ABSTRACT

Subject of master's thesis: Design-theoretical analysis of of thermal condition of explosion-protected asynchronous motors in short-circuit conditions regimes and after them

Lider of work: Bourkovsky A.N.

RUS | UKR | ENG |

Topicality of theme.
The explosion-protected asynchronous motor (AM) makes the base of the electric drive in chemical, oil-and-gas and coal industries, where it operates in different regimes. The especial regime of AM in dangerously explosive factories is a regime with blocked rotor, i.e. short-circuit conditions (SCC) regime. The question of determination of heating of explosion-protected AM windings in SCC regimes and of all the AM construction’s elements after SCC is important for resolving of motors projecting problems, as well as for their application dangerously explosive environments with heightened IIC explosive risk. That is why the problem of determination of thermal condition of explosion-protected AM in SCC regimes and after them is quite important.

Object of Work.
To elaborate methods of determination of thermal condition of explosion-protected AM of different capacities and different structures in SCC regimes and after them. To achieve these objects it is necessary to resolve the following problems: 1. to compose the design procedure of electromagnetic parameters and of stator and rotor current in SCC regimes for the motor with short-circuited rotor with different forms of mortises of rotor of capacity up to 1000 kilowatts. 2. to compose the design procedure of thermal design in SCC regimes and after them for the ribbed winded motors and for explosion-protected motors (6.10 kilowatts) with distributed tubular cooling.

Scientific Novelty.
The scientific novelty consists in the fact that this is the first case of resolving of the problem of the design-theoretical determination of the whole complex of questions as for the determination of the current of both stator and rotor windings (short-circuited single-celled, double-celled, deep-mortised rotors with bottle-shaped mortise) in the short-circuit time function, also as for the heating of the windings at SCC, taking into account current saturation and current displacement; and also as for the researches of the character of thermal changes of all the essential elements of the AM construction when it is switched off or during its operating in nominal regime.

Practical Significance of Work.
There will be elaborated some methods of numeric computation of thermal condition of explosion-protected AM in SCC regimes and after them, that can be used in all the interested bodies.

Work Process.
The electromagnetic parameters of the motor and of the density of losses in windings are changing at SCC under the nonlinear dependence on time. That is why it is appropriate to divide all the time of SCC into the range of intervals with the average values of the indicated magnitudes on every interval. It allows calculating the heating of the stator and rotor windings on each time interval with the average values of the equivalent circuit parameters. The calculation begins with the computation of the stator current magnitude and of the losses of energy, emerged in the stator and rotor windings. Then the calculation of the loss distribution throughout the rotor bar height and the equivalent circuit nodes is made. For the calculation of the distribution of current density the rotor bar is divided along its heightens into a range of layers with conditionally constant current density inside of each layer. Both inductive and active reactances of each layer are calculated, the active reactances being calculated taking into account the real (specified) temperature of this layer. The system of equations of potential drop on each layer is calculated, that allows knowing the current distribution throughout the layers (taking into account the current displacement). Then the losses are distributed throughout the thermal equivalent circuit nodes, and the thermal calculation on each time interval till the expiring of SCC time is made. The calculations of heating in SCC are foreseen to make for the motors with different forms of rotor mortises (simple – without current displacement; double-celled, bottle-shaped). After calculating of heating in SCC, the thermal calculating for determination of the thermal condition of all the motor essential elements in the regime after SCC is made (load run, free run, silent state). This will permit to calculate the rotor parameters on each time interval with the average values of the equivalent circuit parameters. The calculating begins with computing of the magnitude of the stator current and of the energy losses, emerged in the stator and rotor windings. Then the distribution of losses throughout the rotor bar and the equivalent circuit nodes is calculated. For the calculating of distribution of current density, the rotor bar is divided along its height into the range of layers with the conditionally constant current density inside of each layer. Both inductive and active reactances of each layer are calculated, the active reactances being calculated taking into account the real (specified) temperature of this layer. The system of equations of potential drop on each layer is calculated, that allows knowing the current distribution throughout the layers (taking into account the current displacement). Then the losses are distributed throughout the thermal equivalent circuit nodes, and the thermal calculation on each time interval till the expiring of SCC time is made. The calculations of heating in SCC are foreseen to make for the motors with different forms of rotor mortises (simple – without current displacement; double-celled, bottle-shaped). After calculating of heating in SCC, the thermal calculating for determination of the thermal condition of all the motor essential elements in the regime after SCC is made (load run, free run, silent state).

Literature Review.
In the regimes with the stalled rotor, in the stator and rotor windings of the Asynchronous Motor (AM), the starter currents are running, which leads to a very fast heating of the windings. At the same time the convective heat transfer is rather insignificant, that is why the thermal connection between stator and rotor can be neglected. In practical calculations the thermal connection between winding and core is often neglected as well, i.e. the adiabatic heating is being examined. The stator winding heating [5]:

(°К);                     (1)

at  ; ; ;
– the initial current density in the short circuit regime;
– the time of transient process.
The rotor winding heating (material – aluminium):

;                        (2)

at  ; ; ;
For double squirrel-cage, the literature has some more specified functions, taking into account the displacement [6].

Displacement efficiency:                                        ;                    (3)

where , – means the quantity of heat in the starting cage and the total value in the rotor accordingly. Only a part of the emitted heat in the starting cage ( ) produces the cage heating, and the rest of the heat transfers to the iron. So the heating must be calculated on the base of the losses:

;                        (4)

Heating of the starting cage:

 ;                         (5)

where , – means the specific heat and the weight of the starting cage.
The mentioned methods of calculation of the temperature of the stator winding have the following disadvantages: they do not take into account the heat-transfer from the copper to the isolation and packet, and they do not give a possibility to determine the temperature field in SC. The methods of the rotor heating calculation, stated by analogy, do not allow calculating the maximal temperature. Besides, the mentioned methods do not allow to take into account the influence of the rotor heating on the current displacement, which can change to some extent the scheme of the distribution of the heating losses and of the temperatures along the height of the bar. The accuracy of the calculation can be considerably raised by the applying of the equivalent circuit scheme method.

Basic Results and Conclusions.
There is the elaborated methods of calculating of thermal condition of all the essential elements of the explosion-protected AM construction in SCC regimes and after them, taking into account the current saturation and the current displacement in the rotor winding.

Literary Sources.
1. Вольдек А.И. Электрические машины. М-Л. “Энергия”, 1966: с.524-535.
2. Исаченко В.П., Осипова В.А., Сукомел А.С., Теплопередача. М. “Энергия”, 1969: с.7-40, с.92-102.
3. Борисенко А.И., Костиков О.Н., Яковлев А.И. Охлаждение промышленных электрических машин. М. “Энергоатомиздат”, 1983: с. 5-8, с. 10-14, с. 41-65, с.75-76.
4. Борисенко А.И., Данько В.Г., Яковлев А.И. Аэродинамика и теплопередача в электрических машинах. М. “Энергия”, 1974: с.7-12, с. 24-26, с. 75-91, с. 504-511.
5. Сергеев П.С., Виноградов А.В., Горяинов Ф.А. Проектирование электрических машин. М. «Энергия», 1969 г.
6. Е. Вадеман, В. Келленбергер Конструкции электрических машин. Л. «Энегрия», 1972 г.