# Inductor Machines: why is constant flux achieved before reaching nominal speed?

It is observed that an induction machine increases its voltage linearly before reaching its nominal speed. This is achieved due to the emf generated by the rotor movement.

However, I find it difficult to understand why the flux is kept constant while the induction motor increases its speed. See following image:

Could anyone shed some light in this matter?

• Are you sure the diagram isn't for a generator - your words seem to imply this i.e. you say "induction machine increased it's voltage linearly"? – Andy aka Jan 20 at 11:24
• @Andyaka, yes i'm sure. Regarding my sentence I'll rephrase: the total voltage observed on the stator circuit increases linearly as the IM increases its voltage. – Alvaro Jan 20 at 12:50

The curves presented in the question show the typical continuous torque capability, stator voltage and stator flux for an induction motor controlled by a variable frequency drive (VFD). The induction motor does not increase its voltage due to back emf. The VFD is designed to apply stator voltage in proportion to the applied frequency. That is what is required to maintain constant flux in an inductor as the frequency of the applied voltage increases.

The flux is kept constant in order to keep the motor's torque capability constant. It is generally desirable for the motor to be capable of producing as much torque as it can from standstill up to the nominal design speed. The nominal design speed is usually the nominal speed for operation at mains frequency. The flux necessary to produce rated torque capability at rated slip is usually close to the maximum flux supported by the magnetic circuit.

With most design configurations, the VFD can not produce the voltage required to keep the torque constant. Therefore torque capability decreases as speed increases. The decreasing torque is usually sufficient tor produce constant power up to about 150% of nominal speed.

• Thanks! It has become way clearer with your last edit of the explanation :) – Alvaro Jan 20 at 20:50