 12 inch wood cutting circular saw ideally should run around 3000 RPM. Since induction motors depend on frequency to establish speed 3000 is not too feasible, my guess would be 3450 for a 12 inch dewalt but could be 1750 as well, If the RI motor is running 5500 it probably is a 2 pole or 3450 motor and with repair might do just find for abrasive wheels rated 5K.