IEEE Power & Energy Magazine - July/August 2019 - 91

Motors don't produce horsepower, even though they are generally rated in horsepower. Motors produce torque. Rated torque
at rated speed is what gives rated
horsepower. Figure 5 shows speed
torque curves for four types of
dc motors.
Let's first look at torque produced by a shunt motor. As load
torque requirement increases, the
tendency is for the armature to
slow down. This reduction in speed,
in turn, reduces the counter-electromotive force (EMF) generated by
the moving armature conductors
and momentarily allows more
current to flow through the armature, strengthening the armature
field. This stronger field produces
more torque, which causes the
motor to increase speed until a
balance of motor torque and load
torque are reached. This balance
will occur at a speed somewhat
lower than that present when the
additional load torque occurred.
This moderate speed droop is a
characteristic of the shunt motor.
The cumulative compound
motor has a series and a shunt
field. An increasing load torque
causes an increase in armature
current similar to the shunt motor. This increased armature current also flows through the series
field, which strengthens the shunt
field. As a result, the armature's
counter EMF is developed at a
somewhat lower speed than for a
shunt motor. This motor is usually
viewed as a shunt motor with some
series characteristics. It is often
used in elevator service; the series
field is used for starting the elevator car and is frequently shorted
out once it's up to speed.
The series motor's field passes
the same current as that taken by
the armature. As with the other
motors, as load torque increases,
the speed drops, allowing more
current flow. In this case, however,
this current flows through both
the series field and the armature.

Professor of
Energy Systems Analysis
→ The Departments of Mechanical and Process Engineering
(www.mavt.ethz.ch) and Information Technology and Electrical
Engineering (www.ee.ethz.ch) at ETH Zurich and the Paul Scherrer
Institute (www.psi.ch) invite applications for a joint professorship in
Energy Systems Analysis. The new professor will act as Head of the
Energy Systems Analysis Laboratory at PSI, in a joint appointment
with ETH Zurich. The professorship will be associated with one of the
above-mentioned departments and will contribute to the teaching of
ETH Zurich. The main research activities are centred at PSI.
→ This position gives the successful candidate the opportunity to
closely engage in research efforts and collaborate with colleagues
in two of the worldwide leading research institutions in the field
of energy. This special combination and the funding committed to
this position provide excellent conditions to carry out high quality
fundamental but also applied research. Furthermore, ETH Zurich
and the Paul Scherrer Institute are well connected with local industry,
utilities and governmental bodies providing an environment which
not only supports fruitful and relevant research but also provides the
basis to directly impact policy making.
→ The new professor should have demonstrated an exceptional
potential to develop an innovative and collaborative research programme in Energy Systems Analysis. He or she must have an
excellent track record of research in the area of energy systems
analysis, including but not limited to technology, economics, policy,
society and risk. The emphasis is on the analysis of the complex
interdependencies in energy systems which leads to results that
can inform policy makers and stakeholders in their decision making.
Besides scientific leadership, the successful candidate has proven
management and administrative skills and has acquired and coordinated national and international projects with internal and external
participants. Generally, at ETH Zurich undergraduate level courses
are taught in German or English and graduate level courses are
taught in English.
→ Please apply online: www.facultyaffairs.ethz.ch
→ Applications should include a curriculum vitae, a list of publications
and current projects, a statement of future research and teaching interests, and a description of the three most important achievements.
The letter of application should be addressed to the President of ETH
Zurich, Prof. Dr. Joël Mesot. Submissions will be reviewed starting
on 31 July 2019, but applications are welcome until the position
is filled. ETH Zurich is an equal opportunity and family friendly
employer and is responsive to the needs of dual career couples. We
specifically encourage women to apply.

http://www.mavt.ethz.ch http://www.ee.ethz.ch http://www.psi.ch http://www.facultyaffairs.ethz.ch

IEEE Power & Energy Magazine - July/August 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2019