Hello Alex
You can use a 3450 RPM and slow it down with the VFD as long as you do not load it heavily and continuously or it may overheat and become damaged. I was speaking with one of the motor design engineers at work about this issue a couple of weeks ago since I was thinking of doing the same thing. He said that TEFC motors should not be run at less than half speed for extended periods of time with heavy (say over 50% of rated hp) loading. The TEFC motors are Totally Enclosed meaning that no outside air enters the motor. The Fan Cooled part refers the fact that they cool themselves by stirring the air inside using a fan driven by the motor shaft. The heat is transferred from the rotor, on the shaft, to the air then to the housing of the motor. The fins on the outside of the housing disipates this heat into the air surrounding motor. At less than half speed, the fan does not stir the air inside the motor enough to cool it effectively. The amount of heat generated in the motor is related to the load placed on it, not the speed.
A drive (VFD) can be connected to any motor of the same voltage rating of your drive. The hp rating of your motor should be in the range of 20% to 100% of the hp rating of your drive. The drive will typically have trouble keeping accurate control of motor loads less than 20% of it's rating. The drive does not know or care what the actual RPM of your motor is. The drive controls the frequency of the voltage applied to the motor, hence the term Variable Frequency Drive. The output frequency of the drive typically will range from about 1 Hz, or less, up to 400 Hz. The speed of the motor is dependent on the number of "poles" that it has and the input frequency. A 3450 RPM motor achieves that speed with a 60 Hz input. It has twice as many poles as a 1725 RPM motor. To get the 3450 RPM motor to run at 1725 RPM the drive's output frequency would have to be 30 Hz. Your control of the motor speed would not be as fine as with a 1725 RPM motor since your control range is now 0 to 30 Hz instead of 0 to 60 Hz.
You can use two three-pole circuit breakers to divert the output of the drive to the motor that you want to run. Each motor is wired to the output of one of the breakers. The inputs of both breakers are wired to your drive. Only one breaker should be closed at any one time. Don't open or close the breakers while either of the motors is still turning. The breakers also give you some extra protection in case something goes wrong with one of the motors. Please note that with both breakers open, the output of the drive is still energized, and dangerous, unless the input power to the drive is removed. The following is a suitable breaker for your application:
http://www.surpluscenter.com/item.asp?UID=2006030522432722&item=11-2272&catname=electric
These circuit breakers should be mounted in a flame proof cabinet, i.e. a metal box. Do not mount them in or on wood directly since it would be a violation of every electrical code that I have run across (I have lived in three different countries now). If there is ever a fire and your insurance company finds out that you had mounted the breakers or switch gear on a flamable surface, they will probably deny any claims that you try to make.
Sorry for the long post. I hope this helps.
Phil
