Theoretical analysis should be carried out to study torque characteristics and overheating of three-phase induction motor due to fluctuation in frequency and variation in voltage of the motor.
A. When power supply voltage is greater than the rated voltage
According to the equation for induced electromotive force, the frequency is constant, therefore the maximum magnetic flux Φm increases and the over-excitation phenomenon occurs.
Heat is generated because of this excitation current. In a submersible pump, when a thermal protector is built-in in the internal coil for protection, it may activate.
B.When power supply voltage is smaller than the rated voltage
According to the torque equation, when the frequency is constant, the power supply voltage V reduces, so the torque reduces.
C.When the power supply voltage is unbalanced
When the power supply voltage is unbalanced, reverse phase current flows and the temperature increases because of load loss in the coil resistance.
D.When the frequency f is higher than the rated frequency
According to the torque equation, when power supply voltage is constant, the torque reduces.
E.When the frequency f is lower than the rated frequency
When the power supply voltage is constant, the maximum magnetic flux φm increases, and heat is generated because of the excitation current.
According to the torque equation, when the power supply voltage is constant, the torque increases.