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Speed regulation of AM by resistors in rotor circuit




Coordinates regulation of asynchronous motors

 

Above considered characteristics and properties of DCM show that they allow to obtain deep and gradual coordinates regulation. This is the one of the main advantages of DC drives.

However DCM have the range of disadvantages comparatively to AM:

- presence of DC source for the motor powering, because in usual conditions there is AC source of industrial frequency;

- mass-dimensional parameters of DCM are worse than AM, (almost two times);

- the cost of DCM is significantly greater than AM (3 – 5 times);

- construction of DCM is significantly complicated, and the operation reliability is worse.

Thus, AM predominate DCM by exploitation and economical parameters, but by technical characteristics and properties they yield to DCM, however in recent times situation is essentially changing, see the preamble to paragraph 3.10.

The following methods of AM speed regulation obtain the mostly wide application:

- with the help of resistors in rotor and stator circuits;

- changing of pole pairs number;

- changing of stator supply voltage;

- frequency regulation;

- special (multistage, pulse and so on) regulation.

 

It is used for AM with the phase rotor, because such motor has contact rings for the resistor connection. This is the one of widely used methods for speed regulation, because it is simple to realize. Such regulation is called rheostatic (figure 4.24).

 

Figure 4.24 – Scheme with the resistor in rotor circuit.

Characteristics and parameters of regulation

Regulation is realized down from the main, one-regional. The better use of motor will be at speed regulation with constant loading moment, which is equal to nominal, as for DCM IE at connection of resistors into the armature circuit. Smoothness of regulation is defined by the construction of adjustable rheostat (smooth or gradual). The range of regulation depends on load (decreases at load decreasing), and due to significant decreasing of rigidity at increasing of this range is insignificant and is . Characteristics are represented on figure 4.25.

 

 

Figure 4.25 – Mechanical characteristics.

 

At speed regulation with this method significant energy losses appear, because resistor is connected to the circuit of main current – significant rotor current. Therefore it is rational to use it at insignificant range of speed regulation and short term of operation at low speeds. The most rational regulation (from the view of energy losses) is at fan mechanical characteristics of execution mechanism (here with speed decreasing an applied power decreases).

Features of mechanical characteristic:

- speed of ideal idling rate w0 doesn’t change with changing of value ;

- maximal moment of motor at regulation is also practically doesn’t change;

- critical speed (critical sliding) depends on value , critical speed decreases (sliding increases) at increasing of .

The last thesis allows to find by error method such value of additional resistance, at which for the first moment of starting starting moment would be maximal, is the resistance , i.e. to establish the best starting conditions (characteristic 4 on figure 4.25).

Considered method is also used to regulate such motor coordinates as starting current and starting moment.

 

 




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