The output frequency and voltage of an alternator are key parameters that determine the quality of electrical power.
Table of Contents
Output Frequency
Definition
Frequency is the number of cycles per second, measured in Hertz (Hz).
Frequency Equation
\[
f = \frac{P N}{120}
\]
- \(f\): Frequency (Hz)
- \(P\): Number of poles
- \(N\): Speed (rpm)
Key Points
- Depends on speed and poles
- Constant speed → constant frequency
- India: 50 Hz
- USA: 60 Hz
Example
\[
f = \frac{4 \times 1500}{120} = 50 \, \text{Hz}
\]
Output Voltage
Definition
Output voltage is the RMS value of the induced voltage in stator windings.
EMF Equation
\[
E = 4.44 f \phi T k_w
\]
- \(E\): Induced EMF
- \(f\): Frequency
- \(\phi\): Flux per pole
- \(T\): Turns per phase
- \(k_w\): Winding factor
Factors Affecting Voltage
Field Excitation
Increase in excitation increases flux and voltage
Speed
Voltage proportional to frequency
Load
Voltage drops due to internal impedance
Armature Reaction
Affects flux distribution
Voltage Regulation
\[
\text{Voltage Regulation} = \frac{E_{no-load} – E_{full-load}}{E_{full-load}} \times 100
\]
Frequency vs Voltage
- Frequency depends on speed and poles
- Voltage depends on flux, speed, and winding
Control Methods
Frequency Control
- Controlled by prime mover speed
- Using governors
Voltage Control
- Controlled by field excitation
- Using AVR
Important Formulas
\[
f = \frac{P N}{120}
\]
\[
E = 4.44 f \phi T k_w
\]
Conclusion
Frequency is controlled by speed, while voltage is controlled by excitation. Proper control ensures stable and efficient power system operation.