Tips to find the rated voltage of the induction motor without nameplate data
First measure the motor and compare the measurements to NEMA and/or IEC tables of standard dimensions to determine whether it is likely to be a standard NEMA or IEC motor. Determine the standard voltage, frequency, speed and power ratings that might apply to a motor of that size.

Examine the motor leads or terminals. Determine if the number of leads or terminals allows for connecting the motor for more than one voltage. Find standard connection diagrams and connect the motor for the highest possible voltage.

For this kind of project, you should have a variable transformer of sufficient size to supply half of the probable rated current. The motor shaft should not be coupled to any load. Connect the motor at the lowest voltage setting of the transformer. Record the voltage, current and speed of the motor. Plot the voltage and current on a graph. Keep increasing the voltage and plotting the current until you notice the the current is increasing more quickly than the voltage increases. At that point, the applied voltage is probably 20 or 30 percent above the rated voltage. Pick the standard IEC or NEMA voltage that lies on the appropriate area of the curve.

If you are applying 50 Hz to a NEMA motor or 60 Hz to an IEC motor, adjust your estimate using rated 50 Hz voltage = 5/6 X rated 60 Hz voltage.

Added PDF reference for above.

Something to try after you have a preliminary determination of the voltage rating:

If you can run a dynamometer test, you can plot voltage vs. current for a load torque that is carefully held constant. The applied load should be a good estimate of the rated full-load torque. The minimum current should occur at close to the rated voltage. Reference

Added material re induction generator operation:

I found a reference that shows the relationship between voltage and load with various capacitor values.


Al-Saffar, M.A.; Eui-Cheol Nho; Lipo, T.A., "Controlled shunt capacitor self-excited induction generator," in Industry Applications Conference, 1998. Thirty-Third IAS Annual Meeting. The 1998 IEEE , vol.2, no., pp.1486-1490 vol.2, 12-15 Oct. 1998

How to Estimate Rated Torque

I would first measure the motor’s shaft diameter. Compare the diameter with published data for similar motors. That should provide a range of likely values for the rated torque.

Run a dynamometer test to get data to draw a torque vs. speed curve. Before taking data get the motor near it’s rated operating temperature by running it for an hour or more at the minimum torque of the range that you estimate based on shaft diameter. When running the motor above the maximum torque in the estimated range, take data as quickly as you can to avoid overheating the motor. If possible, use automated data acquisition to get data from locked rotor through breakdown. Once you have that data, draw the curve and compare it with published curves or data for motors that might be similar. You can use that method to more closely estimate a range for rated torque.

You might be able to narrow the range by running the motor at various value of torque. At each torque value, run the motor until the temperature stabilizes and perform a temperature rise by resistance test. Unfortunately, that still will not provide an exact value for rated torque unless you know the temperature rating of the insulation used for the windings.