Foil field coil:

1
Enhanced Lundell Alternator
Leandro M. Lorilla, Prof. David J. Perreault, Dr.
Thomas A. Keim, Prof. Jeffrey H.
Lang Massachusetts Institute of Technology,
Laboratory for Electromagnetic and Electronic
Systems

• Foil field coil
• Motivation
• Higher copper packing factor
• Probably no increases in winding cost
• Improved heat transfer
• Allows for more field ampere turns while
  maintaining the same copper losses that can
  result in any of the following
• Improvement in efficiency
• Decrease in size and cost while maintaining
  efficiency
• Improvement in output power if all other
  constraints are met (such as saturation)
• The improved packing factor is achieved below a
  certain
• number of field turns.

Abstract The focus of our research is on
improving the automobile alternator. A load dump
occurs when there is a sudden disconnection to
one of the loads such as a battery, resulting in
a sudden voltage spike at the alternators
output. This overvoltage could be damaging to
other electrical components including
semiconductors. One way of suppressing the
overvoltage is by fast field de-excitation, which
involves the quick removal of field current from
the field winding. The effects of eddy currents,
which tend to limit the rate of field
de-excitation, are studied. We investigate field
de-excitation by the application of a reverse
voltage across the field winding. Allowing for a
reverse field current to flow helps limit the
overvoltage. Another area of research involves
replacing the round wire field winding with
copper foil and evaluating the advantages of
doing so. A foil field winding could lead to
improvements either in efficiency or power.
Current work involves developing circuitry that
achieves higher field currents and fast field
de-excitation while using copper foil for the
field winding.
Field de-excitation by voltage and current
reversal Following the load dump, the voltage
across the field winding is reversed to some
multiple of the original voltage across it.
The field current decay corresponding
to the applied reverse voltage is determined. The
higher the voltage, the faster the
decay. A circuit that reverses the
voltage across the field winding is built and
tested. The overvoltage lasts for around
80ms. A model of the field circuit is
used to predict the required reverse voltage and
current for the overvoltage to last as long as
that achieved by de-excitation through a
capacitor. The simulation shown below corresponds
to a reversal by a factor of 4 that was found to
be necessary to achieve a 50ms overvoltage. The
green curve is the field current while the red
curve is the normalized open circuit voltage.
Eddy current limitations De-excitation through a
capacitor is used to rapidly remove current from
the field winding (inductor) following an opening
of the switch. The field current is removed after
0.24 ms, and the open-circuit voltage (phase to
neutral) decays with two time constants. The
resulting overvoltage lasts for
50ms. The theoretical open circuit
voltage envelope (line-to-line) for a 42V system
is predicted and shown below.

Acknowledgements MIT/Industry Consortium on
Advanced Automotive Electrical/Electronic
Components and Systems