9.2 The Electric Power System. Concepts

9.2 The Electric Power System. Concepts

The following is an overview of a number of concepts that arise in the analysis of asynchronous or induction machines. The stator frequency denotes the frequency of the network powering the stator winding, and the frequency of the current running through this winding. Knowing the stator frequency is valuable because, together with the number of pole pairs in the machine, it determines the value of the synchronous speed. The synchronous speed equates the speed of the rotating field that the inductor creates.

We determine the synchronous speed using one of the following equations:

n₁ = f₁/p where n₁ = rps, f₁ = Hz, p = number of pole pairs (9.1)

Or,

n₁ = 60·f₁/p where n₁ = rpm, f₁ = Hz, p = number of pole pairs (9.2)

The rotational speed refers to the speed of the rotor. In asynchronous machines, it is true that:

n₂ ≠ n₁

By convention, we give a positive sign to the synchronous speed.

For this reason, if the rotor rotates in the same direction as the rotating field, its speed is positive; conversely, if the rotor rotates in the opposite direction, its speed is negative.

Now, we define slip s as:

s = (n₁ - n₂)/n₁, where nₛ = n₁ - n₂ is referred to as slip speed.

Rotor frequency (fₛ) denotes the voltage and current frequency that appear in the rotor windings. The relative speed of the rotating field and the rotor determines its value. We define it as:

fₛ = p·(n₁ - n₂) = p·nₛ (9.3)

where n₁, n₂ and nₛ are given in revolutions per second (rps) and frequency in hertz (Hz).

Remember that 1 rps = 1 Hz.

Therefore, it follows that:

fₛ = p·s·n₁ → fₛ = s·f₁