4. Equations
The asynchronous motor transforms the electrical energy supplied by the single-phase or three-phase alternating current into mechanical energy. It is characterized by input quantities that are electrical and by output quantities that are mechanical. |  |
Definition : The electrical power absorbed by a three-phase asynchronous motor is
with:
: Power in W (Watt);
: Voltage in V (Volts);
: Current in A (Amperes);
: cosine of the phase shift angle between current and voltage
Note: The current and voltage measurements cannot give the power,
can vary between 0.1 and 0.2 empty until 0.9 at full load.
Definition : The mechanical power is that obtained on the motor shaft:
With
: Power in W (watt);
: Engine torque Newton-meter (Nm);
: angular velocity in radian per second (rad / s);
: Rotational speed in revolutions per second (rps).
Definition : Rated power:
This is the mechanical power available on the motor shaft at its rated speed.
Definition : Rated speed:
The nominal speed is the speed at nominal power. We distinguish the speed of synchronize (speed of the rotating field):
With
: synchronism speed;
: frequency of the supply voltage;
: number of pole pairs of the machine.
Definition : Rotation Speed:
The rotation speed of the motor is lower than the speed of synchronism. The difference is characterized by the slip
:
with
-
: Slip in %
-
: Synchronious speed in rps
-
: Real speed in rps
Definition : Nominal current:
This is the value of the current absorbed by the machine at rated power and rated voltage
Definition : Power factor and efficiency:
The efficiency and the power factor change according to the load. They are given for the nominal power of the machine.
With:
: rated useful power;
: voltage between nominal phase;
: rated current;
: power factor;
: efficiency.
Definition : Current ratio:
This is the ratio between the current absorbed at start-up and the rated current. In case of direct start, it varies from 6 to 8. |  |
Definition : Torque ratio:
This is the ratio between the starting torque and the nominal torque. It is between 1.5 and 3 depending on the construction of the machine. |  |
