Cross-Field Theory of Single-Phase Induction Motors — The Engineering Knowledge

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Hi, reader welcomes to onboard. In this post, we will have a detailed look at the Cross-Field Theory of Single-Phase Induction Motors. There are different types of machines that are working in our power system. Some are dc and some are ac in dc motors commonly used motors are dc shunt motor, dc series motor, dc wound motors. These called dc motor since they operate on dc power.

There is another type of motors that operate on ac power called ac motors such as synchronous motors and inductions motor. Induction motors are further divided in three-phase and single-phase induction motors. In this tutorial, we will cover the cross-field theory of single phase motor in the details. So let’s get started.

Cross-Field Theory of Single-Phase Induction Motors

• The double field theory of single induction motor explains that static pulsating magnetic field can be divided in the 2 rotational fields every one has the same magnitude but rotation direction of these is reverse to each other.
• These two fields are entertained by the induction motor and the torque produced by the motor is due to these two fields.
• The below diagram shows that the static pulsating field can be divided in 2 equal and reverse rotatory field .
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• The single-phase induction motor works for every one of these 2 fields existing in the motor therefore net torque in the motor is got by the difference of 2 torque-speed curves.
• This resultant torque can seen here.
• Note that there is not resultant torque for 0 speed there is no starting torque of this motor.
• The above-shown torque sped characterize curve is not good to explain the torque of a single-phase motor.
• It was created with the superposition of 2 three-phase curves and ignoring the factor that both fields are exist at same time in single phase motor.
• If the power supply connected with the 3 phase motor while it is move-in backward its rotor current will have a large value.
• Though the rotor frequency is very large causes the reactance of the rotor to large than resistance.
• As value of rotor reactance is large therefore rotor current will lags behind the voltage of motor to ninety degrees and generate a field that is almost one-eighty degrees from the stator field.
• The torque induced in the motor is directly proportional to the sine of angle among the 2 fields and the sine of an angle close to one-eighty degrees has less value.
• The torque of motor will be less except that a very large rotor current offsets the factor of field angles.
• While in single phase motor forward and reverse fields exist and both generated through a similar current.
• The forward and reverse fields in the motor every provides an element to the net voltage in the stator and in case both are in series combination to each other.
• Since both fields exist the forward-moving fields will restrict the current passing in the stator that generating the fields in forward and reverse directions.
• As the current providing the reverse stator fields is restricted to the smaller value and as the reverse rotor field is at a very high angle according to reverse stator field the torque due to reverse fields is very less close to synchronous speed.
• The better than previous torque-speed character curve can see here for single-phase induction motor.
• For average total torque shown in the above figure. their torque pulses are two times the stationary frequency.
• These torque pulses are results when the forward and reverse fields meet one another in each cycle for two times.
• Though these torque pulses generate no average torque they cause the increment in vibration and they create single phase induction motor noise as compare to 3 phase motor having similar ratings.
• To remove these pulses no method exists because instantaneous power always has pulse form in single-phase circuitry.
• The motor manufacturers should permit fo inherent vibration in the design of the induction motor.

Bs (t) = (Bmax cos wt)J

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That is a detailed post about the Cross-Field Theory of Single-Phase Induction Motors. If you have any further query ask in the comments. See you in the next post.

Author: Henry

http://www.theengineeringknowledge.com

I am a professional engineer and graduate from a reputed engineering university also have experience of working as an engineer in different famous industries. I am also a technical content writer my hobby is to explore new things and share with the world. Through this platform, I am also sharing my professional and technical knowledge to engineering students.

Originally published at https://www.theengineeringknowledge.com on January 12, 2021.