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close this bookElectrical Machines - Basic Vocational Knowledge (IBE - Deutschland; 144 pages)
View the documentIntroduction
Open this folder and view contents1. General information about electrical machines
Open this folder and view contents2. Basic principles
Open this folder and view contents3. Execution of rotating electrical machines
Open this folder and view contents4. Synchronous machines
close this folder5. Asynchronous motors
View the document5.1. Constructional assembly
Open this folder and view contents5.2. Operating principles
close this folder5.3. Operational behaviour
View the document5.3.1. Start
View the document5.3.2. Rating
View the document5.3.3. Speed control
View the document5.3.4. Rotational sense alteration
Open this folder and view contents5.4. Circuit engineering
View the document5.5. Application
View the document5.6. Characteristic values of squirrel cage motors
Open this folder and view contents6. Direct current machines
Open this folder and view contents7. Single-phase alternating current motors
Open this folder and view contents8. Transformer
 

5.3.1. Start

Inrush current origin

As soon as the current is switched on the rotating field rotates at full speed along the rotor bars of the squirrel cage rotor.

The equation

(Cp. Figure 52) indicates that as s = 1 (U = U), a greater inrush current is attained in the rotor which is transmitted transformerwise to the stator side.

Every asynchronous motor accepts a higher current when starting from no-load position.

This inrush current, when utilising the full mains voltage, can be four to eight times as great as the rated current.

This excessive current load can lead to a disruptive voltage drop in the network. Consequently, for example, only motors with a rated performance of up to 2.2 kW may be connected directly to the 380 V network where the making current exceeds the rated current by more than seven times over. Higher powered motors require special measures for cutting back the considerable starting current.

Starting torque

Incorporating the equation

we derive for the rotor current I2

The rotor note only features the small ohmic resistor R2 but also the inductive resistor

XL2 = s • ω1•L2 = s • 2 • π • f1• L2

During the switching torque the resistance attains its maximum value as s = 1 and is therefore greater than the ohmic resistance.


Figure 54 - Indicator diagrams of the rotor circuit resistors

Legend as for Figure 52

The power factor cos φ2 therefore attains a minimal value and there is similarly only a low starting torque

Despite the considerable inrush current the asynchronous motor only evidences a minimal torque when starting from no-load position.

Measures to restrict the starting current

All drive operations presuppose a sound starting up, that is to say, a sufficiently high motor torque. Consequently measures must be undertaken to boost the starting torque. However, the network load which arises during start operations which may be evidenced in a voltage decline or through the inrush current, shall not exceed the prescribed values. It is therefore essential effectively to limit starting current. A simultaneous increase in starting torque is also often requested.

Starting current restriction becomes possible by

- decreasing U2.0: a lesser stator voltage is fed to the motor (U1 ~ U2.0) during starting operations. This leads to a starting procedure for which additional devices are required to connect the short circuit motor.

- increasing R2: increasing the rotor resistance R whilst starting requires a differently constructed rotor. The short-circuit rotor must be replaced by a differently arranged rotor featuring changeable ohmic resistance facilities.

In the equation M = C2• Φ • I2• cos φ2 all physical values have been incorporated which might influence the torque. Such an optimal solution denotes that such values are changed which permit the starting torque to increase without increasing the starting current. This demand is only met if cos φ2 is increased. The power factor is boosted by means of an ohmic resistor at the rotor circuit resistance. This in turn makes necessary a different rotor construction from the short-circuit rotor.

Additional facilities make it possible to decrease the high starting current of the squirrel cage rotor motor (Cp. 5.4.1). A reduction of the starting current whilst simultaneously increasing the starting torque is only possible where differently constructed rotors are used which evidence a greater ohmic resistance during starting operations.
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