Frequency counts complete cycles.

A cycle is one complete repeat of a waveform. Frequency is the number of cycles per second, measured in hertz (Hz). A 50 Hz waveform completes 50 cycles each second.

The period is the time taken for one cycle. At 50 Hz, one cycle takes 1 ÷ 50 second, or 20 milliseconds.

More cycles in the same time.

50 Hz: two cycles in 40 ms

50 Hz: 2 cycles in 40 msA 50 hertz sine wave plotted from 0 to 40 milliseconds and minus 10 to plus 10 volts. It completes 2 full cycles in that interval. One cycle spans 20 milliseconds, marked by an orange bracket. Its 10 volt positive peak is marked by a dot and leader. The companion plot uses exactly the same axes, so the cycle spacing can be compared directly.Voltage (V)010203040+100−10Time (ms)10 V peakOne cycle = 20 ms

100 Hz: four cycles in 40 ms

100 Hz: 4 cycles in 40 msA 100 hertz sine wave plotted from 0 to 40 milliseconds and minus 10 to plus 10 volts. It completes 4 full cycles in that interval. One cycle spans 10 milliseconds, marked by an orange bracket. Its 10 volt positive peak is marked by a dot and leader. The companion plot uses exactly the same axes, so the cycle spacing can be compared directly.Voltage (V)010203040+100−10Time (ms)10 V peakOne cycle = 10 ms
  1. Keep the time window fixed. Both graphs cover 0–40 milliseconds (ms), with identical voltage scales. The second wave repeats twice as often.
  2. Read a complete repeat. The bracket runs from one upward zero crossing to the next upward zero crossing. That is a whole cycle, including its positive and negative halves.
  3. Measure amplitude from zero. Both waves have a peak amplitude of 10 V. Their peak-to-peak range is 20 V, from −10 V to +10 V.
Original plots of ideal sine waves. The 10 V peak is an example chosen to separate waveform size from timing; it is not a mains-supply value.

If both plots show the same time interval, the lower wave has twice the frequency. Their peaks are the same height, so their amplitude is unchanged.

Amplitude and phase describe different things.

Amplitude describes the size of a waveform. Phase describes where it is within its cycle relative to a reference. One complete cycle corresponds to 360 electrical degrees.

Two waves can have the same frequency and amplitude but reach their peaks at different times. That timing relationship is central to three-phase electricity.

A drive has more than one relevant frequency.

The fundamental output frequency is associated with the motor’s rotating magnetic field. The much faster switching frequency describes the electronic switching used to create the output.

Neither is simply the number of lift journeys per minute. The motor and mechanical arrangement determine how electrical operation becomes car movement.

A closer look

A non-sinusoidal repeating waveform can be represented as a fundamental component plus harmonics. Harmonics have frequencies at whole-number multiples of the fundamental; they describe waveform shape, not extra independent mains supplies.

Check your understanding

If frequency doubles from 25 Hz to 50 Hz, what happens to the period?

Reveal the explanation

It halves: from 40 milliseconds to 20 milliseconds. The amplitude does not have to change.

Further reading.

Optional reading to explore this lesson in more detail.

  • OpenStax: period and frequency

    Section 16.2 explains cycles, hertz and the relationship between frequency and period. Example 16.3 shows how to calculate one from the other.

  • OpenStax: reading an electrical sine wave

    For electrical examples, use Figures 20.14–20.16 in section 20.5 to connect amplitude, time and polarity. The subsequent RMS calculation adds more depth.