Principle of Operation

Author: E.E. Kimberly

If the magnetized iron field structure of Fig. 18-1 be caused to rotate counter-clockwise on its axis xX₁t there will be, at the instant shown, electromotive forces induced in the closed loop of wire a; and these electromotive forces will cause a current to flow in the direction shown, according to Faraday's Law. As explained on page 128, the current in the upper horizontal conductor will produce a force toward the left, and the same current in the lower horizontal conductor will produce a force on it toward the right. These forces produce a couple tending to rotate the loop counter-clockwise about the axis xX₁. The loop of wire will tend to rotate in the same direction as does the magnetic field, and it may be called a rotor.

If free to rotate, the rotor would tend to rotate at a speed equal to that of the main field ϕ. If it should succeed, it would be said to run in synchronism with the main field.

Fig. 18-1. Fundamental Elements of Induction Motor

But the rotor could not attain synchronous speed because, for its driving forces, it depends on relative motion between itself and the field. Hence, the rotor must always run at a speed somewhat less than that of the main field, in order that some driving torque may exist. The numerical difference between the speed of the main field (called synchronous speed) and the speed of the rotor is called slip. Slip is usually expressed as a per cent of synchronous speed.