Constant volts/Hz control

The constant volts/Hz control is primarily designed to accommodate variable speed commands by using the inverter to apply a voltage of correct magnitude and frequency so as to approximately achieve commanded speed without the use of speed feedback. Therefore, it is safe to say that the simplest and the least expensive
induction motor drive strategy is constant volts/Hz control.

The speed control strategy relies on two foundations. One of them is that the torque-speed characteristic of an induction machine suggests that the electrical rotor speed of an induction machine is very near to the synchronous speed and hence has a direct relationship to the electrical frequency. Thus, by controlling the frequency, the speed can be controlled. The second foundation is based upon the phase-voltage equation that may be expressed as 

where   Vas    is the phase (a) stator voltage.

             Rs      is the stator phase resistance.

             Ias      is the phase (a) stator current.

             λas    

is the phase (a) stator flux linkage.

For steady-state conditions at intermediate to high speeds wherein then flux-linkage term dominates the resistive term in voltage equation, the magnitude of the applied voltage is related to the magnitude of the stator flux linkage by:

where   Vs      is the stator phase voltage.

              ωs    is the synchronous speed in rad/sec.

               λs      is the stator phase flux linkage.

which suggests that in order to maintain constant flux without any saturation, the stator voltage magnitude should be proportional to the frequency.

The advantages of this control strategy are that it is simple and relatively inexpensive because of being an open loop control solution and that the speed can be controlled to a degree without using speed feedback.

This in turn, increases a drawback of this control strategy; because it is open loop, some error will occur, particularly at low speeds.