Why is it necessary to control the v/f ratio when an inverter drives an inductive load, and how can it be achieved?

 

First off, the fact that you would apply an inverter drive to an induction motor implies that you need to continuously do one of 2 things:
1) change the speed, or
2) change the torque outpout

OK, OK, 3 things, because maybe you want to do both at the same time.

Now look up the formulae for calculating motor speed or torque. If you change the frequency without changing the voltage, the speed changes but the windings go into saturation and produce more heat per unit of work, an undesireable outcome.

If you change the voltage without changing the frequency, the torue falls off by the square of the applied voltage, but slip increases as the motor attempts to turn at the same speed (following the rotating magnetic field), so the current will increase to try to maintain that speed even though less work is being done, again overheating the motor.

If you take a 460V motor designed to run at 60Hz you can calculate the V/Hz ratio as being 460/60, or 7.67 V/Hz. As long as the motor applies 7.67 V/Hz then, the output torque will remain contant at "any" speed (there are other deeper issues that come into play at low low speeds). By the way, if you take a 380V motor designed to run at 50Hz, the V/Hz ratio is, guess what?...7.6 V/Hz. It is not coincidental. So by maintaining the same V/Hz ratio with the inverter you can continuously alter the operating speed without overheating the motor.

If it is torque that you want to control, then you probably have a load that reqires different amounts of torque at different speeds, so you can alter the V/Hz ratio a little to compensate and maximize efficiency, as in a centrifugal pump which will do significantly less work at 1/2 speed than it will at full speed since less of the load (fluid)is coupled to the motor as the impeller slips by without inducing flow. By lowering the V/Hz ratio at low speeds, the motor consumes less energy.

As to how it is done, search the web for PWM Motor Control and you will get plenty of descriptions