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Chapter 10
Measuring Frequency and Duty Cycle
Frequency • Frequency Measurement •
Frequency Measurement Procedures •
Measuring Frequency with Clamp-On
Meter • Duty Cycle • Duty Cycle
Measurement Procedures
Chapter 10 — Measuring Frequency and Duty Cycle
Frequency is the number of cycles produced per
second measured in hertz (Hz).
Chapter 10 — Measuring Frequency and Duty Cycle
Frequency measurement is displayed after connecting
test leads to the circuit when using a DMM with a Hz
setting.
Chapter 10 — Measuring Frequency and Duty Cycle
Frequency measurement using a DMM with a Hz
button requires first measuring AC voltage.
Chapter 10 — Measuring Frequency and Duty Cycle
The frequency
measurement of an
electric motor drive
output current can be
used to calculate the
speed of a motor.
Chapter 10 — Measuring Frequency and Duty Cycle
A load that is turned ON and OFF several times per
second has a duty cycle.
Chapter 10 — Measuring Frequency and Duty Cycle
An automotive
electronic fuel injection
system uses pulsewidth modulation for
precise control of fuel
supplied to the engine.
Chapter 10 — Measuring Frequency and Duty Cycle
When measuring duty
cycle, the DMM
displays the amount of
time the input signal is
above or below a fixed
trigger level.
Chapter 10 — Measuring Frequency and Duty Cycle
On most DMMs, the Hz button is pressed twice for
Duty Cycle mode.
Chapter 10 — Measuring Frequency and Duty Cycle
Review
What are the two standard power frequencies used
throughout the world?
 60 Hz in the United States, Canada, and most of
the Caribbean.
 50 Hz in Europe, Asia, much of South America,
and the rest of the world.
Chapter 10 — Measuring Frequency and Duty Cycle
Review
What procedures are followed when measuring
frequency with a DMM that includes a Hz setting?
 Set the function switch to Hz.
 Plug the black test lead into the common jack.
 Plug the red test lead into the voltage jack.
 Connect the DMM to the test circuit.
 Read the frequency measurement displayed.
Chapter 10 — Measuring Frequency and Duty Cycle
Review
What happens to the speed of an AC motor if the
frequency is decreased or increased?
 If the frequency is decreased, the motor speed is
decreased.
 If the frequency is increased, the motor speed is
increased.
Chapter 10 — Measuring Frequency and Duty Cycle
Review
What is the duty cycle and how is it expressed?
 Duty cycle is an alternative frequency
measurement that is the ratio of time a load or
circuit is ON to the time a load or circuit is OFF.
 Duty cycle is expressed as a percentage.
Chapter 10 — Measuring Frequency and Duty Cycle
Review
What procedures are followed when measuring Duty
Cycle with a DMM?
 Set the DMM to measure duty cycle.
 Plug the black test lead into the common jack.
 Plug the red test lead into the voltage jack.
(continued on next slide…)
Chapter 10 — Measuring Frequency and Duty Cycle
Review
What procedures are followed when measuring Duty
Cycle with a DMM?
 Connect the DMM to the test circuit (match
polarity in DC circuits).
 Read the duty cycle measurement displayed. A
positive symbol (+) indicates ON time percent
voltage measurement. A negative symbol (–)
indicates OFF time percent voltage measurement.