Definition of AM permissible energizing frequency with short-circuited rotor

As the substantial switching frequency occurs the substantial losses in the transient modes occur as well, which involve heating of asynchronous motor that limits amount of switching, breaking and reverse.

These problems are very important at the operation of metal-cutting equipment, press, auxiliary lightning, where frequent switching is the condition of technology process.

So the task is specified to choose minimal allowable duration of operation time, as the over temperature doesn`t ends to permissible.

It is very important task for the short-circuited AM because all heating is radiated into the case of motor in this case, as in case of DC motor or phased AM the greatest part of heating is radiated out of machine. So the short-circuited motor is going to be considered.

As the operation time consists of , operation time of steady modeoperation time at constant load and timeand duration of operation timeconsists of operation time and pause, when the AM is switched off.

Energy losses that are radiating in 1 cycle consist of energy of starting time , breaking time and energy of steady mode with constant load .

Energy losses that are radiating to the environment at the operation time are equal:

,

At the hour of breaking

,

At the starting and breaking time

,

where - factor of heat emission at the switched-off motor;

- factor of heat emission at the time of staring and breaking.

For the further operation time, when quasisteady heating mode occur, amount of radiating energy losses is equal to the amount of energy that radiates to the environment:

. (6.9)

Ifoperationtime, steady-stateandpausetimeare presented as amount of switching per hour it has a look:

(6.10)

Where period of switching on PSo is in relative units.

Substitudevaluesта from (6.10), the following equation is:

. (6.11)

For asynchronous motor the third term can be neglected as:

.

Then 6.11 can be rewritten:

. (6.12)

In the steady mode AM operates with rated power:

,

And 6.12 formula gets simple form:

. (6.13)

From 6.13 it is obvious that the value of switching depends on motor load, on its PSo, on its worse heating factorand energy losses at the transient processes.

Asthe, , aregettinglessthe allowable switching frequency AMis getting greater.

Increasing of allowable switching frequency can be attended by free airing of AM, that acts constantly during the cycle.

Essential allowable switching frequency increasing can be attended by decreasing of losses energy at the transient processes, e.g. using frequency control of AM comparably to the another methods of control.

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