Month: April 2017

Power Factor of Electric Motors

The power that an electric motor provides within its characteristics is called nominal power. The nominal power concept is closely linked to the winding temperature rise. The electric motor will be able to drive loads of powers much bigger than its nominal power, until reaching the maximum conjugate.

If this overload is too high, when the engine is too high, the normal heat will be exceeded by shortening the engine life and may burn quickly. We can understand that the power requested to the motor is defined by the characteristics of the load, regardless of the power of the motor.

An electric motor SUM76004-4 should be supplied to the machine in which it is coupled and it is recommended that we have a margin of slack, as small overloads may occur and are common, and depending on the service regime, the motor will be able to supply the power.

The correction of the power factor is the increase of this factor, carried out with the connection of a capacitive load, will generally be a capacitor or super excited synchronous motor, in parallel with the load.

Reactive power is the portion of the power that “no” performs work, and it is only transferred and stored in the passive elements, such as capacitors and inductors.

Nominal Speed and Rated Current

Speaking of electric motors, nominal speed is the value of the speed (rpm) that the motor was designed to work with its nominal load, ie is the motor speed running at nominal power, under voltage and rated frequencies.

The speed of rotation of an AC motor depends on the frequency of the mains and the slip.

The nominal current is the current that the motor absorbs from the mains when the nominal power operates, under voltage and nominal frequencies. In order to calculate the nominal current of three-phase motors we must consider the mechanical power of the motor, the efficiency and the power factor.

In the starting current of electric motors, a high current will circulate in the stator winding which decreases as the motor speed increases.

It is common to find motors with starting current equal to 7 or 8 times the nominal current. On the nameplate of the motor, we have the necessary data that indicates how many times the starting current is greater than the rated current.

In order to reverse the direction of rotation of three-phase motors, simply reverse two phases that feed the motor.

Magnetic Fields in a Heavy Load Induction Motor

The characteristic curve of electric torque against slip can be divided into approximately three regions.
The first is the region of low slip of the curve, in which it increases with increasing load and the mechanical speed of the rotor decreases with the load. The power factor of the rotor will be almost unitary even if the rotor current increases with slipping. The normal range of operation of the induction motor is included in this linear region of low slip.
The second region of the induction electric motor curve may be called the moderate slip region in which the rotor frequency is greater than before and the rotor reactance is of the same order as the rotor resistance. The maximum torque of the electric motor takes place at the point where, the increase in the rotor current is balanced exactly with the decrease of the power factor of the rotor.

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The third region of the induction motor curve is of high slip, in which the induced torque actually decreases with the increase of the load, because the increase in the rotor current is not remarkable due to the decrease in the power factor of the rotor.