Why Motor never runs exactly at Synchronous Speed?
A motor will never run exactly at synchronous speed (unless in the setting of a synchronous motor under perfect conditions) due to fundamental physical & electrical phenomena.
1). Induction Motors (Common AC Motors)
Induction motors require the rotor to rotate slightly slower than synchronous speed to induce current in rotor windings. This difference is known as slip.
The formula for synchronous speed is:
N s = 120 x f/P (in RPM)
Where,
Ns - Synchronous Speed
f - Supply Frequency
P- Number Of Poles.
Slip:
S = Ns - Nr/100%
Where
Nr - Rotor Speed
s - Slip Percentage
Reason slip is required:
The rotor current is generated by the relative motion of the rotating magnetic field & the rotor.
If the rotor rotated at synchronous speed, there could be no relative motion, no induced EMF, no current & hence no torque.
Typical slip values:
Small motors: 0.5–5%.
Large motors: 1–2%
2). Synchronous Motors
When appropriately excited, a synchronous motor’s rotor is magnetically bound to the rotating magnetic field, allowing it to run at synchronous speeds.
Synchronous motors, on the other hand, typically require an auxiliary motor or a specific starting mechanism at startup since they cannot generate starting torque at synchronous speeds.
3). Other Practical Factors
Even for synchronous motors, accurate synchronous speed is nearly unattainable in reality because of:
Load Changes: Mechanical load fluctuations can create minor speed differences.
Voltage Fluctuations: changes in supply voltage effect torque.
Friction & Windage Losses: Friction & windage losses occur when resistive forces cause the rotor to slow slightly.
Power System Frequency Variations: Even minor changes in grid frequency can have an impact on speed.
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