When the stator and rotor is energized, the rotor magnetic field gets locked with the stator rotational magnetic field, and it rotates at the exact speed of stator field. Due to inertia, the synchronous motor does not immediately starts at the synchronous speed rotational magnetic field.
It acts as an induction motor during the startup. So, it suggests, the synchronous motors are not self-start , they need extra starting mechanism. It can be either separately excited or non-excited motor i. Therefore, the motor speed does not vary with the load. In order to vary the speed of a synchronous motor, the supply frequency must be varied. It is accomplished by using VFD variable frequency drive. The asynchronous motor name suggests that the rotor speed is asynchronous with the rotational speed of the stator magnetic field.
To be exact, the rotor of asynchronous motor rotates with a relatively lesser speed than the stator RMF. It is due to the presence of slip between its stator and rotor speed.
The rotor of the asynchronous motor is either a squirrel cage of wound type rotor. The squirrel cage rotor is constructed using heavy copper bars connected at the end using a conductive ring that electrically short them together. The wound type rotor is made of multiple windings on top of a steel laminated core. The stator rotational magnetic field causes an induced current in the rotor. This induced current flows inside the rotor generating its own magnetic field.
In doing so, the rotor rotates in the direction of the stator RMF. Since it operates on the principle induction, therefore, asynchronous motor is also known as Induction motor. The induction motor can never run on the synchronous speed instead it is always slower than the synchronous speed and it depends on the slip of the motor. The reason is because the induced current in the rotor is generated due to the difference between the stator and rotor field.
Therefore, there would be no magnetic flux to induce current in the rotor. The magnetic flux is necessary for induction motor which is why it has to run at slower speed than its synchronous speed. The squirrel cage rotor has a simpler design and allows induced current to flow through the copper bars. The point is to safely start the motor by reduce the huge starting current drawn by an induction motor.
It is generally done by connecting a variable resistor in series with the rotor windings through the means of slip rings. Synchronous motor is a motor that operates at synchronous speed, i. Asynchronous motor is an AC Induction motor. The rotor of the Asynchronous motor rotates at the speed less than the synchronous speed, i.
This is because the rotor is held electromagnetically in rigid synchronism with the rotating voltage in the stator magnetic locking if you increase the load, the motor will draw more current armature wingdings so synchronous speed is maintained.
It may slow a little while it adjusts but only momentarily. However under certain conditions i. Very helpful, thank you! Is it correct to say that slip ring and squirrel cage induction motors, belong only to asynchronous motor? Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Synchronous motor : Stator poles rotate at the synchronous speed Ns when fed with a three phase supply.
The rotor is fed with a DC supply. The rotor needs to be rotated at a speed near to the synchronous speed during starting. If done so, the rotor poles get magnetically coupled with the rotating stator poles, and thus the rotor starts rotating at the synchronous speed.
Synchronous motor always runs at a speed equal to its synchronous speed. The relative speed between stator's rotating magnetic field and the rotor will cause an induced current in the rotor conductors.
The rotor current gives rise to the rotor flux. According to Lenz's law , the direction of this induced current is such that it will tend to oppose the cause of its production, i.
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I have completed master in Electrical Power System. Thanks for the Excellent explanation. Please share more knowledge regarding the induction motor for an electrical Design point of view.
Thank you so much, Mohammed Imran. Definitely, I will try to publish and share more tutorials and useful technical kinds of stuff. This will help me to continue my services and pay the bills. Dipali Chaudhari. Sharing my knowledge on this blog makes me happy. And sometimes I delve in Python programming.
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