IEEE Electrification Magazine - September 2017 - 84

TECHNOLOGY LEADERS

Technology Leaders
the equivalent dc voltage transformer, or dc-dc converter, remains
a critical technology for dc distribution systems. Watt for watt, the ac
voltage transformer is simpler in
design, more efficient, and more cost
effective than the dc-dc converter,
and this is especially the case for
high power. For high-voltage ratios,
the dc-dc converter still needs an ac
voltage transformer stage, which can
be implemented at high frequencies. Otherwise, the efficiency of the
dc-dc converter is low for high-voltage ratios. Due to these challenges,
the transformer remains a key factor in the decision to distribute dc or
ac power.

Generators
The ac voltage transformer also led
to the adoption of more ac genera-

Figure 2. A photo of Nikola Tesla.

End Rings

Conductors
Rotor
Figure 3. A schematic of an induction motor.

(continued from page 88)

tors rather than dc generators, and
induction motor (Figure 3) required
an ac generator or motor can be rehigh starting currents, and the
garded as a rotating transformer. In
available means of its control were
comparison, typical dc motors and
less efficient than today's controls.
generators need mechanical commuDouble squirrel-cage induction
tators that reverse current direction
motors were designed to solve this
through brushes and copper segissue, an inner high-resistance rotor
ments and require more maintewas used for low speed, and a lownance. In the case of the homopolar
resistance outer rotor was used for
generator, no commutation is rehigh speeds. For the USS Jupiter, which
quired, but extremely high current
was commissioned in 1913 as the
brushes are needed, which also leads
first U.S. Navy surface ship to use
to additional costs
electric drive, phaseand maintenance.
wound induction moThus, using an ac
Thus, using an ac
tors were designed for
generator remains the
generator remains
propulsion, where
only real efficient and
the only real efficient
resistors were conand cost-effective
nected in series with
cost-effective means
means of generatthe rotor windings
of generating
ing electricity today.
through slip rings to
electricity today.
Hence, every electric
achieve low speeds.
ship architecture,
Exactly 100 years later,
whether it is primarthe U.S. Navy DDG
ily dc or ac, must plan for ac-power
1000, the latest combatant ship, was
generation. Future cost-effective fuel
also using an induction motor for
cell technologies may change this
propulsion, albeit with much more
requirement, but, today, fuel cells
advanced controls. For large ocean
remain prohibitively costly.
liner ships, such as the SS Normandie
in the 1920s, propulsion proceedMotors
ed with the more efficient ac synThe ac induction motor, without a
chronous motors, 29 MW on each
need for mechanical commutation,
propeller, and there were four
became the work horse for industry,
motors used. Ship speed was conand Tesla (Figure 2) was a big protrolled by the frequency of the genponent for its use on ships, as he
erators, as no solid-state frequency
wrote in the New York Herald, on 25
changers were invented yet. HowFebruary 1917:
ever, dc propulsion motors were
The ideal simplicity of the
still in use until the 1960s in subinduction motor, its perfect
marines such as the Tullibee and
reversibility and other unique
the Lipscomb.
qualities render it eminently
An efficient use of the induction
suitable for ship propulsion,
motor (Figure 4) in ship propulsion
and ever since I brought my
had to wait for the invention of the
system of power transmission
cycloconverter and the variable freto the attention of the profesquency drive, which required the
sion through the American
silicon controlled rectifier and the
Institute of Electrical Engineers
insulated gate bipolar transistor
I have vigorously insisted on its
(IGBT), and the cycloconverter and
application for that purpose.
the variable frequency drive were
Although simple in design, espedeveloped in the late 1950s and late
cially its squirrel-cage rotor, the
1970s, respectively. In the meantime,

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2017

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2017