---
title: "Frequency standardization of AC systems"
subject: "High Voltage Engineering"
chapter: 1
key: "semester_8/high_voltage_engineering:frequency-standardization-compromise"
source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/frequency-standardization-compromise
site: BCT Engineering
---

# Frequency standardization of AC systems

High Voltage Engineering · Chapter 1: Evolution of Power System

- Asked in: 2071 Bhadra, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/frequency-standardization-compromise
- Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-1

## Question

"The best frequency is a compromise between contradictory requirements". Justify the statement related with frequency standardization of AC system.

## Short answer

Low frequency reduces line reactance and charging current, while high frequency eliminates lamp flicker and reduces machine size. The chosen 50/60 Hz is a compromise that minimizes flicker and keeps transmission losses acceptable.

## Answer

The supply frequency (50 Hz or 60 Hz) was chosen not because it is best for every purpose, but because it is an acceptable middle value between requirements that pull in opposite directions. Low frequency is good for some equipment and high frequency for others.

#### Requirements favouring a LOW frequency
- **Transmission:** line reactance $X = 2\pi fL$ is lower, so voltage drop is smaller, power transfer $P = \frac{V^2}{X}\sin\delta$ and stability limits are higher.
- **Charging current** and Ferranti effect ($I_c = 2\pi fCV$) are smaller in long lines and cables.
- **Skin effect** and eddy currents are lower, so conductor resistance is lower.
- **Commutator machines** (traction motors, rotary converters) commutate better at low frequency; that is why railways used 16⅔ Hz and 25 Hz.
- **Low-speed hydro generators** need fewer poles at a low frequency.

#### Requirements favouring a HIGH frequency
- **Lighting flicker:** incandescent and arc lamps flicker visibly below about 40–50 Hz. This was a major reason to reject 25 Hz.
- **Size of transformers and machines:** $E = 4.44fN\phi_m$, so for the same voltage a higher $f$ needs less flux and iron; transformers and motors become smaller and cheaper.
- **Speed of machines:** $N_s = \frac{120f}{P}$; higher frequency gives higher speed, suitable for steam turbines and small motors.
- **Aircraft and ships** use 400 Hz to save weight.

#### Limits on very high frequency
Iron losses (hysteresis $\propto f$, eddy $\propto f^2$), higher line reactance, larger inductive voltage drop and interference with telephone lines.

| Factor | Better at |
|---|---|
| Line reactance and stability | Low $f$ |
| Charging current | Low $f$ |
| Commutator motors | Low $f$ |
| Lamp flicker | High $f$ |
| Transformer/machine size | High $f$ |
| Iron loss, interference | Low $f$ |

#### Result
Early systems used many frequencies (25, 40, 50, 60, 133 Hz). Around 1900–1920 the industry settled on **50 Hz** (Europe, Asia, Nepal) and **60 Hz** (America), which keep flicker invisible, give reasonably small machines and transformers, and still allow acceptable reactance and losses on transmission lines. So the standard frequency is truly a compromise between contradictory requirements.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does a LOW supply frequency favour long transmission lines?
   A. Line reactance 2*pi*f*L is smaller, so voltage drop is smaller and power transfer limits are higher
   B. Transformer iron becomes smaller
   C. Lamps flicker less
   D. Commutator machines need higher speed
   Answer: A. X = 2*pi*f*L falls with f, giving lower voltage drop and higher P = V^2 sin(delta)/X. (Key point: Why low frequency helps transmission)

2. Which requirement pushes the standard frequency UP and was a main reason for rejecting 25 Hz?
   A. Ferranti effect
   B. Visible flicker of incandescent and arc lamps below about 40-50 Hz
   C. Charging current of cables
   D. Skin effect in conductors
   Answer: B. Lamps flicker visibly at low frequency, so 25 Hz was rejected for lighting. (Key point: Why high frequency is wanted)

3. From E = 4.44 f N phi_m, what happens to transformer size when f is increased at the same voltage?
   A. Size increases because flux rises
   B. Size is unchanged
   C. Less flux and iron are needed, so it becomes smaller and cheaper
   D. Winding turns must be doubled
   Answer: C. For fixed E, phi_m is inversely proportional to f, so core and iron reduce. (Key point: Transformer size and frequency)

4. Why not choose a very high frequency to get the smallest machines?
   A. Hysteresis loss would fall to zero
   B. Iron losses (hysteresis ~f, eddy ~f^2), line reactance and telephone interference rise
   C. Machines would run slower
   D. Flicker would increase
   Answer: B. Higher f raises iron loss, inductive drop and interference, so a middle value is chosen. (Key point: Limit on very high frequency)

## Related in High Voltage Engineering

- [Advantages and disadvantages of HVDC over AC](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-advantages-disadvantages) — High Voltage Engineering, chapter 1, asked 3×
- [Resurgence of HVDC; why AC is inevitable](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-resurgence-ac-inevitable) — High Voltage Engineering, chapter 1
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