Limiting harmonic currents to 3% amplitude is based on keeping the total
harmonic voltage distortion on the ship power system within 5% and the amplitude
of any single harmonic within 3%. There are several undesirable effects of
excessive harmonic voltage distortion.

Upset of Poorly Designed Electronic Systems

Although it is relatively easy to design electronic equipment so that it will
tolerate harmonic voltage distortion greater than 5%, in practice, some systems
are substantially degraded by input power with harmonic distortion of 5% or
greater. In any event, it is necessary to have some limit to which to design.
Five percent total distortion (and 3% maximum for any harmonic) has been the
specified limit since 1963.

Increased Power Losses in Motors and Other Magnetic Devices

Studies for the Department of Energy have shown, both by calculation and
experiment, the losses in motors and other magnetic devices. To limit the
temperature rise in induction motors to +4ºC due to harmonic voltage distortion,
the total harmonic distortion must be less than 2.5% with a 5% limit on the
amplitude of single harmonics (Reference 5).

Production of Harmonic Torques

Harmonic torques can upset the accuracy of some measuring devices such as
induction watt-hour meters. With distortion (and depending on the mix of
harmonics), a wattmeter reads about 1% higher due to accelerating torques
induced by the harmonics (Reference 5).

Perhaps more important, the harmonic-induced torques can set up undesirable
vibration modes that affect the life of bearings, and in the case of Navy ships,
cause an undesired electrical-to-acoustical transformation.

Hull Currents

Harmonic voltages capacitively coupled to the hull through Electro Magnetic
Interference (EMI) filters and stray capacitance are the major driving function
for hull currents. Excessive hull currents can upset ground detectors,
degaussing and some sensitive electronic equipment. Hull currents are best
controlled by limiting the amplitude of harmonic voltages and limiting the
allowed capacitance-to-chassis of shipboard systems.