Answer MET Question 25
Question: What are the factors which determine the synchronous speed of a motor?
Answer:
The synchronous speed is the speed of the revolution of the magnetic
field in the stator winding of the motor. It is the speed at which the
electromotive force is produced by the alternating machine. The
Synchronous Speed is given by the relation shown below.
$\displaystyle \small \mathrm{N_s=\frac{120f}{P}}$
P be the total number of field poles
N is the speed of the field in revolution per minute (r.p.m)
f is the frequency of the generated voltage in hertz.
thus, The synchronous speed of an AC motor is determined by the frequency of the source and the number of poles.
Actual speed of the induction motor will be less than the synchronous speed. The reason for this is if the armature turns exactly at synchronous speed, then the magnetic field of the stator windings no longer cuts the rotor windings. Under this condition, no current would be induced to the rotor windings. With no current flow in the armature, no magnetic field will be developed.
When this happens, there is no longer any torque on the rotor, and its speed drops below the synchronous speed. When the speed is reduced, magnetic flux lines produced by the stator windings again cut the rotor's windings, inducing a current and the resulting magnetic field. Torque is again established.
Actual RPM is approximately 3 to 5% less than synchronous speed.
Slip is the difference between the synchronous speed of a motor and its actual speed. Mathematically it is determined by
Slip = Synchronous Speed − Actual Speed
Synchronous motors are constant speed motors. They run at the synchronous speed of the supply. They are generally used for constant speed operation under no load conditions such as to improve the power factor.
$\displaystyle \small \mathrm{N_s=\frac{120f}{P}}$
P be the total number of field poles
N is the speed of the field in revolution per minute (r.p.m)
f is the frequency of the generated voltage in hertz.
thus, The synchronous speed of an AC motor is determined by the frequency of the source and the number of poles.
Actual speed of the induction motor will be less than the synchronous speed. The reason for this is if the armature turns exactly at synchronous speed, then the magnetic field of the stator windings no longer cuts the rotor windings. Under this condition, no current would be induced to the rotor windings. With no current flow in the armature, no magnetic field will be developed.
When this happens, there is no longer any torque on the rotor, and its speed drops below the synchronous speed. When the speed is reduced, magnetic flux lines produced by the stator windings again cut the rotor's windings, inducing a current and the resulting magnetic field. Torque is again established.
Actual RPM is approximately 3 to 5% less than synchronous speed.
Slip is the difference between the synchronous speed of a motor and its actual speed. Mathematically it is determined by
Slip = Synchronous Speed − Actual Speed
Synchronous motors are constant speed motors. They run at the synchronous speed of the supply. They are generally used for constant speed operation under no load conditions such as to improve the power factor.
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