Operation of electric motors

The condition of electric motors, their control and protection mechanisms must ensure their reliable operation during start-up and in operating modes.

The deviation of the voltage from the nominal value indicated on the nameplate of the electric motor leads to a change in its torque, currents, heating temperatures of the windings and active steel, energy-saving indicators — power factor and efficiency.

The most common squirrel-cage asynchronous motor with a reduction in voltage, the torque decreases in proportion to the square of the voltage, the speed of rotation decreases and, accordingly, the performance of the mechanism decreases.

The reduction of the voltage below 95% of the nominal is characterized by a significant increase in currents and heating of the windings. An increase in the heating temperature has a detrimental effect on the insulation of the stator winding, causing its premature aging.The increase in voltage above 110% of the nominal is accompanied primarily by an increase in the heating of the active steel and a general increase in the heating of the stator winding with increasing current.

Voltage deviations in the range of 95 to 110% of nominal do not cause such serious changes in the parameters of the electric motor and are therefore acceptable. However, the optimum characteristics and characteristics of the electric motor are provided at voltages ranging from 100 to 105% of nominal. In order to maintain the optimal parameters of the electric motor, to create the best conditions for its start, it is necessary to maintain the bus voltage at the upper limit, i.e. 105% of par.

Electric motors and mechanisms driven by them must be marked with arrows showing the direction of rotation. In addition, electric motors and their starters must be marked with the name of the block to which they belong, taking into account the requirements of the PTE.

The functions of most mechanisms are performed with a certain direction of rotation. Therefore, the direction of rotation of the electric motor must be in accordance with the required direction of rotation of the mechanism. It should be borne in mind that a certain direction of rotation for a number of electric motors and mechanisms is mandatory for cooling conditions, lubrication of bearings and other design features.

The tightness of the cooling path (engine housing, air ducts, shock absorbers) should be checked periodically. The separate external cooling fan motors should automatically turn on and off when the main motors are turned on and off.

Blown electric motors installed in dusty rooms and rooms with high humidity must have clean cooling air. This requirement aims to protect electric motors from intensive contamination and wetting of their active parts. The stator winding insulation is primarily exposed to the hazardous effects of dirty and wet environments. Dust falling into the electric motor sharply worsens the conditions for its cooling, causes increased heatingaccelerating the aging of the insulation. Humidification reduces the dielectric strength and causes insulation breakdown. Therefore, supplying clean cooling air through the air ducts to blown electric motors will create normal conditions for their operation.

In the event of a power failure lasting up to 2.5 s, self-starting of the electric motors of critical mechanisms must be ensured.

When the electric motor of the critical mechanism is disconnected from the protective action and there is no spare electric motor, it is allowed to restart the electric motor after external inspection. The list of responsible mechanisms must be approved by the chief energy engineer of the enterprise.

The purpose of self-starting is to restore the normal operation of electric motors after a short power failure, which may be caused by a failure of the working power source, a short circuit in the external network, etc. After loss of power, a shutdown occurs, i.e. reducing the speed of rotation of electric motors. The ability to self-start depends on the duration of the power failure.The longer this interruption, the deeper the stopping of the electric motors and the lower the frequency of their rotation at the time of restoration of the power supply, the greater the total current of self-starting electric motors, which, increasing the drop in the voltage in the power line, reduces the initial voltage of the self-start, which in turn increases the time for the electric motors to run out and restore the operation of the mechanisms.

Electric motors that are in reserve for a long time should be inspected and tested together with mechanisms according to the approved schedule. The continuous operation of the main units of the equipment largely depends on the condition and readiness for operation of the backup electric motors. Engines in standby mode should be considered running.

Supervision of electric motor load, vibration, temperature of bearings and cooling air, maintenance of bearings (maintenance of oil level) and devices for supplying air and water for cooling the windings, as well as starting and stopping operations of motors from the duty staff of the workshop who maintain the mechanisms.

It is allowed to start a squirrel rotor electric motor 2 times in a row from a cold state and 1 time from a hot state.

The frequency of repair of electric motors is not regulated. This makes it possible to repair electric motors in the planned terms for repairing the main units of the equipment. The established frequency and types of repairs must ensure reliable operation of electric motors.

Preventive tests and measurements of electric motors must be carried out in accordance with the Electrical Testing Code.