This system has no feedback.
Exactly. Thats the problem. If you can approximate speed and don't need exact speed then we can make a few calculations to put you in the ball park, probably better than +/- 100 RPM on the motor. Multiply that through your reducer to get an idea of what the final precision will be (+/-3-1/2 RPM). If you need more precise speeds then you must add feedback.
The equation for the motor
synchronous RPM was given in post #6.
RPM = freq*120/p.
A three phase induction motor
CANNOT run at synchronous speed because at synchronous speed the rotating magnetic field matches the rotor and then it cannot induce any current into the rotor windings, causing a loss of magnetic field in the rotor. The exact amount of slip between the magnetic fields will vary with the amount of load on the motor - so your motor speed will ALWAYS be less than what you calculate it to be based on frequency alone.
http://en.wikipedia.org/wiki/Induction_motor
Now you have a gear reducer that you say is a 30:1 ratio. That means that the input shaft speed is 30 times the output shaft speed. BTW, your gear tooth count doesn't jive with what your say the reduction speed is. But running with what you have given us so far, lets do a little math.
Motor_RPM = 30 * Reducer_RPM
Now a little algebra to rearrange the equation from post 6.
step by step:
Motor_RPM = freq*120/p ->
Motor_RPM*p = freq*120/p ->
Motor_RPM*P/120 = freq*120 - >
Freq = Motor_RPM*p/120
Keep in mind that this DOES not account for motor slip. What we know from the nameplate is that slip is 75RPM at the rated horsepower and frequency, but slip under other conditions is unknown. If your drive is a vector drive it may be albe to give you slip feedback, otherwise for exact RPM you will have to add instrumentation.
Not sure which powerflex you are using, but I think that some powerflex models allow you to input the comandspeed in RPM instead of frequency. If your drive supports that then use that option and then you don't have to worry about the conversions.