Can I use a rheostat to slow down the RMP's on a 1/3 hp 110V 2-speed electric motor?

Whether or not you can slow down the motor depends on the type of motor and the type of load. Since this is a 110 volt, 2-speed motor with a top speed of 1750 RPM, it is probably a single phase, 60 Hz, AC induction motor. If it is the shaded pole type, a rheostat or an electronic voltage reduction unit (similar to a lamp dimmer) could be used. However, it would probably only be effective if the load is a fan or centrifugal pump.

There are a few manufacturers making variable frequency drives (VFDs) for use with a shaded pole and permanent split capacitor (PSC) motor with a fan or centrifugal pump load. They may work with other types of loads. Be sure to read the VFD literature carefully. Look at: http://www.anaconsystems.com/text/eagle1.html

VFDs are mostly made for use with 3-phase motors. You can buy VFDs that have single phase 120 VAC input and 3-phase 240 VAC output. The best way to get a variable speed motor for use on single phase power is to buy a 3-phase motor and a 1-Ph in, 3-Ph out VFD. Look at: http://web3.automationdirect.com/adc/Shopping/Cata...

A universal motor, an AC motor with carbon brushes and a commutator, can easily be controlled with a rheostat or electronic voltage control. A 1050/1750 RPM 2-speed motor is unlikely to be a universal motor.

Every motor I had always instructed me to replace the brushes if performance falls / fails. This is why I am very sure it is your brushes, your performance is failing. You should buy new brushes, so you can open it and chech out the other things too but replace the brushes anyway since it is open. If all else looks good close it and try it now...if its the same you now know it is not the brushes any more unless they were installed impropperly..... You could just go the other way.... Current = torque Voltage = speed So if the problems were wth the torque of the motor then we know it is with the current, and that would be Bearings. If the problem were with the speed of the motor then the problem has to be voltage, and that would be brushes If the problem were with both it is understood that one caused the other to fail after failing itself.

Certainly possible, but it would take a big, heavy and expensive rheostat, that would get very hot.

An electronic motor speed control would be cheaper, and work better.

aka AC Motor Speed Controller.

120v Rheostat