Electric motors.

General news. Asynchronous AC induction motors are used to drive gastronomic machines, three-phase and single-phase, with a power of 0.2-5 kW.

DC power P (w wa) taken from the electric network with a voltage of U (in volts V.) and intensity I. (w amperach A) expresses itself in dependence:

P = U · I [W]

1000 W=1 kW;

1 J=1 W · 1 s;

1 kW · h=1000 W · 3600 s = 3 600 000 J;

Due to the phase shift between current and voltage, the AC power is not expressed as U · I.

The power P of a single-phase alternating current is calculated according to the formula:

P= U · I · cos φ

wherein: U - voltage in V,
I - current strength in A.,
cos φ - power factor.

Voltage and current intensity change sinusoidally over time.

Wykresy napięcia i natężenia prądu przemiennego — Charts of alternating current voltage and intensity

Charts of alternating current voltage and intensity: a) at active load, b) with inductive reactive load, c) at capacitive reactive load.

The power factor cos φ takes values from 0 do 1. The nature of the load affects the value of this factor. In a circuit containing only resistance, the phase shift angle φ = 0, so cos φ = 1 i P= U · I. The active power is equal to the apparent power. (The concept of apparent power is used to denote the power of a source, np. generator).

In the case of an inductive load Q = U · I · sin φ. This is called. reactive power.

The current in the network depends on the power factor

When cos φ = 1, the intensity is the lowest; its value grows, when cos φ decreases. The reactive current loads the network wires and causes their heating.

(Power losses are proportional to the square of the current - I² · R).

Thus, the most favorable working conditions are achieved with cos φ = 1.

Electric motors and transformers have significant inductance. The power factor is 0,8-0,9 at full load; cos φ decreases with lower loads. The improvement of the power factor is achieved primarily by selecting motors and transformers of appropriate power and preventing their idling.. Sometimes capacitors are used for this purpose. A capacitor of appropriate capacity connects in parallel with the electric motor. Then the exchange of reactive energy takes place in the motor-capacitor circuit. Three single-phase currents R., S, T form a three-phase current in a suitably associated system. The phases are shifted relative to each other by 1/3 period.

Graph of the intensity of three-phase alternating current

If the receiver is connected between two phases of the electrical network, np. R i S, the voltage at the receiver terminals will be:

Up = √3 · Uf= √3 · 220 V = 380 V.

If the receiver is connected between the phase conductor and the neutral conductor of the network , the voltage obtained at the terminals of the receiver will be Uf = 220 V.

Examples of three-phase associated systems: a) in the star, b) in a triangle

The power of a three-phase current is expressed by the formula:

P = √3 Up · I · cos φ

Exercise:

The OZ-40 potato peeler takes power 0,6 kW. Calculate the electric current in the cables feeding the electric motor of the machine, if the voltage at the motor terminals U = 380 V, a power factor cos φ = 0,9?